647
Discovering the Virus Responsible
for Hepatitis C
You may not be aware that our country is in the midst of
an epidemic of this potentially fatal liver disease. Almost
4 million Americans are infected with the hepatitis C
virus, most of them without knowing it. Some 9000 peo-
ple will die this year in the United States from liver can-
cer and chronic liver failure brought on by the virus, and
the number is expected to triple in the next decade. In
the first years of the new century, the number of annual
U.S. deaths caused by hepatitis C is predicted to overtake
deaths caused by AIDS.
Hepatitis is inflammation of the liver. Researchers in the
1940s identified two distinct forms. One, called infectious
hepatitis or hepatitis A, is transmitted by contact with feces
from infected individuals. A second form of hepatitis, called
serum hepatitis or hepatitis B, is passed only through the
blood. Hepatitis B virus was isolated in the mid-1960s, he-
patitis A virus a decade later. This led in the 1970s to the
development of tests for the two viruses. Disturbingly, a
substantial proportion of hepatitis cases did not appear to
be caused by either of these two viruses.
Clearly another virus was at work. At first, investigators
thought it wouldn't be long before it was isolated. How-
ever, it was not until 1990 that researchers succeeded in
isolating the virus responsible for these "non-A, non-B"
cases, a virus that we now call hepatitis C virus (HCV).
HCV was difficult to isolate because it cannot be grown
reliably in a laboratory culture of cells. Making the prob-
lem even more difficult, HCV is a strictly primate virus. It
infects only humans and our close relatives—chimpanzees
and tamarins. Because it is very expensive to maintain
these animals in research laboratories, only small numbers
of animals can be employed in any one study. Thus, the
virus could not be isolated by the traditional means of pu-
rification from extracts of infected cells. What finally suc-
ceeded, after 15 years of failed attempts at isolation, was
molecular technology. HCV was the first virus isolated en-
tirely by cloning the infectious nucleic acid.
The successful experiment was carried out by Michael
Houghton and fellow researchers at Chiron, a California
biotechnology company. What they did was shotgun clone
the DNA of infected cells, and then screen for HCV.
The genetic material of HCV, like that of many other
viruses, is RNA. So the first step was to convert HCV RNA
to DNA, so that it could be cloned. There was no need to
attempt to achieve entire faithful copies, a touchy and diffi-
cult task, because they did not wish to replicate HCV, only
identify it. So the researchers took the far easier route of
copying the virus RNA as a series of segments, each carry-
ing some part of the virus genome.
Next, they inserted these DNA copies of HCV genes
into a bacteriophage, and allowed the bacteriophage to in-
fect Escherichia coli bacteria. In such a "shotgun" experi-
ment, millions of bacterial cells are infected with bacterio-
phages. The researchers grew individual infected cells to
form discrete colonies on plates of solid culture media. The
colonies together constituted a "clone library." The prob-
lem then is to screen the library for colonies that had suc-
cessfully received HCV.
To understand how they did this, focus on the quarry, a
cell infected with an HCV gene. Once inside a bacterial
cell, an HCV gene fragment becomes just so much more
DNA, not particularly different from all the rest. The cel-
lular machinery of the bacteria reads it just like bacterial
genes, manufacturing the virus protein that the inserted
HCV gene encodes. The secret is to look for cells with
HCV proteins.
How to identify an HCV protein from among a back-
ground of thousands of bacterial proteins? Houghton and
his colleagues tested each colony for its ability to cause a
visible immune reaction with serum isolated from HCV-
infected chimpanzees.
The test is a very simple and powerful one, because its
success does not depend on knowing the identity of the
genes you seek. The serum of HCV-infected animals con-
tains antibodies directed against a broad range of HCV
proteins encountered while combating the animal's HCV
infection. The serum can thus be used as a probe for the
presence of HCV proteins in other cells.
Out of a million bacterial clones tested, just one was
found that reacted with the chimp HCV serum, but not
with serum from the same chimp before infection.
Part
IX
Viruses and Simple
Organisms
Electron
micrograph
of hepatitis
C virus.
648 Part IX Viruses and Simple Organisms
Using this clone as a toehold, the researchers were able
to go back and fish out the rest of the virus genome from
infected cells. From the virus genome, it was a straightfor-
ward matter to develop a diagnostic antibody test for the
presence of the HCV virus.
Using the diagnostic test, researchers found hepatitis C
to be far more common than had been supposed. This is a
problem of major proportions, because hepatitis C virus is
unlike hepatitis A or B in a very important respect: it causes
chronic disease. Most viruses cause a brief, intense infec-
tion and then are done. Hepatitis A, for example, typically
lasts a few weeks. Ninety percent of people with hepatitis C
have it for years, many of them for decades.
All during these long years of infection, damage is being
done to the liver. Cells of the immune system called cyto-
toxic T cells recognize hepatitis C virus proteins on the
surface of liver cells, and kill the infected cells. Over the
years, many dead liver cells accumulate, and in response
the cells around them begin to secrete collagen and other
proteins to cover the mess. This eventually produces pro-
tein fibers interlacing the liver, fibers which disrupt the
flow of materials through the liver's many internal pas-
sages. Imagine dropping bricks and rubble on a highway—
it gets more and more difficult for traffic to move as the
rubble accumulates.
If this fibrosis progresses far enough, it results in com-
plete blockage, cirrhosis, a serious condition which may in-
duce fatal liver failure, and which often induces primary
liver cancer. About 20% of patients develop cirrhosis
within 20 years of infection.
Luckily, hepatitis C is a very difficult virus to transmit.
Direct blood contact is the only known path of direct trans-
mission. Sexual transmission does not seem likely, although
the possibility is still being investigated. Married partners
of infected individuals rarely get the virus, and its incidence
among promiscuous gay men is no higher than among the
population at large.
Why not move vigorously to produce a vaccine directed
against hepatitis C? This turns out to be particularly difficult
for this virus, because antibodies directed against it appear to
be largely ineffective. Those few individuals who do succeed
in clearing the virus from their bodies gain no immunity to
subsequent infection. They produce antibodies directed
against the virus, but the antibodies don't protect them. It ap-
pears that hepatitis C virus evades our antibody defenses by
high mutation rates, just as the AIDS virus does. By the time
antibodies are being produced against one version of the
virus, some of the viruses have already mutated to a different
form that the antibody does not recognize. Like chasing a
burglar who is constantly changing his disguise, the antibod-
ies never learn to recognize the newest version of the virus.
To date, attempts to develop a drug to combat hepatitis
C virus focus on the virus itself. This virus carries just one
gene, a very big one. When it infects liver cells, this gene
is translated into a single immense "polyprotein." Enzymes
then cut the polyprotein into 10 functional pieces. Each
piece plays a key role in building new viruses in infected
liver cells. Some of these proteins form parts of the virus
body, others are enzymes needed to replicate the virus
gene. As you might expect, each of these 10 proteins is
being investigated as a potential target for a drug to fight
the virus, although no success is reported as yet.
Other attempts to fight hepatitis C focus on the part of
our immune system that attacks infected liver cells. Unlike
the ineffective antibody defense, our bodies' cytotoxic T cells
clearly are able to detect and attack cells carrying hepatitis C
proteins. A vaccine that stimulates these cytotoxic T cells
might eliminate all infected cells at the start of an infection,
stopping the disease in its tracks before it got started. A seri-
ous effort is being made to develop such a vaccine.
It doesn't look like an effective remedy is going to be
available anytime soon. In the meantime, as the death rates
from hepatitis C exceed those for AIDS in the next few
years, we can hope research will further intensify.
How the hepatitis C virus was discovered. Michael Houghton and fellow researchers identified the virus responsible for hepatitis C by
making DNA copies of RNA from the cells of infected chimpanzees. They then cloned this DNA, using bacteriophages to carry it into
bacterial cells. Colonies of the bacteria were then tested with serum from infected chimps. Any colony that produced an immune reaction
would have to contain the virus.
649
32
How We Classify
Organisms
Concept Outline
32.1 Biologists name organisms in a systematic way.
The Classification of Organisms. Biologists name
organisms using a binomial system.
Species Names. Every kind of organism is assigned a
unique name.
The Taxonomic Hierarchy. The higher groups into
which an organism is placed reveal a great deal about the
organism.
What Is a Species? Species are groups of similar
organisms that tend not to interbreed with individuals of
other groups.
32.2 Scientists construct phylogenies to understand
the evolutionary relationships among organisms.
Evolutionary Classifications. Traditional and cladistic
interpretations of evolution differ in the emphasis they
place on particular traits.
32.3 All living organisms are grouped into one of a few
major categories.
The Kingdoms of Life. Living organisms are grouped
into three great groups called domains, and within domains
into kingdoms.
Domain Archaea (Archaebacteria). The oldest domain
consists of primitive bacteria that often live in extreme
environments.
Domain Bacteria (Eubacteria). Too small to see with
the unaided eye, eubacteria are more numerous than any
other organism.
Domain Eukarya (Eukaryotes). There are four
kingdoms of eukaryotes, three of them entirely or
predominantly multicellular. Two of the most important
characteristics to have evolved among the eukaryotes are
multicellularity and sexuality.
Viruses: A Special Case. Viruses are not organisms, and
thus do not belong to any kingdom.
A
ll organisms share many biological characteristics.
They are composed of one or more cells, carry out
metabolism and transfer energy with ATP, and encode
hereditary information in DNA. All species have evolved
from simpler forms and continue to evolve. Individuals live
in populations. These populations make up communities
and ecosystems, which provide the overall structure of life
on earth. So far, we have stressed these common themes,
considering the general principles that apply to all organ-
isms. Now we will consider the diversity of the biological
world and focus on the differences among groups of organ-
isms (figure 32.1). For the rest of the text, we will examine
the different kinds of life on earth, from bacteria and amoe-
bas to blue whales and sequoia trees.
FIGURE 32.1
Biological diversity.All living things are assigned to particular
classifications based on characteristics such as their anatomy,
development, mode of nutrition, level of organization, and
biochemical composition. Coral reefs, like the one seen here, are
home to a variety of living things.
books, employed the polynomial system. But as a kind of
shorthand, Linnaeus also included in these books a two-
part name for each species. For example, the honeybee be-
came Apis mellifera. These two-part names, or binomials
(bi, “two”) have become our standard way of designating
species.
A Closer Look at Linnaeus
To illustrate Linnaeus’s work further, let’s consider how he
treated two species of oaks from North America, which by
1753 had been described by scientists. He grouped all oaks in
the genus Quercus, as had been the practice since Roman
times. Linnaeus named the willow oak of the southeastern
United States (figure 32.2a) Quercus foliis lanceolatis inte-
gerrimis glabris (“oak with spear-shaped, smooth leaves with
absolutely no teeth along the margins”). For the common red
oak of eastern temperate North America (figure 32.2b), Lin-
naeus devised a new name, Quercus foliis obtuse-sinuatis
setaceo-mucronatis (“oak with leaves with deep blunt lobes
bearing hairlike bristles”). For each of these species, he also
presented a shorthand designation, the binomial names Quer-
cus phellosand Quercus rubra.These have remained the official
names for these species since 1753, even though Linnaeus did
not intend this when he first used them in his book. He con-
sidered the polynomials the true names of the species.
Two-part (“binomial”) Latin names, first utilized by
Linnaeus, are now universally employed by biologists to
name particular organisms.
650 Part IX Viruses and Simple Organisms
The Classification
of Organisms
Organisms were first classified more than
2000 years ago by the Greek philosopher
Aristotle, who categorized living things as
either plants or animals. He classified ani-
mals as either land, water, or air dwellers,
and he divided plants into three kinds based
on stem differences. This simple classifica-
tion system was expanded by the Greeks and
Romans, who grouped animals and plants
into basic units such as cats, horses, and
oaks. Eventually, these units began to be
called genera (singular, genus), the Latin
word for “groups.” Starting in the Middle
Ages, these names began to be systemati-
cally written down, using Latin, the lan-
guage used by scholars at that time. Thus,
cats were assigned to the genus Felis, horses
to Equus, and oaks to Quercus—names that
the Romans had applied to these groups.
For genera that were not known to the Romans, new
names were invented.
The classification system of the Middle Ages, called the
polynomial system, was used virtually unchanged for hun-
dreds of years.
The Polynomial System
Until the mid-1700s, biologists usually added a series of
descriptive terms to the name of the genus when they
wanted to refer to a particular kind of organism, which
they called a species. These phrases, starting with the
name of the genus, came to be known as polynomials
(poly, “many”; nomial, “name”), strings of Latin words and
phrases consisting of up to 12 or more words. One name
for the European honeybee, for example, was Apis pubes-
cens, thorace subgriseo, abdomine fusco, pedibus posticis glabris
utrinque margine ciliatis. As you can imagine, these poly-
nomial names were cumbersome. Even more worrisome,
the names were altered at will by later authors, so that a
given organism really did not have a single name that was
its alone.
The Binomial System
A much simpler system of naming animals, plants, and
other organisms stems from the work of the Swedish biolo-
gist Carolus Linnaeus (1707–1778). Linnaeus devoted his
life to a challenge that had defeated many biologists before
him—cataloging all the different kinds of organisms. In the
1750s he produced several major works that, like his earlier
32.1 Biologists name organisms in a systematic way.
Quercus phellos
(Willow oak)
Quercus rubra
(Red oak)
FIGURE 32.2
Two species of oaks.(a) Willow oak, Quercus phellos.(b) Red oak, Quercus rubra.
Although they are both oaks (Quercus), these two species differ sharply in leaf shape
and size and in many other features, including geographical range.
Species Names
Taxonomy is the science of classify-
ing living things, and a group of or-
ganisms at a particular level in a clas-
sification system is called a taxon
(plural, taxa). By agreement among
taxonomists throughout the world, no
two organisms can have the same
name. So that no one country is fa-
vored, a language spoken by no coun-
try—Latin—is used for the names.
Because the scientific name of an or-
ganism is the same anywhere in the
world, this system provides a standard
and precise way of communicating,
whether the language of a particular
biologist is Chinese, Arabic, Spanish,
or English. This is a great improve-
ment over the use of common names,
which often vary from one place to
the next. As you can see in figure
32.3, corn in Europe refers to the
plant Americans call wheat; a bear is a
large placental omnivore in the
United States but a koala (a vegetar-
ian marsupial) in Australia; and a
robin is a very different bird in Eu-
rope and North America.
Also by agreement, the first word
of the binomial name is the genus to
which the organism belongs. This
word is always capitalized. The sec-
ond word refers to the particular
species and is not capitalized. The two
words together are called the scien-
tific name and are written in italics or
distinctive print: for example, Homo
sapiens. Once a genus has been used in
the body of a text, it is often abbrevi-
ated in later uses. For example, the di-
nosaur Tyrannosaurus rex becomes T.
rex, and the potentially dangerous
bacterium Escherichia coli is known as
E. coli. The system of naming animals,
plants, and other organisms estab-
lished by Linnaeus has served the sci-
ence of biology well for nearly 230
years.
By convention, the first part of a
binomial species name identifies
the genus to which the species
belongs, and the second part
distinguishes that particular
species from other species in the
genus.
Chapter 32 How We Classify Organisms 651
(a)
(b)
(c)
FIGURE 32.3
Common names make poor labels.The common names corn (a), bear (b), and robin (c)
bring clear images to our minds (photos on left), but the images are very different to
someone living in Europe or Australia (photos on right). There, the same common names
are used to label very different species.
The Taxonomic Hierarchy
In the decades following Linnaeus, taxonomists began to
group organisms into larger, more inclusive categories.
Genera with similar properties were grouped into a cluster
called a family, and similar families were placed into the
same order (figure 32.4). Orders with common properties
were placed into the same class, and classes with similar
characteristics into the same phylum (plural, phyla). For
historical reasons, phyla may also be called divisions among
plants, fungi, and algae. Finally, the phyla were assigned to
one of several great groups, the kingdoms. Biologists cur-
rently recognize six kingdoms: two kinds of bacteria (Ar-
chaebacteria and Eubacteria), a largely unicellular group of
eukaryotes (Protista), and three multicellular groups
(Fungi, Plantae, and Animalia). In order to remember the
seven categories of the taxonomic hierarchy in their proper
order, it may prove useful to memorize a phrase such as
“kindly pay cash or furnish good security” (kingdom–phy-
lum–class–order–family–genus–species).
In addition, an eighth level of classification, called do-
mains, is sometimes used. Biologists recognize three do-
mains, which will be discussed later in this chapter. The
scientific names of the taxonomic units higher than the
genus level are capitalized but not printed distinctively,
italicized, or underlined.
The categories at the different levels may include many,
a few, or only one taxon. For example, there is only one liv-
ing genus of the family Hominidae, but several living gen-
era of Fagaceae. To someone familiar with classification or
with access to the appropriate reference books, each taxon
implies both a set of characteristics and a group of organ-
isms belonging to the taxon. For example, a honeybee has
the species (level 1) name Apis mellifera. Its genus name
(level 2) Apis is a member of the family Apidae (level 3). All
members of this family are bees, some solitary, others liv-
ing in hives as A. mellifera does. Knowledge of its order
(level 4), Hymenoptera, tells you that A. mellifera is likely
able to sting and may live in colonies. Its class (level 5) In-
secta indicates that A. mellifera has three major body seg-
ments, with wings and three pairs of legs attached to the
middle segment. Its phylum (level 6), Arthropoda, tells us
that the honeybee has a hard cuticle of chitin and jointed
appendages. Its kingdom (level 7), Animalia, tells us that A.
mellifera is a multicellular heterotroph whose cells lack cell
walls.
Species are grouped into genera, genera into families,
families into orders, orders into classes, and classes into
phyla. Phyla are the basic units within kingdoms; such a
system is hierarchical.
652 Part IX Viruses and Simple Organisms
Eastern gray squirrel
Sciurus carolinensis
FIGURE 32.4
The hierarchical system
used in classifying an
organism.The organism is
first recognized as a eukaryote
(domain: Eukarya). Second,
within this domain, it is an
animal (kingdom: Animalia).
Among the different phyla of
animals, it is a vertebrate
(phylum: Chordata,
subphylum: Vertebrata). The
organism’s fur characterizes it
as a mammal (class:
Mammalia). Within this class,
it is distinguished by its
gnawing teeth (order:
Rodentia). Next, because it
has four front toes and five
back toes, it is a squirrel
(family: Sciuridae). Within
this family, it is a tree squirrel
(genus: Sciurus), with gray fur
and white-tipped hairs on the
tail (species: Sciurus
carolinensis, the eastern gray
squirrel).
What Is a Species?
In the previous section we discussed how species are named
and grouped, but how do biologists decide when one or-
ganism is distinct enough from another to be called its own
species? In chapter 22, we reviewed the nature of species
and saw there are no absolute criteria for the definition of
this category. Looking different, for example, is not a use-
ful criterion: different individuals that belong to the same
species (for example, dogs) may look very unlike one an-
other, as different as a Chihuahua and a St. Bernard. These
very different-appearing individuals are fully capable of hy-
bridizing with one another.
The biological species concept (figure 32.5) essentially
says that two organisms that cannot interbreed and produce
fertile offspring are different species. This definition of a
species can be useful in describing sexually reproducing
species that regularly outcross—interbreed with individu-
als other than themselves. However, in many groups of or-
ganisms, including bacteria, fungi, and many plants and an-
imals, asexual reproduction—reproduction without
sex—predominates. Among them, hybridization cannot be
used as a criterion for species recognition.
Defining Species
Despite such difficulties, biologists generally agree on the
organisms they classify as species based on the similarity of
morphological features and ecology. As a practical defini-
tion, we can say that species are groups of organisms that
remain relatively constant in their characteristics, can be
distinguished from other species, and do not normally in-
terbreed with other species in nature.
Evolutionary Species Concept
This simple definition of species leaves many problems un-
solved. How, for instance, are we to compare living species
with seemingly similar ones now extinct? Much of the dis-
agreement among alternative species concepts relates to
solving this problem. When do we assign fossil specimens a
unique species name, and when do we assign them to
species living today? If we trace the lineage of two sister
species backwards through time, how far must we go before
the two species converge on their common ancestor? It is
often very hard to know where to draw a sharp line be-
tween two closely related species.
To address this problem, biologists have added an evo-
lutionary time dimension to the biological species concept.
A current definition of an evolutionary species is a single
lineage of populations that maintains its distinctive identity from
other such lineages. Unlike the biological species concept, the
evolutionary species concept applies to both asexual and
sexually reproducing forms. Abrupt changes in diagnostic
features mark the boundaries of different species in evolu-
tionary time.
How Many Species Are There?
Scientists have described and named a total of 1.5 million
species, but doubtless many more actually exist. Some
groups of organisms, such as flowering plants, vertebrate
animals, and butterflies, are relatively well known with an
estimated 90% of the total number of species that actually
exist in these groups having already been described. Many
other groups, however, are very poorly known. It is gener-
ally accepted that only about 5% of all species have been
recognized for bacteria, nematodes (roundworms), fungi,
and mites (a group of organisms related to spiders).
By taking representative samples of organisms from dif-
ferent environments, such as the upper branches of tropical
trees or the deep ocean, scientists have estimated the total
numbers of species that may actually exist to be about 10
million, about 15% of them marine organisms.
Most Species Live in the Tropics
Most species, perhaps 6 or 7 million, are tropical. Presently
only 400,000 species have been named in tropical Asia,
Africa, and Latin America combined, well under 10% of all
species that occur in the tropics. This is an incredible gap
in our knowledge concerning biological diversity in a world
that depends on biodiversity for its sustainability.
These estimates apply to the number of eukaryotic or-
ganisms only. There is no functional way of estimating the
numbers of species of prokaryotic organisms, although it is
clear that only a very small fraction of all species have been
discovered and characterized so far.
Species are groups of organisms that differ from one
another in recognizable ways and generally do not
interbreed with one another in nature.
Chapter 32 How We Classify Organisms 653
(a) (b)
(c)
FIGURE 32.5
The biological species
concept. Horses (a) and
donkeys (b) are not the
same species, because the
offspring they produce
when they interbreed,
mules (c), are sterile.
Evolutionary Classifications
After naming and classifying some 1.5 million organisms,
what have biologists learned? One very important advan-
tage of being able to classify particular species of plants, an-
imals, and other organisms is that individuals of species
that are useful to humans as sources of food and medicine
can be identified. For example, if you cannot tell the fungus
Penicillium from Aspergillus, you have little chance of pro-
ducing the antibiotic penicillin. In a thousand ways, just
having names for organisms is of immense importance in
our modern world.
Taxonomy also enables us to glimpse the evolutionary
history of life on earth. The more similar two taxa are,
the more closely related they are likely to be. By looking
at the differences and similarities between organisms, bi-
ologists can construct an evolutionary tree, or phy-
logeny, inferring which organisms evolved from which
other ones, in what order, and when. The reconstruction
and study of phylogenies is called systematics. Within a
phylogeny, a grouping can be either monophyletic, para-
phyletic, or polyphyletic. A monophyletic group in-
cludes the most recent common ancestor of the group
and all of its descendants. A paraphyletic group includes
the most recent common ancestor of the group but not
all of its descendants. And, a polyphyletic group does
not include the most recent common ancestor of all the
members of the group. Monophyletic groups are com-
monly assigned names, but systematists will not assign a
taxonomic classification to a polyphyletic group. Para-
phyletic groups may be considered taxa by some scien-
tists, although they do not accurately represent the evo-
lutionary relationships among the members of the group
(figure 32.6).
Cladistics
A simple and objective way to construct a phylogenetic
tree is to focus on key characters that a group of organ-
isms share because they have inherited them from a com-
mon ancestor. A clade is a group of organisms related by
descent, and this approach to constructing a phylogeny is
called cladistics. Cladistics infers phylogeny (that is,
builds family trees) according to similarities derived from
a common ancestor, so-called derived characters. A de-
rived character that is unique to a particular clade is
sometimes called a synapomorphy. The key to the ap-
proach is being able to identify morphological, physio-
logical, or behavioral traits that differ among the organ-
isms being studied and can be attributed to a common
ancestor. By examining the distribution of these traits
among the organisms, it is possible to construct a clado-
654 Part IX Viruses and Simple Organisms
32.2 Scientists construct phylogenies to understand the evolutionary
relationships among organisms.
Ray Shark Whale Cow
Orangutan Gorilla Chimpanzee
Human
Monophyletic group
Ray Shark Whale Cow
Orangutan Gorilla Chimpanzee
Human
Paraphyletic group
Ray Shark Whale Cow
Orangutan Gorilla Chimpanzee
Human
Polyphyletic group
(a)
(b)
(c)
FIGURE 32.6
(a) A monophyletic group consists of the most recent common
ancestor and all of its descendants. All taxonomists accept
monophyletic groups in their classifications and in the above
example would give the name “Apes” to the orangutans, gorillas,
chimpanzees, and humans. (b) A paraphyletic group consists of the
most recent common ancestor and some of its descendants.
Taxonomists differ in their acceptance of paraphyletic groups. For
example, some taxonomists arbitrarily group orangutans, gorillas,
and chimpanzees into the paraphyletic family Pongidae, separate
from humans. Other taxonomists do not use the family Pongidae
in their classifications because gorillas and chimpanzees are more
closely related to humans than to orangutans. (c) A polyphyletic
group does not contain the most recent common ancestor of the
group, and taxonomists do not assign taxa to polyphyletic groups.
For example, sharks and whales could be classified in the same
group because they have similar shapes, anatomical features, and
habitats. However, their similarities reflect convergent evolution,
not common ancestry.
gram (figure 32.7), a branching diagram that represents
the phylogeny.
In traditional phylogenies, proposed ancestors will
often be indicated at the nodes between branches, and the
lengths of branches correspond to evolutionary time, with
extinct groups having shorter branches. In contrast, clado-
grams are not true family trees in that they do not identify
ancestors, and the branch lengths do not reflect evolution-
ary time (see figure 32.6). Instead, they convey compara-
tive information about relative relationships. Organisms
that are closer together on a cladogram simply share a
more recent common ancestor than those that are farther
apart. Because the analysis is comparative, it is necessary to
have something to anchor the comparison to, some solid
ground against which the comparisons can be made. To
achieve this, each cladogram must contain an outgroup, a
rather different organism (but not too different) to serve as
a baseline for comparisons among the other organisms
being evaluated, the ingroup. For example, in figure 32.7,
the lamprey is the outgroup to the clade of animals that
have jaws.
Cladistics is a relatively new approach in biology and has
become popular among students of evolution. This is be-
cause it does a very good job of portraying the order in
which a series of evolutionary events have occurred. The
great strength of a cladogram is that it can be completely
objective. In fact, most cladistic analyses involve many
characters, and computers are required to make the com-
parisons.
Sometime it is necessary to “weight” characters, or take
into account the variation in the “strength” of a character,
such as the size or location of a fin or the effectiveness of
a lung. To reduce a systematist’s bias even more, many
analyses will be run through the computer with the traits
weighted differently each time. Under this procedure,
several different cladograms will be constructed, the goal
being to choose the one that is the most parsimonious,
or simplest and thus most likely. Reflecting the impor-
tance of evolutionary processes to all fields of biology,
most taxonomy today includes at least some element of
cladistic analysis.
Chapter 32 How We Classify Organisms 655
Lamprey Tiger Gorilla Human
Jaws
Lungs
Amniotic
membrane
Hair
No tail
Bipedal
LizardSalamanderShark
Traits:
Organism
Jaws Lungs Amniotic
membrane
Hair No tail Bipedal
Lamprey
Shark
Salamander
Lizard
Tiger
Gorilla
Human
00 0 0 0 0
10 0 0 0 0
11 0 0 0 0
11 1 0 0 0
11 1 1 0 0
11 1 1 1 0
11 1 1 1 1
FIGURE 32.7
A cladogram.Morphological data for a group
of seven vertebrates is tabulated. A “1”
indicates the presence of a trait, or derived
character, and a “0” indicates the absence of
the trait. A tree, or cladogram, diagrams the
proposed evolutionary relationships among
the organisms based on the presence of
derived characters. The derived characters
between the cladogram branch points are
shared by all organisms above the branch
point and are not present in any below it. The
outgroup, in this case the lamprey, does not
possess any of the derived characters.
Traditional Taxonomy
Weighting characters lies at the core of traditional taxon-
omy. In this approach, taxa are assigned based on a vast
amount of information about the morphology and biology
of the organism gathered over a long period of time. Tradi-
tional taxonomists consider both the common descent and
amount of adaptive evolutionary change when grouping or-
ganisms. The large amount of information used by tradi-
tional taxonomists permits a knowledgeable weighting of
characters according to their biological significance. In tra-
ditional taxonomy, the full observational power and judg-
ment of the biologist is brought to bear—and also any bi-
ases he or she may have. For example, in classifying the
terrestrial vertebrates, traditional taxonomists place birds in
their own class (Aves), giving great weight to the characters
that made powered flight possible, such as feathers. How-
ever, cladists (figure 32.8) lumps birds in among the rep-
tiles with crocodiles. This accurately reflects their true an-
cestry but ignores the immense evolutionary impact of a
derived character such as feathers.
Overall, classifications based on traditional taxonomy
are information-rich, while classifications based on clado-
grams need not be. Traditional taxonomy is often used
when a great deal of information is available to guide char-
acter weighting, while cladistics is a good approach when
little information is available about how the character af-
fects the life of the organism. DNA sequence comparisons,
for example, lend themselves well to cladistics—you have a
great many derived characters (DNA sequence differences)
but little or no idea of what impact the sequence differ-
ences have on the organism.
A phylogeny may be represented as a cladogram based
on the order in which groups evolved. Traditional
taxonomists weight characters according to assumed
importance.
656 Part IX Viruses and Simple Organisms
Mammals
Mammals
Turtles
Turtles
Crocodilians
Crocodilians
Birds
Birds
Dinosaurs
Dinosaurs
Lizards and
snakes
Lizards and
snakes
Early reptiles
Class
Mammalia
Class Reptilia
Class Aves
Mammalia Reptilia
Archosaurs
(a) Traditional phylogeny and taxonomic classification (b) Cladogram and cladistic classification
FIGURE 32.8
Traditional and cladistic interpretations of vertebrate classification.Traditional and cladistic taxonomic analyses of the same set of
data often produce different results: in these two classifications of vertebrates, notice particularly the placement of the birds. (a) In the
traditional analysis, key characteristics such as feathers and hollow bones are weighted more heavily than others, placing the birds in their
own group and the reptiles in a paraphyletic group. (b) Cladistic analysis gives equal weight to these and many other characters and places
birds in the same grouping with crocodiles, reflecting the close evolutionary relationship between the two. Also, in the traditional
phylogeny, the branch leading to the dinosaurs is shorter because the length corresponds to evolutionary time. In cladograms, branch
lengths do not correspond to evolutionary time.
The Kingdoms of Life
The earliest classification systems recognized only two
kingdoms of living things: animals and plants (figure
32.9a). But as biologists discovered microorganisms and
learned more about other organisms, they added kingdoms
in recognition of fundamental differences discovered
among organisms (figure 32.9b). Most biologists now use a
six-kingdom system first proposed by Carl Woese of the
University of Illinois (figure 32.9c).
In this system, four kingdoms consist of eukaryotic or-
ganisms. The two most familiar kingdoms, Animalia and
Plantae, contain only organisms that are multicellular dur-
ing most of their life cycle. The kingdom Fungi contains
multicellular forms and single-celled yeasts, which are
thought to have multicellular ancestors. Fundamental dif-
ferences divide these three kingdoms. Plants are mainly sta-
tionary, but some have motile sperm; fungi have no motile
cells; animals are mainly motile. Animals ingest their food,
plants manufacture it, and fungi digest it by means of se-
creted extracellular enzymes. Each of these kingdoms prob-
ably evolved from a different single-celled ancestor.
The large number of unicellular eukaryotes are arbitrar-
ily grouped into a single kingdom called Protista (see
chapter 35). This kingdom includes the algae, all of which
are unicellular during parts of their life cycle.
The remaining two kingdoms, Archaebacteria and Eu-
bacteria, consist of prokaryotic organisms, which are vastly
different from all other living things (see chapter 34). Ar-
chaebacteria are a diverse group including the
methanogens and extreme thermophiles, and differ from
the other bacteria, members of the kingdom Eubacteria.
Domains
As biologists have learned more about the archaebacteria, it
has become increasingly clear that this ancient group is
very different from all other organisms. When the full ge-
nomic DNA sequences of an archaebacterium and a eubac-
terium were first compared in 1996, the differences proved
striking. Archaebacteria are as different from eubacteria as
eubacteria are from eukaryotes. Recognizing this, biologists
are increasingly adopting a classification of living organ-
isms that recognizes three domains, a taxonomic level
higher than kingdom (figure 32.9d). Archaebacteria are in
one domain, eubacteria in a second, and eukaryotes in the
third.
Living organisms are grouped into three general
categories called domains. One of the domains, the
eukaryotes, is subdivided into four kingdoms: protists,
fungi, plants, and animals.
Chapter 32 How We Classify Organisms 657
32.3 All living organisms are grouped into one of a few major categories.
Animalia
AnimaliaPlantae
Plantae
FungiProtistaArchaebacteria
AnimaliaPlantaeFungiProtistaMonera
(a) A two-kingdom system—Linnaeus
(b) A five-kingdom system—Whittaker
(c) A six-kingdom system—Woese
(d) A three-domain system—Woese
EukaryaArchaeaBacteria
Eubacteria
FIGURE 32.9
Different approaches to classifying living organisms.(a) Linnaeus popularized a two-kingdom approach, in which the fungi and the
photosynthetic protists were classified as plants, and the nonphotosynthetic protists as animals; when bacteria were described, they too
were considered plants. (b) Whittaker in 1969 proposed a five-kingdom system that soon became widely accepted. (c) Woese has
championed splitting the bacteria into two kingdoms for a total of six kingdoms, or even assigning them separate domains (d).
Domain Archaea (Archaebacteria)
The term archaebacteria (Greek, archaio, ancient) refers to
the ancient origin of this group of bacteria, which seem to
have diverged very early from the eubacteria (figure
32.10). This conclusion comes largely from comparisons
of genes that encode ribosomal RNAs. The last several
years have seen an explosion of DNA sequence informa-
tion from microorganisms, information which paints a
more complex picture. It had been thought that by se-
quencing numerous microbes we could eventually come up
with an accurate picture of the phylogeny of the earliest
organisms on earth. The new whole-genome DNA se-
quence data described in chapter 19 tells us that it will not
be that simple. Comparing whole-genome sequences leads
evolutionary biologists to a variety of trees, some of which
contradict each other. It appears that during their early
evolution microorganisms have swapped genetic informa-
tion, making constructing phylogenetic trees very difficult.
As an example of the problem, we can look at Thermo-
toga, a thermophile found on Volcano Island off Italy. The
sequence of one of its RNAs places it squarely within the
eubacteria near an ancient microbe called Aquifex. Recent
DNA sequencing, however, fails to support any consistent
relationship between the two microbes. There is disagree-
ment as to the serious effect of gene swapping on the abil-
ity of evolutionary biologists to provide accurate phyloge-
nies from molecular data. For now, we will provisionally
accept the tree presented in figure 32.10. Over the next few
years we can expect to see considerable change in accepted
viewpoints as more and more data is brought to bear.
Today, archaebacteria inhabit some of the most extreme
environments on earth. Though a diverse group, all archae-
bacteria share certain key characteristics (table 32.1). Their
cell walls lack peptidoglycan (an important component of
the cell walls of eubacteria), the lipids in the cell mem-
branes of archaebacteria have a different structure than
those in all other organisms, and archaebacteria have dis-
tinctive ribosomal RNA sequences. Some of their genes
possess introns, unlike those of other bacteria.
The archaebacteria are grouped into three general cate-
gories, methanogens, extremophiles, and nonextreme ar-
chaebacteria, based primarily on the environments in which
they live or their specialized metabolic pathways.
Methanogens obtain their energy by using hydrogen
gas (H
2
) to reduce carbon dioxide (CO
2
) to methane gas
(CH
4
). They are strict anaerobes, poisoned by even traces
of oxygen. They live in swamps, marshes, and the intestines
of mammals. Methanogens release about 2 billion tons of
methane gas into the atmosphere each year.
Extremophiles are able to grow under conditions that
seem extreme to us.
Thermophiles (“heat lovers”) live in very hot places, typi-
cally from 60o to 80oC. Many thermophiles are au-
totrophs and have metabolisms based on sulfur. Some
thermophilic archaebacteria form the basis of food webs
around deep-sea thermal vents where they must with-
stand extreme temperatures and pressures. Other types,
like Sulfolobus, inhabit the hot sulfur springs of Yellow-
stone National Park at 70o to 75oC. The recently de-
scribed Pyrolobus fumarii holds the current record for
heat stability, with a 106oC temperature optimum and
113oC maximum—it is so heat tolerant that it is not
killed by a one-hour treatment in an autoclave (121oC)!
Halophiles (“salt lovers”) live in very salty places like the
Great Salt Lake in Utah, Mono Lake in California, and
the Dead Sea in Israel. Whereas the salinity of seawater
is around 3%, these bacteria thrive in, and indeed re-
quire, water with a salinity of 15 to 20%.
pH-tolerant archaebacteria grow in highly acidic (pH =
0.7) and very basic (pH = 11) environments.
Pressure-tolerant archaebacteria have been isolated from
ocean depths that require at least 300 atmospheres of
pressure to survive, and tolerate up to 800 atmospheres!
Nonextreme archaebacteria grow in the same envi-
ronments eubacteria do. As the genomes of archaebacteria
have become better known, microbiologists have been able
to identify signature sequences of DNA present in all ar-
chaebacteria and in no other organisms. When samples
from soil or seawater are tested for genes matching these
signal sequences, many of the bacteria living there prove to
be archaebacteria. Clearly, archaebacteria are not restricted
to extreme habitats, as microbiologists used to think.
Archaebacteria are poorly understood bacteria that
inhabit diverse environments, some of them extreme.
658 Part IX Viruses and Simple Organisms
Domain
Bacteria
(Eubacteria)
Domain
Archaea
(Archaebacteria)
Common ancestor
Domain
Eukarya
(Eukaryotes)
FIGURE 32.10
An evolutionary relationship among the three domains.
Eubacteria are thought to have diverged early from the
evolutionary line that gave rise to the archaebacteria and
eukaryotes.
Domain Bacteria (Eubacteria)
The eubacteria are the most abundant organisms on earth.
There are more living eubacteria in your mouth than there
are mammals living on earth. Although too tiny to see with
the unaided eye, eubacteria play critical roles throughout
the biosphere. They extract from the air all the nitrogen
used by organisms, and play key roles in cycling carbon and
sulfur. Much of the world’s photosynthesis is carried out by
eubacteria. However, certain groups of eubacteria are also
responsible for many forms of disease. Understanding their
metabolism and genetics is a critical part of modern medi-
cine.
There are many different kinds of eubacteria, and the
evolutionary links between them are not well understood.
While there is considerable disagreement among taxono-
mists about the details of bacterial classification, most rec-
ognize 12 to 15 major groups of eubacteria. Comparisons
of the nucleotide sequences of ribosomal RNA (rRNA)
molecules are beginning to reveal how these groups are re-
lated to one another and to the other two domains. One
view of our current understanding of the “Tree of Life” is
presented in figure 32.11. The oldest divergences represent
the deepest rooted branches in the tree. The root of the
tree is within the eubacterial domain. The archaebacteria
and eukaryotes are more closely related to each other than
to eubacteria and are on a separate evolutionary branch of
the tree, even though archaebacteria and eubacteria are
both prokaryotes.
Eubacteria are as different from archaebacteria as from
eukaryotes.
Chapter 32 How We Classify Organisms 659
BACTERIA
Purple bacteria
Common ancestor
Cyanobacteria
Flavobacteria
Thermotoga
Pyrodictium
Thermoproteus
Methanobacterium
Methanopyrus
Thermoplasma
Methano-
coccus
Thermo-
coccus
Halobacterium
Aquifex
Gram-positive
bacteria
Entamoebae
Slime molds
Animals
Fungi
Plants
Ciliates
Flagellates
Diplomonads
Microsporidia
ARCHAEA EUKARYA
FIGURE 32.11
A tree of life.This phylogeny, prepared from rRNA analyses, shows the evolutionary relationships among the three domains. The base of
the tree was determined by examining genes that are duplicated in all three domains, the duplication presumably having occurred in the
common ancestor. When one of the duplicates is used to construct the tree, the other can be used to root it. This approach clearly
indicates that the root of the tree is within the eubacterial domain. Archaebacteria and eukaryotes diverged later and are more closely
related to each other than either is to eubacteria.
Table 32.1 Features of the Domains of Life
Domain
Feature Archaea Bacteria Eukarya
Amino acid Methionine Formyl- Methionine
that initiates methionine
protein
synthesis
Introns Present in Absent Present
some genes
Membrane- Absent Absent Present
bounded
organelles
Membrane Branched Unbranched Unbranched
lipid
structure
Nuclear Absent Absent Present
envelope
Number of Several One Several
different
RNA
polymerases
Peptidoglycan Absent Present Absent
in cell wall
Response Growth Growth Growth not
to the not inhibited inhibited inhibited
antibiotics
streptomycin
and chloram-
phenicol
Domain Eukarya (Eukaryotes)
For at least 2 billion years, bacteria ruled the earth. No
other organisms existed to eat them or compete with
them, and their tiny cells formed the world’s oldest fossils.
The third great domain of life, the eukaryotes, appear in
the fossil record much later, only about 1.5 billion years
ago. Metabolically, eukaryotes are more uniform than
bacteria. Each of the two domains of prokaryotic organ-
isms has far more metabolic diversity than all eukaryotic
organisms taken together. However, despite the metabolic
similarity of eukaryotic cells, their structure and function
allowed larger cell sizes and, eventually, multicellular life
to evolve.
Four Kingdoms of Eukaryotes
The first eukaryotes were unicellular organisms. A wide
variety of unicellular eukaryotes exist today, grouped to-
gether in the kingdom Protista on the basis that they do
not fit into any of the other three kingdoms of eukaryotes.
Protists are a fascinating group containing many organ-
isms of intense interest and great biological significance.
They vary from the relatively simple, single-celled
amoeba to multicellular organisms like kelp that can be 20
meters long.
Fungi, plants, and animals are largely multicellular king-
doms, each a distinct evolutionary line from a single-celled
ancestor that would be classified in the kingdom Protista.
Because of the size and ecological dominance of plants, ani-
mals, and fungi, and because they are predominantly multi-
cellular, we recognize them as kingdoms distinct from Pro-
tista, even though the amount of diversity among the
protists is much greater than that within or between the
fungi, plants, and animals.
Symbiosis and the Origin of Eukaryotes
The hallmark of eukaryotes is complex cellular organiza-
tion, highlighted by an extensive endomembrane system
that subdivides the eukaryotic cell into functional compart-
ments. Not all of these compartments, however, are de-
rived from the endomembrane system. With few excep-
tions, all modern eukaryotic cells possess energy-producing
organelles, the mitochondria, and some eukaryotic cells
possess chloroplasts, which are energy-harvesting or-
ganelles. Mitochondria and chloroplasts are both believed
to have entered early eukaryotic cells by a process called
endosymbiosis (endo, inside). We discussed the theory of
the endosymbiotic origin of mitochondria and chloroplasts
in chapter 5; also see figure 32.12. Both organelles contain
their own ribosomes, which are more similar to bacterial ri-
bosomes than to eukaryotic cytoplasmic ribosomes. They
manufacture their own inner membranes. They divide in-
dependently of the cell and contain chromosomes similar
to those in bacteria. Mitochondria are about the size of
bacteria and contain DNA. Comparison of the nucleotide
sequence of this DNA with that of a variety of organisms
660 Part IX Viruses and Simple Organisms
Thermophiles
Halophiles
Methanogens
Purple bacteria
Photosynthetic bacteria
Photosynthetic
protists
Nonphotosynthetic protists
Brown algae
Animalia
Fungi
Protista
Plantae
Eubacteria
Red algae
Green algae
Other bacteria
Archaebacteria
Ancestral
eukaryotic
cell
Original
cell
Mitochondria
Chloroplasts
FIGURE 32.12
Diagram of the evolutionary relationships among the six kingdoms of organisms.The colored lines indicate symbiotic events.
indicates clearly that mitochondria are
the descendants of purple bacteria that
were incorporated into eukaryotic cells
early in the history of the group. Chloro-
plasts are derived from cyanobacteria
that became symbiotic in several groups
of protists early in their history.
Some biologists suggest that basal
bodies, centrioles, flagella, and cilia
may have arisen from endosymbiotic
spirochaete-like bacteria. Even today,
so many bacteria and unicellular pro-
tists form symbiotic alliances that the
incorporation of smaller organisms with
desirable features into eukaryotic cells
appears to be a relatively common
process.
Key Characteristics
of Eukaryotes
Multicellularity. The unicellular
body plan has been tremendously suc-
cessful, with unicellular prokaryotes
and eukaryotes constituting about half of the biomass on
earth. Yet a single cell has limits. The evolution of multi-
cellularity allowed organisms to deal with their environ-
ments in novel ways. Distinct types of cells, tissues, and
organs can be differentiated within the complex bodies of
multicellular organisms. With such a functional division
within its body, a multicellular organism can do many
things, like protect itself, resist drought efficiently, regu-
late its internal conditions, move about, seek mates and
prey, and carry out other activities on a scale and with a
complexity that would be impossible for its unicellular
ancestors. With all these advantages, it is not surprising
that multicellularity has arisen independently so many
times.
True multicellularity, in which the activities of individ-
ual cells are coordinated and the cells themselves are in
contact, occurs only in eukaryotes and is one of their
major characteristics. The cell walls of bacteria occasion-
ally adhere to one another, and bacterial cells may also be
held together within a common sheath. Some bacteria
form filaments, sheets, or three-dimensional aggregates
(figure 32.13), but the individual cells remain independent
of each other, reproducing and carrying on their meta-
bolic functions and without coordinating with the other
cells. Such bacteria are considered colonial, but none are
truly multicellular. Many protists also form similar colo-
nial aggregates of many cells with little differentiation or
integration.
Other protists—the red, brown, and green algae, for ex-
ample—have independently attained multicellularity. Cer-
tain forms of multicellular green algae
were ancestors of the plants (see chapters
35 and 37), and, like the other photosyn-
thetic protists, are considered plants in
some classification schemes. In the sys-
tem adopted here, the plant kingdom in-
cludes only multicellular land plants, a
group that arose from a single ancestor
in terrestrial habitats and that has a
unique set of characteristics. Aquatic
plants are recent derivatives.
Fungi and animals arose from unicel-
lular protist ancestors with different
characteristics. As we will see in subse-
quent chapters, the groups that seem to
have given rise to each of these king-
doms are still in existence.
Sexuality. Another major characteris-
tic of eukaryotic organisms as a group is
sexuality. Although some interchange of
genetic material occurs in bacteria (see
chapter 34), it is certainly not a regular,
predictable mechanism in the same
sense that sex is in eukaryotes. The sex-
ual cycle characteristic of eukaryotes alternates between
syngamy, the union of male and female gametes producing
a cell with two sets of chromosomes, and meiosis, cell divi-
sion producing daughter cells with one set of chromo-
somes. This cycle differs sharply from any exchange of ge-
netic material found in bacteria.
Except for gametes, the cells of most animals and plants
are diploid, containing two sets of chromosomes, during
some part of their life cycle. A few eukaryotes complete
their life cycle in the haploid condition, with only one set
of chromosomes in each cell. As we have seen, in diploid
cells, one set of chromosomes comes from the male parent
and one from the female parent. These chromosomes seg-
regate during meiosis. Because crossing over frequently oc-
curs during meiosis (see chapter 12), no two products of a
single meiotic event are ever identical. As a result, the off-
spring of sexual, eukaryotic organisms vary widely, thus
providing the raw material for evolution.
Sexual reproduction, with its regular alternation be-
tween syngamy and meiosis, produces genetic variation.
Sexual organisms can adapt to the demands of their envi-
ronments because they produce a variety of progeny.
In many of the unicellular phyla of protists, sexual re-
production occurs only occasionally. Meiosis may have
originally evolved as a means of repairing damage to DNA,
producing an organism better adapted to survive changing
environmental conditions. The first eukaryotes were prob-
ably haploid. Diploids seem to have arisen on a number of
separate occasions by the fusion of haploid cells, which
then eventually divided by meiosis.
Chapter 32 How We Classify Organisms 661
FIGURE 32.13
Colonial bacteria.No bacteria are
truly multicellular. These gliding
bacteria, Stigmatella aurantiaca,have
aggregated into a structure called a
fruiting body; within, some cells
transform into spores.
Eukaryotic Life Cycles
Eukaryotes are characterized by three major types of life
cycles (figure 32.14):
1. In the simplest cycle, found in algae, the zygote is the
only diploid cell. Such a life cycle is said to be charac-
terized by zygotic meiosis, because the zygote im-
mediately undergoes meiosis.
2. In most animals, the gametes are the only haploid
cells. Animals exhibit gametic meiosis, meiosis pro-
ducing gametes which fuse, giving rise to a zygote.
3. Plants show a regular alternation of generations be-
tween a multicellular haploid phase and a multicel-
lular diploid phase. The diploid phase undergoes
meiosis producing haploid spores that give rise to
the haploid phase, and the haploid phase produces
gametes that fuse to form the zygote. The zygote is
the first cell of the multicellular diploid phase. This
kind of life cycle is characterized by alternation of
generations and has sporic meiosis.
The characteristics of the six kingdoms are outlined in
table 32.2.
Eukaryotic cells acquired mitochondria and chloroplasts
by endosymbiosis, mitochondria being derived from
purple bacteria and chloroplasts from cyanobacteria.
The complex differentiation that we associate with
advanced life-forms depends on multicellularity and
sexuality, which must have been highly advantageous to
have evolved independently so often.
662 Part IX Viruses and Simple Organisms
Table 32.2 Characteristics of the Six Kingdoms
Nuclear
Kingdom Cell Type Envelope Mitochondria Chloroplasts Cell Wall
Archaebacteria
and Eubacteria
Protista
Fungi
Plantae
Animalia
Prokaryotic
Eukaryotic
Eukaryotic
Eukaryotic
Eukaryotic
Absent
Present
Present
Present
Present
Absent
Present or absent
Present or absent
Present
Present
None (photosynthetic
membranes in some types)
Present (some forms)
Absent
Present
Absent
Noncellulose (polysaccharide
plus amino acids)
Present in some forms,
various types
Chitin and other noncellulose
polysaccharides
Cellulose and other
polysaccharides
Absent
2n
Zygote
n
+
+
–
–
Haploid
individuals
–
+
Meiosis
Syngamy
Haploid
cells
Gametes
(a) Zygotic meiosis
Haploid DiploidKey:
+
–
–
–
+
Syngamy
Gametes
Gametes
(b) Gametic meiosis (c) Sporic meiosis
n
2n
Reproductive
cell2n
2n
Diploid
individual
Zygote
Meiosis
n
+
+
–
–
–
+
Meiosis
Syngamy
Spores
Gametes
n
Gametophytes
(haploid)
2n
Spore-forming
cell2n
2n
Sporophyte
(diploid)
Zygote
+
FIGURE 32.14
Diagrams of the three major kinds of life cycles in eukaryotes.(a) Zygotic meiosis, (b) gametic meiosis, and (c) sporic meiosis.
Viruses: A Special Case
Viruses pose a challenge to biologists as they do not possess
the fundamental characteristics of living organisms. Viruses
appear to be fragments of nucleic acids originally derived
from the genome of a living cell. Unlike all living organ-
isms, viruses are acellular—that is, they are not cells and do
not consist of cells. They do not have a metabolism; in
other words, viruses do not carry out photosynthesis, cellu-
lar respiration, or fermentation. The one characteristic of
life that they do display is reproduction, which they do by
hijacking the metabolism of living cells.
Viruses thus present a special classification problem. Be-
cause they are not organisms, we cannot logically place
them in any of the kingdoms. Viruses are really just com-
plicated associations of molecules, bits of nucleic acids usu-
ally surrounded by a protein coat. But, despite their sim-
plicity, viruses are able to invade cells and direct the genetic
machinery of these cells to manufacture more of the mole-
cules that make up the virus (figure 32.15). Viruses can in-
fect organisms at all taxonomic levels.
Viruses are not organisms and are not classified in the
kingdoms of life.
Chapter 32 How We Classify Organisms 663
Table 32.2 Characteristics of the Six Kingdoms
Means of Genetic
Recombination, Mode of Nervous
if Present Nutrition Motility Multicellularity System
Conjugation, transduction,
transformation
Fertilization and meiosis
Fertilization and meiosis
Fertilization and meiosis
Fertilization and meiosis
Autotrophic (chemo-
synthetic, photosyn-
thetic) or heterotrophic
Photosynthetic or het-
erotrophic, or combina-
tion of both
Absorption
Photosynthetic
chlorophylls aand b
Digestion
Bacterial flagella, gliding
or nonmotile
9 + 2 cilia and flagella;
amoeboid, contractile fibrils
Nonmotile
None in most forms,
9 + 2 cilia and flagella in
gametes of some forms
9 + 2 cilia and flagella,
contractile fibrils
Absent
Absent in most forms
Present in most forms
Present in all forms
Present in all forms
None
Primitive mechanisms
for conducting stimuli
in some forms
None
None
Present, often complex
FIGURE 32.15
Viruses are cell parasites.In this micrograph, several T4
bacteriophages (viruses) are attacking an Escherichia colibacterium.
Some of the viruses have already entered the cell and are
reproducing within it.
664 Part IX Viruses and Simple Organisms
? A fundamental division among organisms is between
prokaryotes, which lack a true nucleus, and
eukaryotes, which have a true nucleus and several
membrane-bound organelles.
? Prokaryotes, or bacteria, are assigned to two quite
different kingdoms, Archaebacteria and Eubacteria.
? The eukaryotic kingdoms are more closely related
than are the two kingdoms of prokaryotes. Many
distinctive evolutionary lines of unicellular eukaryotes
exist, most are in the Protista kingdom.
? Three of the major evolutionary lines of eukaryotic
organisms that consist principally or entirely of
multicellular organisms are recognized as separate
kingdoms: Plantae, Animalia, and Fungi.
? True multicellularity and sexuality are found only
among eukaryotes. Multicellularity confers the
advantages of functional specialization. Sexuality
permits genetic variation among descendants.
? Viruses are not organisms and are not included in the
classification of organisms. They are self-replicating
portions of the genomes of organisms.
5.Is there a greater fundamental
difference between plants and
animals or between prokaryotes
and eukaryotes? Explain.
6.From which of the four
eukaryotic kingdoms have the
other three evolved?
7.What is the apparent origin of
the organelles found in almost
all eukaryotes?
8.What defines if a collection of
cells is truly multicellular? Did
multicellularity arise once or
many times in the evolutionary
process? What advantages do
multicellular organisms have
over unicellular ones?
9.What are the three major
types of life cycles in eukaryotes?
Describe the major events of
each.
32.3 All living organisms are grouped into one of a few major categories.
Chapter 32
Summary Questions Media Resources
32.1 Biologists name organisms in a systematic way.
? Biologists give every species a two-part (binomial)
name that consists of the name of its genus plus a
distinctive specific epithet.
? In the hierarchical system of classification used in
biology, genera are grouped into families, families
into orders, orders into classes, classes into phyla, and
phyla into kingdoms.
? There are perhaps 10 million species of plants,
animals, fungi, and eukaryotic microorganisms, but
only about 1.5 million of them have been assigned
names. About 15% of the total number of species are
marine; the remainder are mostly terrestrial.
1.What was the polynomial
system? Why didn’t this system
become the standard for naming
particular species?
2.From the most specific to the
most general, what are the
names of the groups in the
hierarchical taxonomic system?
Which two are given special
consideration in the way in
which they are printed? What
are these distinctions?
? Taxonomists may use different approaches to classify
organisms.
? Cladistic systems of classification arrange organisms
according to evolutionary relatedness based on the
presence of shared, derived traits.
? Traditional taxonomy classifies organisms based on
large amounts of information, giving due weight to
the evolutionary significance of certain characters.
3.What types of features are
emphasized in a cladistic
classification system? What is
the resulting relationship of
organisms that are classified in
this manner?
4.What does it mean when
characters are weighted?
32.2 Scientists construct phylogenies to understand the evolutionary relationships among organisms.
www.mhhe.com/raven6e www.biocourse.com
? Hierarchies
? Book Reviews:
Ship Feverby Barrett
? Art Activity:
Organism
Classification
? Kingdoms
? Three Domains
? Phylogeny
? Book Review:
Thowim Way Leg
by Flannery
665
33
Viruses
Concept Outline
33.1 Viruses are strands of nucleic acid encased within
a protein coat.
The Discovery of Viruses. The first virus to be isolated
proved to consist of two chemicals, one a protein and the
other a nucleic acid.
The Nature of Viruses. Viruses occur in all organisms.
Able to reproduce only within living cells, viruses are not
themselves alive.
33.2 Bacterial viruses exhibit two sorts of reproductive
cycles.
Bacteriophages. Some bacterial viruses, called
bacteriophages, rupture the cells they infect, while others
integrate themselves into the bacterial chromosome to
become a stable part of the bacterial genome.
Cell Transformation and Phage Conversion.
Integrated bacteriophages sometimes modify the host
bacterium they infect.
33.3 HIV is a complex animal virus.
AIDS. The animal virus HIV infects certain key cells of
the immune system, destroying the ability of the body to
defend itself from cancer and disease. The HIV infection
cycle is typically a lytic cycle, in which the HIV RNA first
directs the production of a corresponding DNA, and this
DNA then directs the production of progeny virus
particles.
The Future of HIV Treatment. Combination therapies
and chemokines offer promising avenues of AIDS therapy.
33.4 Nonliving infectious agents are responsible for
many human diseases.
Disease Viruses. Some of the most serious viral diseases
have only recently infected human populations, the result
of transfer from other hosts.
Prions and Viroids. In some instances, proteins and
“naked” RNA molecules can also transmit diseases.
W
e start our exploration of the diversity of life with
viruses. Viruses are genetic elements enclosed in
protein and are not considered to be organisms, as they
cannot reproduce independently. Because of their disease-
producing potential, viruses are important biological enti-
ties. The virus particles you see in figure 33.1 produce the
important disease influenza. Other viruses cause AIDS,
polio, flu, and some can lead to cancer. Many scientists
have attempted to unravel the nature of viral genes and
how they work. For more than four decades, viral studies
have been thoroughly intertwined with those of genetics
and molecular biology. In the future, it is expected that
viruses will be one of the principal tools used to experimen-
tally carry genes from one organism to another. Already,
viruses are being employed in the treatment of human ge-
netic diseases.
FIGURE 33.1
Influenza viruses. A virus has been referred to as “a piece of bad
news wrapped up in a protein.” How can something as “simple” as
a virus have such a profound effect on living organisms? (30,000H11003)
lar but rather chemical. Each particle of TMV virus is in
fact a mixture of two chemicals: RNA and protein. The
TMV virus has the structure of a Twinkie, a tube made of
an RNA core surrounded by a coat of protein. Later work-
ers were able to separate the RNA from the protein and
purify and store each chemical. Then, when they reassem-
bled the two components, the reconstructed TMV particles
were fully able to infect healthy tobacco plants and so
clearly were the virus itself, not merely chemicals derived
from it. Further experiments carried out on other viruses
yielded similar results.
Viruses are chemical assemblies that can infect cells and
replicate within them. They are not alive.
666 Part IX Viruses and Simple Organisms
The Discovery of Viruses
The border between the living and the nonliving is very
clear to a biologist. Living organisms are cellular and able
to grow and reproduce independently, guided by informa-
tion encoded within DNA. The simplest creatures living on
earth today that satisfy these criteria are bacteria. Even
simpler than bacteria are viruses. As you will learn in this
section, viruses are so simple that they do not satisfy the
criteria for “living.”
Viruses possess only a portion of the properties of or-
ganisms. Viruses are literally “parasitic” chemicals, seg-
ments of DNA or RNA wrapped in a protein coat. They
cannot reproduce on their own, and for this reason they are
not considered alive by biologists. They can, however, re-
produce within cells, often with disastrous results to the
host organism. Earlier theories that viruses represent a kind
of halfway point between life and nonlife have largely been
abandoned. Instead, viruses are now viewed as detached
fragments of the genomes of organisms due to the high de-
gree of similarity found among some viral and eukaryotic
genes.
Viruses vary greatly in appearance and size. The smallest
are only about 17 nanometers in diameter, and the largest
are up to 1000 nanometers (1 micrometer) in their greatest
dimension (figure 33.2). The largest viruses are barely visi-
ble with a light microscope, but viral morphology is best
revealed using the electron microscope. Viruses are so
small that they are comparable to molecules in size; a hy-
drogen atom is about 0.1 nanometer in diameter, and a
large protein molecule is several hundred nanometers in its
greatest dimension.
Biologists first began to suspect the existence of
viruses near the end of the nineteenth century. European
scientists attempting to isolate the infectious agent re-
sponsible for hoof-and-mouth disease in cattle concluded
that it was smaller than a bacterium. Investigating the
agent further, the scientists found that it could not multi-
ply in solution—it could only reproduce itself within liv-
ing host cells that it infected. The infecting agents were
called viruses.
The true nature of viruses was discovered in 1933,
when the biologist Wendell Stanley prepared an extract of
a plant virus called tobacco mosaic virus (TMV) and at-
tempted to purify it. To his great surprise, the purified
TMV preparation precipitated (that is, separated from so-
lution) in the form of crystals. This was surprising because
precipitation is something that only chemicals do—the
TMV virus was acting like a chemical off the shelf rather
than an organism. Stanley concluded that TMV is best re-
garded as just that—chemical matter rather than a living
organism.
Within a few years, scientists disassembled the TMV
virus and found that Stanley was right. TMV was not cellu-
33.1 Viruses are strands of nucleic acid encased within a protein coat.
Vaccinia virus
(cowpox)
Influenza
virus
T4 bacteriophage
HIV-1
(AIDS)
Tobacco mosaic
virus (TMV)
Herpes simplex
virus
Rhinovirus
(common
cold)
Adenovirus
(respiratory
virus)
Poliovirus
(polio)
Ebola virus
100 nm
FIGURE 33.2
Viral diversity. A sample of the extensive diversity and small size
viruses is depicted. At the scale these viruses are shown, a human
hair would be nearly 8 meters thick.
The Nature of Viruses
Viral Structure
All viruses have the same basic struc-
ture: a core of nucleic acid surrounded
by protein. Individual viruses contain
only a single type of nucleic acid, either
DNA or RNA. The DNA or RNA
genome may be linear or circular, and
single-stranded or double-stranded.
Viruses are frequently classified by the
nature of their genomes. RNA-based
viruses are known as retroviruses.
Nearly all viruses form a protein
sheath, or capsid, around their nucleic
acid core. The capsid is composed of
one to a few different protein molecules
repeated many times (figure 33.3) In
some viruses, specialized enzymes are
stored within the capsid. Many animal
viruses form an envelope around the
capsid rich in proteins, lipids, and glyco-
protein molecules. While some of the
material of the envelope is derived from the host cell’s
membrane, the envelope does contain proteins derived
from viral genes as well.
Viruses occur in virtually every kind of organism that
has been investigated for their presence. However, each
type of virus can replicate in only a very limited number
of cell types. The suitable cells for a particular virus are
collectively referred to as its host range. The size of the
host range reflects the coevolved histories of the virus and
its potential hosts. A recently discovered herpesvirus
turned lethal when it expanded its host range from the
African elephant to the Indian elephant, a situation made
possible through cross-species contacts between elephants
in zoos. Some viruses wreak havoc on the cells they infect;
many others produce no disease or other outward sign of
their infection. Still other viruses remain dormant for
years until a specific signal triggers their expression. A
given organism often has more than one kind of virus.
This suggests that there may be many more kinds of
viruses than there are kinds of organisms—perhaps mil-
lions of them. Only a few thousand viruses have been de-
scribed at this point.
Viral Replication
An infecting virus can be thought of as a set of instruc-
tions, not unlike a computer program. A computer’s oper-
ation is directed by the instructions in its operating pro-
gram, just as a cell is directed by DNA-encoded
instructions. A new program can be introduced into the
computer that will cause the computer to cease what it is
doing and devote all of its energies to another activity,
such as making copies of the introduced program. The
new program is not itself a computer and cannot make
copies of itself when it is outside the computer, lying on
the desk. The introduced program, like a virus, is simply a
set of instructions.
Viruses can reproduce only when they enter cells and
utilize the cellular machinery of their hosts. Viruses code
their genes on a single type of nucleic acid, either DNA or
RNA, but viruses lack ribosomes and the enzymes neces-
sary for protein synthesis. Viruses are able to reproduce be-
cause their genes are translated into proteins by the cell’s
genetic machinery. These proteins lead to the production
of more viruses.
Viral Shape
Most viruses have an overall structure that is either helical
or isometric. Helical viruses, such as the tobacco mosaic
virus, have a rodlike or threadlike appearance. Isometric
viruses have a roughly spherical shape whose geometry is
revealed only under the highest magnification.
The only structural pattern found so far among isomet-
ric viruses is the icosahedron, a structure with 20 equilat-
eral triangular facets, like the adenovirus shown in figure
33.2. Most viruses are icosahedral in basic structure. The
icosahedron is the basic design of the geodesic dome. It is
the most efficient symmetrical arrangement that linear
subunits can take to form a shell with maximum internal
capacity.
Viruses occur in all organisms and can only reproduce
within living cells. Most are icosahedral in structure.
Chapter 33 Viruses 667
Capsid (protein sheath)
DNA
Envelope
protein
Envelope
Capsid
Enzyme
RNA
(a) Bacteriophage (b) Tobacco mosaic virus
(TMV)
(c) Human immunodeficiency
virus (HIV)
RNA
Proteins
FIGURE 33.3
The structure of a bacterial, plant, and animal virus. (a) Bacterial viruses, called
bacteriophages, often have a complex structure. (b) TMV infects plants and consists of
2130 identical protein molecules (purple) that form a cylindrical coat around the single
strand of RNA (green). The RNA backbone determines the shape of the virus and is
protected by the identical protein molecules packed tightly around it. (c) In the human
immunodeficiency virus (HIV), the RNA core is held within a capsid that is encased by a
protein envelope.
Bacteriophages
Bacteriophages are viruses that infect bacteria. They are
diverse both structurally and functionally, and are united
solely by their occurrence in bacterial hosts. Many of these
bacteriophages, called phages for short, are large and com-
plex, with relatively large amounts of DNA and proteins.
Some of them have been named as members of a “T” series
(T1, T2, and so forth); others have been given different
kinds of names. To illustrate the diversity of these viruses,
T3 and T7 phages are icosahedral and have short tails. In
contrast, the so-called T-even phages (T2, T4, and T6)
have an icosahedral head, a capsid that consists primarily of
three proteins, a connecting neck with a collar and long
“whiskers,” a long tail, and a complex base plate (figure
33.4).
The Lytic Cycle
During the process of bacterial infection by T4 phage, at
least one of the tail fibers of the phage—they are normally
held near the phage head by the “whiskers”—contacts the
lipoproteins of the host bacterial cell wall. The other tail
fibers set the phage perpendicular to the surface of the bac-
terium and bring the base plate into contact with the cell
surface. The tail contracts, and the tail tube passes through
an opening that appears in the base plate, piercing the bac-
terial cell wall. The contents of the head, mostly DNA, are
then injected into the host cytoplasm.
When a virus kills the infected host cell in which it is
replicating, the reproductive cycle is referred to as a lytic
cycle (figure 33.5). The T-series bacteriophages are all vir-
ulent viruses, multiplying within infected cells and even-
tually lysing (rupturing) them. However, they vary consid-
erably as to when they become virulent within their host
cells.
The Lysogenic Cycle
Many bacteriophages do not immediately kill the cells they
infect, instead integrating their nucleic acid into the
genome of the infected host cell. While residing there, it is
called a prophage. Among the bacteriophages that do this
is the lambda (H9261) phage of Escherichia coli. We know as
much about this bacteriophage as we do about virtually any
other biological particle; the complete sequence of its
48,502 bases has been determined. At least 23 proteins are
associated with the development and maturation of lambda
phage, and many other enzymes are involved in the inte-
gration of these viruses into the host genome.
The integration of a virus into a cellular genome is
called lysogeny. At a later time, the prophage may exit the
genome and initiate virus replication. This sort of repro-
ductive cycle, involving a period of genome integration, is
called a lysogenic cycle. Viruses that become stably inte-
grated within the genome of their host cells are called lyso-
genic viruses or temperate viruses.
Bacteriophages are a diverse group of viruses that
attack bacteria. Some kill their host in a lytic cycle;
others integrate into the host’s genome, initiating a
lysogenic cycle.
668 Part IX Viruses and Simple Organisms
33.2 Bacterial viruses exhibit two sorts of reproductive cycles.
.05 μm
(b)
Head
Capsid
(protein sheath)
DNA
Whiskers
Tail
Tail fiber
Base
plate
Neck
FIGURE 33.4
A bacterial virus.
Bacteriophages exhibit a
complex structure. (a)
Electron micrograph and
(b) diagram of the
structure of a T4
bacteriophage.(a)
Cell Transformation and Phage
Conversion
During the integrated portion of a lysogenic reproductive
cycle, virus genes are often expressed. The RNA poly-
merase of the host cell reads the viral genes just as if they
were host genes. Sometimes, expression of these genes has
an important effect on the host cell, altering it in novel
ways. The genetic alteration of a cell’s genome by the in-
troduction of foreign DNA is called transformation.
When the foreign DNA is contributed by a bacterial virus,
the alteration is called phage conversion.
Phage Conversion of the Cholera-Causing
Bacterium
An important example of this sort of phage conversion di-
rected by viral genes is provided by the bacterium responsi-
ble for an often-fatal human disease. The disease-causing
bacteria Vibrio cholerae usually exists in a harmless form, but
a second disease-causing, virulent form also occurs. In this
latter form, the bacterium causes the deadly disease
cholera, but how the bacteria changed from harmless to
deadly was not known until recently. Research now shows
that a bacteriophage that infects V. cholerae introduces into
the host bacterial cell a gene that codes for the cholera
toxin. This gene becomes incorporated into the bacterial
chromosome, where it is translated along with the other
host genes, thereby converting the benign bacterium to a
disease-causing agent. The transfer occurs through bacter-
ial pili (see chapter 34); in further experiments, mutant bac-
teria that did not have pili were resistant to infection by the
bacteriophage. This discovery has important implications
in efforts to develop vaccines against cholera, which have
been unsuccessful up to this point.
Bacteriophages convert Vibrio cholerae bacteria from
harmless gut residents into disease-causing agents.
Chapter 33 Viruses 669
Lysis of
cell
Uninfected cell
Virus attaching
to cell wall
Bacterial
chromosome
Viral DNA
injected into cell
Viral DNA integrated
into bacterial chromosome
Reproduction of lysogenic bacteria
Lysogenic
cycle
Lytic
cycle
Reduction to
prophage
Induction of
prophage to
vegetative virus
Replication of
vegetative
virus
Assembly
of new
viruses
using
bacterial
cell
machinery
FIGURE 33.5
Lytic and lysogenic cycles of a
bacteriophage. In the lytic cycle,
the bacteriophage exists as viral
DNA free in the bacterial host
cell’s cytoplasm; the viral DNA
directs the production of new
viral particles by the host cell
until the virus kills the cell by
lysis. In the lysogenic cycle, the
bacteriophage DNA is integrated
into the large, circular DNA
molecule of the host bacterium
and is reproduced along with the
host DNA as the bacterium
replicates. It may continue to
replicate and produce lysogenic
bacteria or enter the lytic cycle
and kill the cell. Bacteriophages
are much smaller relative to their
hosts than illustrated in this
diagram.
AIDS
A diverse array of viruses occur among animals. A good
way to gain a general idea of what they are like is to look at
one animal virus in detail. Here we will look at the virus
responsible for a comparatively new and fatal viral disease,
acquired immunodeficiency syndrome (AIDS). AIDS was
first reported in the United States in 1981. It was not long
before the infectious agent, a retrovirus called human im-
munodeficiency virus (HIV), was identified by laboratories
in France and the United States. Study of HIV revealed it
to be closely related to a chimpanzee virus, suggesting a
recent host expansion to humans in central Africa from
chimpanzees.
Infected humans have little resistance to infection, and
nearly all of them eventually die of diseases that nonin-
fected individuals easily ward off. Few who contract
AIDS survive more than a few years untreated. The risk
of HIV transmission from an infected individual to a
healthy one in the course of day-to-day contact is essen-
tially nonexistent. However, the transfer of body fluids,
such as blood, semen, or vaginal fluid, or the use of non-
sterile needles, between infected and healthy individuals
poses a severe risk. In addition, HIV-infected mothers
can pass the virus on to their unborn children during
fetal development.
The incidence of AIDS is growing very rapidly in the
United States. It is estimated that over 33 million people
worldwide are infected with HIV. Many—perhaps all of
them—will eventually come down with AIDS. Over 16
million people have died already since the outbreak of the
epidemic. AIDS incidence is already very high in many
African countries and is growing at 20% worldwide. The
AIDS epidemic is discussed further in chapter 57.
How HIV Compromises the Immune System
In normal individuals, an army of specialized cells patrols
the bloodstream, attacking and destroying any invading
bacteria or viruses. In AIDS patients, this army of de-
fenders is vanquished. One special kind of white blood
cell, called a CD4
+
T cell (discussed further in chapter
57) is required to rouse the defending cells to action. In
AIDS patients, the virus homes in on CD4
+
T cells, in-
fecting and killing them until none are left (figure 33.6).
Without these crucial immune system cells, the body
cannot mount a defense against invading bacteria or
viruses. AIDS patients die of infections that a healthy
person could fight off.
Clinical symptoms typically do not begin to develop until
after a long latency period, generally 8 to 10 years after the
initial infection with HIV. During this long interval, carriers
of HIV have no clinical symptoms but are apparently fully in-
fectious, which makes the spread of HIV very difficult to con-
trol. The reason why HIV remains hidden for so long seems
to be that its infection cycle continues throughout the 8- to
10-year latent period without doing serious harm to the in-
fected person. Eventually, however, a random mutational
event in the virus allows it to quickly overcome the immune
defense, starting AIDS.
The HIV Infection Cycle
The HIV virus infects and eliminates key cells of the im-
mune system, destroying the body’s ability to defend itself
from cancer and infection. The way HIV infects humans
(figure 33.7) provides a good example of how animal
viruses replicate. Most other viral infections follow a simi-
lar course, although the details of entry and replication dif-
fer in individual cases.
Attachment. When HIV is introduced into the human
bloodstream, the virus particle circulates throughout the
entire body but will only infect CD4
+
cells. Most other ani-
mal viruses are similarly narrow in their requirements; he-
patitis goes only to the liver, and rabies to the brain.
How does a virus such as HIV recognize a specific kind
of target cell? Recall from chapter 7 that every kind of cell
in the human body has a specific array of cell-surface glyco-
protein markers that serve to identify them to other, similar
cells. Each HIV particle possesses a glycoprotein (called
gp120) on its surface that precisely fits a cell-surface
marker protein called CD4 on the surfaces of immune sys-
tem cells called macrophages and T cells. Macrophages are
infected first.
670 Part IX Viruses and Simple Organisms
33.3 HIV is a complex animal virus.
FIGURE 33.6
The AIDS virus. HIV particles exit an infected CD4
+
T cell
(both shown in false color). The free virus particles are able to
infect neighboring CD4
+
T cells.
Entry into Macrophages. After docking onto the CD4
receptor of a macrophage, HIV requires a second
macrophage receptor, called CCR5, to pull itself across the
cell membrane. After gp120 binds to CD4, it goes through a
conformational change that allows it to bind to CCR5. The
current model suggests that after the conformational change,
the second receptor passes the gp120-CD4 complex through
the cell membrane, triggering passage of the contents of the
HIV virus into the cell by endocytosis, with the cell mem-
brane folding inward to form a deep cavity around the virus.
Replication. Once inside the macrophage, the HIV parti-
cle sheds its protective coat. This leaves virus RNA floating
in the cytoplasm, along with a virus enzyme that was also
within the virus shell. This enzyme, called reverse tran-
scriptase, synthesizes a double strand of DNA complemen-
tary to the virus RNA, often making mistakes and so intro-
ducing new mutations. This double-stranded DNA directs
the host cell machinery to produce many copies of the virus.
HIV does not rupture and kill the macrophage cells it in-
fects. Instead, the new viruses are released from the cell by
exocytosis. HIV synthesizes large numbers of viruses in this
way, challenging the immune system over a period of years.
Entry into T Cells. During this time, HIV is con-
stantly replicating and mutating. Eventually, by chance,
HIV alters the gene for gp120 in a way that causes the
gp120 protein to change its second-receptor allegiance.
This new form of gp120 protein prefers to bind instead
to a different second receptor, CXCR4, a receptor that
occurs on the surface of T lymphocyte CD4
+
cells. Soon
the body’s T lymphocytes become infected with HIV.
This has deadly consequences, as new viruses exit the cell
by rupturing the cell membrane, effectively killing the in-
fected T cell. Thus, the shift to the CXCR4 second re-
ceptor is followed swiftly by a steep drop in the number
of T cells. This destruction of the body’s T cells blocks
the immune response and leads directly to the onset of
AIDS, with cancers and opportunistic infections free to
invade the defenseless body.
HIV, the virus that causes AIDS, is an RNA virus that
replicates inside human cells by first making a DNA
copy of itself. It is only able to gain entrance to those
cells possessing a particular cell surface marker
recognized by a glycoprotein on its own surface.
Chapter 33 Viruses 671
1
2
3 4
Reverse transcriptase catalyzes the synthesis of a DNA copy of
the viral RNA. The host cell then synthesizes a complementary
strand of DNA.
The gp120 glycoprotein on the surface of HIV attaches to CD4 and
one of two coreceptors on the surface of a CD4
+
cell. The viral
contents enter the cell by endocytosis.
The double-stranded DNA directs the synthesis
of both HIV RNA and HIV proteins.
Complete HIV particles are assembled. In macrophages, HIV buds
out of the cell by exocytosis. In T cells, however, HIV ruptures the
cell, releasing free HIV back into the bloodstream.
DNA
Viral
RNA
Reverse
transcriptase
Double-
stranded
DNA
Viral
RNA
CD4
receptor
gp120
HIV
CD4
+
cell
CCR5 or CXCR4
coreceptor
DNA
Viral
RNA
Viral
proteins
Viral exit by
exocytosis in
macrophages
Viral exit by
cell lysis in
T cells
FIGURE 33.7
The HIV infection cycle. The cycle begins and ends with free HIV particles present in the bloodstream of its human host. These free
viruses infect white blood cells called CD4
H11001
T cells.
The Future of HIV Treatment
New discoveries of how HIV works continue to fuel re-
search on devising ways to counter HIV. For example, sci-
entists are testing drugs and vaccines that act on HIV re-
ceptors, researching the possibility of blocking CCR5, and
looking for defects in the structures of HIV receptors in in-
dividuals that are infected with HIV but have not devel-
oped AIDS. Figure 33.8 summarizes some of the recent de-
velopments and discoveries.
Combination Drug Therapy
A variety of drugs inhibit HIV in the test tube. These in-
clude AZT and its analogs (which inhibit virus nucleic acid
replication) and protease inhibitors (which inhibit the
cleavage of the large polyproteins encoded by gag, poll, and
env genes into functional capsid, enzyme, and envelope seg-
ments). When combinations of these drugs were adminis-
tered to people with HIV in controlled studies, their condi-
tion improved. A combination of a protease inhibitor and
two AZT analog drugs entirely eliminated the HIV virus
from many of the patients’ bloodstreams. Importantly, all
of these patients began to receive the drug therapy within
three months of contracting the virus, before their bodies
had an opportunity to develop tolerance to any one of
them. Widespread use of this combination therapy has
cut the U.S. AIDS death rate by three-fourths since its in-
troduction in the mid-1990s, from 49,000 AIDS deaths in
1995 to 36,000 in 1996, and just over 10,000 in 1999.
Unfortunately, this sort of combination therapy does
not appear to actually succeed in eliminating HIV from
the body. While the virus disappears from the blood-
stream, traces of it can still be detected in lymph tissue of
the patients. When combination therapy is discontinued,
virus levels in the bloodstream once again rise. Because
of demanding therapy schedules and many side effects,
long-term combination therapy does not seem a promis-
ing approach.
Using a Defective HIV Gene to Combat AIDS
Recently, five people in Australia who are HIV-positive but
have not developed AIDS in 14 years were found to have all
received a blood transfusion from the same HIV-positive
person, who also has not developed AIDS. This led scien-
tists to believe that the strain of virus transmitted to these
people has some sort of genetic defect that prevents it from
effectively disabling the human immune system. In subse-
quent research, a defect was found in one of the nine genes
present in this strain of HIV. This gene is called nef, named
for “negative factor,” and the defective version of nef in the
HIV strain that infected the six Australians seems to be
missing some pieces. Viruses with the defective gene may
have reduced reproductive capability, allowing the immune
system to keep the virus in check.
This finding has exciting implications for developing a
vaccine against AIDS. Before this, scientists have been un-
successful in trying to produce a harmless strain of AIDS
that can elicit an effective immune response. The Aus-
tralian strain with the defective nef gene has the potential to
be used in a vaccine that would arm the immune system
against this and other strains of HIV.
Another potential application of this discovery is its use
in developing drugs that inhibit HIV proteins that speed
virus replication. It seems that the protein produced from
the nef gene is one of these critical HIV proteins, because
viruses with defective forms of nef do not reproduce, as
seen in the cases of the six Australians. Research is cur-
rently underway to develop a drug that targets the nef
protein.
Chemokines and CAF
In the laboratory, chemicals called chemokines appear to
inhibit HIV infection by binding to and blocking the
CCR5 and CXCR4 coreceptors. As you might expect, peo-
ple long infected with the HIV virus who have not devel-
oped AIDS prove to have high levels of chemokines in their
blood.
The search for HIV-inhibiting chemokines is intense.
Not all results are promising. Researchers report that in
their tests, the levels of chemokines were not different be-
tween patients in which the disease was not progressing
and those in which it was rapidly progressing. More
promising, levels of another factor called CAF (CD8
+
cell
antiviral factor) are different between these two groups. Re-
searchers have not yet succeeded in isolating CAF, which
seems not to block receptors that HIV uses to gain entry to
cells, but, instead, to prevent replication of the virus once it
has infected the cells. Research continues on the use of
chemokines in treatments for HIV infection, either in-
creasing the amount of chemokines or disabling the CCR5
receptor. However, promising research on CAF suggests
that it may be an even better target for treatment and pre-
vention of AIDS.
One problem with using chemokines as drugs is that
they are also involved in the inflammatory response of
the immune system. The function of chemokines is to at-
tract white blood cells to areas of infection. Chemokines
work beautifully in small amounts and in local areas, but
chemokines in mass numbers can cause an inflammatory
response that is worse than the original infection. Injec-
tions of chemokines may hinder the immune system’s
ability to respond to local chemokines, or they may even
trigger an out-of-control inflammatory response. Thus,
scientists caution that injection of chemokines could
make patients more susceptible to infections, and they
continue to research other methods of using chemokines
to treat AIDS.
672 Part IX Viruses and Simple Organisms
Disabling Receptors
A 32-base-pair deletion in the gene that codes for the CCR5
receptor appears to block HIV infection. Individuals at high
risk of HIV infection who are homozygous for this mutation
do not seem to develop AIDS. In one study of 1955 people,
scientists found no individuals who were infected and ho-
mozygous for the mutated allele. The allele seems to be
more common in Caucasian populations (10 to 11%) than in
African-American populations (2%), and absent in African
and Asian populations. Treatment for AIDS involving dis-
ruption of CCR5 looks promising, as research indicates that
people live perfectly well without CCR5. Attempts to block
or disable CCR5 are being sought in numerous laboratories.
A cure for AIDS is not yet in hand, but many new
approaches look promising.
Chapter 33 Viruses 673
Blocking Receptors
gp120
Viral
RNA
HIV
CD4
CCR5
or
CXCR4Chemokine
blocking receptor
Mutated
coreceptor
Disabling Receptors
CCR5
or
CXCR4
Blocking Replication with CAF
Replication
CAF
3 4 5
CD4
CD4
+
cell
Envelope
proteins
tat or rev
gag
pol
vif
vpr
vpu
env
nef
Replication
AZT
Protease
inhibitors
Capsid
proteins
Replication
proteins
Critical
protein
nef protein
inhibitor
Vaccine incorporating
defective nef
Combination Therapy
1
Vaccine or
Drug Therapy
2
HIV
RNA
FIGURE 33.8
Research is currently underway to develop new treatments for HIV. Among them are these five: (1) Combination therapy involves
using two drugs, AZT to block replication of the virus and protease inhibitors to block the production of critical viral proteins. (2) Using a
defective form of the viral gene nef, scientists may be able to construct an HIV vaccine. Also, drug therapy that inhibits nef’s protein product
is being tested. (3) Other research focuses on the use of chemokine chemicals to block receptors (CXCR4 and CCR5), thereby disabling the
mechanism HIV uses to enter CD4
H11001
T cells. (4) Producing mutations that will disable receptors may also be possible. (5) Lastly, CAF, an
antiviral factor which acts inside the CD4
H11001
T cell, may be able to block replication of HIV.
Disease Viruses
Humans have known and feared diseases caused by viruses
for thousands of years. Among the diseases that viruses
cause (table 33.1) are influenza, smallpox, infectious hepati-
tis, yellow fever, polio, rabies, and AIDS, as well as many
other diseases not as well known. In addition, viruses have
been implicated in some cancers and leukemias. For many
autoimmune diseases, such as multiple sclerosis and
rheumatoid arthritis, and for diabetes, specific viruses have
been found associated with certain cases. In view of their
effects, it is easy to see why the late Sir Peter Medawar,
Nobel laureate in Physiology or Medicine, wrote, “A virus
is a piece of bad news wrapped in protein.” Viruses not
only cause many human diseases, but also cause major
losses in agriculture, forestry, and in the productivity of
natural ecosystems.
Influenza
Perhaps the most lethal virus in human history has been
the influenza virus. Some 22 million Americans and Euro-
peans died of flu within 18 months in 1918 and 1919, an as-
tonishing number.
Types. Flu viruses are animal retroviruses. An individ-
ual flu virus resembles a rod studded with spikes com-
posed of two kinds of protein (figure 33.9). There are
three general “types” of flu virus, distinguished by their
capsid (inner membrane) protein, which is different for
each type: Type A flu virus causes most of the serious flu
epidemics in humans, and also occurs in mammals and
birds. Type B and Type C viruses, with narrower host
ranges, are restricted to humans and rarely cause serious
health problems.
Subtypes. Different strains of flu virus, called subtypes,
differ in their protein spikes. One of these proteins,
hemagglutinin (H) aids the virus in gaining access to the
cell interior. The other, neuraminidase (N) helps the
daughter virus break free of the host cell once virus repli-
cation has been completed. Parts of the H molecule con-
tain “hot spots” that display an unusual tendency to
change as a result of mutation of the virus RNA during
imprecise replication. Point mutations cause changes in
these spike proteins in 1 of 100,000 viruses during the
course of each generation. These highly variable seg-
ments of the H molecule are targets against which the
body’s antibodies are directed. The high variability of
these targets improves the reproductive capacity of the
virus and hinders our ability to make perfect vaccines.
Because of accumulating changes in the H and N mole-
cules, different flu vaccines are required to protect
against different subtypes. Type A flu viruses are cur-
rently classified into 13 distinct H subtypes and 9 distinct
N subtypes, each of which requires a different vaccine to
protect against infection. Thus, the type A virus that
caused the Hong Kong flu epidemic of 1968 has type 3 H
molecules and type 2 N molecules, and is called
A(H3N2).
674 Part IX Viruses and Simple Organisms
33.4 Nonliving infectious agents are responsible for many human diseases.
Envelope (outer
lipid membrane)
Capsid (inner protein
membrane)
Hemagglutinin
Coils of RNA
Neuraminidase
(b)
(a)
FIGURE 33.9
The influenza virus. (a) TEM of the so-called “bird flu” influenza virus, A(H5N1), which first infected humans in Hong Kong in 1997.
(b) Diagram of an influenza virus. The coiled RNA has been revealed by cutting through the outer lipid-rich envelope, with its two kinds
of projecting spikes, and the inner protein capsid.
Importance of Recombination. The greatest problem
in combating flu viruses arises not through mutation, but
through recombination. Viral genes are readily reas-
sorted by genetic recombination, sometimes putting to-
gether novel combinations of H and N spikes unrecog-
nizable by human antibodies specific for the old
configuration. Viral recombination of this kind seems to
have been responsible for the three major flu pandemics
(that is, worldwide epidemics) that occurred in the last
century, by producing drastic shifts in H N combina-
tions. The “killer flu” of 1918, A(H1N1), killed 40 mil-
lion people. The Asian flu of 1957, A(H2N2), killed over
100,000 Americans. The Hong Kong flu of 1968,
A(H3N2), infected 50 million people in the United States
alone, of which 70,000 died.
Chapter 33 Viruses 675
Table 33.1 Important Human Viral Diseases
Disease Pathogen Reservoir Vector/Epidemiology
AIDS HIV STD Destroys immune defenses, resulting in death by infection
or cancer. Over 33 million cases worldwide by 1998.
Chicken pox Human herpes- Humans Spread through contact with infected individuals. No cure.
virus 3 (varicella- Rarely fatal. Vaccine approved in U.S. in early 1995.
zoster)
Ebola Filoviruses Unknown Acute hemorrhagic fever; virus attacks connective tissue,
leading to massive hemorrhaging and death. Peak mortality
is 50–90% if the disease goes untreated. Outbreaks
confined to local regions of central Africa.
Hepatitus B Hepatitis B virus Humans Highly infectious through contact with infected body fluids.
(viral) (HBV) Approximately 1% of U.S. population infected. Vaccine
available, no cure. Can be fatal.
Herpes Herpes simplex Humans Fever blisters; spread primarily through contact with
virus (HSV) infected saliva. Very prevalent worldwide. No cure.
Exhibits latency—the disease can be dormant for
several years.
Influenza Influenza viruses Humans, ducks Historically a major killer (22 million died in 18 months in
1918–19); wild Asian ducks, chickens, and pigs are major
reservoirs. The ducks are not affected by the flu virus, which
shuffles its antigen genes while multiplying within them,
leading to new flu strains.
Measles Paramyxoviruses Humans Extremely contagious through contact with infected
individuals. Vaccine available. Usually contracted in
childhood, when it is not serious; more dangerous to adults.
Mononucleosis Epstein-Barr Humans Spread through contact with infected saliva. May last several
virus (EBV) weeks; common in young adults. No cure. Rarely fatal.
Mumps Paramyxovirus Humans Spread through contact with infected saliva. Vaccine
available; rarely fatal. No cure.
Pneumonia Influenza virus Humans Acute infection of the lungs, often fatal without treatment.
Polio Poliovirus Humans Acute viral infection of the CNS that can lead to paralysis
and is often fatal. Prior to the development of Salk’s vaccine
in 1954, 60,000 people a year contracted the disease in the
U.S. alone.
Rabies Rhabdovirus Wild and domestic Canidae An acute viral encephalomyelitis transmitted by the bite of
(dogs, foxes, wolves, an infected animal. Fatal if untreated.
coyotes), bats, and raccoons
Smallpox Variola virus Formerly humans, now Historically a major killer; the last recorded case of smallpox
only exists in two research was in 1977. A worldwide vaccination campaign wiped out
labs—may be eliminated the disease completely.
Yellow fever Flavivirus Humans, mosquitoes Spread from individual to individual by mosquito bites; a
notable cause of death during the construction of the
Panama Canal. If untreated, this disease has a peak
mortality rate of 60%.
It is no accident that new strains of flu usually originate
in the far east. The most common hosts of influenza virus
are ducks, chickens, and pigs, which in Asia often live in
close proximity to each other and to humans. Pigs are sub-
ject to infection by both bird and human strains of the
virus, and individual animals are often simultaneously in-
fected with multiple strains. This creates conditions favor-
ing genetic recombination between strains, producing new
combinations of H and N subtypes. The Hong Kong flu,
for example, arose from recombination between A(H3N8)
[from ducks] and A(H2N2) [from humans]. The new strain
of influenza, in this case A(H3N2), then passed back to hu-
mans, creating an epidemic because the human population
has never experienced that H N combination before.
A potentially deadly new strain of flu virus emerged in
Hong Kong in 1997, A(H5N1). Unlike all previous in-
stances of new flu strains, A(H5N1) passed to humans di-
rectly from birds, in this case chickens. A(H5N1) was first
identified in chickens in 1961, and in the spring of 1997
devastated flocks of chickens in Hong Kong. Fortunately,
this strain of flu virus does not appear to spread easily from
person to person, and the number of human infections by
A(H5N1) remains small. Public health officials remain con-
cerned that the genes of A(H5N1) could yet mix with those
of a human strain to create a new strain that could spread
widely in the human population, and to prevent this or-
dered the killing of all 1.2 million chickens in Hong Kong
in 1997.
Emerging Viruses
Sometimes viruses that originate in one organism pass to
another, thus expanding their host range. Often, this ex-
pansion is deadly to the new host. HIV, for example, arose
in chimpanzees and relatively recently passed to humans.
Influenza is fundamentally a bird virus. Viruses that origi-
nate in one organism and then pass to another and cause
disease are called emerging viruses and represent a con-
siderable threat in an age when airplane travel potentially
allows infected individuals to move about the world
quickly, spreading an infection.
Among the most lethal of emerging viruses are a collec-
tion of filamentous viruses arising in central Africa that
cause severe hemorrhagic fever. With lethality rates in ex-
cess of 50%, these so-called filoviruses are among the most
lethal infectious diseases known. One, Ebola virus (figure
33.10), has exhibited lethality rates in excess of 90% in iso-
lated outbreaks in central Africa. The outbreak of Ebola
virus in the summer of 1995 in Zaire killed 245 people out
of 316 infected—a mortality rate of 78%. The latest out-
break occurred in Gabon, West Africa, in February 1996.
The natural host of Ebola is unknown.
Another type of emerging virus caused a sudden out-
break of a hemorrhagic-type infection in the southwestern
United States in 1993. This highly fatal disease was soon
attributed to the hantavirus, a single-stranded RNA virus
associated with rodents. The hantavirus is transmitted to
humans through rodent fecal contamination in areas of
human habitation. Although hantavirus has been known for
some period of time, this particular outbreak was attributed
to the presence of an unusually large rodent population in
the area following a higher than normal amount of rainfall
the previous winter.
Viruses and Cancer
Through epidemiological studies and research, scientists
have established a link between some viral infections and
the subsequent development of cancer. Examples include
the association between chronic hepatitis B infections and
the development of liver cancer and the development of
cervical carcinoma following infections with certain strains
of papillomaviruses. It has been suggested that viruses con-
tribute to about 15% of all human cancer cases worldwide.
Viruses are capable of altering the growth properties of
human cells they infect by triggering the expression of
oncogenes (cancer-causing genes). Certain viruses can ei-
ther activate host proto-oncogenes (see chapter 18) or
bring in viral oncogenes that become incorporated into the
host genome. Virus-induced cancer is not simply a matter
of infection. The disease involves complex interactions with
cellular genes and requires a series of events in order to de-
velop.
Viruses are responsible for some of the most lethal
diseases of humans. Some of the most serious examples
are viruses that have transferred to humans from some
other host. Influenza, a bird virus, has been responsible
for the most devastating epidemics in human history.
Newly emerging viruses such as Ebola have received
considerable public attention.
676 Part IX Viruses and Simple Organisms
FIGURE 33.10
The Ebola virus. This virus, with a fatality rate that can exceed
90%, appears sporadically in West Africa. Health professionals are
scrambling to identify the natural host of the virus, which is
unknown, so they can devise strategies to combat transmission of
the disease.
Prions and Viroids
For decades scientists have been fascinated by a peculiar
group of fatal brain diseases. These diseases have the un-
usual property that it is years and often decades after infec-
tion before the disease is detected in infected individuals.
The brains of infected individuals develop numerous small
cavities as neurons die, producing a marked spongy appear-
ance. Called transmissible spongiform encephalopathies
(TSEs), these diseases include scrapie in sheep, “mad cow”
disease in cattle, and kuru and Creutzfeldt-Jakob disease in
humans.
TSEs can be transmitted by injecting infected brain tis-
sue into a recipient animal’s brain. TSEs can also spread via
tissue transplants and, apparently, food. Kuru was common
in the Fore people of Papua New Guinea, when they prac-
ticed ritual cannibalism, literally eating the brains of in-
fected individuals. Mad cow disease spread widely among
the cattle herds of England in the 1990s because cows were
fed bone meal prepared from cattle carcasses to increase
the protein content of their diet. Like the Fore, the British
cattle were literally eating the tissue of cattle that had died
of the disease.
A Heretical Suggestion
In the 1960s, British researchers T. Alper and J. Griffith
noted that infectious TSE preparations remained infectious
even after exposed to radiation that would destroy DNA or
RNA. They suggested that the infectious agent was a pro-
tein. Perhaps, they speculated, the protein usually preferred
one folding pattern, but could sometimes misfold, and then
catalyze other proteins to do the same, the misfolding
spreading like a chain reaction. This heretical suggestion
was not accepted by the scientific community, as it violates
a key tenant of molecular biology: only DNA or RNA act
as hereditary material, transmitting information from one
generation to the next.
Prusiner’s Prions
In the early 1970s, physician Stanley Prusiner, moved by
the death of a patient from Creutzfeldt-Jakob disease,
began to study TSEs. Prusiner became fascinated with
Alper and Griffith’s hypothesis. Try as he might, Prusiner
could find no evidence of nucleic acids or viruses in the in-
fectious TSE preparations, and concluded, as Alper and
Griffith had, that the infectious agent was a protein, which
in a 1982 paper he named a prion, for “proteinaceous in-
fectious particle.”
Prusiner went on to isolate a distinctive prion protein,
and for two decades continued to amass evidence that pri-
ons play a key role in triggering TSEs. The scientific com-
munity resisted Prusiner’s renegade conclusions, but even-
tually experiments done in Prusiner’s and other
laboratories began to convince many. For example, when
Prusiner injected prions of a different abnormal conforma-
tion into several different hosts, these hosts developed pri-
ons with the same abnormal conformations as the parent
prions (figure 33.11). In another important experiment,
Charles Weissmann showed that mice genetically engi-
neered to lack Prusiner’s prion protein are immune to TSE
infection. However, if brain tissue with the prion protein is
grafted into the mice, the grafted tissue—but not the rest
of the brain—can then be infected with TSE. In 1997,
Prusiner was awarded the Nobel Prize in Physiology or
Medicine for his work on prions.
Viroids
Viroids are tiny, naked molecules of RNA, only a few hun-
dred nucleotides long, that are important infectious disease
agents in plants. A recent viroid outbreak killed over ten
million coconut palms in the Philippines. It is not clear
how viroids cause disease. One clue is that viroid nu-
cleotide sequences resemble the sequences of introns
within ribosomal RNA genes. These sequences are capable
of catalyzing excision from DNA—perhaps the viroids are
catalyzing the destruction of chromosomal integrity.
Prions are infectious proteins that some scientists
believe are responsible for serious brain diseases. In
plants, naked RNA molecules called viroids can also
transmit disease.
Chapter 33 Viruses 677
Misfolded prion
proteins
Normal prion
proteins
Neuron
FIGURE 33.11
How prions arise. Misfolded prions seem to cause normal prion
protein to misfold simply by contacting them. When prions
misfolded in different ways (blue) contact normal prion protein
(purple), the normal prion protein misfolds in the same way.
678 Part IX Viruses and Simple Organisms
Chapter 33
Summary Questions Media Resources
33.1 Viruses are strands of nucleic acid encased within a protein coat.
? Viruses are fragments of DNA or RNA surrounded
by protein that are able to replicate within cells by
using the genetic machinery of those cells.
? The simplest viruses use the enzymes of the host cell
for both protein synthesis and gene replication; the
more complex ones contain up to 200 genes and are
capable of synthesizing many structural proteins and
enzymes.
? Viruses are basically either helical or isometric. Most
isometric viruses are icosahedral in shape.
1. Why are viruses not
considered to be living
organisms?
2. How did early scientists come
to the conclusion that the
infectious agents associated with
hoof-and-mouth disease in cattle
were not bacteria?
3. What is the approximate size
range of viruses and type of
microscope is generally required
to visualize viruses?
? Virulent bacteriophages infect bacterial cells by
injecting their viral DNA or RNA into the cell, where
it directs the production of new virus particles,
ultimately lysing the cell.
? Temperate bacteriophages, upon entering a bacterial
cell, insert their DNA into the cell genome, where
they may remain integrated into the bacterial genome
as a prophage for many generations.
4. What is a bacteriophage?
How does a T4 phage infect a
host cell?
33.2 Bacterial viruses exhibit two sorts of reproductive cycles.
? AIDS, a viral infection that destroys the immune
system, is caused by HIV (human immunodeficiency
virus). After docking on a specific protein called CD4,
HIV enters the cell and replicates, destroying the cell.
? Considerable progress has been made in the
treatment of AIDS, particularly with drugs such as
protease inhibitors that block cleavage of HIV
polyproteins into functional segments.
5. What specific type of human
cell does the AIDS virus infect?
How does it recognize this
specific kind of cell?
6. How do many animal viruses
penetrate the host cell? How
does a plant virus infect its host?
How does a bacterial virus infect
its host?
33.3 HIV is a complex animal virus.
? Viruses are responsible for many serious human
diseases. Some of the most serious, like AIDS and
Ebola, have only recently transferred to humans from
some other animal host.
? Proteins called prions may transmit serious brain
diseases from one individual to another.
7. Why is it so much more
difficult to treat a viral infection
than a bacterial one? Is this
different from treating bacterial
infections?
8. What is a prion? How does it
integrate into living systems?
33.4 Nonliving infectious agents are responsible for many human diseases.
www.mhhe.com/raven6e www.biocourse.com
? Characteristics of
Viruses
? Life Cycle of Viruses
? Bioethics Case Study:
AIDS Vaccine
On Science Articles:
? HIV’s Waiting Game
? Drug Therapy for
AIDS
? Curing AIDS Just Got
Harder
? HIV Delivery Protein
? Scientists on Science:
Prions
? Book Review: The
Coming Plague by
Garrett
On Science Articles:
? Smallpox:
Tomorrow’s
Nightmare?
? Smallpox Questions
? Mad Cows and Prions
? Prions and Blood
Supply
? Hepatitis C
? Increasing Mad Cow
Diseases
679
34
Bacteria
Concept Outline
34.1 Bacteria are the smallest and most numerous
organisms.
The Prevalence of Bacteria. The simplest of organisms,
bacteria are thought to be the most ancient. They are the
most abundant living organisms. Bacteria lack the high
degree of internal compartmentalization characteristic of
eukaryotes.
34.2 Bacterial cell structure is more complex than
commonly supposed.
The Bacterial Surface. Some bacteria have a secondary
membranelike covering outside of their cell wall.
The Cell Interior. While bacteria lack extensive internal
compartments, they may have complex internal
membranes.
34.3 Bacteria exhibit considerable diversity in both
structure and metabolism.
Bacterial Diversity. There are at least 16 phyla of
bacteria, although many more remain to be discovered.
Bacterial Variation. Mutation and recombination
generate enormous variation within bacterial populations.
Bacterial Metabolism. Bacteria obtain carbon atoms and
energy from a wide array of sources. Some can thrive in the
absence of other organisms, while others must obtain their
energy and carbon atoms from other organisms.
34.4 Bacteria are responsible for many diseases but
also make important contributions to ecosystems.
Human Bacterial Diseases. Many serious human
diseases are caused by bacteria, some of them responsible
for millions of deaths each year.
Importance of Bacteria. Bacteria have had a profound
impact on the world’s ecology, and play a major role in
modern medicine and agriculture.
T
he simplest organisms living on earth today are bacte-
ria, and biologists think they closely resemble the first
organisms to evolve on earth. Too small to see with the un-
aided eye, bacteria are the most abundant of all organisms
(figure 34.1) and are the only ones characterized by
prokaryotic cellular organization. Life on earth could not
exist without bacteria because bacteria make possible many
of the essential functions of ecosystems, including the cap-
ture of nitrogen from the atmosphere, decomposition of
organic matter, and, in many aquatic communities, photo-
synthesis. Indeed, bacterial photosynthesis is thought to
have been the source for much of the oxygen in the earth’s
atmosphere. Bacterial research continues to provide extra-
ordinary insights into genetics, ecology, and disease. An
understanding of bacteria is thus essential.
FIGURE 34.1
A colony of bacteria. With their enormous adaptability and
metabolic versatility, bacteria are found in every habitat on earth,
carrying out many of the vital processes of ecosystems, including
photosynthesis, nitrogen fixation, and decomposition.
Bacterial Form
Bacteria are mostly simple in form and exhibit one of three
basic structures: bacillus (plural, bacilli) straight and rod-
shaped, coccus (plural, cocci) spherical-shaped, and spiril-
lus (plural, spirilla) long and helical-shaped, also called
spirochetes. Spirilla bacteria generally do not form associa-
tions with other cells and swim singly through their envi-
ronments. They have a complex structure within their cell
membranes that allow them to spin their corkscrew-shaped
bodies which propels them along. Some rod-shaped and
spherical bacteria form colonies, adhering end-to-end after
they have divided, forming chains (see figure 34.2). Some
bacterial colonies change into stalked structures, grow
long, branched filaments, or form erect structures that re-
lease spores, single-celled bodies that grow into new bacte-
rial individuals. Some filamentous bacteria are capable of
gliding motion, often combined with rotation around a
longitudinal axis. Biologists have not yet determined the
mechanism by which they move.
Prokaryotes versus Eukaryotes
Prokaryotes—eubacteria and archaea—differ from eukary-
otes in numerous important features. These differences
represent some of the most fundamental distinctions that
separate any groups of organisms.
1. Multicellularity. All prokaryotes are fundamentally
single-celled. In some types, individual cells adhere to
680 Part IX Viruses and Simple Organisms
The Prevalence
of Bacteria
Bacteria are the oldest, structurally
simplest, and the most abundant
forms of life on earth. They are also
the only organisms with prokaryotic
cellular organization. Represented in
the oldest rocks from which fossils
have been obtained, 3.5 to 3.8 billion
years old, bacteria were abundant for
over 2 billion years before eukaryotes
appeared in the world (see figure
4.11). Early photosynthetic bacteria
(cyanobacteria) altered the earth’s at-
mosphere with the production of oxy-
gen which lead to extreme bacterial
and eukaryotic diversity. Bacteria play
a vital role both in productivity and in
cycling the substances essential to all
other life-forms. Bacteria are the only
organisms capable of fixing atmos-
pheric nitrogen.
About 5000 different kinds of bac-
teria are currently recognized, but there are doubtless
many thousands more awaiting proper identification (figure
34.2). Every place microbiologists look, new species are
being discovered, in some cases altering the way we think
about bacteria. In the 1970s and 80s a new type of bac-
terium was analyzed that eventually lead to the classifica-
tion of a new prokaryotic cell type, the archeabacteria (or
Archaea). Even when viewed with an electron microscope,
the structural differences between different bacteria are
minor compared to other groups of organisms. Because the
structural differences are so slight, bacteria are classified
based primarily upon their metabolic and genetic charac-
teristics. Bacteria can be characterized properly only when
they are grown on a defined medium because the charac-
teristics of these organisms often change, depending on
their growth conditions.
Bacteria are ubiquitous on Earth, and live everywhere
eukaryotes do. Many of the other more extreme environ-
ments in which bacteria are found would be lethal to any
other form of life. Bacteria live in hot springs that would
cook other organisms, hypersaline environments that
would dehydrate other cells, and in atmospheres rich in
toxic gases like methane or hydrogen sulfide that would kill
most other organisms. These harsh environments may be
similar to the conditions present on the early Earth, when
life first began. It is likely that bacteria evolved to dwell in
these harsh conditions early on and have retained the abil-
ity to exploit these areas as the rest of the atmosphere has
changed.
34.1 Bacteria are the smallest and most numerous organisms.
(a) (b) (c)
FIGURE 34.2
The diversity of bacteria. (a) Pseudomonas aeruginosa, a rod-shaped, flagellated bacterium
(bacillus). Pseudomonas includes the bacteria that cause many of the most serious plant
diseases. (b) Streptococcus. The spherical individual bacteria (cocci) adhere in chains in the
members of this genus (34,000H11003). (c) Spirillum volutans, one of the spirilla. This large
bacterium, which occurs in stagnant fresh water, has a tuft of flagella at each end (500H11003).
each other within a matrix and form filaments, how-
ever the cells retain their individuality. Cyanobacte-
ria, in particular, are likely to form such associations
but their cytoplasm is not directly interconnected, as
often is the case in multicellular eukaryotes. The ac-
tivities of a bacterial colony are less integrated and
coordinated than those in multicellular eukaryotes. A
primitive form of colonial organization occurs in
gliding bacteria, which move together and form
spore-bearing structures (figure 34.3). Such coordi-
nated multicellular forms are rare among bacteria.
2. Cell size. As new species of bacteria are discovered,
we are finding that the size of prokaryotic cells varies
tremendously, by as much as five orders of magni-
tude. Most prokaryotic cells are only 1 micrometer or
less in diameter. Most eukaryotic cells are well over
10 times that size.
3. Chromosomes. Eukaryotic cells have a membrane-
bound nucleus containing chromosomes made up of
both nucleic acids and proteins. Bacteria do not have
membrane-bound nuclei, nor do they have chromo-
somes of the kind present in eukaryotes, in which
DNA forms a structural complex with proteins. In-
stead, their naked circular DNA is localized in a zone
of the cytoplasm called the nucleoid.
4. Cell division and genetic recombination. Cell divi-
sion in eukaryotes takes place by mitosis and involves
spindles made up of microtubules. Cell division in bac-
teria takes place mainly by binary fission (see chapter
11). True sexual reproduction occurs only in eukaryotes
and involves syngamy and meiosis, with an alternation
of diploid and haploid forms. Despite their lack of sex-
ual reproduction, bacteria do have mechanisms that
lead to the transfer of genetic material. These mecha-
nisms are far less regular than those of eukaryotes and
do not involve the equal participation of the individuals
between which the genetic material is transferred.
5. Internal compartmentalization. In eukaryotes,
the enzymes for cellular respiration are packaged in
mitochondria. In bacteria, the corresponding en-
zymes are not packaged separately but are bound to
the cell membranes (see chapters 5 and 9). The cyto-
plasm of bacteria, unlike that of eukaryotes, contains
no internal compartments or cytoskeleton and no or-
ganelles except ribosomes.
6. Flagella. Bacterial flagella are simple in structure,
composed of a single fiber of the protein flagellin
(figure 34.4; see also chapter 5). Eukaryotic flagella
and cilia are complex and have a 9 + 2 structure of
microtubules (see figure 5.27). Bacterial flagella also
function differently, spinning like propellers, while
eukaryotic flagella have a whiplike motion.
7. Metabolic diversity. Only one kind of photosyn-
thesis occurs in eukaryotes, and it involves the release
of oxygen. Photosynthetic bacteria have several dif-
ferent patterns of anaerobic and aerobic photosynthe-
sis, involving the formation of end products such as
sulfur, sulfate, and oxygen (see chapter 10). Prokary-
otic cells can also be chemoautotrophic, using the en-
ergy stored in chemical bonds of inorganic molecules
to synthesize carbohydrates; eukaryotes are not capa-
ble of this metabolic process.
Bacteria are the oldest and most abundant organisms on
earth. Bacteria, or prokaryotes, differ from eukaryotes
in a wide variety of characteristics, a degree of
difference as great as any that separates any groups of
organisms.
Chapter 34 Bacteria 681
FIGURE 34.3
Approaches to
multicellularity
in bacteria.
Chondromyces
crocatus, one of
the gliding
bacteria. The
rod-shaped
individuals move
together, forming
the composite
spore-bearing
structures shown
here. Millions of
spores, which are
basically
individual
bacteria, are
released from
these structures.
FIGURE 34.4
Flagella in the common intestinal bacterium, Escherichia coli.
The long strands are flagella, while the shorter hairlike outgrowths
are called pili.
The Bacterial Surface
The bacterial cell wall is an important structure because it
maintains the shape of the cell and protects the cell from
swelling and rupturing. The cell wall usually consists of
peptidoglycan, a network of polysaccharide molecules
connected by polypeptide cross-links. In some bacteria, the
peptidoglycan forms a thick, complex network around the
outer surface of the cell. This network is interlaced with
peptide chains. In other bacteria a thin layer of peptidogly-
can is found sandwiched between two plasma membranes.
The outer membrane contains large molecules of lipopoly-
saccharide, lipids with polysaccharide chains attached.
These two major types of bacteria can be identified using a
staining process called a Gram stain. Gram-positive bac-
teria have the thicker peptidoglycan wall and stain a purple
color (figure 34.5). The more common gram-negative
bacteria contain less peptidoglycan and do not retain the
purple-colored dye. Gram-negative bacteria stain red. The
outer membrane layer makes gram-negative bacteria resis-
tant to many antibiotics that interfere with cell wall synthe-
sis in gram-positive bacteria. In some kinds of bacteria, an
additional gelatinous layer, the capsule, surrounds the cell
wall.
Many kinds of bacteria have slender, rigid, helical fla-
gella (singular, flagellum) composed of the protein fla-
gellin (figure 34.6). These flagella range from 3 to 12 mi-
crometers in length and are very thin—only 10 to 20
nanometers thick. They are anchored in the cell wall and
spin, pulling the bacteria through the water like a
propeller.
Pili (singular, pilus) are other hairlike structures that
occur on the cells of some bacteria (see figure 34.4). They
are shorter than bacterial flagella, up to several microme-
ters long, and about 7.5 to 10 nanometers thick. Pili help
the bacterial cells attach to appropriate substrates and ex-
change genetic information.
Some bacteria form thick-walled endospores around
their chromosome and a small portion of the surrounding
cytoplasm when they are exposed to nutrient-poor condi-
tions. These endospores are highly resistant to environ-
mental stress, especially heat, and can germinate to form
new individuals after decades or even centuries.
Bacteria are encased within a cell wall composed of one
or more polysaccharide layers. They also may contain
external structures such as flagella and pili.
682 Part IX Viruses and Simple Organisms
34.2 Bacterial cell structure is more complex than commonly supposed.
Peptidoglycan
Peptide side
chains
Cell wall
(peptidoglycan)
Cell wall
Plasma
membrane
Plasma
membrane
Protein
Outer
membrane
Gram-positive
bacteria
Gram-negative
bacteria
Lipopolysaccharides
FIGURE 34.5
The Gram stain. The peptidoglycan layer encasing gram-positive bacteria traps crystal violet dye, so the bacteria appear purple in a
Gram-stained smear (named after Hans Christian Gram, who developed the technique). Because gram-negative bacteria have much less
peptidoglycan (located between the plasma membrane and an outer membrane), they do not retain the crystal violet dye and so exhibit the
red background stain (usually a safranin dye).
The Cell Interior
The most fundamental characteristic of bacterial cells is
their prokaryotic organization. Bacterial cells lack the ex-
tensive functional compartmentalization seen within eu-
karyotic cells.
Internal membranes. Many bacteria possess invagi-
nated regions of the plasma membrane that function in
respiration or photosynthesis (figure 34.7).
Nucleoid region. Bacteria lack nuclei and do not pos-
sess the complex chromosomes characteristic of eukary-
otes. Instead, their genes are encoded within a single
double-stranded ring of DNA that is crammed into one
region of the cell known as the nucleoid region. Many
bacterial cells also possess small, independently replicat-
ing circles of DNA called plasmids. Plasmids contain
only a few genes, usually not essential for the cell’s sur-
vival. They are best thought of as an excised portion of
the bacterial chromosome.
Ribosomes. Bacterial ribosomes are smaller than
those of eukaryotes and differ in protein and RNA con-
tent. Antibiotics such as tetracycline and chlorampheni-
col can tell the difference—they bind to bacterial ribo-
somes and block protein synthesis, but do not bind to
eukaryotic ribosomes.
The interior of a bacterial cell may possess internal
membranes and a nucleoid region.
Chapter 34 Bacteria 683
Flagellum
Filament
Sleeve
Hook
Outer membrane
Peptidoglycan portion
of cell wall
Rod
H
+
Plasma membrane
Outer protein ring
Inner protein ring H
+
FIGURE 34.6
The flagellar motor of a gram-negative bacterium. A protein filament, composed of the protein flagellin, is attached to a protein shaft
that passes through a sleeve in the outer membrane and through a hole in the peptidoglycan layer to rings of protein anchored in the cell
wall and plasma membrane, like rings of ballbearings. The shaft rotates when the inner protein ring attached to the shaft turns with
respect to the outer ring fixed to the cell wall. The inner ring is an H
+
ion channel, a proton pump that uses the passage of protons into the
cell to power the movement of the inner ring past the outer one.
FIGURE 34.7
Bacterial cells often have complex internal membranes. This
aerobic bacterium (a) exhibits extensive respiratory membranes
within its cytoplasm not unlike those seen in mitochondria. This
cyanobacterium (b) has thylakoid-like membranes that provide a
site for photosynthesis.
(a) (b)
Bacterial Diversity
Bacteria are not easily classified according to their
forms, and only recently has enough been learned about
their biochemical and metabolic characteristics to de-
velop a satisfactory overall classification comparable to
that used for other organisms. Early systems for classify-
ing bacteria relied on differential stains such as the
Gram stain. Key bacterial characteristics used in classify-
ing bacteria were:
1. Photosynthetic or nonphotosynthetic
2. Motile or nonmotile
3. Unicellular or multicellular
4. Formation of spores or dividing by transverse
binary fission
With the development of genetic and molecular ap-
proaches, bacterial classifications can at last reflect true
evolutionary relatedness. Molecular approaches include:
(1) the analysis of the amino acid sequences of key pro-
teins; (2) the analysis of nucleic acid base sequences by
establishing the percent of guanine (G) and cytosine (C);
(3) nucleic acid hybridization, which is essentially the
mixing of single-stranded DNA from two species and
determining the amount of base-pairing (closely related
species will have more bases pairing); and (4) nucleic
acid sequencing especially looking at ribosomal RNA.
Lynn Margulis and Karlene Schwartz proposed a useful
classification system that divides bacteria into 16 phyla,
according to their most significant features. Table 34.1
outlines some of the major features of the phyla we
describe.
Kinds of Bacteria
Although they lack the structural complexity of eukary-
otes, bacteria have diverse internal chemistries, metabo-
lisms and unique functions. Bacteria have adapted to
many kinds of environments, including some you might
consider harsh. They have successfully invaded very salty
waters, very acidic or alkaline environments, and very hot
or cold areas. They are found in hot springs where the
temperatures exceed 78°C (172°F) and have been recov-
ered living beneath 435 meters of ice in Antarctica!
Much of what we know of bacteria we have learned
from studies in the laboratory. It is important to under-
stand the limits this has placed on our knowledge: we have
only been able to study those bacteria that can be cultured
in laboratories. Field studies suggest that these represent
but a small fraction of the kinds of bacteria that occur in
soil, most of which cannot be cultured with existing tech-
niques. We clearly have only scraped the surface of bacte-
rial diversity.
As we learned in chapter 32, bacteria split into two
lines early in the history of life, so different in structure
and metabolism that they are as different from each other
as either is from eukaryotes. The differences are so fun-
damental that biologists assign the two groups of bacteria
to separate domains. One domain, the Archaea, consists
of the archaebacteria (“ancient bacteria”—although they
are actually not as ancient as the other bacterial domain).
It was once thought that survivors of this group were
confined to extreme environments that may resemble
habitats on the early earth. However, the use of genetic
screening has revealed that these “ancient” bacteria live
in nonextreme environments as well. The other more an-
cient domain, the Bacteria, consists of the eubacteria
(“true bacteria”). It includes nearly all of the named
species of bacteria.
Comparing Archaebacteria and Eubacteria
Archaebacteria and eubacteria are similar in that they both
have a prokaryotic cellular but they vary considerably at the
biochemical and molecular level. There are four key areas
in which they differ:
1. Cell wall. Both kinds of bacteria typically have
cell walls covering the plasma membrane that
strengthen the cell. The cell walls of eubacteria are
constructed of carbohydrate-protein complexes
called peptidoglycan, which link together to create
a strong mesh that gives the eubacterial cell wall
great strength. The cell walls of archaebacteria lack
peptidoglycan.
2. Plasma membranes. All bacteria have plasma
membranes with a lipid-bilayer architecture (as de-
scribed in chapter 6). The plasma membranes of eu-
bacteria and archaebacteria, however, are made of
very different kinds of lipids.
3. Gene translation machinery. Eubacteria possess
ribosomal proteins and an RNA polymerase that
are distinctly different from those of eukaryotes.
However, the ribosomal proteins and RNA of
archaebacteria are very similar to those of
eukaryotes.
4. Gene architecture. The genes of eubacteria are
not interrupted by introns, while at least some of the
genes of archaebacteria do possess introns.
While superficially similar, bacteria differ from one
another in a wide variety of characteristics.
684 Part IX Viruses and Simple Organisms
34.3 Bacteria exhibit considerable diversity in both structure and metabolism.
Chapter 34 Bacteria 685
Table 34.1 Bacteria
Major Group Typical Examples Key Characteristics
ARCHAEBACTERIA
Archaebacteria Methanogens,
thermophiles,
halophiles
EUBACTERIA
Actinomycetes Streptomyces,
Actinomyces
Chemoautotrophs Sulfur bacteria,
Nitrobacter,
Nitrosomonas
Cyanobacteria Anabaena,
Nostoc
Enterobacteria Escherichia coli,
Salmonella,
Vibrio
Gliding and Myxobacteria,
budding bacteria Chondromyces
Pseudomonads Pseudomonas
Rickettsias and Rickettsia,
chlamydias Chlamydia
Spirochaetes Treponema
Bacteria that are not members of the kingdom Eubacteria.
Mostly anaerobic with unusual cell walls. Some produce
methane. Others reduce sulfur.
Gram-positive bacteria. Form branching filaments and produce
spores; often mistaken for fungi. Produce many commonly used
antibiotics, including streptomycin and tetracycline. One of the
most common types of soil bacteria; also common in dental
plaque.
Bacteria able to obtain their energy from inorganic chemicals.
Most extract chemical energy from reduced gases such as H
2
S
(hydrogen sulfide), NH
3
(ammonia), and CH
4
(methane). Play a
key role in the nitrogen cycle.
A form of photosynthetic bacteria common in both marine and
freshwater environments. Deeply pigmented; often responsible
for “blooms” in polluted waters.
Gram-negative, rod-shaped bacteria. Do not form spores; usually
aerobic heterotrophs; cause many important diseases, including
bubonic plague and cholera.
Gram-negative bacteria. Exhibit gliding motility by secreting
slimy polysaccharides over which masses of cells glide; some
groups form upright multicelluar structures carrying spores
called fruiting bodies.
Gram-negative heterotrophic rods with polar flagella. Very
common form of soil bacteria; also contain many important plant
pathogens.
Small, gram-negative intracelluar parasites. Rickettsia life cycle
involves both mammals and arthropods such as fleas and ticks;
Rickettsia are responsible for many fatal human diseases,
including typhus (Rickettsia prowazekii) and Rocky Mountain
spotted fever. Chlamydial infections are one of the most
common sexually transmitted diseases.
Long, coil-shaped cells. Common in aquatic environments; a
parasitic form is responsible for the disease syphilis.
Bacterial Variation
Bacteria reproduce rapidly, allowing
genetic variations to spread quickly
through a population. Two processes
create variation among bacteria: mu-
tation and genetic recombination.
Mutation
Mutations can arise spontaneously
in bacteria as errors in DNA replica-
tion occur. Certain factors tend to
increase the likelihood of errors oc-
curring such as radiation, ultraviolet
light, and various chemicals. In a
typical bacterium such as Escherichia
coli there are about 5000 genes. It is
highly probably that one mutation
will occur by chance in one out of
every million copies of a gene. With
5000 genes in a bacterium, the laws
of probability predict that 1 out of
every 200 bacteria will have a muta-
tion (figure 34.8). A spoonful of soil
typically contains over a billion bac-
teria and therefore should contain
something on the order of 5 million
mutant individuals!
With adequate food and nutri-
ents, a population of E. coli can dou-
ble in under 20 minutes. Because
bacteria multiply so rapidly, mutations can spread rapidly
in a population and can change the characteristics of that
population.
The ability of bacteria to change rapidly in response to
new challenges often has adverse effects on humans. Re-
cently a number of strains of Staphylococcus aureus associated
with serious infections in hospitalized patients have ap-
peared, some of them with alarming frequency. Unfortu-
nately, these strains have acquired resistance to penicillin and
a wide variety of other antibiotics, so that infections caused
by them are very difficult to treat. Staphylococcus infections
provide an excellent example of the way in which mutation
and intensive selection can bring about rapid change in bac-
terial populations. Such changes have serious medical impli-
cations when, as in the case of Staphylococcus, strains of bacte-
ria emerge that are resistant to a variety of antibiotics.
Recently, concern has arisen over the prevalence of an-
tibacterial soaps in the marketplace. They are marketed as a
means of protecting your family from harmful bacteria;
however, it is likely that their routine use will favor bacteria
that have mutations making them immune to the antibi-
otics contained in them. Ultimately, extensive use of an-
tibacterial soaps could have an adverse effect on our ability
to treat common bacterial infections.
Genetic Recombination
Another source of genetic variation in populations of bac-
teria is recombination, discussed in detail in chapter 18.
Bacterial recombination occurs by the transfer of genes
from one cell to another by viruses, or through conjuga-
tion. The rapid transfer of newly produced, antibiotic-
resistant genes by plasmids has been an important factor
in the appearance of the resistant strains of Staphylococcus
aureus discussed earlier. An even more important example
in terms of human health involves the Enterobacteriaceae,
the family of bacteria to which the common intestinal
bacterium, Escherichia coli, belongs. In this family, there
are many important pathogenic bacteria, including the or-
ganisms that cause dysentery, typhoid, and other major
diseases. At times, some of the genetic material from these
pathogenic species is exchanged with or transferred to E.
coli by plasmids. Because of its abundance in the human
digestive tract, E. coli poses a special threat if it acquires
harmful traits.
Because of the short generation time of bacteria,
mutation and recombination play an important role in
generating genetic diversity.
686 Part IX Viruses and Simple Organisms
Incubate
Incubate
Velveteen
Cells lifted
from colonies
Colonies
absent
Medium
lacking
growth
factor
Mutagen-treated
bacteria are added
Supplemented
medium
Bacterial
colony
A
A
A
B
Bacterial cells
are spread
B
FIGURE 34.8
A mutant hunt in bacteria. Mutations in bacteria can be detected by a technique called
replica plating, which allows the genetic characteristics of the colonies to be investigated
without destroying them. The bacterial colonies, growing on a semisolid agar medium, are
transferred from A to B using a sterile velveteen disc pressed on the plate. Plate A has a
medium that includes special growth factors, while B has a medium that lacks some of these
growth factors. Bacteria that are not mutated can produce their own growth factors and do
not require them to be added to the medium. The colonies absent in B were unable to grow
on the deficient medium and were thus mutant colonies; they were already present but
undetected in A.
Bacterial Metabolism
Bacteria have evolved many mechanisms to acquire the en-
ergy and nutrients they need for growth and reproduction.
Many are autotrophs, organisms that obtain their carbon
from inorganic CO
2
. Autotrophs that obtain their energy
from sunlight are called photoautotrophs, while those that
harvest energy from inorganic chemicals are called
chemoautotrophs. Other bacteria are heterotrophs, organ-
isms that obtain at least some of their carbon from organic
molecules like glucose. Heterotrophs that obtain their en-
ergy from sunlight are called photoheterotrophs, while those
that harvest energy from organic molecules are called
chemoheterotrophs.
Photoautotrophs. Many bacteria carry out photosyn-
thesis, using the energy of sunlight to build organic mol-
ecules from carbon dioxide. The cyanobacteria use
chlorophyll a as the key light-capturing pigment and use
H
2
O as an electron donor, releasing oxygen gas as a by-
product. Other bacteria use bacteriochlorophyll as their
pigment and H
2
S as an electron donor, leaving elemen-
tal sulfur as the by-product.
Chemoautotrophs. Some bacteria obtain their energy
by oxidizing inorganic substances. Nitrifiers, for exam-
ple, oxidize ammonia or nitrite to obtain energy, pro-
ducing the nitrate that is taken up by plants. This
process is called nitrogen fixation and is essential in ter-
restrial ecosystems as plants can only absorb nitrogen in
the form of nitrate. Other bacteria oxidize sulfur, hydro-
gen gas, and other inorganic molecules. On the dark
ocean floor at depths of 2500 meters, entire ecosystems
subsist on bacteria that oxidize hydrogen sulfide as it es-
capes from thermal vents.
Photoheterotrophs. The so-called purple nonsulfur
bacteria use light as their source of energy but obtain
carbon from organic molecules such as carbohydrates or
alcohols that have been produced by other organisms.
Chemoheterotrophs. Most bacteria obtain both car-
bon atoms and energy from organic molecules. These
include decomposers and most pathogens.
How Heterotrophs Infect Host Organisms
In the 1980s, researchers studying the disease-causing
species of Yersinia, a group of gram-negative bacteria,
found that they produced and secreted large amounts of
proteins. Most proteins secreted by gram-negative bac-
teria have special signal sequences that allow them to
pass through the bacterium’s double membrane. This
key signal sequence was missing the proteins being se-
creted by Yersinia. These proteins lacked a signal-
sequence that two known secretion mechanisms require
for transport across the double membrane of gram-
negative bacteria. The proteins must therefore have
been secreted by a third type of system, which re-
searchers called the type III system.
As more bacteria species are studied, the genes coding
for the type III system are turning up in other gram-
negative animal pathogens, and even in more distantly
related plant pathogens. The genes seem to be more
closely related to one another than do the bacteria. Fur-
thermore, the genes are similar to those that code for
bacterial flagella.
The role of these proteins is still under investigation,
but it seems that some of the proteins are used to transfer
other virulence proteins into nearby eukaryotic cells.
Given the similarity of the type III genes to the genes
that code for flagella, some scientists hypothesize that the
transfer proteins may form a flagellum-like structure that
shoots virulence proteins into the host cells. Once in the
eukaryotic cells, the virulence proteins may determine
the host’s response to the pathogens. In Yersinia, proteins
secreted by the type III system are injected into
macrophages; they disrupt signals that tell the
macrophages to engulf bacteria. Salmonella and Shigella
use their type III proteins to enter the cytoplasm of eu-
karyotic cells and thus are protected from the immune
system of their host. The proteins secreted by E. coli alter
the cytoskeleton of nearby intestinal eukaryotic cells, re-
sulting in a bulge onto which the bacterial cells can
tightly bind.
Currently, researchers are looking for a way to disarm
the bacteria using knowledge of their internal machinery,
possibly by causing the bacteria to release the virulence
proteins before they are near eukaryotic cells. Others are
studying the eukaryotic target proteins and the process by
which they are affected.
Bacteria as Plant Pathogens
Many costly diseases of plants are associated with partic-
ular heterotrophic bacteria. Almost every kind of plant is
susceptible to one or more kinds of bacterial disease.
The symptoms of these plant diseases vary, but they are
commonly manifested as spots of various sizes on the
stems, leaves, flowers, or fruits. Other common and de-
structive diseases of plants, including blights, soft rots,
and wilts, also are associated with bacteria. Fire blight,
which destroys pears, apple trees, and related plants, is a
well-known example of bacterial disease. Most bacteria
that cause plant diseases are members of the group of
rod-shaped bacteria known as pseudomonads (see
figure 34.2a).
While bacteria obtain carbon and energy in many
ways, most are chemoheterotrophs. Some
heterotrophs have evolved sophisticated ways to infect
their hosts.
Chapter 34 Bacteria 687
688 Part IX Viruses and Simple Organisms
Human Bacterial Diseases
Bacteria cause many diseases in humans, including cholera,
leprosy, tetanus, bacterial pneumonia, whooping cough,
diphtheria and lyme disease (table 34.2). Members of the
genus Streptococcus (see figure 34.2b) are associated with
scarlet fever, rheumatic fever, pneumonia, and other infec-
tions. Tuberculosis (TB), another bacterial disease, is still a
leading cause of death in humans. Some of these diseases
like TB are mostly spread through the air in water vapor.
Other bacterial diseases are dispersed in food or water, in-
cluding typhoid fever, paratyphoid fever, and bacillary
dysentery. Typhus is spread among rodents and humans by
insect vectors.
Tuberculosis
Tuberculosis has been one of the great killer diseases for
thousands of years. Currently, about one-third of all people
worldwide are infected with Mycobacterium tuberculosis, the
tuberculosis bacterium (figure 34.9). Eight million new
cases crop up each year, with about 3 million people dying
from the disease annually (the World Health Organization
predicts 4 million deaths a year by 2005). In fact, in 1997,
TB was the leading cause of death from a single infectious
agent worldwide. Since the mid-1980s, the United States
has been experiencing a dramatic resurgence of tuberculo-
sis. TB afflicts the respiratory system and is easily transmit-
ted from person to person through the air. The causes of
this current resurgence of TB include social factors such as
poverty, crowding, homelessness, and incarceration (these
factors have always promoted the spread of TB). The in-
creasing prevalence of HIV infections is also a significant
contributing factor. People with AIDS are much more
likely to develop TB than people with healthy immune sys-
tems.
In addition to the increased numbers of cases—more
than 25,000 nationally as of March 1995—there have been
alarming outbreaks of multidrug-resistant strains of tuber-
culosis—strains resistant to the best available anti-TB med-
ications. Multidrug-resistant TB is particularly concerning
because it requires much more time to treat, is more expen-
sive to treat, and may prove to be fatal.
The basic principles of TB treatment and control are to
make sure all patients complete a full course of medication
so that all of the bacteria causing the infection are killed
and drug-resistant strains do not develop. Great efforts are
being made to ensure that high-risk individuals who are in-
fected but not yet sick receive preventative therapy, which
is 90% effective in reducing the likelihood of developing
active TB.
Dental Caries
One human disease we do not usually consider bacterial in
origin arises in the film on our teeth. This film, or plaque,
consists largely of bacterial cells surrounded by a polysac-
charide matrix. Most of the bacteria in plaque are filaments
of rod-shaped cells classified as various species of Actino-
myces, which extend out perpendicular to the surface of the
tooth. Many other bacterial species are also present in
plaque. Tooth decay, or dental caries, is caused by the
bacteria present in the plaque, which persists especially in
places that are difficult to reach with a toothbrush. Diets
that are high in sugars are especially harmful to teeth be-
cause lactic acid bacteria (especially Streptococcus sanguis and
S. mutans) ferment the sugars to lactic acid, a substance that
reduces the pH of the mouth, causing the local loss of cal-
cium from the teeth. Frequent eating of sugary snacks or
sucking on candy over a period of time keeps the pH level
of the mouth low resulting in the steady degeneration of
the tooth enamel. As the calcium is removed from the
tooth, the remaining soft matrix of the tooth becomes vul-
nerable to attack by bacteria which begin to break down its
proteins and tooth decay progresses rapidly. Fluoride
makes the teeth more resistant to decay because it retards
the loss of calcium. It was first realized that bacteria cause
tooth decay when germ-free animals were raised. Their
teeth do not decay even if they are fed sugary diets.
34.4 Bacteria are responsible for many diseases but also make important
contributions to ecosystems.
FIGURE 34.9
Mycobacterium tuberculosis. This color-enhanced image shows
the rod-shaped bacterium responsible for tuberculosis in humans.
Chapter 34 Bacteria 689
Table 34.2 Important Human Bacterial Diseases
Disease Pathogen Vector/Reservoir Epidemiology
Anthrax Bacillus anthracis Animals, including Bacterial infection that can be transmitted through
processed skins contact or ingested. Rare except in sporadic
outbreaks. May be fatal.
Botulism Clostridium botulinum Improperly prepared food Contracted through ingestion or contact with wound.
Produces acute toxic poison; can be fatal.
Chlamydia Chlamydia trachomatis Humans, STD Urogenital infections with possible spread to
eyes and respiratory tract. Occurs worldwide;
increasingly common over past 20 years.
Cholera Vibrio cholerae Human feces, plankton Causes severe diarrhea that can lead to death
by dehydration; 50% peak mortality if the disease
goes untreated. A major killer in times of crowding
and poor sanitation; over 100,000 died in Rwanda in
1994 during a cholera outbreak.
Dental caries Streptococcus Humans A dense collection of this bacteria on the surface of
teeth leads to secretion of acids that destroy minerals
in tooth enamel—sugar alone will not cause caries.
Diphtheria Corynebacterium Humans Acute inflammation and lesions of mucous
diphtheriae membranes. Spread through contact with infected
individual. Vaccine available.
Gonorrhea Neisseria gonorrhoeae Humans only STD, on the increase worldwide. Usually not fatal.
Hansen’s disease Mycobacterium leprae Humans, feral armadillos Chronic infection of the skin; worldwide incidence
(leprosy) about 10–12 million, especially in Southeast Asia.
Spread through contact with infected individuals.
Lyme disease Borrelia bergdorferi Ticks, deer, small rodents Spread through bite of infected tick. Lesion
followed by malaise, fever, fatigue, pain, stiff neck,
and headache.
Peptic ulcers Helicobacter pylori Humans Originally thought to be caused by stress or diet, most
peptic ulcers now appear to be caused by this
bacterium; good news for ulcer sufferers as it can be
treated with antibiotics.
Plague Yersinia pestis Fleas of wild rodents: rats Killed
1
?4 of the population of Europe in the 14th
and squirrels century; endemic in wild rodent populations of the
western U.S. today.
Pneumonia Streptococcus, Humans Acute infection of the lungs, often fatal without
Mycoplasma, Chlamydia treatment
Tuberculosis Mycobacterium Humans An acute bacterial infection of the lungs, lymph, and
tuberculosis meninges. Its incidence is on the rise, complicated by
the development of new strains of the bacteria that
are resistant to antibiotics.
Typhoid fever Salmonella typhi Humans A systemic bacterial disease of worldwide incidence.
Less than 500 cases a year are reported in the U.S.
The disease is spread through contaminated water or
foods (such as improperly washed fruits and
vegetables). Vaccines are available for travelers.
Typhus Rickettsia typhi Lice, rat fleas, humans Historically a major killer in times of crowding and
poor sanitation; transmitted from human to human
through the bite of infected lice and fleas. Typhus has
a peak untreated mortality rate of 70%.
Sexually Transmitted Diseases
A number of bacteria cause sexually transmitted diseases
(STDs). Three are particularly important (figure 34.10).
Gonorrhea. Gonorrhea is one of the most prevalent
communicable diseases in North America. Caused by the
bacterium Neisseria gonorrhoeae, gonorrhea can be transmit-
ted through sexual intercourse or any other sexual contacts
in which body fluids are exchanged, such as oral or anal in-
tercourse. Gonorrhea can infect the throat, urethra, cervix,
or rectum and can spread to the eyes and internal organs,
causing conjunctivitis (a severe infection of the eyes) and
arthritic meningitis (an infection of the joints). Left un-
treated in women, gonorrhea can cause pelvic inflamma-
tory disease (PID), a condition in which the fallopian tubes
become scarred and blocked. PID can eventually lead to
sterility. The incidence of gonorrhea has been on the de-
cline, but it remains a serious threat.
Syphilis. Syphilis, a very destructive STD, was once
prevalent but is now less common due to the advent of
blood-screening procedures and antibiotics. Syphilis is
caused by a spirochete bacterium, Treponema pallidum, that
is transmitted during sexual intercourse or through direct
contact with an open syphilis sore. The bacterium can also
be transmitted from a mother to her fetus, often causing
damage to the heart, eyes, and nervous system of the baby.
Once inside the body, the disease progresses in four dis-
tinct stages. The first, or primary stage, is characterized by
the appearance of a small, painless, often unnoticed sore
called a chancre. The chancre resembles a blister and oc-
curs at the location where the bacterium entered the body
about three weeks following exposure. This stage of the
disease is highly infectious, and an infected person may un-
wittingly transmit the disease to others.
The second stage of syphilis is marked by a rash, a sore
throat, and sores in the mouth. The bacteria can be trans-
mitted at this stage through kissing or contact with an open
sore.
The third stage of syphilis is symptomless. This stage
may last for several years, and at this point, the person is no
longer infectious but the bacteria are still present in the
body, attacking the internal organs. The final stage of
syphilis is the most debilitating, however, as the damage
done by the bacteria in the third stage becomes evident.
Sufferers at this stage of syphilis experience heart disease,
mental deficiency, and nerve damage, which may include a
loss of motor functions or blindness.
Chlamydia. Sometimes called the “silent STD,” chlamy-
dia is caused by an unusual bacterium, Chlamydia trachoma-
tis, that has both bacterial and viral characteristics. Like a
bacterium, it is susceptible to antibiotics, and, like a virus, it
depends on its host to replicate its genetic material; it is an
obligate internal parasite. The bacterium is transmitted
through vaginal, anal, or oral intercourse with an infected
person.
Chlamydia is called the “silent STD” because women
usually experience no symptoms until after the infection
has become established. In part because of this symptom-
less nature, the incidence of chlamydia has skyrocketed, in-
creasing by more than sevenfold nationally since 1984. The
effects of an established chlamydia infection on the female
body are extremely serious. Chlamydia can cause pelvic in-
flammatory disease (PID), which can lead to sterility.
It has recently been established that infection of the re-
productive tract by chlamydia can cause heart disease.
Chlamydia produce a peptide similar to one produced by
cardiac muscle. As the body’s immune system tries to fight
off the infection, it recognizes this peptide. The similarity
between the bacterial and cardiac peptides confuses the im-
mune system and T cells attack cardiac muscle fibers, inad-
vertently causing inflammation of the heart and other
problems.
Within the last few years, two types of tests for chlamy-
dia have been developed that look for the presence of the
bacteria in the discharge from men and women. The treat-
ment for chlamydia is antibiotics, usually tetracycline (peni-
cillin is not effective against chlamydia). Any woman who
experiences the symptoms associated with this STD should
be tested for the presence of the chlamydia bacterium; oth-
erwise, her fertility may be at risk.
This discussion of STDs may give the impression that
sexual activity is fraught with danger, and in a way, it is. It
is folly not to take precautions to avoid STDs. The best
way to do this is to know one’s sexual partners well enough
to discuss the possible presence of an STD. Condom use
can also prevent transmission of most of the diseases. Re-
sponsibility for protection lies with each individual.
Bacterial diseases have a major impact worldwide.
Sexually transmitted diseases (STDs) are becoming
increasingly widespread among Americans as sexual
activity increases.
690 Part IX Viruses and Simple Organisms
1984
400
0
50
100
150
200
250
300
350
Year
Syphilis
Number of cases
(per 100,000 people)
1986 1988 1990 1992 1994 1996
Gonorrhea
Chlamydia
FIGURE 34.10
Trends in sexually transmitted diseases in the U.S.
Source: CDC, Atlanta, GA.
Importance of Bacteria
Bacteria were largely responsible for creating the proper-
ties of the atmosphere and the soil over billions of years.
They are metabolically much more diverse than eukary-
otes, which is why they are able to exist in such a wide
range of habitats. The many autotrophic bacteria—either
photosynthetic or chemoautotrophic—make major contri-
butions to the carbon balance in terrestrial, freshwater, and
marine habitats. Other heterotrophic bacteria play a key
role in world ecology by breaking down organic com-
pounds. One of the most important roles of bacteria in the
global ecosystem relates to the fact that only a few genera
of bacteria—and no other organisms—have the ability to
fix atmospheric nitrogen and thus make it available for use
by other organisms (see chapter 28).
Bacteria are very important in many industrial
processes. Bacteria are used in the production of acetic
acid and vinegar, various amino acids and enzymes, and
especially in the fermentation of lactose into lactic acid,
which coagulates milk proteins and is used in the produc-
tion of almost all cheeses, yogurt, and similar products. In
the production of bread and other foods, the addition of
certain strains of bacteria can lead to the enrichment of
the final product with respect to its mix of amino acids, a
key factor in its nutritive value. Many products tradition-
ally manufactured using yeasts, such as ethanol, can also
be made using bacteria. The comparative economics of
these processes will determine which group of organisms
is used in the future. Many of the most widely used antibi-
otics, including streptomycin, aureomycin, erythromycin,
and chloromycetin, are derived from bacteria. Most an-
tibiotics seem to be substances used by bacteria to com-
pete with one another and fungi in nature, allowing one
species to exclude others from a favored habitat. Bacteria
can also play a part in removing environmental pollutants
(figure 34.11)
Bacteria and Genetic Engineering
Applying genetic engineering methods to produce im-
proved strains of bacteria for commercial use, as discussed
in chapter 19, holds enormous promise for the future. Bac-
teria are under intense investigation, for example, as non-
polluting insect control agents. Bacillus thuringiensis attacks
insects in nature, and improved, highly specific strains of B.
thuringiensis have greatly increased its usefulness as a bio-
logical control agent. Bacteria have also been extraordinar-
ily useful in our attempts to understand genetics and mole-
cular biology.
Bacteria play a major role in modern medicine and
agriculture, and have profound ecological impact.
Chapter 34 Bacteria 691
FIGURE 34.11
Using bacteria to clean up oil spills. Bacteria can often be used to remove environmental pollutants, such as petroleum hydrocarbons
and chlorinated compounds. In areas contaminated by the Exxon Valdez oil spill (rocks on the left), oil-degrading bacteria produced
dramatic results (rocks on the right).
692 Part IX Viruses and Simple Organisms
Chapter 34
Summary Questions Media Resources
34.1 Bacteria are the smallest and most numerous organisms.
? Bacteria are the oldest and simplest organisms, but
they are metabolically much more diverse than all
other life-forms combined.
? Bacteria differ from eukaryotes in many ways, the
most important of which concern the degree of
internal organization within the cell.
1. Structural differences among
bacteria are not great. How are
different species of bacteria
recognized?
2. In what seven ways do
prokaryotes differ substantially
from eukaryotes?
? Most bacteria have cell walls that consist of a network
of polysaccharide molecules connected by
polypeptide cross-links.
? A bacterial cell does not possess specialized
compartments or a membrane-bounded nucleus, but
it may exhibit a nucleoid region where the bacterial
DNA is located.
3. What is the structure of the
bacterial cell wall? How does the
cell wall differ between gram-
positive and gram-negative
bacteria? In general, which type
of bacteria is more resistant to
the action of most antibiotics?
Why?
34.2 Bacterial cell structure is more complex than commonly supposed.
? The two bacterial kingdoms, Archaebacteria and
Eubacteria, are made up of prokaryotes, with about
5000 species named so far.
? The Archaebacteria differ markedly from Eubacteria
and from eukaryotes in their ribosomal sequences and
in other respects.
? Mutation and genetic recombination are important
sources of variability in bacteria.
? Many bacteria are autotrophic and make major
contributions to the world carbon balance. Others are
heterotrophic and play a key role in world ecology by
breaking down organic compounds.
? Some heterotrophic bacteria cause major diseases in
plants and animals.
4. How do the Archaebacteria
differ from the Eubacteria?
What unique metabolism do
they exhibit?
5. Why does mutation play such
an important role in creating
genetic diversity in bacteria?
6. How do heterotrophic
bacteria that are successful
pathogens overcome the many
defenses the human body uses to
ward off disease?
34.3 Bacteria exhibit considerable diversity in both structure and metabolism.
? Human diseases caused by heterotrophic bacteria
include many fatal diseases that have had major
impacts on human history, including tuberculosis,
cholera, plague, and typhus.
? Bacteria play vital roles in cycling nutrients within
ecosystems. Certain bacteria are the only organisms
able to fix atmospheric nitrogen into organic
molecules, a process on which all life depends.
7. What are STDs? How are
they transmitted? Which STDs
are caused by viruses and which
are caused by bacteria? Why is
the cause of chlamydia unusual?
34.4 Bacteria are responsible for many diseases but also make important contributions to ecosystems.
? Enhancement
Chapter:
Extremophilic
Bacteria, Introduction
and Section 1
? Characteristics of
Bacteria
? Enhancement
Chapter:
Extremophilic
Bacteria, Section 2
? Bacteria Diversity
? Scientists on Science:
Marine
Biotechnology
? Enhancement
Chapter:
Extremophilic
Bacteria, Section 3
? Student Research:
Improving Antibiotics
? On Science Article:
Antibiotic Resistance
www.mhhe.com/raven6e www.biocourse.com
693
35
Protists
Concept Outline
35.1 Eukaryotes probably arose by endosymbiosis.
Endosymbiosis. Mitochondria and chloroplasts are
thought to have arisen by endosymbiosis from aerobic
bacteria.
35.2 The kingdom Protista is by far the most diverse
of any kingdom.
The Challenge of Classifying the Protists. There is no
general agreement among taxonomists about how to
classify the protists.
General Biology of the Protists. Protista contains
members exhibiting a wide range of methods of
locomotion, nutrition, and reproduction.
35.3 Protists can be categorized into five groups.
Five Groups of Protists. The 15 major phyla of protists
can be conveniently discussed in seven general groups that
share certain characteristics.
Heterotrophs with No Permanent Locomotor
Apparatus. Amoebas and other sarcodines have no
permanent locomotor apparatus.
Photosynthetic Protists. The flagellates are
photosynthesizers that propel themselves through the water
with flagella. Diatoms are photosynthesizers with hard
shells of silica. Algae are photosynthetic protists, some are
multicellular.
Heterotrophs with Flagella. Flagellates propel
themselves through the water. Single cells with many cilia,
the ciliates possess highly complex and specialized
organelles.
Nonmotile Spore-Formers. The sporozoans are
nonmotile parasites that spread by forming spores.
Heterotrophs with Restricted Mobility. Heterotrophs
with restricted mobility, molds have cell walls made of
carbohydrate.
F
or more than half of the long history of life on earth,
all life was microscopic in size. The biggest organisms
that existed for over 2 billion years were single-celled bac-
teria fewer than 6 micrometers thick. The first evidence of
a different kind of organism is found in tiny fossils in rock
1.5 billion years old. These fossil cells are much larger than
bacteria (some as big as 60 micrometers in diameter) and
have internal membranes and what appear to be small,
membrane-bounded structures. Many have elaborate
shapes, and some exhibit spines or filaments. These new,
larger fossil organisms mark one of the most important
events in the evolution of life, the appearance of a new kind
of organism, the eukaryote (figure 35.1). Flexible and
adaptable, the eukaryotes rapidly evolved to produce all of
the diverse large organisms that populate the earth today,
including ourselves—indeed, all organisms other than bac-
teria are eukaryotes.
FIGURE 35.1
Volvox, a colonial protist. The protists are a large, diverse group
of primarily single-celled organisms, a group from which the
other three eukaryotic kingdoms each evolved.
contained a complex system of inter-
nal membranes. The inner membrane
of mitochondria is folded into numer-
ous layers, resembling the folded
membranes of nonsulfur purple bacte-
ria; embedded within this membrane
are the proteins that carry out oxida-
tive metabolism. The engulfed bacte-
ria became the interior portion of the
mitochondria we see today. Host cells
were unable to carry out the Krebs
cycle or other metabolic reactions
necessary for living in an atmosphere
that contained increasing amounts of
oxygen before they had acquired these
bacteria.
During the billion and a half years in
which mitochondria have existed as en-
dosymbionts within eukaryotic cells,
most of their genes have been trans-
ferred to the chromosomes of the host
cells—but not all. Each mitochondrion
still has its own genome, a circular,
closed molecule of DNA similar to that
found in eubacteria, on which is located
genes encoding the essential proteins of
oxidative metabolism. These genes are
transcribed within the mitochondrion,
using mitochondrial ribosomes that are
smaller than those of eukaryotic cells, very much like bacte-
rial ribosomes in size and structure. Mitochondria divide by
simple fission, just as bacteria do, replicating and sorting
their DNA much as bacteria do. However, nuclear genes
direct the process, and mitochondria cannot be grown out-
side of the eukaryotic cell, in cell-free culture.
The theory of endosymbiosis has had a controversial
history but has now been accepted by all but a few biolo-
gists. The evidence supporting the theory is so extensive
that in this text we will treat it as established.
What of mitosis, the other typical eukaryotic process
that Pelomyxa lacks? The mechanism of mitosis, now so
common among eukaryotes, did not evolve all at once.
Traces of very different, and possibly intermediate,
mechanisms survive today in some of the eukaryotes. In
fungi and some groups of protists, for example, the nu-
clear membrane does not dissolve and mitosis is confined
to the nucleus. When mitosis is complete in these organ-
isms, the nucleus divides into two daughter nuclei, and
only then does the rest of the cell divide. This separate
nuclear division phase of mitosis does not occur in most
protists, or in plants or animals. We do not know if it
represents an intermediate step on the evolutionary jour-
ney to the form of mitosis that is characteristic of most
694 Part IX Viruses and Simple Organisms
Endosymbiosis
What was the first eukaryote like? We
cannot be sure, but a good model is
Pelomyxa palustris, a single-celled,
nonphotosynthetic organism that ap-
pears to represent an early stage in
the evolution of eukaryotic cells (fig-
ure 35.2). The cells of Pelomyxa are
much larger than bacterial cells and
contain a complex system of internal
membranes. Although they resemble
some of the largest early fossil eukary-
otes, these cells are unlike those of
any other eukaryote: Pelomyxa lacks
mitochondria and does not undergo
mitosis. Its nuclei divide somewhat as
do those of bacteria, by pinching apart
into two daughter nuclei, around
which new membranes form. Al-
though Pelomyxa cells lack mitochon-
dria, two kinds of bacteria living
within them may play the same role
that mitochondria do in all other eu-
karyotes. This primitive eukaryote is
so distinctive that it is assigned a phy-
lum all its own, Caryoblastea.
Biologists know very little of the
origin of Pelomyxa, except that in many
of its fundamental characteristics it resembles the archae-
bacteria far more than the eubacteria. Because of this gen-
eral resemblance, it is widely assumed that the first eukary-
otic cells were nonphotosynthetic descendants of
archaebacteria.
What about the wide gap between Pelomyxa and all
other eukaryotes? Where did mitochondria come from?
Most biologists agree with the theory of endosymbiosis,
which proposes that mitochondria originated as symbiotic,
aerobic (oxygen-requiring) eubacteria (figure 35.3). Sym-
biosis (Greek, syn, “together with” + bios, “life”) means liv-
ing together in close association. Recall from chapter 5 that
mitochondria are sausage-shaped organelles 1 to 3 microm-
eters long, about the same size as most eubacteria. Mito-
chondria are bounded by two membranes. Aerobic eubac-
teria are thought to have become mitochondria when they
were engulfed by ancestral eukaryotic cells, much like
Pelomyxa, early in the history of eukaryotes.
The most similar eubacteria to mitochondria today
are the nonsulfur purple bacteria, which are able to carry
out oxidative metabolism (described in chapter 9). In mi-
tochondria, the outer membrane is smooth and is
thought to be derived from the endoplasmic reticulum of
the host cell, which, like Pelomyxa, may have already
35.1 Eukaryotes probably arose by endosymbiosis.
FIGURE 35.2
Pelomyxa palustris.This unique, amoeba-
like protist lacks mitochondria and does not
undergo mitosis. Pelomyxamay represent a
very early stage in the evolution of
eukaryotic cells.
eukaryotes today or if it is simply a different way of solv-
ing the same problem. There are no fossils in which we
can see the interiors of dividing cells well enough to be
able to trace the history of mitosis.
Endosymbiosis Is Not Rare
Many eukaryotic cells contain other endosymbiotic bacteria
in addition to mitochondria. Plants and algae contain
chloroplasts, bacteria-like organelles that were apparently
derived from symbiotic photosynthetic bacteria. Chloro-
plasts have a complex system of inner membranes and a cir-
cle of DNA. Centrioles, organelles associated with the as-
sembly of microtubules, resemble in many respects
spirochete bacteria, and they contain bacteria-like DNA in-
volved in the production of their structural proteins.
While all mitochondria are thought to have arisen from a
single symbiotic event, it is difficult to be sure with chloro-
plasts. Three biochemically distinct classes of chloroplasts
exist, each resembling a different bacterial ancestor. Red
algae possess pigments similar to those of cyanobacteria;
plants and green algae more closely resemble the photosyn-
thetic bacteria Prochloron; while brown algae and other pho-
tosynthetic protists resemble a third group of bacteria. This
diversity of chloroplasts has led to the widely held belief that
eukaryotic cells acquired chloroplasts by symbiosis at least
three different times. Recent comparisons of chloroplast
DNA sequences, however, suggest a single origin of chloro-
plasts, followed by very different postendosymbiotic histo-
ries. For example, in each of the three main lines, different
genes became relocated to the nucleus, lost, or modified.
The theory of endosymbiosis proposes that
mitochondria originated as symbiotic aerobic
eubacteria.
Chapter 35 Protists 695
Photosynthetic
bacterium
Ancestral eukaryotic cell
Eukaryotic cell with mitochondrion
Internal
membrane
system
Aerobic
bacterium
Mitochondrion
Chloroplast
Eukaryotic cell with chloroplasts
Endosymbiosis
Endosymbiosis
FIGURE 35.3
The theory of endosymbiosis.Scientists propose that ancestral eukaryotic cells, which already had an internal system of membranes,
engulfed aerobic eubacteria, which then became mitochondria in the eukaryotic cell. Chloroplasts may also have originated this way, with
eukaryotic cells engulfing photosynthetic eubacteria.
The Challenge of Classifying
the Protists
Protists are the most diverse of the four kingdoms in the
domain Eukaryota. The kingdom Protista contains many
unicellular, colonial, and multicellular groups. Probably the
most important statement we can make about the kingdom
Protista is that it is an artificial group; as a matter of conve-
nience, single-celled eukaryotic organisms have typically
been grouped together into this kingdom. This lumps
many very different and only distantly related forms to-
gether. The “single-kingdom” classification of the Protista
is not representative of any evolutionary relationships. The
phyla of protists are, with very few exceptions, only dis-
tantly related to one another.
New applications of a wide variety of molecular methods
are providing important insights into the relationships
among the protists. Of all the groups of organisms biolo-
gists study, protists are probably in the greatest state of flux
when it comes to classification. There is little consensus,
even among experts, as to how the different kinds of protists
should be classified. Are they a single, very diverse kingdom,
or are they better considered as several different kingdoms,
each of equal rank with animals, plants, and fungi?
Because the Protista are still predominantly considered
part of one diverse, nonunified group, that is how we will
treat them in this chapter, bearing in mind that biologists
are rapidly gaining a better understanding of the evolution-
ary relationships among members of the kingdom Protista
(figure 35.4). It seems likely that within a few years, the tra-
ditional kingdom Protista will be replaced by another more
illuminating arrangement.
The taxonomy of the protists is in a state of flux as new
information shapes our understanding of this kingdom.
696 Part IX Viruses and Simple Organisms
35.2 The kingdom Protista is by far the most diverse of any kingdom.
Animals
Ciliates and dinoflagellates
Euglenoids, cellular slime
molds, water molds,
trypanosomes
Rhizopods, plasmodial
slime molds, fungi
(basidiomycetes)
Plants
Fungi (ascomycetes)
Red algae
Analysis Based on Ribosomal Subunits
Red algae
KINGDOM
CHROMISTA
10 protist phyla,
including the more
familiar 7 “heterokont”
phyla: brown algae,
slime nets, and diatoms
Photosynthetic protists
(Volvox, Spirogyra)
and plants
Most rhizopods, water molds,
diatoms, brown algae,
heliozoans, slime nets
Heterotrophic symbiotic
flagellates (Trichomonas,
Trichonympha, Giardia)
Choanoflagellates and animals
Amoeboflagellates and
cellular slime molds
Euglenoids
Dinoflagellates and ciliates
Cladistic Analysis
(Prokaryotic Ancestor)
Six Eukaryotic Kingdoms
KINGDOM ARCHEZOA
Primitive amitochondrial
forms, including Pelomyxa,
Giardia
KINGDOM FUNGI
Fungi and 1 phylum
of saprophytic protists
(chytridiomycota)
KINGDOM ANIMALIA
(no protist phyla)
KINGDOM PLANTAE
Plants and 5 protist
phyla, including the
green algae (Volvox,
Ulva, Spirogyra) and
the red algae
KINGDOM PROTOZOA
14 protist phyla, including
hypermastigotes (Trichonympha),
euglenoids, slime molds,
choanoflagellates, dinoflagellates,
ciliates, apicomplexans, rhizopods,
heliozoans, foraminiferans, and
radiolarians
FIGURE 35.4
The challenge of protistan classification. Three different
suggestions for protistan classification are presented, each adapted
from the work of an authority in the field. Their great differences
attest to the wide divergence of opinion within the field itself.
The classification on the top is based on molecular variation in
ribosomal subunits. The classification in the middle presents a
cladistic analysis of a broad range of characters (including
ribosomal subunits). The classification on the bottom outlines a
more revolutionary reevaluation of the protists. Comparison of
the three schemes reveals that some groups are commonly
recognized as related (like ciliates and dinoflagellates), while the
classification of others (like Giardia) is clearly in a state of flux.
General Biology
of the Protists
Protists are united on the basis of a
single negative characteristic: they
are not fungi, plants, or animals. In
all other respects they are highly
variable with no uniting features.
Many are unicellular (figure 35.5),
but there are numerous colonial and
multicellular groups. Most are mi-
croscopic, but some are as large as
trees. They represent all symmetries,
and exhibit all types of nutrition.
The Cell Surface
Protists possess a varied array of cell
surfaces. Some protists, like amoebas,
are surrounded only by their plasma
membranes. All other protists have a
plasma membrane but some, like
algae and molds, are encased within
strong cell walls. Still others, like di-
atoms and forams, secrete glassy
shells of silica.
Locomotor Organelles
Movement in protists is also accomplished by diverse mech-
anisms. Protists move chiefly by either flagellar rotation or
pseudopodial movement. Many protists wave one or more
flagella to propel themselves through the water, while oth-
ers use banks of short, flagella-like structures called cilia to
create water currents for their feeding or propulsion.
Pseudopodia are the chief means of locomotion among
amoeba, whose pseudopods are large, blunt extensions of
the cell body called lobopodia. Other related protists extend
thin, branching protrusions called filopodia. Still other pro-
tists extend long, thin pseudopodia called axopodia sup-
ported by axial rods of microtubules. Axopodia can be ex-
tended or retracted. Because the tips can adhere to adjacent
surfaces, the cell can move by a rolling motion, shortening
the axopodia in front and extending those in the rear.
Cyst Formation
Many protists with delicate surfaces are successful in quite
harsh habitats. How do they manage to survive so well?
They survive inhospitable conditions by forming cysts. A
cyst is a dormant form of a cell with a resistant outer cover-
ing in which cell metabolism is more or less completely
shut down. Not all cysts are so sturdy. Vertebrate parasitic
amoebae, for example, form cysts that are quite resistant to
gastric acidity, but will not tolerate desiccation or high
temperature.
Nutrition
Protists employ every form of nutri-
tional acquisition except chemoau-
totrophic, which has so far been ob-
served only in bacteria. Some protists
are photosynthetic autotrophs and are
called phototrophs. Others are het-
erotrophs that obtain energy from or-
ganic molecules synthesized by other or-
ganisms. Among heterotrophic protists,
those that ingest visible particles of food
are called phagotrophs, or holozoic
feeders. Those ingesting food in soluble
form are called osmotrophs, or sapro-
zoic feeders.
Phagotrophs ingest food particles into
intracellular vesicles called food vac-
uoles or phagosomes. Lysosomes fuse
with the food vacuoles, introducing en-
zymes that digest the food particles
within. As the digested molecules are ab-
sorbed across the vacuolar membrane,
the food vacuole becomes progressively
smaller.
Reproduction
Protists typically reproduce asexually, reproducing sexu-
ally only in times of stress. Asexual reproduction involves
mitosis, but the process is often somewhat different from
the mitosis that occurs in multicellular animals. The nu-
clear membrane, for example, often persists throughout
mitosis, with the microtubular spindle forming within it.
In some groups, asexual reproduction involves spore for-
mation, in others fission. The most common type of fis-
sion is binary, in which a cell simply splits into nearly
equal halves. When the progeny cell is considerably
smaller than its parent, and then grows to adult size, the
fission is called budding. In multiple fission, or schizo-
gony, common among some protists, fission is preceded
by several nuclear divisions, so that fission produces sev-
eral individuals almost simultaneously.
Sexual reproduction also takes place in many forms
among the protists. In ciliates and some flagellates, ga-
metic meiosis occurs just before gamete formation, as it
does in metazoans. In the sporozoans, zygotic meiosis oc-
curs directly after fertilization, and all the individuals that
are produced are haploid until the next zygote is formed. In
algae, there is intermediary meiosis, producing an alter-
nation of generations similar to that seen in plants, with
significant portions of the life cycle spent as haploid as well
as diploid.
Protists exhibit a wide range of forms, locomotion,
nutrition and reproduction.
Chapter 35 Protists 697
FIGURE 35.5
A unicellular protist. The protist kingdom
is a catch-all kingdom for many different
groups of unicellular organisms, such as this
Vorticella(phylum Ciliophora), which is
heterotrophic, feeds on bacteria, and has a
retractable stalk.
Five Groups of Protists
There are some 15 major phyla of protists. It is difficult to
encompass their great diversity with any simple scheme.
Traditionally, texts have grouped them artificially (as was
done in the nineteenth century) into photosynthesizers
(algae), heterotrophs (protozoa), and absorbers (funguslike
protists).
In this text, we will group the protists into five gen-
eral groups according to some of the major shared char-
acteristics (figure 35.6). These are characteristics that
taxonomists are using today in broad attempts to classify
the kingdom Protista. These include (1) the presence or
absence and type of cilia or flagella, (2) the presence and
kinds of pigments, (3) the type of mitosis, (4) the kinds
of cristae present in the mitochondria, (5) the molecular
genetics of the ribosomal “S” subunit, (6) the kind of in-
clusions the protist may have, (7) overall body form
(amoeboid, coccoid, and so forth), (8) whether the pro-
tist has any kind of shell or other body “armor,” and
(9)modes of nutrition and movement. These represent
only some of the characters used to define phylogenetic
relationships.
The five criteria we have chosen to define groups are
not the only ones that might be chosen, and there is no
broad agreement among biologists as to which set of crite-
ria is preferable. As molecular analysis gives us a clearer
picture of the phylogenetic relationships among the pro-
tists, more evolutionarily suitable groupings will without a
doubt replace the one represented here. Table 35.1 sum-
marizes some of the general characteristics and groupings
of the 15 major phyla of protists. It is important to remem-
ber that while the phyla of protists discussed here are gen-
erally accepted taxa, the larger groupings of phyla pre-
sented are functional groupings.
The 15 major protist phyla can be conveniently
categorized into five groups according to major shared
characteristics.
698 Part IX Viruses and Simple Organisms
35.3 Protists can be categorized into five groups.
Rhizopoda
Actinopoda
Foraminifera
Pyrrhophyta
Euglenophyta
Chrysophyta
Rhodophyta
Phaeophyta
Chlorophyta
Sarcomastigophora
Ciliophora
Apicomplexa
Oomycota
Acrasiomycota
Myxomycota
Protists
Heterotrophs with no permanent
locomotor apparatus
Photosynthetic protists
Heterotrophs with flagella
Nonmotile spore-formers
Heterotrophs with restricted mobility
FIGURE 35.6
Five general groups of protists. This text presents the 15 major phyla of protists in five groups that share major characteristics.
Chapter 35 Protists 699
Table 35.1 Kinds of Protists
Typical
Group Phylum Examples Key Characteristics
HETEROTROPHS WITH NO PERMANENT LOCOMOTOR APPARATUS
Amoebas Rhizopoda Amoeba Move by pseudopodia
Radiolarians Actinopoda Radiolarians Glassy skeletons; needlelike pseudopods
Forams Foraminifera Forams Rigid shells; move by protoplasmic
streaming
PHOTOSYNTHETIC PROTISTS
Dinoflagellates Pyrrhophyta Red tides Photosynthetic; unicellular; two flagella;
contain chlorophylls aand b
Euglenoids Euglenophyta Euglena Some photosynthetic; others
heterotrophic; unicellular; contain
chlorophylls aand b or none
Diatoms Chrysophyta Diatoma Unicellular; manufacture the
carbohydrate chrysolaminarin; unique
double shells of silica; contain
chlorophylls aand c
Golden algae Chrysophyta Golden algae Unicellular, but often colonial;
manufacture the carbohydrate
chrysolaminarin; contain
chlorophylls aand c
Red Rhodophyta Coralline algae Most multicellular; contain
chlorophyll aand a red pigment
Brown Phaeophyta Kelp Multicellular; contain chlorophylls aand c
Green Chlorophyta Chlamydomonas Unicellular or multicellular; contain
chlorophylls aand b
HETEROTROPHS WITH FLAGELLA
Zoomastigotes Sarcomastigophora Trypanosomes Heterotrophic; unicellular
Ciliates Ciliophora Paramecium Heterotrophic unicellular protists with
cells of fixed shape possessing two nuclei
and many cilia; many cells also contain
highly complex and specialized organelles
NONMOTILE SPORE-FORMERS
Sporozoans Apicomplexa Plasmodium Nonmotile; unicellular; the apical end
of the spores contains a complex mass
of organelles
HETEROTROPHS WITH RESTRICTED MOBILITY
Water molds Oomycota Water molds, Terrestrial and freshwater
rusts, and mildew
Cellular slime Acrasiomycota Dictyostelium Colonial aggregations of individual cells;
molds most closely related to amoebas
Plasmodial Myxomycota Fuligo Stream along as a multinucleate
slime molds mass of cytoplasm
Heterotrophs with No
Permanent Locomotor
Apparatus
The largest of the five general groups
of protists are primarily unicellular or-
ganisms with amoeboid forms. There
are three principle phyla: the forams
and the radiolarians have carbonate
shells and the rhizopods lack shells.
Rhizopoda: The Amoebas
Hundreds of species of amoebas are
found throughout the world in both
fresh and salt waters. They are also
abundant in soil. Many kinds of amoe-
bas are parasites of animals. Reproduc-
tion in amoebas occurs by fission, or
the direct division into two cells of
equal volume. Amoebas of the phylum
Rhizopoda lack cell walls, flagella,
meiosis, and any form of sexuality.
They do undergo mitosis, with a spin-
dle apparatus that resembles that of
other eukaryotes.
Amoebas move from place to place
by means of their pseudopods, from
the Greek words for “false” and “foot”
(figure 35.7). Pseudopods are flowing
projections of cytoplasm that extend
and pull the amoeba forward or engulf
food particles, a process called cyto-
plasmic streaming. An amoeba puts a
pseudopod forward and then flows into
it. Microfilaments of actin and myosin
similar to those found in muscles are
associated with these movements. The
pseudopodia can form at any point on
the cell body so that it can move in any
direction.
Some kinds of amoebas form resis-
tant cysts. In parasitic species such as
Entamoeba histolytica, which causes
amoebic dysentery, cysts enable the amoebas to resist di-
gestion by their animal hosts. Mitotic division takes place
within the cysts, which ultimately rupture and release
four, eight, or even more amoebas within the digestive
tracts of their host animals. The primary infection takes
place in the intestine, but it often moves into the liver and
other parts of the body. The cysts are dispersed in the
feces and may be transmitted from person to person in in-
fected food or water, or by flies. It is estimated that up to
10 million people in the United States have infections of
parasitic amoebas, and some 2 million show symptoms of
the disease, ranging from abdominal discomfort with
slight diarrhea to much more serious
conditions. In some tropical areas,
more than half of the population may
be infected. The spread of amoebic
dysentery can be limited by proper
sanitation and hygiene.
Actinopoda: The Radiolarians
The pseudopodia of amoeboid cells
give them truly amorphous bodies.
One group, however, have more dis-
tinct structures. Members of the phy-
lum Actinopoda, often called radiolari-
ans, secrete glassy exoskeletons made
of silica. These skeletons give the uni-
cellular organisms a distinct shape, ex-
hibiting either bilateral or radial sym-
metry. The shells of different species
form many elaborate and beautiful
shapes and its pseudopodia extrude
outward along spiky projections of the
skeleton (figure 35.8). Microtubules
support these cytoplasmic projections.
Foraminifera: Forams
Members of the phylum Foraminifera
are heterotrophic marine protists.
They range in diameter from about 20
micrometers to several centimeters.
Characteristic of the group are pore-
studded shells (called tests) composed
of organic materials usually reinforced
with grains of inorganic matter. These
grains may be calcium carbonate, sand,
or even plates from the shells of echin-
oderms or spicules (minute needles of
calcium carbonate) from sponge skele-
tons. Depending on the building mate-
rials they use, foraminifera—often in-
formally called “forams”—may have
shells of very different appearance.
Some of them are brilliantly colored
red, salmon, or yellow-brown.
Most foraminifera live in sand or are attached to other
organisms, but two families consist of free-floating plank-
tonic organisms. Their tests may be single-chambered but
more often are multichambered, and they sometimes have a
spiral shape resembling that of a tiny snail. Thin cytoplas-
mic projections called podia emerge through openings in
the tests (figure 35.9). Podia are used for swimming, gath-
ering materials for the tests, and feeding. Forams eat a wide
variety of small organisms.
The life cycles of foraminifera are extremely complex,
involving an alternation between haploid and diploid gen-
erations (sporic meiosis). Forams have contributed massive
700 Part IX Viruses and Simple Organisms
FIGURE 35.7
Amoeba proteus. This relatively large
amoeba is commonly used in teaching and
for research in cell biology. The projections
are pseudopods; an amoeba moves by
flowing into them. The nucleus of the
amoeba is plainly visible.
Heterotrophs with no permanent
locomotor apparatus
Photosynthetic protists
Heterotrophs with flagella
Nonmotile spore-formers
Heterotrophs with restricted mobility
accumulations of their tests to the fossil record for more
than 200 million years. Because of the excellent preserva-
tion of their tests and the often striking differences among
them, forams are very important as geological markers.
The pattern of occurrence of different forams is often used
as a guide in searching for oil-bearing strata. Limestones
all over the world, including the famous white cliffs of
Dover in southern England, are often rich in forams (fig-
ure 35.10).
Amoebas, radiolarians, and forams are unicellular,
heterotrophic protists that lack cell walls, flagella,
meiosis, and sexuality. Amoebas move from place to
place by means of extensions called pseudopodia. The
pore-studded tests, or shells, of the forams have
openings through which podia extend that are used for
locomotion.
Chapter 35 Protists 701
FIGURE 35.8
Actinosphaerium, a protist of the phylum Actinopoda (300×).
This amoeba-like radiolarian has striking needlelike pseudopods.
FIGURE 35.9
A representative of the Foraminifera (90×). A living foram with
podia, thin cytoplasmic projections that extend through pores in
the calcareous test, or shell, of the organism.
FIGURE 35.10
White cliffs of Dover. The limestone that forms these cliffs is
composed almost entirely of fossil shells of protists, including
coccolithophores (a type of algae) and foraminifera.
Photosynthetic Protists
Pyrrhophyta: The
Dinoflagellates
The dinoflagellates consist of about
2100 known species of primarily uni-
cellular, photosynthetic organisms,
most of which have two flagella. A
majority of the dinoflagellates are ma-
rine, and they are often abundant in
the plankton, but some occur in fresh
water. Some planktonic dinoflagel-
lates are luminous and contribute to
the twinkling or flashing effects that
we sometimes see in the sea at night,
especially in the tropics.
The flagella, protective coats, and
biochemistry of dinoflagellates are
distinctive, and they do not appear to
be directly related to any other phy-
lum. Plates made of a cellulose-like
material encase the cells. Grooves
form at the junctures of these plates
and the flagella are usually located
within these grooves, one encircling
the body like a belt, and the other
perpendicular to it. By beating in
their respective grooves, these fla-
gella cause the dinoflagellate to ro-
tate like a top as it moves. The di-
noflagellates that are clad in stiff
cellulose plates, often encrusted with
silica, may have a very unusual ap-
pearance (figure 35.11). Most have
chlorophylls a and c, in addition to
carotenoids, so that in the biochem-
istry of their chloroplasts, they re-
semble the diatoms and the brown
algae, possibly acquiring such chloro-
plasts by forming endosymbiotic re-
lationships with members of those
groups.
Some dinoflagellates occur as sym-
bionts in many other groups of or-
ganisms, including jellyfish, sea
anemones, mollusks, and corals.
When dinoflagellates grow as sym-
bionts within other cells, they lack
their characteristic cellulose plates
and flagella, appearing as spherical,
golden-brown globules in their host
cells. In such a state they are called
zooxanthellae. Photosynthetic
zooxanthellae provide their hosts
with nutrients. It is the photosynthe-
sis conducted by zooxanthellae that
makes coral reefs one of the most pro-
ductive ecosystems on earth. Corals
primarily live in warm tropical seas
that are typically extremely low in nu-
trients; without the aid of their photo-
synthetic endosymbionts, they would
not be able to form large reefs in the
nutrient-poor environment. Most of
the carbon that the zooxanthellae fix is
translocated to the host corals.
The poisonous and destructive “red
tides” that occur frequently in coastal
areas are often associated with great
population explosions, or “blooms,” of
dinoflagellates. The pigments in the in-
dividual, microscopic cells of the di-
noflagellates are responsible for the
color of the water. Red tides have a pro-
found, detrimental effect on the fishing
industry in the United States. Some 20
species of dinoflagellates are known to
produce powerful toxins that inhibit the
diaphragm and cause respiratory failure
in many vertebrates. When the toxic di-
noflagellates are abundant, fishes, birds,
and marine mammals may die in large
numbers.
More recently, a particularly dan-
gerous toxic dinoflagellate called Pfies-
teria piscicida is reported to be a carniv-
orous, ambush predator. During
blooms, it stuns fish with its toxin and
then feeds on the prey’s body fluids.
Dinoflagellates reproduce primarily
by asexual cell division. But sexual re-
production has been reported to occur
under starvation conditions. They
have a unique form of mitosis in which
the permanently condensed chromo-
somes divide longitudinally within the
confines of a permanent nuclear enve-
lope. After the numerous chromo-
somes duplicate, the nucleus divides
into two daughter nuclei. Also the di-
noflagellate chromosome is unique
among eukaryotes in that the DNA is
not complexed with histone proteins.
In all other eukaryotes, the chromoso-
mal DNA is complexed with histones
to form nucleosomes, which represents
the first order of DNA packaging in
the nucleus. How dinoflagellates are
able to maintain distinct chromosomes
without histones and nucleosomes re-
mains a mystery.
702 Part IX Viruses and Simple Organisms
Noctiluca Ptychodiscus
Ceratium Gonyaulax
Heterotrophs with no permanent
locomotor apparatus
Photosynthetic protists
Heterotrophs with flagella
Nonmotile spore-formers
Heterotrophs with restricted mobility
FIGURE 35.11
Some dinoflagellates: Noctiluca,
Ptychodiscus, Ceratium, and Gonyaulax.
Noctiluca,which lacks the heavy cellulose
armor characteristic of most dinoflagellates,
is one of the bioluminescent organisms that
causes the waves to sparkle in warm seas. In
the other three genera, the shorter, encircling
flagellum is seen in its groove, with the
longer one projecting away from the body of
the dinoflagellate. (Not drawn to scale.)
Euglenophyta: The Euglenoids
Most of the approximately 1000 known species of eugle-
noids live in fresh water. The members of this phylum
clearly illustrate the impossibility of distinguishing “plants”
from “animals” among the protists. About a third of the ap-
proximately 40 genera of euglenoids have chloroplasts and
are fully autotrophic; the others lack chloroplasts, ingest
their food, and are heterotrophic. These organisms are not
significantly different from some groups of zoomastigotes
(see next section), and many biologists believe that the two
phyla should be merged into one.
Some euglenoids with chloroplasts may become het-
erotrophic if the organisms are kept in the dark; the
chloroplasts become small and nonfunctional. If they are
put back in the light, they may become green within a few
hours. Normally photosynthetic euglenoids may sometimes
feed on dissolved or particulate food.
Individual euglenoids range from 10 to 500 micrometers
long and are highly variable in form. Interlocking proteina-
ceous strips arranged in a helical pattern form a flexible
structure called the pellicle, which lies within the cell
membrane of the euglenoids. Because its pellicle is flexible,
a euglenoid is able to change its shape. Reproduction in
this phylum occurs by mitotic cell division. The nuclear en-
velope remains intact throughout the process of mitosis.
No sexual reproduction is known to occur in this group.
In Euglena (figure 35.12), the genus for which the phy-
lum is named, two flagella are attached at the base of a
flask-shaped opening called the reservoir, which is located
at the anterior end of the cell. One of the flagella is long
and has a row of very fine, short, hairlike projections along
one side. A second, shorter flagellum is located within the
reservoir but does not emerge from it. Contractile vacuoles
collect excess water from all parts of the organism and
empty it into the reservoir, which apparently helps regulate
the osmotic pressure within the organism. The stigma, an
organ that also occurs in the green algae (phylum Chloro-
phyta), is light-sensitive and aids these photosynthetic or-
ganisms to move toward light.
Cells of Euglena contain numerous small chloroplasts.
These chloroplasts, like those of the green algae and plants,
contain chlorophylls aand b,together with carotenoids. Al-
though the chloroplasts of euglenoids differ somewhat in
structure from those of green algae, they probably had a
common origin. It seems likely that euglenoid chloroplasts
ultimately evolved from a symbiotic relationship through
ingestion of green algae.
Chapter 35 Protists 703
(a)
Reservoir
Pellicle
Basal body
Contractile
vacuole
Second
flagellum
Stigma
Flagellum
Nucleus
Chloroplast
Paramylon
granule
(b)
FIGURE 35.12
Euglenoids. (a) Micrograph of individuals of the genus Euglena
(Euglenophyta). (b) Diagram of Euglena.Paramylon granules are
areas where food reserves are stored.
Chrysophyta: The Diatoms and Golden Algae
The Diatoms. Diatoms, members of the phylum Chrys-
ophyta, are photosynthetic, unicellular organisms with
unique double shells made of opaline silica, which are often
strikingly and characteristically marked. The shells of di-
atoms are like small boxes with lids, one half of the shell fit-
ting inside the other. Their chloroplasts, with chlorophylls
aand c,as well as carotenoids, resemble those of the brown
algae and dinoflagellates. In other respects, however, there
are few similarities between these groups, and they proba-
bly do not share an immediate common ancestor. Another
member of the phylum Chrysophyta is the golden algae.
Diatoms and golden algae are grouped together because
they both produce a unique carbohydrate called chrysolam-
inarin.
There are more than 11,500 living species of diatoms,
with many more known in the fossil record. The shells of
fossil diatoms often form very thick deposits, which are
sometimes mined commercially. The resulting “diatoma-
ceous earth” is used as an abrasive or to add the sparkling
quality to the paint used on roads, among other purposes.
Living diatoms are often abundant both in the sea and in
fresh water, where they are important food producers. Di-
atoms occur in the plankton and are attached to submerged
objects in relatively shallow water. Many species are able to
move by means of a secretion that is produced from a fine
groove along each shell. The diatoms exude and perhaps
also retract this secretion as they move.
There are two major groups of diatoms, one with radial
symmetry (like a wheel) and the other with bilateral (two-
sided) symmetry (figure 35.13). Diatom shells are rigid, and
the organisms reproduce asexually by separating the two
halves of the shell, each half then regenerating another half
shell within it. Because of this mode of reproduction, there
is a tendency for the shells, and consequently the individual
diatoms, to get smaller and smaller with each asexual re-
production. When the resulting individuals have dimin-
ished to about 30% of their original size, one may slip out
of its shell, grow to full size, and regenerate a full-sized pair
of new shells.
Individual diatoms are diploid. Meiosis occurs more fre-
quently under conditions of starvation. Some marine di-
atoms produce numerous sperm and others a single egg. If
fusion occurs, the resulting zygote regenerates a full-sized
individual. In some freshwater diatoms, the gametes are
amoeboid and similar in appearance.
The Golden Algae. Also included within the Chryso-
phyta are the golden algae, named for the yellow and
brown carotenoid and xanthophyll accessory pigments in
their chloroplasts, which give them a golden color. Unicel-
lular but often colonial, these freshwater protists typically
have two flagella, both attached near the same end of the
cell. When ponds and lakes dry out in summer, golden
algae form resistant cysts. Viable cells emerge from these
cysts when wetter conditions recur in the fall.
704 Part IX Viruses and Simple Organisms
FIGURE 35.13
Diatoms (Chrysophyta). Several different centric (radially symmetrical) diatoms.
Rhodophyta: The Red Algae
Along with green algae and brown
algae, red algae are the seaweeds we
see cast up along shores and on
beaches. Their characteristic colors re-
sult from phycoerythrin, a type of phy-
cobilin pigment. Phycobilins are re-
sponsible for the colors of the
cyanobacteria. Chlorophyll a also oc-
curs with the phycobilins in red algae,
just as it does in cyanobacteria. These
similarities with cyanobacteria make it
likely that the rhodophyta evolved
when their heterotrophic eukaryotic
ancestor developed an endosymbiotic
relationship with a cyanobacteria
which eventually gave rise to their
chloroplasts.
The great majority of the estimated
4000 species of red algae occur in the
sea, and almost all are multicellular.
Red algae have complex bodies
made up of interwoven filaments of
cells. In the cell walls of many red
algae are sulfated polysaccharides such
as agar and carrageenan, which make
these algae important economically.
Agar is used to make gel capsules, as
material for dental impressions, and as
a base for cosmetics. It is also the basis
of the laboratory media on which bac-
teria, fungi, and other organisms are
often grown. In addition, agar is used
to prevent baked goods from drying
out, for rapid-setting jellies, and as a
temporary preservative for meat and
fish in warm regions. Carrageenan is
used mainly to stabilize emulsions
such as paints, cosmetics, and dairy
products such as ice cream. In addition
to these uses, red algae such as Por-
phyra, called “nori,” are eaten and, in
Japan, are even cultivated as a human
food crop.
The life cycles of red algae are com-
plex but usually involve an alternation
of generations (sporic meiosis). None
of the red algae have flagella or cilia at
any stage in their life cycle, and they
may have descended directly from an-
cestors that never had them, especially
as the red algae also lack centrioles. To-
gether with the fungi, which also lack
flagella and centrioles, the red algae
may be one of the most ancient groups
of eukaryotes.
Phaeophyta: The Brown Algae
The phaeophyta, or brown algae, con-
sist of about 1500 species of multicel-
lular protists, almost exclusively ma-
rine. They are the most conspicuous
seaweeds in many northern regions,
dominating rocky shores almost every-
where in temperate North America. In
habitats where large brown algae
known as kelps (order Laminariales)
occur abundantly in so-called kelp
forests (figure 35.14), they are respon-
sible for most of the food production
through photosynthesis. Many kelps
are conspicuously differentiated into
flattened blades, stalks, and grasping
basal portions that anchor them to the
rocks.
Among the larger brown algae are
genera such as Macrocystis, in which
some individuals may reach 100 me-
ters in length. The flattened blades of
this kelp float out on the surface of
the water, while the base is anchored
tens of meters below the surface. An-
other ecologically important member
of this phylum is sargasso weed, Sar-
gassum, which forms huge floating
masses that dominate the vast Sar-
gasso Sea, an area of the Atlantic
Ocean northeast of the Caribbean.
The stalks of the larger brown algae
often exhibit a complex internal dif-
ferentiation of conducting tissues
analogous to that of plants.
The life cycle of the brown algae is
marked by an alternation of genera-
tions between a sporophyte and a ga-
metophyte. The large individuals we
recognize, such as the kelps, are
sporophytes. The gametophytes are
often much smaller, filamentous indi-
viduals, perhaps a few centimeters
across. Sporangia, which produce
haploid, swimming spores after meio-
sis, are formed on the sporophytes.
These spores divide by mitosis, giving
rise to individual gametophytes.
There are two kinds of gametophytes
in the kelps; one produces sperm, and
the other produces eggs. If sperm and
eggs fuse, the resulting zygotes grow
into the mature kelp sporophytes,
provided that they reach a favorable
site.
FIGURE 35.14
Brown algae (Phaeophyta). The massive
“groves” of giant kelp that occur in
relatively shallow water along the coasts of
the world provide food and shelter for
many different kinds of organisms.
Chlorophyta: The Green
Algae
Green algae are an extremely varied
group of more than 7000 species.
The chlorophytes have an extensive
fossil record dating back 900 million
years. They are mostly aquatic, but
some are semiterrestrial in moist
places, such as on tree trunks or in
soil. Many are microscopic and uni-
cellular, but some, such as sea let-
tuce, Ulva (see figure 35.16), are
tens of centimeters across and easily
visible on rocks and pilings around
the coasts.
Green algae are of special inter-
est, both because of their unusual di-
versity and because the ancestors of
the plant kingdom were clearly mul-
ticellular green algae. Many features
of modern green algae closely re-
semble plants, especially their
chloroplasts which are biochemically
similar to those of the plants. They
contain chlorophylls a and b, as well
as carotenoids. Green algae include
a very wide array of both unicellular
and multicellular organisms.
Among the unicellular green
algae, Chlamydomonas (figure 35.15)
is a well-known genus. Individuals
are microscopic (usually less than 25
micrometers long), green, rounded,
and have two flagella at the anterior
end. They move rapidly in water by beating their flagella in
opposite directions. Each individual has an eyespot, which
contains about 100,000 molecules of rhodopsin, the same
pigment employed in vertebrate eyes. Light received by
this eyespot is used by the alga to help direct its swimming.
Most individuals of Chlamydomonas are haploid. Chlamy-
domonas reproduces asexually (by cell division) as well as
sexually. In sexual reproduction, two haploid individuals
fuse to form a four-flagellated zygote. The zygote ulti-
mately enters a resting phase, called the zygospore, in
which the flagella disappear and a tough protective coat is
formed. Meiosis occurs at the end of this resting period and
results in the production of four haploid cells.
Chlamydomonas probably represents a primitive state for
green algae and several lines of evolutionary specialization
have been derived from organisms like it. The first is the
evolution of nonmotile, unicellular green algae. Chlamy-
domonas is capable of retracting its flagella and settling
down as an immobile unicellular organism if the ponds in
which it lives dry out. Some common algae of soil and bark,
such as Chlorella, are essentially like Chlamydomonas in this
trait, but do not have the ability to form flagella. Chlorellais
widespread in both fresh and salt water as well as soil and is
only known to reproduce asexually. Recently, Chlorella has
been widely investigated as a possible food source for hu-
mans and other animals, and pilot farms have been estab-
lished in Israel, the United States, Germany, and Japan.
Another major line of specialization from cells like
Chlamydomonas concerns the formation of motile, colonial
organisms. In these genera of green algae, the Chlamy-
domonas-like cells retain some of their individuality. The
most elaborate of these organisms is Volvox(see figure 35.1),
a hollow sphere made up of a single layer of 500 to 60,000
individual cells, each cell with two flagella. Only a small
number of the cells are reproductive. The colony has defi-
nite anterior and posterior ends, and the flagella of all of the
cells beat in such a way as to rotate the colony in a clockwise
direction as it moves forward through the water. The repro-
ductive cells of Volvox are located mainly at the posterior
end of the colony. Some may divide asexually, bulge inward,
and give rise to new colonies that initially remain within the
parent colony. Others produce gametes. In some species of
706 Part IX Viruses and Simple Organisms
– Strain
– Strain
– Gamete
+ Gamete
SYNGAMY
MEIOSIS
Zygospore (diploid)
Asexual
reproduction
+ Strain
+ Strain
n
2n
Pairing of
positive and
negative
strains
FIGURE 35.15
Life cycle of Chlamydomonas (Chlorophyta). Individual cells of this microscopic,
biflagellated alga, which are haploid, divide asexually, producing identical copies of themselves.
At times, such haploid cells act as gametes—fusing, as shown in the lower right-hand side of the
diagram, to produce a zygote. The zygote develops a thick, resistant wall, becoming a
zygospore; this is the only diploid cell in the entire life cycle. Within this diploid zygospore,
meiosis takes place, ultimately resulting in the release of four haploid individuals. Because of the
segregation during meiosis, two of these individuals are called the (+) strain, the other two the
(–) strain. Only + and – individuals are capable of mating with each other when syngamy does
take place, although both may divide asexually to reproduce themselves.
Volvox, there is a true division of labor among the different
types of cells, which are specialized in relation to their ulti-
mate function throughout the development of the organism.
In addition to these two lines of specialization from
Chlamydomonas-like cells, there are many other kinds of
green algae of less certain derivation. Many filamentous
genera, such as Spirogyra, with its ribbon-like chloro-
plasts, differ substantially from the remainder of the green
algae in their modes of cell division and reproduction.
Some of these genera have even been placed in separate
phyla. The study of the green algae, involving modern
methods of electron microscopy and biochemistry, is be-
ginning to reveal unexpected new relationships within this
phylum.
Ulva, or sea lettuce (figure 35.16), is a genus of marine
green algae that is extremely widespread. The glistening
individuals of this genus, often more than 10 centimeters
across, consist of undulating sheets only two cells thick.
Sea lettuce attaches by protuberances of the basal cells to
rocks or other substrates. The reproductive cycle of Ulva
involves an alternation of generations (sporic meiosis;
figure 35.16) as is typical among green algae. Unlike
most organisms that undergo sporic meiosis, however,
the gametophytes (haploid phase) and sporophytes
(diploid phase) resemble one another closely.
The stoneworts, a group of about 250 living species of
green algae, many of them in the genera Chara and Nitella,
have complex structures. Whorls of short branches arise
regularly at their nodes, and the gametangia (structures
that give rise to gametes) are complex and multicellular.
Stoneworts are often abundant in fresh to brackish water
and are common as fossils.
Dinoflagellates are primarily unicellular,
photosynthetic, and flagellated. Euglenoids (phylum
Euglenophyta) consist of about 40 genera, about a third
of which have chloroplasts similar biochemically to
those of green algae and plants. Diatoms and golden
algae are unicellular, photosynthetic organisms that
produce a unique carbohydrate. Diatoms have double
shells made of opaline silica. Nonmotile, unicellular
algae and multicellular, flagellated colonies have been
derived from green algae like Chlamydomonas—a
biflagellated, unicellular organism. The life cycle of
brown algae is marked by an alternation of generations
between the diploid phase, or sporophyte, and the
haploid phase, or gametophyte.
Chapter 35 Protists 707
Spores
MEIOSIS
Sporangia
Sporophyte
(2n)
Germinating
zygote
Zygote
SYNGAMY
Gametes
+ –
+ Gametangia
+ Gametophyte
(n)
– Gametangia
– Gametophyte (n)
2n
n
Gametes fuse
FIGURE 35.16
Life cycle of Ulva.In this green alga, the
gametophyte and the sporophyte are identical in
appearance and consist of flattened sheets two
cells thick. In the haploid (n) gametophyte,
gametangia give rise to haploid gametes, which
fuse to form a diploid (2n) zygote. The zygote
germinates to form the diploid sporophyte.
Sporangia within the sporophyte give rise to
haploid spores by meiosis. The haploid spores
develop into haploid gametophytes.
Heterotrophs with
Flagella
The phylum Sarcomastigophora con-
tains a diverse group of protists com-
bined into one phylum because they all
possess a single kind of nucleus and use
flagella or pseudopodia (or both) for
locomotion. We will focus on the class
Zoomastigophora.
Zoomastigophora: The
Zoomastigotes
The class Zoomastigophora is com-
posed of unicellular, heterotrophic or-
ganisms that are highly variable in
form (figure 35.17). Each has at least
one flagellum, with some species hav-
ing thousands. They include both free-
living and parasitic organisms. Many
zoomastigotes apparently reproduce
only asexually, but sexual reproduction
occurs in some species. The members
of one order, the kinetoplastids, in-
clude the genera Trypanosoma (figure
35.17c) and Crithidia, pathogens of hu-
mans and domestic animals. The eug-
lenoids could be viewed as a special-
ized group of zoomastigotes, some of
which acquired chloroplasts during the
course of evolution.
Trypanosomes cause many serious
human diseases, the most familiar of
which is trypanosomiasis also known as
African sleeping sickness (figure
35.18). Trypanosomes cause many
other diseases including East Coast
fever, leishmaniasis, and Chagas’ dis-
ease, all of great importance in tropical
areas where they afflict millions of
people each year. Leishmaniasis,
which is transmitted by sand flies, af-
flicts about 4 million people a year.
The effects of these diseases range
from extreme fatigue and lethargy in
sleeping sickness to skin sores and
deep eroding lesions that can almost
obliterate the face in leishmaniasis.
The trypanosomes that cause these
diseases are spread by biting insects,
including tsetse flies and assassin bugs.
A serious effort is now under way to
produce a vaccine for trypanosome-
caused diseases. These diseases make it
impossible to raise domestic cattle for
meat or milk in a large portion of
Africa. Control is especially difficult
because of the unique attributes of
these organisms. For example, tsetse
fly-transmitted trypanosomes have
evolved an elaborate genetic mecha-
nism for repeatedly changing the anti-
genic nature of their protective glyco-
protein coat, thus dodging the
antibodies their hosts produce against
them (see chapter 57). Only a single
one out of some 1000 to 2000 variable
antigen genes is expressed at a time.
Rearrangements of these genes during
the asexual cycle of the organism allow
for the expression of a seemingly end-
less variety of different antigen genes
that maintain infectivity by the try-
panosomes.
When the trypanosomes are in-
gested by a tsetse fly, they embark on a
complicated cycle of development and
multiplication, first in the fly’s gut and
later in its salivary glands. It is their
position in the salivary glands that al-
lows them to move into their verte-
brate host. Recombination has been
observed between different strains of
trypanosomes introduced into a single
fly, thus suggesting that mating, syn-
gamy, and meiosis occur, even though
they have not been observed directly.
Although most trypanosome repro-
duction is asexual, this sexual cycle, re-
ported for the first time in 1986, af-
fords still further possibilities for
recombination in these organisms.
In the guts of the flies that spread
them, trypanosomes are noninfective.
When they are ready to transfer to the
skin or bloodstream of their host, try-
panosomes migrate to the salivary
glands and acquire the thick coat of
glycoprotein antigens that protect
them from the host’s antibodies. When
they are taken up by a fly, the try-
panosomes again shed their coats. The
production of vaccines against such a
system is complex, but tests are under-
way. Releasing sterilized flies to im-
pede the reproduction of populations is
another technique used to try to con-
trol the fly population. Traps made of
dark cloth and scented like cows, but
708 Part IX Viruses and Simple Organisms
Trypanosoma
(c)
Codosiga
(a)
Trichonympha
(b)
Heterotrophs with no permanent
locomotor apparatus
Photosynthetic protists
Heterotrophs with flagella
Nonmotile spore-formers
Heterotrophs with restricted mobility
FIGURE 35.17
Three genera of zoomastigotes
(Zoomastigophora), a highly diverse
group. (a) Codosiga,a colonial
choanoflagellate that remains attached to its
substrate; other colonial choanoflagellates
swim around as a colony, resembling the
green alga Volvoxin this respect. (b)
Trichonympha,one of the zoomastigotes that
inhabits the guts of termites and wood-
feeding cockroaches and digests cellulose
there. Trichonymphahas rows of flagella in its
anterior regions. (c) Trypanosoma,which
causes sleeping sickness, an important
tropical disease. It has a single, anterior
flagellum.
poisoned with insecticides, have like-
wise proved effective. Research is pro-
ceeding rapidly because the presence
of tsetse flies with their associated try-
panosomes blocks the use of some 11
million square kilometers of potential
grazing land in Africa.
Some zoomastigotes occur in the
guts of termites and other wood-eat-
ing insects. They possess enzymes that
allow them to digest the wood and
thus make the components of the
wood available to their hosts. The re-
lationship is similar to that between
certain bacteria and protozoa that
function in the rumens of cattle and
related mammals (see chapter 51).
Another order of zoomastigotes,
the choanoflagellates, is most likely
the group from which the sponges (phylum Porifera) and
probably all animals arose. Choanoflagellates have a single
emergent flagellum surrounded by a funnel-shaped, con-
tractile collar composed of closely placed filaments, a
unique structure that is exactly matched in the sponges.
These protists feed on bacteria strained out of the water by
the collar.
Hiker’s Diarrhea. Giardia lamblia is a flagellate protist
(belonging to a small order called diplomonads) found
throughout the world, including all parts of the United
States and Canada (figure 35.19). It occurs in water, includ-
ing the clear water of mountain streams and the water sup-
plies of some cities. It infects at least 40 species of wild and
domesticated animals in addition to humans. In 1984 in
Pittsburgh, 175,000 people had to boil their drinking water
for several days following the appearance of Giardia in the
city’s water system. Although most individuals exhibit no
symptoms if they drink water infested with Giardia, many
suffer nausea, cramps, bloating, vomiting, and diarrhea.
Only 35 years ago, Giardia was thought to be harmless;
today, it is estimated that at least 16 million residents of the
United States are infected by it.
Giardia lives in the upper small intestine of its host. It
occurs there in a motile form that cannot survive outside
the host’s body. It is spread in the feces of infected individ-
uals in the form of dormant, football-shaped cysts—some-
times at levels as high as 300 million individuals per gram
of feces. These cysts can survive at least two months in cool
water, such as that of mountain streams. They are relatively
resistant to the usual water-treatment agents such as chlo-
rine and iodine but are killed at temperatures greater than
about 65°C. Apparently, pollution by humans seems to be
the main way Giardia is released into stream water. There
are at least three species of Giardia and many distinct
strains; how many of them attack humans and under what
circumstances are not known with certainty.
In the wilderness, good sanitation is important in pre-
venting the spread of Giardia. Dogs, which readily contract
and spread the disease, should not be taken into pristine
wilderness areas. Drinking water should be filtered—the
filter must be capable of eliminating particles as small as 1
micrometer in diameter—or boiled for at least one minute.
Water from natural streams or lakes should never be con-
sumed directly, regardless of how clean it looks. In other
regions, good sanitation methods are important to prevent
not only Giardiainfection but also other diseases.
Chapter 35 Protists 709
FIGURE 35.18
Trypanosoma is the zoomastigote that causes sleeping sickness. (a) Trypanosomaamong
red blood cells. The nuclei (dark-staining bodies), anterior flagella, and undulating,
changeable shape of the trypanosomes are visible in this photograph (500×). (b) The tsetse
FIGURE 35.19
Giardia lamblia. Giardiaare flagellated unicellular parasites that
infect the human intestine. Giardiaare very primitive, having only
a rudimentary cytoskeleton and lacking mitochondria and
chloroplasts. Sequencing of ribosomal RNA suggests that Giardia
and Pelomyxa,the eukaryotes most closely related to prokaryotes,
should be grouped together. The name Archezoa (Greek arkhaios,
“ancient”) has been suggested for the group, stressing its early
divergence from bacteria as long as 2 billion years ago.
20 μm
(a) (b)
Ciliophora: The Ciliates
As the name indicates, most members of the Ciliophora
feature large numbers of cilia. These heterotrophic, unicel-
lular protists range in size from 10 to 3000 micrometers
long. About 8000 species have been named. Despite their
unicellularity, ciliates are extremely complex organisms, in-
spiring some biologists to consider them organisms without
cell boundaries rather than single cells.
Their most characteristic feature, cilia, are usually
arranged either in longitudinal rows or in spirals around
the body of the organism (figure 35.20). Cilia are anchored
to microtubules beneath the cell membrane, and they beat
in a coordinated fashion. In some groups, the cilia have
specialized locomotory and feeding functions, becoming
fused into sheets, spikes, and rods which may then function
as mouths, paddles, teeth, or feet. The ciliates have a tough
but flexible outer covering called the pellicle that enables
the organism to squeeze through or move around many
kinds of obstacles.
All ciliates that have been studied have two very differ-
ent types of nuclei within their cells, small micronuclei
and larger macronuclei (figure 35.21). The micronuclei,
which contain apparently normal diploid chromosomes, di-
vide by meiosis and are able to undergo genetic recombina-
tion. Macronuclei are derived from certain micronuclei in a
complex series of steps. Within the macronuclei are multi-
ple copies of the genome, and the DNA is divided into
small pieces—smaller than individual chromosomes. In one
group of ciliates, these are equivalent to single genes.
Macronuclei divide by elongating and constricting and play
an essential role in routine cellular functions, such as the
production of mRNA to direct protein synthesis for growth
and regeneration.
Ciliates form vacuoles for ingesting food and regulating
their water balance. Food first enters the gullet, which in
the well-known ciliate Paramecium is lined with cilia fused
into a membrane (figure 35.21). From the gullet, the food
passes into food vacuoles, where enzymes and hydrochloric
acid aid in its digestion. After the digested material has
been completely absorbed, the vacuole empties its waste
contents through a special pore in the pellicle known as the
cytoproct. The cytoproct is essentially an exocytotic vesi-
cle that appears periodically when solid particles are ready
to be expelled. The contractile vacuoles, which function in
the regulation of water balance, periodically expand and
contract as they empty their contents to the outside of the
organism.
Ciliates usually reproduce by transverse fission of the
parent cell across its short axis, thus forming two identical
individuals (figure 35.22a). In this process of cell division,
the mitosis of the micronuclei proceeds normally, and the
macronuclei divide as just described.
In Paramecium, the cells divide asexually for about 700
generations and then die if sexual reproduction has not oc-
curred. Like most ciliates, Paramecium has a sexual process
called conjugation, in which two individual cells remain
attached to each other for up to several hours (figure
35.22b,c). Only cells of two different genetically determined
mating types, oddand even,are able to conjugate. Meiosis in
the micronuclei of each individual produces several haploid
micronuclei, and the two partners exchange a pair of these
micronuclei through a cytoplasmic bridge that appears be-
tween the two partners.
In each conjugating individual, the new micronucleus
fuses with one of the micronuclei already present in that in-
dividual, resulting in the production of a new diploid mi-
cronucleus in each individual. After conjugation, the
macronucleus in each cell disintegrates, while the new
diploid micronucleus undergoes mitosis, thus giving rise to
two new identical diploid micronuclei within each individ-
ual. One of these micronuclei becomes the precursor of the
future micronuclei of that cell, while the other micronu-
cleus undergoes multiple rounds of DNA replication, be-
coming the new macronucleus. This kind of complete seg-
regation of the genetic material is a unique feature of the
710 Part IX Viruses and Simple Organisms
Anterior contractile vacuole
Micronucleus
Macronucleus
Pellicle
Posterior
contractile
vacuole
Food vacuole
Gullet
Cilia
Cytoproct
FIGURE 35.20
A ciliate
(Ciliophora).
Stentor,a funnel-
shaped ciliate,
showing spirally
arranged cilia
(120×).
FIGURE 35.21
Paramecium. The main features of this familiar ciliate are shown.
ciliates and makes them ideal organisms for the study of
certain aspects of genetics.
Progeny from a sexual division in Paramecium must go
through about 50 asexual divisions before they are able to
conjugate. When they do so, their biological clocks are
restarted, and they can conjugate again. After about 600
asexual divisions, however, Paramecium loses the protein
molecules around the gullet that enable it to recognize an
appropriate mating partner. As a result, the individuals are
unable to mate, and death follows about 100 generations
later. The exact mechanisms producing these unusual
events are unknown, but they involve the accumulation of a
protein, which is now being studied.
The zoomastigotes are a highly diverse group of
flagellated unicellular heterotrophs, containing among
their members the ancestors of animals as well as the
very primitive Giardia. Ciliates possess characteristic
cilia, and have two types of nuclei. The macronuclei
contain multiple copies of certain genes, while the
micronuclei contain multigene chromosomes.
Chapter 35 Protists 711
Two Paramecium
individuals of different
mating types come
into contact.
The diploid
micronucleus in each
divides by meiosis to
produce four haploid
micronuclei.
Three of the haploid
micronuclei
degenerate. The
remaining
micronucleus in each
divides by mitosis.
Mates exchange
micronuclei.
In each individual, the
new micronucleus
fuses with the
micronucleus already
present, forming a
diploid micronucleus.
The macronucleus
disintegrates, and the
diploid micronucleus
divides by mitosis to
produce two identical
diploid micronuclei
within each individual.
One of these
micronuclei is the
precursor of the
micronucleus for that
cell, and the other
eventually gives rise to
the macronucleus.
Macronucleus
Micronucleus (2n)
(c)
Diploid
micronucleus
(2n)
Haploid
micronucleus (n)
(a)
(b)
FIGURE 35.22
Life cycle of Paramecium. (a) When Parameciumreproduces
asexually, a mature individual divides, and two complete
individuals result. (b,c) In sexual reproduction, two mature cells
fuse in a process called conjugation (100×).
Nonmotile Spore-
Formers
Apicomplexa: The Sporozoans
All sporozoans are nonmotile, spore-
forming parasites of animals. Their
spores are small, infective bodies that
are transmitted from host to host.
These organisms are distinguished by a
unique arrangement of fibrils, micro-
tubules, vacuoles, and other cell or-
ganelles at one end of the cell. There
are 3900 described species of this phy-
lum; best known among them is the
malarial parasite, Plasmodium.
Sporozoans have complex life cycles
that involve both asexual and sexual
phases. Sexual reproduction involves
an alternation of haploid and diploid
generations. Both haploid and diploid
individuals can also divide rapidly by mitosis, thus produc-
ing a large number of small infective individuals. Sexual re-
production involves the fertilization of a large female ga-
mete by a small, flagellated male gamete. The zygote that
results soon becomes an oocyst. Within the oocyst, mei-
otic divisions produce infective haploid spores called
sporozoites.
An alternation between different hosts often occurs in
the life cycles of sporozoans. Sporozoans of the genus Plas-
modium are spread from person to person by mosquitoes of
the genus Anopheles (figure 35.23); at least 65 different
species of this genus are involved. When an Anopheles mos-
quito penetrates human skin to obtain blood, it injects
saliva mixed with an anticoagulant. If the mosquito is in-
fected with Plasmodium, it will also inject the elongated
sporozoites into the bloodstream of its victim. The parasite
makes its way through the bloodstream to the liver, where
it rapidly divides asexually. After this division phase, mero-
zoites, the next stage of the life cycle, form, either reinvad-
ing other liver cells or entering the host’s bloodstream. In
the bloodstream, they invade the red blood cells, dividing
rapidly within them and causing them to become enlarged
and ultimately to rupture. This event releases toxic sub-
stances throughout the body of the host, bringing about
the well-known cycle of fever and chills that is characteris-
tic of malaria. The cycle repeats itself regularly every 48
hours, 72 hours, or longer.
Plasmodium enters a sexual phase when some mero-
zoites develop into gametocytes, cells capable of produc-
ing gametes. There are two types of gametocytes: male
and female. Gametocytes are incapable of producing ga-
metes within their human hosts and do so only when they
are extracted from an infected human by a mosquito.
Within the gut of the mosquito, the male and female ga-
metocytes form sperm and eggs, respectively. Zygotes de-
velop within the mosquito’s intestinal
walls and ultimately differentiate into
oocysts. Within the oocysts, repeated
mitotic divisions take place, produc-
ing large numbers of sporozoites.
These sporozoites migrate to the sali-
vary glands of the mosquito, and
from there they are injected by the
mosquito into the bloodstream of a
human, thus starting the life cycle of
the parasite again.
Malaria. Malaria, caused by infec-
tions by the sporozoan Plasmodium,is
one of the most serious diseases in
the world. According to the World
Health Organization, about 500 mil-
lion people are affected by it at any
one time, and approximately 2 mil-
lion of them, mostly children, die
each year. Malaria kills most children
under five years old who contract it. In areas where
malaria is prevalent, most survivors more than five or six
years old do not become seriously ill again from malaria
infections. The symptoms, familiar throughout the trop-
ics, include severe chills, fever, and sweating, an enlarged
and tender spleen, confusion, and great thirst. Ulti-
mately, a victim of malaria may die of anemia, kidney
failure, or brain damage. The disease may be brought
under control by the person’s immune system or by
drugs. As discussed in chapter 21, some individuals are
genetically resistant to malaria. Other persons develop
immunity to it.
Efforts to eradicate malaria have focused on (1) the
elimination of the mosquito vectors; (2) the development of
drugs to poison the parasites once they have entered the
human body; and (3) the development of vaccines. The
widescale applications of DDT from the 1940s to the 1960s
led to the elimination of the mosquito vectors in the
United States, Italy, Greece, and certain areas of Latin
America. For a time, the worldwide elimination of malaria
appeared possible, but this hope was soon crushed by the
development of DDT-resistant strains of malaria-carrying
mosquitoes in many regions; no fewer than 64 resistant
strains were identified in a 1980 survey. Even though the
worldwide use of DDT, long banned in the United States,
nearly doubled from its 1974 level to more than 30,000
metric tons in 1984, its effectiveness in controlling mosqui-
toes is dropping. Further, there are serious environmental
concerns about the use of this long-lasting chemical any-
where in the world. In addition to the problems with resis-
tant strains of mosquitoes, strains of Plasmodium have ap-
peared that are resistant to the drugs that have historically
been used to kill them.
As a result of these problems, the number of new cases
of malaria per year roughly doubled from the mid-1970s to
712 Part IX Viruses and Simple Organisms
Heterotrophs with no permanent
locomotor apparatus
Photosynthetic protists
Heterotrophs with flagella
Nonmotile spore-formers
Heterotrophs with restricted mobility
the mid-1980s, largely because of the spread of resistant
strains of the mosquito and the parasite. In many tropical
regions, malaria is blocking permanent settlement. Scien-
tists have therefore redoubled their efforts to produce an
effective vaccine. Antibodies to the parasites have been iso-
lated and produced by genetic engineering techniques, and
they are starting to produce promising results.
Vaccines against Malaria. The three different stages of
the Plasmodium life cycle each produce different antigens,
and they are sensitive to different antibodies. The gene
encoding the sporozoite antigen was cloned in 1984, but
it is not certain how effective a vaccine against sporozoites
might be. When a mosquito inserts its proboscis into a
human blood vessel, it injects about a thousand sporo-
zoites. They travel to the liver within a few minutes,
where they are no longer exposed to antibodies circulat-
ing in the blood. If even one sporozoite reaches the liver,
it will multiply rapidly there and cause malaria. The num-
ber of malaria parasites increases roughly eightfold every
24 hours after they enter the host’s body. A compound
vaccination against sporozoites, merozoites, and gameto-
cytes would probably be the most effective preventive
measure, but such a compound vaccine has proven diffi-
cult to develop.
However, research completed in 1997 brings a glim-
mer of hope. An experimental vaccine containing one of
the surface proteins of the disease-causing parasite, P. fal-
ciparum, seems to induce the immune system to produce
defenses that are able to destroy the parasite in future in-
fections. In tests, six out of seven vaccinated people did
not get malaria after being bitten by mosquitoes that car-
ried P. falciparum. Although research is still underway,
many are hopeful that this new vaccine may be able to
fight malaria, especially in Africa, where it takes a devas-
tating toll.
The best known of the sporozoans is the malarial
parasite Plasmodium. Like other sporozoans,
Plasmodium has a complex life cycle involving sexual
and asexual phases and alternation between different
hosts, in this case mosquitoes and humans. Malaria kills
about 2 million people each year.
Chapter 35 Protists 713
Mosquito injects
sporozoites
Gametocytes
ingested by
mosquito
Oocysts
Sporozoites
form within
mosquito
Gametocytes
Certain
merozoites
develop into
gametocytes
Sporozoites
Stages
in liver
Merozoites
Stages in
red blood
cells
Zygote
6
1
2
3
4
5
FIGURE 35.23
The life cycle of Plasmodium, the sporozoan that causes malaria. Plasmodiumhas a complex life cycle that alternates between
mosquitoes and mammals.
Heterotrophs with
Restricted Mobility
Oomycota
The oomycetes comprise about 580
species, among them the water molds,
white rusts, and downy mildews. All of
the members of this group are either
parasites or saprobes (organisms that
live by feeding on dead organic mat-
ter). The cell walls of the oomycetes
are composed of cellulose or polymers
that resemble cellulose. They differ re-
markably from the chitin cell walls of
fungi, with which the oomycetes have
at times been grouped. Oomycete life
cycles are characterized by gametic
meiosis and a diploid phase; this also
differs from fungi. Mitosis in the
oomycetes resembles that in most
other organisms, while mitosis in fungi has a number of un-
usual features, as you will see in chapter 36. Filamentous
structures of fungi and, by convention, those of oomycetes,
are called hyphae. Most oomycetes live in fresh or salt
water or in soil, but some are plant parasites that depend on
the wind to spread their spores. A few aquatic oomycetes
are animal parasites.
Oomycetes are distinguished from
other protists by the structure of their
motile spores, or zoospores, which
bear two unequal flagella, one of
which is directed forward, the other
backward. Such zoospores are pro-
duced asexually in a sporangium. Sex-
ual reproduction in the group involves
gametangia (singular, ga-
metangium)—gamete-producing
structures—of two different kinds.
The female gametangium is called an
oogonium, and the male ga-
metangium is called an antheridium.
The antheridia contain numerous
male nuclei, which are the functional
male gametes; the oogonia contain
from one to eight eggs, which are the
female gametes. When the contents of
an antheridium flow into an oogo-
nium, it leads to the individual fusion
of male nuclei with eggs. This is followed by the thickening
of the cell wall around the resulting zygote or zygotes. This
produces a special kind of thick-walled cell called an
oospore, the structure that gives the phylum its name. De-
tails from the life cycle of one of the oomycetes, Saproleg-
nia,are shown in figure 35.24.
714 Part IX Viruses and Simple Organisms
Heterotrophs with no permanent
locomotor apparatus
Photosynthetic protists
Heterotrophs with flagella
Nonmotile spore-formers
Heterotrophs with restricted mobility
Sexual
reproduction
Asexual
reproduction
Oospores
Encysted
primary
zoospore
Germination of
secondary zoospore
Encysted
secondary zoospore
Sperm
nuclei
fertilize
eggs
Hyphae
Secondary
zoospore
with lateral
flagella
Primary zoospore
with apical flagella Sporangium
Antheridium
Oogonium
Eggs
Sperm nucleus
in fertilization tube
MEIOSIS
FIGURE 35.24
Life cycle of Saprolegnia, an oomycete. Asexual reproduction by means of flagellated zoospores is shown at left, sexual reproduction at
right. Hyphae with diploid nuclei are produced by germination of both zoospores and oospores.
Aquatic oomycetes, or water molds, are common and
easily cultured. Some water molds cause fish diseases, pro-
ducing a kind of white fuzz on aquarium fishes. Among
their terrestrial relatives are oomycetes of great importance
as plant pathogens, including Plasmopara viticola, which
causes downy mildew of grapes, and Phytophthora infestans,
which causes the late blight of potatoes. This oomycete was
responsible for the Irish potato famine of 1845 and 1847,
during which about 400,000 people starved to death or died
of diseases complicated by starvation. Millions of Irish peo-
ple emigrated to the United States and elsewhere as a result
of this disaster.
Acrasiomycota: The Cellular Slime Molds
There are about 70 species of cellular slime molds. This
phylum has extraordinarily interesting features and was
once thought to be related to fungi, “mold” being a general
term for funguslike organisms. In fact, the cellular slime
molds are probably more closely related to amoebas (phy-
lum Rhizopoda) than to any other group, but they have
many special features that mark them as distinct. Cellular
slime molds are common in fresh water, damp soil, and on
rotting vegetation, especially fallen logs. They have be-
come one of the most important groups of organisms for
studies of differentiation because of their relatively simple
developmental systems and the ease of analyzing them (fig-
ure 35.25).
The individual organisms of this group behave as sepa-
rate amoebas, moving through the soil or other substrate
and ingesting bacteria and other smaller organisms. At a
certain phase of their life cycle, the individual organisms
aggregate and form a moving mass, the “slug,” that eventu-
ally transforms itself into a spore-containing mass, the
sorocarp. In the sorocarp the amoebas become encysted as
spores. Some of the amoebas fuse sexually to form macro-
cysts, which have diploid nuclei; meiosis occurs in them
after a short period (zygotic meiosis). The sporocarp de-
velops a stalked structure with a chamber at the top which
releases the spores. Other amoebas are released directly,
eventually aggregating again to form a new slug.
The development of Dictyostelium discoideum, a cellular
slime mold, has been studied extensively because of the im-
plication its unusual life cycle has for understanding the de-
velopmental process in general. When the individual amoe-
bas of this species exhaust the supply of bacteria in a given
area and are near starvation, they aggregate and form a
compound, motile mass. The aggregation of the individual
amoebas is induced by pulses of cyclic adenosine
monophosphate (cAMP), which the cells begin to secrete
when they are starving. The cells form an aggregate organ-
ism that moves to a new area where food is more plentiful.
In the new area, the colony differentiates into a multicellu-
lar sorocarp within which spores differentiate. Each of
these spores, if it falls into a suitably moist habitat, releases
a new amoeba, which begins to feed, and the cycle is
started again.
Chapter 35 Protists 715
Slug begins
to right itself.
Slug is transformed
into spore-forming
body, the sorocarp.
Spores
Free-living
amoeba is
released.
Amoeba mass
forms. Amoebas begin
to congregate.
Moving
amoeba
mass is
called a
slug.
(c)
(b)
(a)
(f)
(d)
(e)
FIGURE 35.25
Development in Dictyostelium
discoideum, a cellular slime mold. (a)
First, a spore germinates, forming amoebas.
These amoebas feed and reproduce until the
food runs out. (b) The amoebas aggregate
and move toward a fixed center. (c) Next,
they form a multicellular “slug” 2 to 3 mm
long that migrates toward light. (d) The slug
stops moving and begins to differentiate into
a spore-forming body, called a sorocarp (e).
(f) Within heads of the sorocarps, amoebas
become encysted as spores.
Myxomycota: The Plasmodial Slime Molds
Plasmodial slime molds are a group of about 500 species.
These bizarre organisms stream along as a plasmodium, a
nonwalled, multinucleate mass of cytoplasm, that resembles
a moving mass of slime (figure 35.26). This is called the
feeding phase, and the plasmodia may be orange, yellow, or
another color. Plasmodia show a back-and-forth streaming
of cytoplasm that is very conspicuous, especially under a mi-
croscope. They are able to pass through the mesh in cloth
or simply flow around or through other obstacles. As they
move, they engulf and digest bacteria, yeasts, and other
small particles of organic matter. Plasmodia contain many
nuclei (multinucleate), but these are not separated by cell
membranes. The nuclei undergo mitosis synchronously,
with the nuclear envelope breaking down, but only at late
anaphase or telophase. Centrioles are lacking in cellular
slime molds. Although they have similar common names,
there is no evidence that the plasmodial slime molds are
closely related to the cellular slime molds; they differ in
most features of their structure and life cycles (figure 35.27).
When either food or moisture is in short supply, the
plasmodium migrates relatively rapidly to a new area. Here
it stops moving and either forms a mass in which spores dif-
ferentiate or divides into a large number of small mounds,
each of which produces a single, mature sporangium, the
structure in which spores are produced. These sporangia are
often extremely complex in form and beautiful (figure
35.28). The spores can be either diploid or haploid. In most
species of plasmodial slime molds with a diploid plasmod-
ium, meiosis occurs in the spores within 24 hours of their
formation. Three of the four nuclei in each spore disinte-
grate, leaving each spore with a single haploid nucleus.
The spores are highly resistant to unfavorable environ-
mental influences and may last for years if kept dry. When
conditions are favorable, they split open and release their
protoplast, the contents of the individual spore. The proto-
plast may be amoeboid or bear two flagella. These two
stages appear to be interchangeable, and conversions in ei-
ther direction occur readily. Later, after the fusion of hap-
loid protoplasts (gametes), a usually diploid plasmodium
may be reconstituted by repeated mitotic divisions.
Molds are heterotrophic protists, many of which are
capable of amoeba-like streaming. The feeding phase of
plasmodial slime molds consists of a multinucleate mass
of protoplasm; a plasmodium can flow through a cloth
mesh and around obstacles. If the plasmodium begins
to dry out or is starving, it forms often elaborate
sporangia. Meiosis occurs in the spores once they have
formed within the sporangium.
716 Part IX Viruses and Simple Organisms
FIGURE 35.26
A plasmodial protist. This multinucleate plasmodium moves about in search of the bacteria and other organic particles that it ingests.
Chapter 35 Protists 717
MEIOSIS
Spores
(n)
Mature spore
Mature
sporangium
Germinating
spore
Amoeboid
gametes
Flagellated
gametes
SYNGAMY
Zygote
Diploid
plasmodium (2n)
Initiation of
sporangium formation
Young
sporangium
n2n
FIGURE 35.27
Life cycle of a plasmodial slime mold. When food or moisture is scarce, a diploid plasmodium stops moving and forms sporangia.
Haploid spores form by meiosis. The spores wait until conditions are favorable to germinate. Spores can give rise to flagellated or
amoeboid gametes; the two forms convert from one to the other readily. Fusion of the gametes forms the diploid zygote, which gives rise
to the mobile, feeding plasmodium by mitosis.
FIGURE 35.28
Sporangia of three genera of plasmodial slime molds (phylum Myxomycota). (a) Arcyria. (b) Fuligo.(c) Developing sporangia of
Tubifera.
(a) (b) (c)
718 Part IX Viruses and Simple Organisms
Chapter 35
Summary Questions Media Resources
35.1 Eukaryotes probably arose by endosymbiosis.
? The theory of endosymbiosis, accepted by almost all
biologists, proposes that mitochondria and
chloroplasts were once aerobic eubacteria that were
engulfed by ancestral eukaryotes.
? There is some suggestion that centrioles may also
have an endosymbiotic origin.
1.What kind of bacteria most
likely gave rise to the
chloroplasts in the eukaryotic
cells of plants and some algae?
? The kingdom Protista consists of predominantly
unicellular phyla, together with three phyla that
include large numbers of multicellular organisms.
? The catch-all kingdom Protista includes all
eukaryotic organisms except animals, plants, and
fungi.
2.Why is the kingdom Protista
said to be an artificial group?
How is this different from the
other kingdoms?
35.2 The kingdom Protista is by far the most diverse of any kingdom.
? Dinoflagellates (phylum Dinoflagellata) are a major
phylum of primarily unicellular organisms that have
unique chromosomes and a very unusual form of
mitosis. They are the only eukaryotes known to lack
histones and nucleosomes.
? Euglenoids (phylum Euglenophyta) have chloroplasts
that share the biochemical features of those found in
green algae and plants.
? Diatoms (phylum Chrysophyta) are unicellular,
photosynthetic protists with opaline silica shells.
They include the golden algae.
? Brown algae (phylum Phaeophyta) are multicellular,
marine protists, some reaching 100 meters in length.
The kelps contribute greatly to the productivity of
the sea, especially along the coasts in relatively
shallow areas.
? The zoomastigotes (phylum Sarcomastigophora) are
a group of heterotrophic, mostly unicellular protists
that includes the organism responsible for sleeping
sickness.
? There are about 8000 named species of ciliates
(phylum Ciliophora); these protists have a very
complex morphology with numerous cilia.
? The malarial parasite, Plasmodium,is a member of the
phylum Apicomplexa. Carried by mosquitoes, it
multiplies rapidly in the liver of humans and other
primates and brings about the cyclical fevers
characteristic of malaria by releasing toxins into the
bloodstream of its host.
3.Why is mitosis in
dinoflagellates unique? What
are zooxanthellae?
4.What determines whether a
collection of individuals is truly
multicellular?
5.What unique characteristic
differentiates the members of
Ciliophora from other protists?
What is the function of two
vacuoles exhibited by most
members of Ciliophora?
6.Why has it been so difficult to
produce a vaccine for
trypanosome-caused diseases?
7.What differentiates the
oomycetes from the kingdom
Fungi, in which they were
previously placed? What is the
feeding strategy of this phylum?
Why are these organisms
generally considered harmful?
35.3 Protists can be categorized into five groups.
www.mhhe.com/raven6e www.biocourse.com
? Characteristics of
Protists
? Protozoa
? Photosynthetic
Protists
? Fungus-like Protists
719
36
Fungi
Concept Outline
36.1 Fungi are unlike any other kind of organism.
A Fungus Is Not a Plant. Unlike any plant, all fungi are
filamentous heterotrophs with cell walls made of chitin.
The Body of a Fungus. Cytoplasm flows from one cell
to another within the filamentous body of a fungus.
How Fungi Reproduce. Fungi reproduce sexually when
filaments of different fungi encounter one another and fuse.
How Fungi Obtain Nutrients. Fungi secrete digestive
enzymes and then absorb the products of the digestion.
Ecology of Fungi. Fungi are among the most important
decomposers in terrestrial ecosystems.
36.2 Fungi are classified by their reproductive
structures.
The Three Phyla of Fungi. There are three phyla of
fungi, distinguished by their reproductive structures.
Phylum Zygomycota. In zygomycetes, the fusion of
hyphae leads directly to the formation of a zygote.
Phylum Ascomycota. In ascomycetes, hyphal fusion
leads to stable dikaryons that grow into massive webs of
hyphae that form zygotes within a characteristic saclike
structure, the ascus. Yeasts are unicellular fungi, mostly
ascomycetes, that play many important commercial and
medical roles.
Phylum Basidiomycota. In basidiomycetes, dikaryons
also form, but zygotes are produced within reproductive
structures called basidia.
The Imperfect Fungi. Fungi that have not been
observed to reproduce sexually cannot be classified into one
of the three phyla.
36.3 Fungi form two key mutualistic symbiotic
associations.
Lichens. A lichen is a mutualistic symbiotic association
between a fungus and a photosynthetic alga or
cyanobacterium.
Mycorrhizae. Mycorrhizae are mutualistic symbiotic
associations between fungi and the roots of plants.
O
f all the bewildering variety of organisms that live on
earth, perhaps the most unusual, the most peculiarly
different from ourselves, are the fungi (figure 36.1). Mush-
rooms and toadstools are fungi, multicellular creatures that
grow so rapidly in size that they seem to appear overnight
on our lawns. At first glance, a mushroom looks like a
funny kind of plant growing up out of the soil. However,
when you look more closely, fungi turn out to have nothing
in common with plants except that they are multicellular
and grow in the ground. As you will see, the more you ex-
amine fungi, the more unusual they are.
FIGURE 36.1
Spores exploding from the surface of a puffball fungus. The
fungi constitute a unique kingdom of heterotrophic organisms.
Along with bacteria, they are important decomposers and disease-
causing organisms.
Most fungi reproduce sexually with nuclear exchange
rather than gametes.
4. Fungi have cell walls made of chitin. The cell
walls of fungi are built of polysaccharides (chains of
sugars) and chitin, the same tough material a crab
shell is made of. The cell walls of plants are made of
cellulose, also a strong building material.
5. Fungi have nuclear mitosis. Mitosis in fungi is
different from that in plants or most other eukaryotes
in one key respect: the nuclear envelope does not
break down and re-form. Instead, mitosis takes place
withinthe nucleus. A spindle apparatus forms there,
dragging chromosomes to opposite poles of the nu-
cleus(not the cell, as in most other eukaryotes).
You could build a much longer list, but already the
take-home lesson is clear: fungi are not like plants at all!
Their many unique features are strong evidence that fungi
are not closely related to any other group of organisms.
DNA studies confirm significant differences from other
eukaryotes.
Fungi absorb their food after digesting it with secreted
enzymes. This mode of nutrition, combined with a
filamentous growth form, nuclear mitosis, and other
traits, makes the members of this kingdom highly
distinctive.
720 Part IX Viruses and Simple Organisms
A Fungus Is Not a Plant
The fungi are a distinct kingdom of organisms, comprising
about 77,000 named species (figure 36.2). Mycologists,
scientists who study fungi, believe there may be many more
species in existence, as many as 1.2 million. Although fungi
have traditionally been included in the plant kingdom, they
lack chlorophyll and resemble plants only in their general
appearance and lack of mobility. Significant differences be-
tween fungi and plants include the following:
1. Fungi are heterotrophs. Perhaps most obviously,
a mushroom is not green. Virtually all plants are pho-
tosynthesizers, while no fungi have chlorophyll or
carry out photosynthesis. Instead, fungi obtain their
food by secreting digestive enzymes onto the sub-
strate, and then absorbing the organic molecules that
are released by the enzymes.
2. Fungi have filamentous bodies. Fungi are basi-
cally filamentous in their growth form (that is, their
bodies consist of long slender filaments called hy-
phae), even though these hyphae may be packed to-
gether to form complex structures like the mush-
room. Plants, in contrast, are made of several types of
cells organized into tissues and organs.
3. Fungi have unusual reproductive modes. Some
plants have motile sperm with flagella. No fungi do.
36.1 Fungi are unlike any other kind of organism.
FIGURE 36.2
Representatives of the three phyla of fungi. (a) A cup fungus,
Cookeina tricholoma,an ascomycete, from the rain forest of Costa
Rica. (b) Amanita muscaria,the fly agaric, a toxic basidiomycete. In
the cup fungi, the spore-producing structures line the cup; in
basidiomycetes that form mushrooms, like Amanita,they line the
gills beneath the cap of the mushroom. All visible structures of
fleshy fungi, such as the ones shown here, arise from an extensive
network of filamentous hyphae that penetrates and is interwoven
with the substrate on which they grow. (c) Pilobolus, a zygomycete
that grows on animal feces. Stalks about 10 millimeters long
contain dark spore-bearing sacs.
(a)
(b)
(c)
The Body of a Fungus
Fungi exist mainly in the form of slender filaments, barely
visible to the naked eye, which are called hyphae (singular,
hypha). These hyphae are typically made up of long chains
of cells joined end-to-end divided by cross-walls called
septa (singular, septum). The septa rarely form a complete
barrier, except when they separate the reproductive cells.
Cytoplasm characteristically flows or streams freely
throughout the hyphae, passing right through major pores
in the septa (figure 36.3). Because of this streaming, pro-
teins synthesized throughout the hyphae may be carried to
their actively growing tips. As a result, fungal hyphae may
grow very rapidly when food and water are abundant and
the temperature is optimum.
A mass of connected hyphae is called a mycelium
(plural, mycelia). This word and the term mycologistare
both derived from the Greek word for fungus, myketos.The
mycelium of a fungus (figure 36.4) constitutes a system that
may, in the aggregate, be many meters long. This
mycelium grows through and penetrates its substrate, re-
sulting in a unique relationship between the fungus and its
environment. All parts of such a fungus are metabolically
active, continually interacting with the soil, wood, or other
material in which the mycelium is growing.
In two of the three phyla of fungi, reproductive struc-
tures formed of interwoven hyphae, such as mushrooms,
puffballs, and morels, are produced at certain stages of the
life cycle. These structures expand rapidly because of rapid
elongation of the hyphae. For this reason, mushrooms can
appear suddenly on your lawn.
The cell walls of fungi are formed of polysaccharides
and chitin, not cellulose like those of plants and many
groups of protists. Chitin is the same material that makes
up the major portion of the hard shells, or exoskeletons, of
arthropods, a group of animals that includes insects and
crustaceans (see chapter 46). The commonality of chitin is
one of the traits that has led scientists to believe that fungi
and animals share a common ancestor.
Mitosis in fungi differs from that in most other organ-
isms. Because of the linked nature of the cells, the cell it-
self is not the relevant unit of reproduction; instead, the
nucleus is. The nuclear envelope does not break down and
re-form; instead, the spindle apparatus is formed withinit.
Centrioles are lacking in all fungi; instead, fungi regulate
the formation of microtubules during mitosis with small,
relatively amorphous structures called spindle plaques.
This unique combination of features strongly suggests that
fungi originated from some unknown group of single-
celled eukaryotes with these characteristics.
Fungi exist primarily in the form of filamentous hyphae,
typically with incomplete division into individual cells
by septa. These and other unique features indicate that
fungi are not closely related to any other group of
organisms.
Chapter 36 Fungi 721
FIGURE 36.3
A septum (45,000×). This transmission electron micrograph of a
section through a hypha of the basidiomycete Inonotus tomentosus
shows a pore through which the cytoplasm streams.
FIGURE 36.4
Fungal mycelium. This mycelium, composed of hyphae, is
growing through leaves on the forest floor in Maryland.
How Fungi Reproduce
Fungi are capable of both sexual and asexual reproduc-
tion. When a fungus reproduces sexually it forms a
diploid zygote, as do animals and plants. Unlike animals
and plants, all fungal nuclei except for the zygote are hap-
loid, and there are many haploid nuclei in the common
cytoplasm of a fungal mycelium. When fungi reproduce
sexually, hyphae of two genetically different mating types
come together and fuse. In two of the three phyla of
fungi, the genetically different nuclei that are associated
in a common cytoplasm after fusion do not combine im-
mediately. Instead, the two types of nuclei coexist for
most of the life of the fungus. A fungal hypha containing
nuclei derived from two genetically distinct individuals is
called a heterokaryotic hypha. If all of the nuclei are ge-
netically similar to one another, the hypha is said to be
homokaryotic. If there are two distinct nuclei within
each compartment of the hyphae, they are dikaryotic. If
each compartment has only a single nucleus, it is
monokaryotic. Dikaryotic hyphae have some of the ge-
netic properties of diploids, because both genomes are
transcribed. These distinctions are important in under-
standing the life cycles of the individual groups.
Cytoplasm in fungal hyphae normally flows through
perforated septa or moves freely in their absence. Repro-
ductive structures are an important exception to this gen-
eral pattern. When reproductive structures form, they are
cut off by complete septa that lack perforations or have
perforations that soon become blocked. Three kinds of re-
productive structures occur in fungi: (1) sporangia, which
are involved in the formation of spores; (2) gametangia,
structures within which gametes form; and (3) conidio-
phores, structures that produce conidia, multinucleate
asexual spores.
Spores are a common means of reproduction among
fungi. They may form as a result of either asexual or sexual
processes and are always nonmotile, being dispersed by
wind. When spores land in a suitable place, they germinate,
giving rise to a new fungal hypha. Because the spores are
very small, they can remain suspended in the air for long
periods of time. Because of this, fungal spores may be
blown great distances from their place of origin, a factor in
the extremely wide distributions of many kinds of fungi.
Unfortunately, many of the fungi that cause diseases in
plants and animals are spread rapidly and widely by such
means. The spores of other fungi are routinely dispersed by
insects and other small animals.
Fungi reproduce sexually after two hyphae of opposite
mating type fuse. Asexual reproduction by spores is a
second common means of reproduction.
How Fungi Obtain Nutrients
All fungi obtain their food by secreting digestive enzymes
into their surroundings and then absorbing back into the
fungus the organic molecules produced by this external di-
gestion. The significance of the fungal body plan reflects
this approach, the extensive network of hyphae providing
an enormous surface area for absorption. Many fungi are
able to break down the cellulose in wood, cleaving the link-
ages between glucose subunits and then absorbing the glu-
cose molecules as food. That is why fungi so often grow on
dead trees.
It might surprise you to know that some fungi are
predatory (figure 36.5). For example, the mycelium of the
edible oyster fungus, Pleurotus ostreatus,excretes a sub-
stance that anesthetizes tiny roundworms known as nema-
todes (see chapter 44) that feed on the fungus. When the
worms become sluggish and inactive, the fungal hyphae
envelop and penetrate their bodies and absorb their nutri-
tious contents. The fungus usually grows within living
trees or on old stumps, obtaining the bulk of its glucose
through the enzymatic digestion of cellulose from the
wood, so that the nematodes it consumes apparently serve
mainly as a source of nitrogen—a substance almost always
in short supply in biological systems. Other fungi are even
more active predators than Pleurotus,snaring, trapping, or
firing projectiles into nematodes, rotifers, and other small
animals on which they prey.
Fungi secrete digestive enzymes onto organic matter
and then absorb the products of the digestion.
722 Part IX Viruses and Simple Organisms
FIGURE 36.5
A carnivorous fungus. The oyster mushroom, Pleurotus ostreatus,
not only decomposes wood but also immobilizes nematodes,
which the fungus uses as a source of nitrogen.
Ecology of Fungi
Fungi, together with bacteria, are the principal decom-
posers in the biosphere. They break down organic materi-
als and return the substances locked in those molecules to
circulation in the ecosystem. Fungi are virtually the only
organisms capable of breaking down lignin, one of the
major constituents of wood. By breaking down such sub-
stances, fungi release critical building blocks, such as car-
bon, nitrogen, and phosphorus, from the bodies of dead or-
ganisms and make them available to other organisms.
In breaking down organic matter, some fungi attack liv-
ing plants and animals as a source of organic molecules,
while others attack dead ones. Fungi often act as disease-
causing organisms for both plants (figure 36.6) and animals,
and they are responsible for billions of dollars in agricul-
tural losses every year. Not only are fungi the most harmful
pests of living plants, but they also attack food products
once they have been harvested and stored. In addition,
fungi often secrete substances into the foods that they are
attacking that make these foods unpalatable, carcinogenic,
or poisonous.
The same aggressive metabolism that makes fungi eco-
logically important has been put to commercial use in
many ways. The manufacture of both bread and beer de-
pends on the biochemical activities of yeasts, single-celled
fungi that produce abundant quantities of ethanol and car-
bon dioxide. Cheese and wine achieve their delicate flavors
because of the metabolic processes of certain fungi, and
others make possible the manufacture of soy sauce and
other fermented foods. Vast industries depend on the bio-
chemical manufacture of organic substances such as citric
acid by fungi in culture, and yeasts are now used on a large
scale to produce protein for the enrichment of animal food.
Many antibiotics, including the first one that was used on a
wide scale, penicillin, are derived from fungi.
Some fungi are used to convert one complex organic
molecule into another, cleaning up toxic substances in the
environment. For example, at least three species of fungi
have been isolated that combine selenium, accumulated at
the San Luis National Wildlife Refuge in California’s San
Joaquin Valley, with harmless volatile chemicals—thus re-
moving excess selenium from the soil.
Two kinds of mutualistic associations between fungi and
autotrophic organisms are ecologically important. Lichens
are mutualistic symbiotic associations between fungi and
either green algae or cyanobacteria. They are prominent
nearly everywhere in the world, especially in unusually
harsh habitats such as bare rock. Mycorrhizae, specialized
mutualistic symbiotic associations between the roots of
plants and fungi, are characteristic of about 90% of all
plants. In each of them, the photosynthetic organisms fix
atmospheric carbon dioxide and thus make organic material
available to the fungi. The metabolic activities of the fungi,
in turn, enhance the overall ability of the symbiotic associa-
tion to exist in a particular habitat. In the case of mycor-
rhizae, the fungal partner expedites the plant’s absorption
of essential nutrients such as phosphorus. Both of these as-
sociations will be discussed further in this chapter.
Fungi are key decomposers and symbionts within
almost all terrestrial ecosystems and play many other
important ecological and commercial roles.
Chapter 36 Fungi 723
FIGURE 36.6
World’s largest organism?
Armillaria,a pathogenic fungus shown
here afflicting three discrete regions
of coniferous forest in Montana,
grows out from a central focus as a
single circular clone. The large patch
at the bottom of the picture is almost
8 hectares in diameter. The largest
clone measured so far has been 15
hectares in diameter—pretty
impressive for a single individual!
The Three Phyla of Fungi
There are three phyla but actually four groups of fungi:
phylum Zygomycota, the zygomycetes; phylum Ascomy-
cota, the ascomycetes; phylum Basidiomycota, the basid-
iomycetes, and the imperfect fungi (figure 36.7 and table
36.1). Several other groups that historically have been asso-
ciated with fungi, such as the slime molds and water molds
(phylum Oomycota; see chapter 35), now are considered to
be protists, not fungi. Oomycetes are sharply distinct from
fungi in their (1) motile spores; (2) cellulose-rich cell walls;
(3) pattern of mitosis; and (4) diploid hyphae.
The three phyla of fungi are distinguished primarily by
their sexual reproductive structures. In the zygomycetes,
the fusion of hyphae leads directly to the formation of a zy-
gote, which divides by meiosis when it germinates. In the
other two phyla, an extensive growth of dikaryotic hyphae
may lead to the formation of structures of interwoven hy-
phae within which are formed the distinctive kind of repro-
ductive cell characteristic of that particular group. Nuclear
fusion, followed by meiosis, occurs within these cells. The
imperfect fungi are either asexual or the sexual reproduc-
tive structures have not been identified.
Sexual reproductive structures distinguish the three
phyla of fungi.
724 Part IX Viruses and Simple Organisms
36.2 Fungi are classified by their reproductive structures.
Zygomycota (zygomycetes)
Imperfect fungi
Ascomycota (ascomycetes)
Basidiomycota (basidiomycetes)
Fungi
FIGURE 36.7
The four major groups of fungi.The imperfect fungi are not a
true phylum, but rather a collection of fungi in which sexual
structures have not been identified.
Table 36.1 Fungi
Approximate
Number of
Phylum Typical Examples Key Characteristics Living Species
Ascomycota Yeasts, truffles, Develop by sexual means; ascospores are 32,000
morels formed inside a sac called an ascus;
asexual reproduction is also common
Imperfect Aspergillus, Sexual reproduction has not been observed; 17,000
fungi Penicillium most are thought to be ascomycetes that
have lost the ability to reproduce sexually
Basidiomycota Mushrooms, Develop by sexual means; basidiospores are 22,000
toadstools, rusts borne on club-shaped structures called basidia;
the terminal hyphal cell that produces spores is
called a basidium; asexual reproduction occurs
occasionally
Zygomycota Rhizopus Develop sexually and asexually; multinucleate 1050
(black bread mold) hyphae lack septa, except for reproductive
structures; fusion of hyphae leads directly to
formation of a zygote, in which meiosis occurs
just before it germinates
Phylum Zygomycota
The zygomycetes (phylum Zygomycota)
lack septa in their hyphae except when they
form sporangia or gametangia. Zy-
gomycetes are by far the smallest of the
three phyla of fungi, with only about 1050
named species. Included among them are
some of the more common bread molds
(figure 36.8), as well as a variety of other
microscopic fungi found on decaying or-
ganic material. The group is named after a
characteristic feature of the life cycle of its
members, the production of temporarily
dormant structures called zygosporangia.
In the life cycle of the zygomycetes (fig-
ure 36.8b), sexual reproduction occurs by
the fusion of gametangia, which contain numerous nuclei.
The gametangia are cut off from the hyphae by complete
septa. These gametangia may be formed on hyphae of dif-
ferent mating types or on a single hypha. If both + and –
mating strains are present in a colony, they may grow to-
gether and their nuclei may fuse. Once the haploid nuclei
have fused, forming diploid zygote nuclei, the area where
the fusion has taken place develops into an often massive
and elaborate zygosporangium. A zygosporangium may
contain one or more diploid nuclei and acquires a thick
coat. The zygosporangium helps the
species survive conditions not favorable for
growth. Meiosis occurs during the germi-
nation of the zygosporangium. Normal,
haploid hyphae grow from the haploid
cells that result from this process. Except
for the zygote nuclei, all nuclei of the zy-
gomycetes are haploid.
Asexual reproduction occurs much more
frequently than sexual reproduction in the
zygomycetes. During asexual reproduction,
hyphae grow over the surface of the bread
or other material on which the fungus
feeds and produce clumps of erect stalks,
called sporangiophores. The tips of the
sporangiophores form sporangia, which
are separated by septa. Thin-walled hap-
loid spores are produced within the sporangia. Their spores
are thus shed above the substrate, in a position where they
may be picked up by the wind and dispersed to a new food
source.
Zygomycetes form characteristic resting structures,
called zygosporangia, which contain one or more
zygotic nuclei. The hyphae of zygomycetes are
multinucleate, with septa only where gametangia or
sporangia are separated.
Chapter 36 Fungi 725
Imperfect fungi
Ascomycetes
Basidiomycetes
Zygomycetes
FIGURE 36.8
Rhizopus, a zygomycete that grows on
moist bread and other similar
substrates. (a) The dark, spherical,
spore-producing sporangia are on
hyphae about a centimeter tall. The
rootlike hyphae anchor the sporangia.
(b) Life cycle of Rhizopus,a zygomycete.
This phylum is named for its
characteristic zygosporangia.
Hypha
n2n
MEIOSIS
(occurs
during
germination)
Rhizoid
Sporangiophore
Sporangium
Mating
strain
–
Mating
strain
+
Gametangia
FUSION OF
GAMETANGIA
+
–
Zygosporangium
Germinating
zygosporangium
+
–
Sporangium
Spores
(a)
(b)
Phylum Ascomycota
The second phylum of fungi, the as-
comycetes (phylum Ascomycota), is a very
large group of about 32,000 named species,
with more being discovered each year.
Among the ascomycetes are such familiar
and economically important fungi as yeasts,
common molds, morels (figure 36.9a,b), and
truffles. Also included in this phylum are
many serious plant pathogens, including the
chestnut blight, Cryphonectria parasitica,and
Dutch elm disease, Ophiostoma ulmi.
The ascomycetes are named for their
characteristic reproductive structure, the
microscopic, saclike ascus (plural, asci).
The zygotic nucleus, which is the only
diploid nucleus of the ascomycete life cycle (figure 36.9c), is
formed within the ascus. The asci are differentiated within
a structure made up of densely interwoven hyphae, corre-
sponding to the visible portions of a morel or cup fungus,
called the ascocarp.
Asexual reproduction is very common in the as-
comycetes. It takes place by means of conidia (singular,
conidium), spores cut off by septa at the
ends of modified hyphae called conidio-
phores. Conidia allow for the rapid colo-
nization of a new food source. Many coni-
dia are multinucleate. The hyphae of
ascomycetes are divided by septa, but the
septa are perforated and the cytoplasm
flows along the length of each hypha. The
septa that cut off the asci and conidia are
initially perforated, but later become
blocked.
The cells of ascomycete hyphae may
contain from several to many nuclei. The
hyphae may be either homokaryotic or
heterokaryotic. Female gametangia,
called ascogonia, each have a beaklike
outgrowth called a trichogyne. When
the antheridium, or male gametangium, forms, it fuses
with the trichogyne of an adjacent ascogonium. Initially,
both kinds of gametangia contain a number of nuclei.
Nuclei from the antheridium then migrate through the
trichogyne into the ascogonium and pair with nuclei of
the opposite mating type. Dikaryotic hyphae then arise
from the area of the fusion. Throughout such hyphae,
726 Part IX Viruses and Simple Organisms
Imperfect fungi
Basidiomycetes
Zygomycetes Ascomycetes
n
2n
+
Strain
Ascospore
Each haploid
nucleus divides
once by mitosis
Trichogyne
Antheridium
Plasmogamy
(cytoplasmic
bridge allows
strain nuclei
to pass into
ascogonium)
Dikaryotic
hyphae
form from
ascogonium
Karyogamy
(formation of
young ascus)
Zygote
Young
ascus
Asexual reproduction by spores (conidia)
Fully developed ascocarp composed of dikaryotic
(ascogenic) hyphae and sterile hyphae
Dikaryotic
Monokaryotic
MEIOSIS
–
–
Strain
Ascogonium
(c)
FIGURE 36.9
An ascomycetes. (a) This morel, Morchella esculenta,is a delicious edible ascomycete that
appears in early spring. (b) A cup fungus. (c) Life cycle of an ascomycete. The zygote forms
within the ascus.
(a)
(b)
nuclei that represent the two different original mating
types occur. These hyphae are thus both dikaryotic and
heterokaryotic.
Asci are formed at the tips of the dikaryotic hyphae and
are separated by the formation of septa. There are two
haploid nuclei within each ascus, one of each mating type
represented in the dikaryotic hypha. Fusion of these two
nuclei occurs within each ascus, forming a zygote. Each
zygote divides immediately by meiosis, forming four hap-
loid daughter nuclei. These usually divide again by mito-
sis, producing eight haploid nuclei that become walled as-
cospores. In many ascomycetes, the ascus becomes highly
turgid at maturity and ultimately bursts, often at a pre-
formed area. When this occurs, the ascospores may be
thrown as far as 30 centimeters, an amazing distance con-
sidering that most ascospores are only about 10 microme-
ters long. This would be equivalent to throwing a baseball
(diameter 7.5 centimeters) 1.25 kilometers—about 10
times the length of a home run!
Yeasts
Yeasts, which are unicellular, are one of the most interest-
ing and economically important groups of microscopic
fungi, usually ascomycetes. Most of their reproduction is
asexual and takes place by cell fission or budding, when a
smaller cell forms from a larger one (figure 36.10).
Sometimes two yeast cells will fuse, forming one cell
containing two nuclei. This cell may then function as an
ascus, with syngamy followed immediately by meiosis.
The resulting ascospores function directly as new yeast
cells.
Because they are single-celled, yeasts were once con-
sidered primitive fungi. However, it appears that they
are actually reduced in structure and were originally de-
rived from multicellular ancestors. The word yeastactu-
ally signifies only that these fungi are single-celled.
Some yeasts have been derived from each of the three
phyla of fungi, although ascomycetes are best repre-
sented. Even yeasts that were derived from ascomycetes
are not necessarily directly related to one another, but
instead seem to have been derived from different groups
of ascomycetes.
Putting Yeasts to Work. The ability of yeasts to fer-
ment carbohydrates, breaking down glucose to produce
ethanol and carbon dioxide, is fundamental in the produc-
tion of bread, beer, and wine. Many different strains of
yeast have been domesticated and selected for these
processes. Wild yeasts—ones that occur naturally in the
areas where wine is made—were important in wine mak-
ing historically, but domesticated yeasts are normally used
now. The most important yeast in all these processes is
Saccharomyces cerevisiae.This yeast has been used by hu-
mans throughout recorded history. Other yeasts are im-
portant pathogens and cause diseases such as thrush and
cryptococcosis; one of them, Candida,causes common
oral or vaginal infections.
Over the past few decades, yeasts have become increas-
ingly important in genetic research. They were the first
eukaryotes to be manipulated extensively by the tech-
niques of genetic engineering, and they still play the lead-
ing role as models for research in eukaryotic cells. In
1983, investigators synthesized a functional artificial chro-
mosome in Saccharomyces cerevisiaeby assembling the ap-
propriate DNA molecule chemically; this has not yet been
possible in any other eukaryote. In 1996, the genome se-
quence of S. cerevisiae,the first eukaryote to be sequenced
entirely, was completed. With their rapid generation time
and a rapidly increasing pool of genetic and biochemical
information, the yeasts in general and S. cerevisiaein par-
ticular are becoming the eukaryotic cells of choice for
many types of experiments in molecular and cellular biol-
ogy. Yeasts have become, in this respect, comparable to
Escherichia coliamong the bacteria, and they are continu-
ing to provide significant insights into the functioning of
eukaryotic systems.
Ascomycetes form their zygotes within a characteristic
saclike structure, the ascus. Meiosis follows, resulting
in the production of ascospores. Yeasts are unicellular
fungi, mainly ascomycetes, that have evolved from
hypha-forming ancestors; not all yeasts are directly
related to one another. Long useful for baking,
brewing, and wine making, yeasts are now becoming
very important in genetic research.
Chapter 36 Fungi 727
FIGURE 36.10
Scanning electron micrograph of a yeast, showing the
characteristic cell division method of budding (19,000×).
The cells tend to hang together in chains, a feature that calls to
mind the derivation of single-celled yeasts from multicellular
ancestors.
Phylum Basidiomycota
The third phylum of fungi, the basid-
iomycetes (phylum Basidiomycota), has
about 22,000 named species. These are
among the most familiar fungi. Among the
basidiomycetes are not only the mush-
rooms, toadstools, puffballs, jelly fungi, and
shelf fungi, but also many important plant
pathogens including rusts and smuts (figure
36.11). Many mushrooms are used as food,
but others are deadly poisonous.
Basidiomycetes are named for their char-
acteristic sexual reproductive structure, the
basidium (plural, basidia). A basidium is
club-shaped. Karyogamy occurs within the
basidium, giving rise to the zygote, the only
diploid cell of the life cycle (figure 36.11b). As in all fungi,
meiosis occurs immediately after the formation of the zy-
gote. In the basidiomycetes, the four haploid products of
meiosis are incorporated into basidiospores. In most
members of this phylum, the basidiospores are borne at the
end of the basidia on slender projections called sterigmata
(singular, sterigma). Thus the structure of a basidium dif-
fers from that of an ascus, although functionally the two are
identical. Recall that the ascospores of the ascomycetes are
borne internally in asci.
The life cycle of a basidiomycete contin-
ues with the production of homokaryotic
hyphae after spore germination. These hy-
phae lack septa at first. Eventually, septa
form between the nuclei of the monokary-
otic hyphae. A basidiomycete mycelium
made up of monokaryotic hyphae is called
a primary mycelium. Different mating
types of monokaryotic hyphae may fuse,
forming a dikaryotic or secondary
mycelium. Such a mycelium is heterokary-
otic, with two nuclei, representing the two
different mating types, between each pair
of septa. The maintenance of two genomes
in the heterokaryon allows for more ge-
netic plasticity than in a diploid cell with
one nucleus. One genome may compen-
sate for mutations in the other. The basidiocarps, or
mushrooms, are formed entirely of secondary (dikaryotic)
mycelium. Gills on the undersurface of the cap of a mush-
room form vast numbers of minute spores. It has been esti-
mated that a mushroom with a cap that is 7.5 centimeters
across produces as many as 40 million spores per hour!
Most basidiomycete hyphae are dikaryotic. Ultimately,
the hyphae fuse to form basidiocarps, with basidia lining
the gills on the underside. Meiosis immediately follows
syngamy in these basidia.
728 Part IX Viruses and Simple Organisms
Imperfect fungi
Ascomycetes
Zygomycetes
Basidiomycetes
FIGURE 36.11
Basidiomycetes. (a) Death cap
mushroom, Amanita phalloides.When
eaten, these mushrooms are usually
fatal. (b) Life cycle of a basidiomycete.
The basidium is the reproductive
structure where syngamy occurs.
Fusion of
and
hyphae
–+
Basidiocarp
MEIOSIS
Formation of
basidiospores
Basidiospores
Primary mycelium
(monokaryotic)
Secondary mycelium
(dikaryotic)
Gills lined
with basidia
Basidia
Sterigma
Zygote
n
n + n
Strain
Strain
–
n2
+
–
+
(a)
(b)
The Imperfect Fungi
Most of the so-called imperfect fungi, a
group also called deuteromycetes, are those
in which the sexual reproductive stages
have not been observed. Most of these ap-
pear to be related to ascomycetes although
some have clear affinities to the other
phyla. The group of fungi from which a
particular nonsexual strain has been derived
usually can be determined by the features
of its hyphae and asexual reproduction. It
cannot, however, be classified by the stan-
dards of that group because the classifica-
tion systems are based on the features re-
lated to sexual reproduction. One
consequence of this system is that as sexual
reproduction is discovered in an imperfect fungus, it may
have two names assigned to different stages of its life cycle.
There are some 17,000 described species of imperfect
fungi (figure 36.12). Even though sexual reproduction is
absent among imperfect fungi, a certain amount of ge-
netic recombination occurs. This becomes possible when
hyphae of different genetic types fuse, as sometimes hap-
pens spontaneously. Within the heterokaryotic hyphae
that arise from such fusion, a special kind of genetic re-
combination called parasexuality may occur. In parasex-
uality, genetically distinct nuclei within a common hypha
exchange portions of chromosomes. Recombination of
this sort also occurs in other groups of fungi and seems to
be responsible for some of the new pathogenic strains of
wheat rust.
Economic Importance
Among the economically important genera
of the imperfect fungi are Penicilliumand
Aspergillus.Some species of Penicilliumare
sources of the well-known antibiotic peni-
cillin, and other species of the genus give
the characteristic flavors and aromas to
cheeses such as Roquefort and Camembert.
Species of Aspergillusare used to ferment
soy sauce and soy paste, processes in which
certain bacteria and yeasts also play impor-
tant roles. Citric acid is produced commer-
cially with members of this genus under
highly acidic conditions. Some species of
both Penicilliumand Aspergillusform asco-
carps, but the genera are still classified pri-
marily as imperfect fungi because the asco-
carps are found rarely in only a few species. Most of the
fungi that cause skin diseases in humans, including athlete’s
foot and ringworm, are also imperfect fungi.
A number of imperfect fungi occur widely on food.
Fusariumspecies growing on spoiled food produce highly
toxic substances such as trichothecenes. Aflatoxins, among
the most carcinogenic compounds known, are produced by
some Aspergillus flavusstrains growing on corn, peanuts,
etc. Most developed countries have legal limits on the con-
centration of aflatoxin permitted in different foods.
Imperfect fungi are fungi in which no sexual
reproduction has been observed. For this reason, they
cannot be classified by the standards applied to the
three phyla of fungi. The great majority of the
imperfect fungi are clearly ascomycetes.
Chapter 36 Fungi 729
Ascomycetes
Basidiomycetes
Zygomycetes
Imperfect fungi
FIGURE 36.12
The imperfect fungi. (a) Verticillium alboatrum(1350×), an important pathogen of alfalfa,
has whorled conidiophores. The single-celled conidia of this member of the imperfect
fungi are borne at the ends of the conidiophores. (b) In Tolypocladium inflatum,the conidia
arise along the branches. This fungus is one of the sources of cyclosporin, a drug that
suppresses immune reactions and thus assists in making human organ grafts possible; the
drug was put on the market in 1979. (c) This scanning electron micrograph of Aspergillus
shows conidia, the spheres at the end of the hyphae.(a)
(b)
(c)
Lichens
Lichens (figure 36.13) are symbiotic
associations between a fungus and a
photosynthetic partner. They pro-
vide an outstanding example of mu-
tualism, the kind of symbiotic asso-
ciation that benefits both partners.
Ascomycetes (including some imper-
fect fungi) are the fungal partners in
all but about 20 of the approxi-
mately 15,000 species of lichens esti-
mated to exist; the exceptions,
mostly tropical, are basidiomycetes.
Most of the visible body of a lichen
consists of its fungus, but within the
tissues of that fungus are found
cyanobacteria, green algae, or some-
times both (figure 36.14). Special-
ized fungal hyphae penetrate or envelop the photosyn-
thetic cells within them and transfer nutrients directly to
the fungal partner. Biochemical “signals” sent out by the
fungus apparently direct its cyanobacterial or green algal
component to produce metabolic substances that it does
not produce when growing independently of the fungus.
The photosynthetic member of the association is nor-
mally held between thick layers of interwoven fungal hy-
phae and is not directly exposed to the light, but enough
light penetrates the translucent layers of fungal hyphae to
make photosynthesis possible. The fungi in lichens are
unable to grow normally without their photosynthetic
partners and the fungi protect their partners from strong
light and desiccation.
The durable construction of the fungus, combined with
the photosynthetic properties of its partner, has enabled
lichens to invade the harshest habitats at the tops of moun-
tains, in the farthest northern and southern latitudes, and
on dry, bare rock faces in the desert. In harsh, exposed
areas, lichens are often the first colonists, breaking down
the rocks and setting the stage for the invasion of other
organisms.
Lichens are often strikingly colored because of the pres-
ence of pigments that probably play a role in protecting
the photosynthetic partner from the destructive action of
the sun’s rays. These same pigments may be extracted
from the lichens and used as natural dyes. The traditional
method of manufacturing Scotland’s famous Harris tweed
used fungal dyes.
Lichens are extremely sensitive to pollutants in the at-
mosphere, and thus they can be used as bioindicators of air
quality. Their sensitivity results from their ability to ab-
sorb substances dissolved in rain and dew. Lichens are
generally absent in and around cities because of automo-
bile traffic and industrial activity, even though suitable
substrates exist.
Lichens are symbiotic associations between a fungus—
an ascomycete in all but a very few instances—and a
photosynthetic partner, which may be a green alga or a
cyanobacterium or both.
730 Part IX Viruses and Simple Organisms
36.3 Fungi form two key mutualistic symbiotic associations.
(a) (b)
FIGURE 36.13
Lichens are found in a variety of habitats. (a) A fruticose lichen, Cladina evansii,growing
on the ground in Florida. (b) A foliose (“leafy”) lichen, Parmotrema gardneri,growing on the
bark of a tree in a mountain forest in Panama.
Fungal
hyphae
Algal
cells
FIGURE 36.14
Stained section of a lichen (250×). This section shows fungal
hyphae (purple) more densely packed into a protective layer on the
top and, especially, the bottom layer of the lichen. The blue cells
near the upper surface of the lichen are those of a green alga.
These cells supply carbohydrate to the fungus.
Mycorrhizae
The roots of about 90% of all kinds of vascular plants nor-
mally are involved in mutualistic symbiotic relationships
with certain kinds of fungi. It has been estimated that these
fungi probably amount to 15% of the total weight of the
world’s plant roots. Associations of this kind are termed
mycorrhizae (from the Greek words for “fungus” and
“roots”). The fungi in mycorrhizae associations function as
extensions of the root system. The fungal hyphae dramati-
cally increase the amount of soil contact and total surface
area for absorption. When mycorrhizae are present, they
aid in the direct transfer of phosphorus, zinc, copper, and
other nutrients from the soil into the roots. The plant, on
the other hand, supplies organic carbon to the fungus, so
the system is an example of mutualism.
There are two principal types of mycorrhizae (figure
36.15): endomycorrhizae, in which the fungal hyphae
penetrate the outer cells of the plant root, forming coils,
swellings, and minute branches, and also extend out into
the surrounding soil; and ectomycorrhizae, in which the
hyphae surround but do not penetrate the cell walls of the
roots. In both kinds of mycorrhizae, the mycelium extends
far out into the soil.
Endomycorrhizae
Endomycorrhizae are by far the more common of these
two types. The fungal component in them is a zy-
gomycete. Only about 100 species of zygomycetes are
known to be involved in such relationships throughout the
world. These few species of zygomycetes are associated
with more than 200,000 species of plants. Endomycor-
rhizal fungi are being studied intensively because they are
potentially capable of increasing crop yields with lower
phosphate and energy inputs.
The earliest fossil plants often show endomycorrhizal
roots. Such associations may have played an important role
in allowing plants to colonize land. The soils available at
such times would have been sterile and completely lacking
in organic matter. Plants that form mycorrhizal associa-
tions are particularly successful in infertile soils; consider-
ing the fossil evidence, the suggestion that mycorrhizal as-
sociations found in the earliest plants helped them succeed
on such soils seems reasonable. In addition, the most prim-
itive vascular plants surviving today continue to depend
strongly on mycorrhizae.
Ectomycorrhizae
Ectomycorrhizae (figure 36.15b) involve far fewer kinds of
plants than do endomycorrhizae, perhaps a few thousand.
They are characteristic of certain groups of trees and
shrubs, particularly those of temperate regions, including
pines, firs, oaks, beeches, and willows. The fungal compo-
nents in most ectomycorrhizae are basidiomycetes, but
some are ascomycetes. Several different kinds of ectomyc-
orrhizal fungi may form mycorrhizal associations with one
plant. Different combinations have different effects on the
physiological characteristics of the plant and its ability to
survive under different environmental conditions. At least
5000 species of fungi are involved in ectomycorrhizal rela-
tionships, and many of them are restricted to a single
species of plant.
Mycorrhizae are symbiotic associations between plants
and fungi.
Chapter 36 Fungi 731
FIGURE 36.15
Endomycorrhizae and ectomycorrhizae. (a) In
endomycorrhizae, fungal hyphae penetrate and branch out in the
root cells of plants. In ectomycorrhizae, fungal hyphae do not
penetrate root cells but grow around and extend between the cells.
(b) Ectomycorrhizae on roots of pines. From left to right are
yellow-brown mycorrhizae formed by Pisolithus,white
mycorrhizae formed by Rhizopagon,and pine roots not associated
with a fungus.
Ectomycorrhizae
Endomycorrhizae
(a)
(b)
732 Part IX Viruses and Simple Organisms
Chapter 36
Summary Questions Media Resources
36.1 Fungi are unlike any other kind of organism.
? The fungi are a distinct kingdom of eukaryotic
organisms characterized by a filamentous growth
form, lack of chlorophyll and motile cells, chitin-rich
cell walls, and external digestion of food by the
secretion of enzymes.
? Fungal filaments, called hyphae, collectively make up
the fungus body, which is called the mycelium.
? In many fungi, the two kinds of nuclei that will
eventually undergo syngamy occur together in
hyphae for a long period before they fuse. Meiosis
occurs immediately after the formation of the zygote
in all fungi; the zygote, therefore, is the only diploid
nucleus of the entire life cycle in these organisms.
1.What is a hypha? What is
the advantage to having
incomplete septa?
2.What is the composition of
the fungal cell wall? Why is this
composition an advantage to the
fungi?
3.Which fungal nuclei are
diploid? Which are haploid? To
what do the following terms
refer: heterokaryotic, homokaryotic,
dikaryotic, andmonokaryotic?
? There are three phyla of fungi: Zygomycota, the
zygomycetes; Ascomycota, the ascomycetes; and
Basidiomycota, the basidiomycetes.
? Zygomycetes form septa only when gametangia or
sporangia are cut off at the ends of their hyphae;
otherwise, their hyphae are multinucleate. Most
hyphae of ascomycetes and basidiomycetes have
perforated septa through which the cytoplasm, but
not necessarily the nuclei, flows freely.
? Cells within the heterokaryotic hyphae of
ascomycetes are multinucleate; those within the
heterokaryotic hyphae of the basidiomycetes are
dikaryotic. Zygotes in ascomycetes form within sac-
like structures known as asci, and those in
basidiomycetes form within structures known as
basidia.
? Asexual reproduction in zygomycetes takes place by
means of spores from multinucleate sporangia; in
ascomycetes, it takes place by means of conidia.
Asexual reproduction in basidiomycetes is rare.
4.What are the three
reproductive structures that
occur in fungi? How do they
differ?
5.Fungi are nonmotile. How
are they dispersed to new areas?
6.What are the ascomycete
asexual spores called? Do the
nonreproductive hyphae of this
division have septa?
7.To what phyla do the yeasts
belong? How do they differ from
other fungi? Is it more likely that
this characteristic is primitive or
degenerate?
8.What are the imperfect
fungi? Which phylum seems to
be best represented in this
group? By what means can
individuals in this group be
classified?
36.2 Fungi are classified by their reproductive structures.
? Lichens are mutualistic symbiotic systems involving
fungi (almost always ascomycetes), which derive their
nutrients from green algae, cyanobacteria, or both.
? Mycorrhizae are mutualistic symbiotic associations
between fungi and plants. Endomycorrhizae, more
common, involve zygomycetes, while ectomycor-
rhizal fungi are mainly basidiomycetes.
9.What are lichens? Which
fungal phylum is best
represented in the lichens?
10.What are mycorrhizae? How
do endomycorrhizae and
ectomycorrhizae differ?
36.3 Fungi form two key mutualistic symbiotic associations.
www.mhhe.com/raven6e www.biocourse.com
? Characteristics of
Fungi
? Diversity of Fungi
? Student Research:
Mushroom Spore
Germination
733
Part Opener Title
Text to come.
Part
X
Plant Form and Function
Part opener figure 1 title. Figure legend.
734 Part X Plant Form and Function
Part opener figure 2 title. Figure legend.
735
37
Evolutionary History
of Plants
Concept Outline
37.1 Plants have multicellular haploid and diploid
stages in their life cycles.
The Evolutionary Origins of Plants. Plants evolved
from freshwater green algae and eventually developed
cuticles, stomata, conducting systems, and reproductive
strategies that adapt them well for life on land.
Plant Life Cycles. Plants have haplodiplontic life cycles.
Diploid sporophytes produce haploid spores which develop
into haploid gametophytes that produce haploid gametes.
37.2 Nonvascular plants are relatively unspecialized,
but successful in many terrestrial environments.
Mosses, Liverworts, and Hornworts. The most
conspicuous part of a nonvascular plant is the green
photosynthetic gametophyte, which supports the smaller
sporophyte nutritionally.
37.3 Seedless vascular plants have well-developed
conducting tissues in their sporophytes.
Features of Vascular Plants. In vascular plants,
specialized tissue called xylem conducts water and dissolved
minerals within the plant, and tissue called phloem conducts
sucrose and plant growth regulators within the plant.
Seedless Vascular Plants. Seedless vascular plants have a
much more conspicuous sporophyte than nonvascular
plants do, and many have well-developed conducting
systems in stem, roots, and leaves.
37.4 Seeds protect and aid in the dispersal of plant
embryos.
Seed Plants. In seed plants, the sporophyte is dominant.
Male and female gametophytes develop within the
sporophyte and depend on it for food. Seeds allow embryos
to germinate when conditions are favorable.
Gymnosperms. In gymnosperms, the female
gametophyte (ovule) is not completely enclosed by
sporophyte tissue at the time of pollination by male
gametophytes (pollen).
Angiosperms. In angiosperms, the ovule is completely
enclosed by sporophyte tissue at the time of pollination.
Angiosperms, by far the most successful plant group,
produce flowers.
P
lant evolution is the story of the conquest of land by
green algal ancestors. For about 500 million years,
algae were confined to a watery domain, limited by the
need for water to reproduce, provide structural support,
prevent water loss, and provide some protection from the
sun’s ultraviolet irradiation. Numerous evolutionary solu-
tions to these challenges have resulted in over 300,000
species of plants dominating all terrestrial communities
today, from forests to alpine tundra, from agricultural
fields to deserts (figure 37.1). Most plants are photosyn-
thetic, converting light energy into chemical-bond energy
and providing oxygen for all aerobic organisms. We rely
on plants for food, clothing, wood for shelter and fuel,
chemicals, and many medicines. This chapter explores the
evolutionary history and strategies that have allowed
plants to inhabit most terrestrial environments over mil-
lions of years.
FIGURE 37.1
An arctic tundra. This is one of the harshest environments on
earth, yet a diversity of plants have made this home. These
ecosystems are fragile and particularly susceptible to global
change.
Adaptations to Land
Plants and fungi are the only major groups of organisms
that are primarily terrestrial. Most plants are protected
from desiccation—the tendency of organisms to lose water
to the air—by a waxy cuticle that is secreted onto their ex-
posed surfaces. The cuticle is relatively impermeable and
provides an effective barrier to water loss.
This solution creates another problem by limiting gas
exchange essential for respiration and photosynthesis.
Water and gas diffusion into and out of a plant occurs
through tiny mouth-shaped openings called stomata (sin-
gular, stoma).
The evolution of leaves resulted in increased photo-
synthetic surface area. The shift to a dominant diploid
generation, accompanied by the structural support of vas-
cular tissue, allowed plants to take advantage of the verti-
cal dimension of the terrestrial environment, resulting in
trees.
Plants evolved from freshwater green algae and
eventually developed cuticles, stomata, conducting
systems, and reproductive strategies that adapt them
well for life on land.
736 Part X Plant Form and Function
The Evolutionary Origins of Plants
Biologists have long suspected that plants are the evolu-
tionary descendants of green algae. Now we are sure. The
evolutionary history of plants was laid bare at the 1999 In-
ternational Botanical Congress held in St. Louis, Mis-
souri, by a team of 200 biologists from 12 countries that
had been working together for five years with U.S. federal
funding. Their project, Deep Green (more formally
known as The Green Plant Phylogeny Research Coordi-
nation Group), coordinated the efforts of laboratories
using molecular, morphological, and anatomical traits to
create a new “Tree of Life.” Deep Green confirmed the
long-standing claim that green algae were ancestral to
plants. More surprising was the finding that just a single
species of green algae gave rise to the entire terrestrial
plant lineage from moss through the flowering plants (an-
giosperms). Exactly what this ancestral alga was is still a
mystery, but close relatives are believed to exist in fresh-
water lakes today. DNA sequence data is consistent with
the claim that a single “Eve” gave rise to the entire king-
dom Plantae 450 million years ago. At each subsequent
step in evolution, the evidence suggests that only a single
family of plants made the transition. The fungi appear to
have branched later than the plants and are more closely
related to us.
There are 12 plant phyla, all of which afford some pro-
tection to their embryos. All plants also have a haploid and
a diploid stage that is multicellular. The trend over time
has been toward increasing embryo protection and a
smaller haploid stage in the life cycle. The plants are di-
vided into two groups based on the presence or absence of
vascular tissues which facilitate the transport of water and
nutrients in plants. Three phyla (mosses, liverworts, and
hornworts) lack vascular tissue and are referred to as the
nonvascular plants. Members of 9 of the 12 plant phyla
are collectively called vascular plants, and include, among
others, the ferns, conifers, and flowering plants. Vascular
plants have water-conducting xylem and food-conducting
phloem strands of tissues in their stems, roots, and leaves.
Vascular plants can be further grouped based on how much
protection embryos have. The seedless vascular plants
(ferns) provide less protection than the seeds of the gym-
nosperms (conifers) and angiosperms (flowering plants)
(figure 37.2). About 150 million years ago the angiosperms
arose with further innovations—flowers to attract pollina-
tors and fruit surrounding the seed to protect the embryo
and aid in seed dispersal. Many of these lineages have per-
sisted. If you could travel back 65 million years to the di-
nosaur era, you would encounter oak, walnut, and
sycamore trees!
37.1 Plants have multicellular haploid and diploid stages in their life cycles.
Nonvascular plants
Seedless vascular plants
Gymnosperms
Angiosperms
Plants
FIGURE 37.2
The four major groups of plants. In this chapter we will discuss
four major groups of plants. The green algae, discussed in chapter
35, are the protists from which plants are thought to have evolved.
Plant Life Cycles
All plants undergo mitosis after both gamete
fusion and meiosis. The result is a multicellu-
lar haploid and a multicellular diploid individ-
ual, unlike us where gamete fusion directly fol-
lows meiosis. We have a diplontic life cycle
(only the diploid stage is multicellular), but the
plant life cycle is haplodiplontic (with multi-
cellular haploid and diploid stages). The basic
haplodiplontic cycle is summarized in figure
37.3. Brown, red, and green algae are also hap-
lodiplontic (see chapter 35). While we produce
gametes via meiosis, plants actually produce
gametes by mitosis in a multicellular, haploid
individual. The diploid generation, or sporo-
phyte, alternates with the haploid generation,
or gametophyte. Sporophyte means “spore
plant,” and gametophyte means “gamete
plant.” These terms indicate the kinds of re-
productive cells the respective generations
produce.
The diploid sporophyte undergoes meiosis
to produce haploid spores (not gametes).
Meiosis takes place in structures called spo-
rangia, where diploid spore mother cells
(sporocytes) undergo meiosis, each producing
four haploid spores. Spores divide by mitosis,
producing a multicellular, haploid gameto-
phyte. Spores are the first cells of the gameto-
phyte generation.
In turn, the haploid gametophyte produces haploid ga-
metes by mitosis. When the gametes fuse, the zygote
they form is diploid and is the first cell of the next sporo-
phyte generation. The zygote grows into a diploid sporo-
phyte that produces sporangia in which meiosis ulti-
mately occurs.
While all plants are haplodiplontic, the haploid genera-
tion consumes a much larger chunk of the life cycle in
mosses than in gymnosperms and angiosperms. In mosses,
liverworts, and ferns, the gametophyte is photosynthetic
and free-living; in other plants it is either nutritionally de-
pendent on the sporophyte, or saprobic (deriving its en-
ergy directly from nonliving organic matter). When you
look at moss, what you see is largely gametophyte tissue;
their sporophytes are usually smaller, brownish or yellow-
ish structures attached to or enclosed within tissues of the
gametophyte. In most vascular plants the gametophytes are
much smaller than the sporophytes. In seed plants, the ga-
metophytes are nutritionally dependent on the sporophytes
and are enclosed within their tissues. When you look at a
gymnosperm or angiosperm, what you see, with rare excep-
tions, is a sporophyte.
The difference between dominant gametophytes and
sporophytes is key to understanding why there are no
moss trees. What we identify as a moss plant is a gameto-
phyte and it produces gametes at its tip. The egg is sta-
tionery and sperm lands near the egg in a droplet of
water. If the moss were the height of a sequoia, not only
would it need vascular tissue for conduction and support,
the sperm would have to swim up the tree! In contrast,
the fern gametophyte develops on the forest floor where
gametes can meet. Fern trees abound in Australia and the
haploid spores fall to the ground and develop into game-
tophytes.
Having completed our overview of plant life cycles, we
will consider the major plant groups. As we do, we will
see a progressive reduction of the gametophyte from
group to group, a loss of multicellular gametangia
(structures in which gametes are produced), and increas-
ing specialization for life on the land, culminating with
the remarkable structural adaptations of the flowering
plants, the dominant plants today. Similar trends must
have characterized the progression to seed plants over the
hundreds of millions of years since a freshwater alga
made the move onto land.
Plants have haplodiplontic life cycles. Diploid
sporophytes produce haploid spores which develop into
haploid gametophytes that produce haploid gametes.
Chapter 37 Evolutionary History of Plants 737
2n
2n
Egg
Mitosis
Spore
2n
Sporophyte
(2n)
Sporangia
Spore mother
cell
Spores
n
n
n
n
Gametophyte
(n)
Gamete fusion
Sperm
Embryo
Zygote
Meiosis
Haploid
Diploid
FIGURE 37.3
A generalized plant life cycle. Anything yellow is haploid and anything blue is
diploid. Note that both haploid and diploid individuals can be multicellular. Also,
spores are produced by meiosis while gametes are produced by mitosis.
Mosses, Liverworts, and
Hornworts
There are about 24,700 bryophytes—
mosses, liverworts, and hornworts—
that are simply but highly adapted to a
diversity of terrestrial environments
(even deserts!). Scientists now agree
that bryophytes consist of three quite
distinct phyla of relatively unspecial-
ized plants. Their gametophytes are
photosynthetic. Sporophytes are at-
tached to the gametophytes and de-
pend on them nutritionally to varying
degrees. Bryophytes, like ferns and cer-
tain other vascular plants, require
water (for example, rainwater) to re-
produce sexually. It is not surprising
that they are especially common in
moist places, both in the tropics and tem-
perate regions.
Most bryophytes are small; few exceed 7
centimeters in height. The gametophytes
are more conspicuous than the sporo-
phytes. Some of the sporophytes are com-
pletely enclosed within gametophyte tissue;
others are not and usually turn brownish or
straw-colored at maturity.
Mosses (Bryophyta)
The gametophytes of mosses typically
consist of small leaflike structures
arranged spirally or alternately around a
stemlike axis (figure 37.4); the axis is an-
chored to its substrate by means of rhi-
zoids. Each rhizoid consists of several
cells that absorb water, but nothing like
the volume of water absorbed by a vascu-
lar plant root. Moss “leaves” have little in
common with true leaves, except for the
superficial appearance of the green, flat-
tened blade and slightly thickened midrib
that runs lengthwise down the middle.
They are only one cell thick (except at the
midrib), lack vascular strands and stomata, and all the
cells are haploid.
Water may rise up a strand of specialized cells in the
center of a moss gametophyte axis, but most water used by
the plant travels up the outside of the plant. Some mosses
also have specialized food-conducting
cells surrounding those that conduct
water.
Multicellular gametangia are formed
at the tips of the leafy gametophytes
(figure 37.5). Female gametangia
(archegonia) may develop either on the
same gametophyte as the male ga-
metangia (antheridia) or on separate
plants. A single egg is produced in the
swollen lower part of an archegonium
while numerous sperm are produced in
an antheridium. When sperm are re-
leased from an antheridium, they swim
with the aid of flagella through a film of
dew or rainwater to the archegonia.
One sperm (which is haploid) unites
with an egg (also haploid), forming a
diploid zygote. The zygote divides by
mitosis and develops into the sporo-
phyte, a slender, basal stalk with a swollen
capsule, the sporangium, at its tip. As the
sporophyte develops, its base becomes em-
bedded in gametophyte tissue, its nutri-
tional source. The sporangium is often
cylindrical or club-shaped. Spore mother
cells within the sporangium undergo meio-
sis, each becoming four haploid spores. At
maturity, the top of the sporangium pops
off, and the spores are released. A spore
that lands in a suitable damp location may
germinate and grow into a threadlike struc-
ture that branches to form rhizoids and
“buds” that grow upright. Each bud devel-
ops into a new gametophyte plant consist-
ing of a leafy axis.
In the Arctic and the Antarctic, mosses
are the most abundant plants, boasting not
only the largest number of individuals in
these harsh regions, but also the largest
number of species. Many mosses are able to
withstand prolonged periods of drought, al-
though they are not common in deserts.
Most are remarkably sensitive to air pollu-
tion and are rarely found in abundance in
or near cities or other areas with high levels of air pollu-
tion. Some mosses, such as the peat mosses (Sphagnum),
can absorb up to 25 times their weight in water and are
valuable commercially as a soil conditioner, or as a fuel
when dry.
738 Part X Plant Form and Function
37.2 Nonvascular plants are relatively unspecialized, but successful in many
terrestrial environments.
Nonvascular plants
Seedless vascular plants
Gymnosperms
Angiosperms
FIGURE 37.4
A hair-cup moss, Polytrichum
(phylum Bryophyta). The
“leaves” belong to the
gametophyte. Each of the
yellowish-brown stalks, with the
capsule, or sporangium, at its
summit, is a sporophyte.
Liverworts (Hepaticophyta)
The old English word wyrt means
“plant” or “herb.” Some common liver-
worts have flattened gametophytes with
lobes resembling those of liver—hence
the combination “liverwort.” Although
the lobed liverworts are the best-known
representatives of this phylum, they
constitute only about 20% of the
species (figure 37.6). The other 80%
are leafy and superficially resemble
mosses. Liverworts are less complex
than mosses. Gametophytes are pros-
trate instead of erect, and the rhizoids
are one-celled.
Some liverworts have air chambers
containing upright, branching rows of
photosynthetic cells, each chamber
having a pore at the top to facilitate gas
exchange. Unlike stomata, the pores
are fixed open and cannot close.
Sexual reproduction in liverworts is
similar to that in mosses. Lobed liver-
worts form gametangia in umbrella-
like structures. Asexual reproduction
occurs when lens-shaped pieces of
tissue that are released from the
gametophyte grow to form new
gametophytes.
Hornworts (Anthocerotophyta)
The origins of hornworts are a puzzle.
They are most likely among the earliest
land plants, yet the earliest fossil spores
date from the Cretaceous period, 65 to
145 million years ago, when an-
giosperms were emerging.
The small hornwort sporophytes re-
semble tiny green broom handles rising
from filmy gametophytes usually less
than 2 centimeters in diameter (figure
37.7). The sporophyte base is embed-
ded in gametophyte tissue, from which
it derives some of its nutrition. How-
ever, the sporophyte has stomata, is
photosynthetic, and provides much of
the energy needed for growth and re-
production. Hornwort cells usually
have a single chloroplast.
The three major phyla of
nonvascular plants are all relatively
unspecialized, but well suited for
diverse terrestrial environments.
Chapter 37 Evolutionary History of Plants 739
Sperm
FERTILIZATION
Zygote
Developing
sporophyte in
archegonium
Mature
sporophyte
Parent
gametophyte
Sporangium
Antheridium
Egg
Archegonium
Male Female
Gametophytes
Bud
Rhizoid
Germinating
spores
Spores
MEIOSIS
n 2n
Mitosis
FIGURE 37.5
Life cycle of a typical moss. The majority of the life cycle of a moss is in the haploid
state. The leafy gametophyte is photosynthetic, while the smaller sporophyte is not, and
is nutritionally dependent on the gametophyte. Water is required to carry sperm to
the egg.
FIGURE 37.6
A common liverwort, Marchantia
(phylum Marchantiophyta). The
sporophytes are borne within the tissues of
the umbrella-shaped structures that arise
from the surface of the flat, green, creeping
gametophyte.
FIGURE 37.7
Hornworts (phylum Anthocerotophyta).
Hornwort sporophytes are seen in this
photo. Unlike the sporophytes of other
bryophytes, these are photosynthetic, but
they also depend on the gametophyte for
nutrition.
Features of Vascular Plants
The first vascular plants for which we have a relatively
complete record belonged to the phylum Rhyniophyta;
they flourished some 410 million years ago but are now ex-
tinct. We are not certain what the very earliest of these vas-
cular plants looked like, but fossils of Cooksonia provide
some insight into their characteristics (figure 37.8). Cookso-
nia, the first known vascular land plant, appeared in the
late Silurian period about 420 million years ago. It was suc-
cessful partly because it encountered little competition as it
spread out over vast tracts of land. The plants were only a
few centimeters tall and had no roots or leaves. They con-
sisted of little more than a branching axis, the branches
forking evenly and expanding slightly toward the tips. They
were homosporous (producing only one type of spore).
Sporangia formed at branch tips. Other ancient vascular
plants that followed evolved more complex arrangements
of sporangia. Leaves began to appear as protuberances
from stems.
Cooksonia and the other early plants that followed it be-
came successful colonizers of the land through the develop-
ment of efficient water- and food-conducting systems
known as vascular tissues. The word vascular comes from
the Latin vasculum, meaning a “vessel or duct.” These tis-
sues consist of strands of specialized cylindrical or elon-
gated cells that form a network throughout a plant, extend-
ing from near the tips of the roots, through the stems, and
into true leaves. One type of vascular tissue, the xylem, con-
ducts water and dissolved minerals upward from the roots;
another type of tissue, phloem, conducts sucrose and hor-
monal signals throughout the plant. It is important to note
that vascular tissue developed in the sporophyte, but (with
few exceptions) not the gametophyte. (See the discussion of
vascular tissue structure in chapter 38.) The presence of a
cuticle and stomata are also characteristic of vascular
plants.
The nine phyla of vascular plants (table 37.1) dominate
terrestrial habitats everywhere, except for the highest
mountains and the tundra. The haplodiplontic life cycle
persists, but the gametophyte has been reduced during evo-
lution of vascular plants. A similar reduction in multicellu-
lar gametangia has occurred.
Accompanying this reduction in size and complexity of
the gametophytes has been the appearance of the seed.
Seeds are highly resistant structures well suited to protect
a plant embryo from drought and to some extent from
predators. In addition, most seeds contain a supply of
food for the young plant. Seeds occur only in het-
erosporous plants (plants that produce two types of
spores). Heterospory is believed to have arisen multiple
times in the plants. Fruits in the flowering plants add a
layer of protection to seeds and attract animals that assist
in seed dispersal, expanding the potential range of the
species. Flowers, which evolved among the angiosperms,
attract pollinators. Flowers allow plants to overcome lim-
itations of their rooted, immobile nature and secure the
benefits of wide outcrossing in promoting genetic
diversity.
Most vascular plants have well-developed conducting
tissues, specialized stems, leaves, roots, cuticles, and
stomata. Many have seeds which protect embryos until
conditions are suitable for further development.
740 Part X Plant Form and Function
37.3 Seedless vascular plants have well-developed conducting tissues in their
sporophytes.
Sporangia
FIGURE 37.8
Cooksonia, the first known vascular land plant. Its upright,
branched stems, which were no more than a few centimeters tall,
terminated in sporangia, as seen here. It probably lived in moist
environments such as mudflats, had a resistant cuticle, and
produced spores typical of vascular plants. This fossil represents a
plant that lived some 410 million years ago. Cooksonia belongs to
phylum Rhyniophyta, consisting entirely of extinct plants.
Chapter 37 Evolutionary History of Plants 741
Table 37.1 The Nine Phyla of Extant Vascular Plants
Approximate
Number of
Phylum Examples Key Characteristics Living Species
Heterosporous seed plants. Sperm not motile; conducted 250,000
to egg by a pollen tube. Seeds enclosed within a fruit.
Leaves greatly varied in size and form. Herbs, vines,
shrubs, trees. About 14,000 genera.
Primarily homosporous (a few heterosporous) vascular 11,000
plants. Sperm motile. External water necessary for
fertilization. Leaves are megaphylls that uncoil as they
mature. Sporophytes and virtually all gametophytes
photosynthetic. About 365 genera.
Homosporous or heterosporous vascular plants. Sperm 1,150
motile. External water necessary for fertilization. Leaves
are microphylls. About 12–13 genera.
Heterosporous seed plants. Sperm not motile; conducted 601
to egg by a pollen tube. Leaves mostly needlelike or
scalelike. Trees, shrubs. About 50 genera.
Heterosporous vascular seed plants. Sperm flagellated and 206
motile but confined within a pollen tube that grows to the
vicinity of the egg. Palmlike plants with pinnate leaves.
Secondary growth slow compared with that of the conifers.
Ten genera.
Heterosporous vascular seed plants. Sperm not motile; 65
conducted to egg by a pollen tube. The only gymnosperms
with vessels. Trees, shrubs, vines. Three very diverse genera
(Ephedra, Gnetum, Welwitschia).
Homosporous vascular plants. Sperm motile. External 15
water necessary for fertilization. Stems ribbed, jointed,
either photosynthetic or nonphotosynthetic. Leaves
scalelike, in whorls, nonphotosynthetic at maturity.
One genus.
Homosporous vascular plants. Sperm motile. External 6
water necessary for fertilization. No differentiation
between root and shoot. No leaves; one of the two genera
has scalelike enations and the other leaflike appendages.
Heterosporous vascular seed plants. Sperm flagellated and 1
motile but conducted to the vicinity of the egg by a pollen
tube. Deciduous tree with fan-shaped leaves that have
evenly forking veins. Seeds resemble a small plum with
fleshy, ill-scented outer covering.
Anthophyta
Pterophyta
Lycophyta
Coniferophyta
Cycadophyta
Gnetophyta
Arthrophyta
Psilophyta
Ginkgophyta
Flowering plants
(angiosperms)
Ferns
Club mosses
Conifers
(including
pines,
spruces,
firs, yews,
redwoods,
and others)
Cycads
Gnetophytes
Horsetails
Whisk ferns
Ginkgo
Seedless Vascular Plants
The earliest vascular plants lacked
seeds. Members of four phyla of living
vascular plants lack seeds, as do at least
three other phyla known only from fos-
sils. As we explore the adaptations of the
vascular plants, we focus on both repro-
ductive strategies and the advantages of
increasingly complex transport systems.
We will begin with the most familiar
phylum of seedless vascular plants, the
ferns.
Ferns (Pterophyta)
Ferns are the most abundant group of
seedless vascular plants, with about
12,000 living species. The fossil record
indicates that they originated during the
Devonian period about 350 million years
ago and became abundant and varied in form during the
next 50 million years. Their apparent ancestors had no
broad leaves and were established on land as much as 375
million years ago.
Today, ferns flourish in a wide range of habitats
throughout the world; about 75% of the species, however,
occur in the tropics. The conspicuous sporophytes may be
less than a centimeter in diameter—as seen in small aquatic
ferns such as Azolla—or more than 24 meters tall and with
leaves up to 5 meters or more long in the tree ferns (figure
37.9). The sporophytes and the smaller gametophytes,
which rarely reach 6 millimeters in diameter, are both pho-
tosynthetic. The fern life cycle differs from that of a moss
primarily in the much greater development, independence,
and dominance of the fern’s sporophyte. The fern’s sporo-
phyte is much more complex than that of the moss’s; the
fern sporophyte has vascular tissue and well-differentiated
roots, stems, and leaves. The gametophyte, however, lacks
vascular tissue.
Fern sporophytes typically have a horizontal under-
ground stem called a rhizome, with roots emerging from
the sides. The leaves, referred to as fronds, usually de-
velop at the tip of the rhizome as tightly rolled-up coils
(“fiddleheads”) that unroll and expand. Many fronds are
highly dissected and feathery, making the ferns that pro-
duce them prized as ornamentals. Some ferns, such as
Marsilea, have fronds that resemble a four-leaf clover,
but Marsilea fronds still begin as coiled fiddleheads.
Other ferns produce a mixture of photosynthetic fronds
and nonphotosynthetic reproductive fronds that tend to
be brownish in color.
Most ferns are homosporous, producing distinctive, spo-
rangia, usually in clusters called sori, typically on the backs
of the fronds. Sori are often protected during their devel-
opment by a transparent, umbrella-like covering. At first
glance, one might mistake the sori for an
infection on the plant. Diploid spore
mother cells in each sporangium un-
dergo meiosis, producing haploid spores.
At maturity, the spores are catapulted
from the sporangium by a snapping ac-
tion, and those that land in suitable
damp locations may germinate, produc-
ing gametophytes which are often heart-
shaped, are only one cell thick (except in
the center) and have rhizoids that anchor
them to their substrate. These rhizoids
are not true roots as they lack vascular
tissue, but as with many of the nonvascu-
lar plants they do aid in transporting
water and nutrients from the soil. Flask-
shaped archegonia and globular an-
theridia are produced on either the same
or different gametophyte.
742 Part X Plant Form and Function
Nonvascular plants
Gymnosperms
Angiosperms
Seedless vascular plants
FIGURE 37.9
A tree fern (phylum Pterophyta) in the forests of Malaysia.
The ferns are by far the largest group of seedless vascular plants.
Trees in other phyla are now extinct.
The sperm formed in the antheridia have flagella, with
which they swim toward the archegonia when water is pre-
sent, often in response to a chemical signal secreted by the
archegonia. One sperm unites with the single egg toward
the base of an archegonium, forming a zygote. The zygote
then develops into a new sporophyte, completing the life
cycle (figure 37.10). There are still multicellular gametan-
gia. As discussed earlier, the shift to a dominant sporophyte
generation allows ferns to achieve significant height with-
out interfering with sperm swimming efficiently to the egg.
The multicellular archegonia provide some protection for
the developing embryo.
Chapter 37 Evolutionary History of Plants 743
Archegonium
Egg
Antheridium
Sperm
FERTILIZATION
Embryo
Leaf of young
sporophyte
Gametophyte
Rhizome
Mature
frond
Mature
sporangium
MEIOSIS
Spore
Rhizoids
Gametophyte
n
2n
Adult
sporophyte
Mitosis
Sorus (cluster
of sporangia)
FIGURE 37.10
Life cycle of a typical fern. Both the gametophyte and
sporophyte are photosynthetic and can live independently.
Water is necessary for fertilization. The gametes are released
on the underside of the gametophyte and swim in moist soil to
neighboring gametophytes. Spores are dispersed by wind.
Whisk Ferns (Psilophyta)
The three other phyla of seedless vascular plants, the Psilo-
phyta, (whisk ferns), Lycophyta (club mosses), and Arthro-
phyta (horsetails), have many features in common with
ferns. For example, they all form antheridia and archego-
nia. Free water is required for the process of fertilization,
during which the sperm, which have flagella, swim to and
unite with the eggs. In contrast, most seed plants have non-
flagellated sperm; none form antheridia, although a few
form archegonia.
The origins of the two genera of whisk ferns, which
occur in the tropics and subtropics, are not clear, but
they are considered to be living remnants of the very ear-
liest vascular plants. Certainly they are the simplest of all
extant vascular plants, consisting merely of evenly forking
green stems without roots or leaves. The two or three
species of the genus Psilotum do, however, have tiny,
green, spirally arranged, flaps of tissue lacking veins and
stomata. Another genus, Tmespiteris, has leaflike
appendages.
The gametophytes of whisk ferns were unknown for
many years until their discovery in the soil beneath the
sporophytes. They are essentially colorless and are less than
2 millimeters in diameter, but they can be up to 18 mil-
limeters long. They form saprobic or parasitic associations
with fungi, which furnish their nutrients. Some develop el-
ements of vascular tissue and have the distinction of being
the only gametophytes known to do so.
Club Mosses (Lycophyta)
The club mosses are worldwide in distribution but are
most abundant in the tropics and moist temperate re-
gions. Several genera of club mosses, some of them tree-
like, became extinct about 270 million years ago. Mem-
bers of the four genera and nearly 1000 living species of
club mosses superficially resemble true mosses, but once
their internal structure and reproductive processes be-
came known it was clear that these vascular plants are
quite unrelated to mosses. Modern club mosses are either
homosporous or heterosporous. The sporophytes have
leafy stems that are seldom more than 30 centimeters
long.
Horsetails (Arthrophyta)
The 15 living species of horsetails, also called scouring
rushes, are all heterosporous and herbaceous. They con-
stitute a single genus, Equisetum. Fossil forms of Equise-
tum extend back 300 million years to an era when some of
their relatives were treelike. Today, they are widely scat-
tered around the world, mostly in damp places. Some that
grow among the coastal redwoods of California may reach
a height of 3 meters, but most are less than a meter tall
(figure 37.11).
Horsetail sporophytes consist of ribbed, jointed, photo-
synthetic stems that arise from branching underground rhi-
zomes with roots at their nodes. A whorl of nonphotosyn-
thetic, scalelike leaves emerges at each node. The stems,
which are hollow in the center, have silica deposits in the
epidermal cells of the ribs, and the interior parts of the
stems have two sets of vertical, somewhat tubular canals.
The larger outer canals, which alternate with the ribs, con-
tain air, while the smaller inner canals opposite the ribs
contain water.
Ferns and other seedless vascular plants have a much
larger and more conspicuous sporophyte, with vascular
tissue. Many have well-differentiated roots, stem, and
leaves. The shift to a dominant sporophyte lead to the
evolution of trees.
744 Part X Plant Form and Function
FIGURE 37.11
A horsetail, Equisetum telmateia, a representative of the only
living genus of the phylum Arthrophyta. This species forms
two kinds of erect stems; one is green and photosynthetic, and the
other, which terminates in a spore-producing “cone,” is mostly
light brown.
Seed Plants
Seed plants first appeared about 425 million years ago.
Their ancestors appear to have been spore-bearing plants
known as progymnosperms. Progymnosperms shared sev-
eral features with modern gymnosperms, including sec-
ondary xylem and phloem (which allows for an increase in
girth later in development). Some progymnosperms had
leaves. Their reproduction was very simple, and it is not
certain which particular group of progymnosperms gave
rise to seed plants.
From an evolutionary and ecological perspective, the
seed represents an important advance. The embryo is pro-
tected by an extra layer of sporophyte tissue creating the
ovule. During development this tissue hardens to produce
the seed coat. In addition to protection from drought, dis-
persal is enhanced. Perhaps even more significantly, a dor-
mant phase is introduced into the life cycle that allows the
embryo to survive until environmental conditions are fa-
vorable for further growth.
Seed plants produce two kinds of gametophytes—male
and female, each of which consists of just a few cells. Pollen
grains, multicellular male gametophytes, are conveyed to
the egg in the female gametophyte by wind or a pollinator.
The sperm move toward the egg through a growing pollen
tube. This eliminates the need for water. In contrast to the
seedless plants, the whole male gametophyte rather than
just the sperm moves to the female gametophyte. A female
gametophyte develops within an ovule. In flowering plants
(angiosperms), the ovules are completely enclosed within
diploid sporophyte tissue (ovaries which develop into the
fruit). In gymnosperms (mostly cone-bearing seed plants),
the ovules are not completely enclosed by sporophyte tissue
at the time of pollination.
A common ancestor with seeds gave rise to the
gymnosperms and the angiosperms. Seeds can allow for
a pause in the life cycle until environmental conditions
are more optimal.
Chapter 37 Evolutionary History of Plants 745
37.4 Seeds protect and aid in the dispersal of plant embryos.
filament The stalklike structure that sup-
ports the anther of a stamen.
gametophyte The multicellular, haploid
phase of a plant life cycle in which gametes
are produced by mitosis.
gynoecium The carpel(s) of a flower.
heterosporous Refers to a plant that pro-
duces two types of spores: microspores and
megaspores.
homosporous Refers to a plant that pro-
duces only one type of spore.
integument The outer layer(s) of an
ovule; integuments become the seed coat of
a seed.
micropyle The opening in the ovule in-
tegument through which the pollen tube
grows.
nucellus The tissue of an ovule in which
an embryo sac develops.
ovary The basal, swollen portion of a
carpel (gynoecium); it contains the ovules
and develops into the fruit.
ovule A seed plant structure within an ovary;
it contains a female gametophyte surrounded
by the nucellus and one or two integuments.
At maturity, an ovule becomes a seed.
pollen grain A binucleate or trinucleate
seed plant structure produced from a mi-
crospore in a microsporangium.
pollination The transfer of a pollen grain
from an anther to a stigma in angiosperms,
or to the vicinity of the ovule in gym-
nosperms.
primary endosperm nucleus The
triploid nucleus resulting from the fusion of
a single sperm with the two polar nuclei.
seed A reproductive structure that devel-
ops from an ovule in seed plants. It consists
of an embryo and a food supply surrounded
by a seed coat.
seed coat The protective layer of a seed; it
develops from the integument or integu-
ments.
spore A haploid reproductive cell, pro-
duced when a diploid spore mother cell un-
dergoes meiosis; it gives rise by mitosis to a
gametophyte.
sporophyte The multicellular, diploid
phase of a plant life cycle; it is the genera-
tion that ultimately produces spores.
stamen A unit of an androecium; it con-
sists of a pollen-bearing anther and usually a
stalklike filament.
stigma The uppermost pollen-receptive
portion of a gynoecium.
androecium The stamens of a flower.
anther The pollen-producing portion of a
stamen. This is a sporophyte structure
where male gametophytes are produced by
meiosis.
antheridium The male sperm-producing
structure found in the gametophytes of
seedless plants and certain fungi.
archegonium The multicellular egg-
producing structure in the gametophytes of
seedless plants and gymnosperms.
carpel A leaflike organ in angiosperms
that encloses one or more ovules; a unit of a
gynoecium.
double fertilization The process, unique
to angiosperms, in which one sperm fuses
with the egg, forming a zygote, and the
other sperm fuses with the two polar nuclei,
forming the primary endosperm nucleus.
endosperm The usually triploid (although
it can have a much higher ploidy level) food
supply of some angiosperm seeds.
A Vocabulary of
Plant Terms
Gymnosperms
There are several groups of living gym-
nosperms (conifers, cycads, gneto-
phytes, and Ginkgo), none of which are
directly related to one another, but all
of which lack the flowers and fruits of
angiosperms. In all of them the ovule,
which becomes a seed, rests exposed on
a scale (modified leaf) and is not com-
pletely enclosed by sporophyte tissues at
the time of pollination. The name gym-
nosperm combines the Greek root gym-
nos, or “naked,” with sperma, or “seed.”
In other words, gymnosperms are
naked-seeded plants. However, al-
though the ovules are naked at the time
of pollination, the seeds of gym-
nosperms are sometimes enclosed by
other sporophyte tissues by the time
they are mature.
Details of reproduction vary somewhat in gym-
nosperms, and their forms vary greatly. For example, cy-
cads and Ginkgo have motile sperm, even though the
sperm are borne within a pollen tube, while many others
have sperm with no flagella. The female cones range from
tiny woody structures weighing less than 25 grams with a
diameter of a few millimeters, to massive structures
weighing more than 45 kilograms growing to lengths
more than a meter.
Conifers (Coniferophyta)
The most familiar gymnosperms are conifers (phylum
Coniferophyta), which include pines (figure 37.12),
spruces, firs, cedars, hemlocks, yews, larches, cypresses, and
others. The coastal redwood (Sequoia sempervirens), a
conifer native to northwestern California and southwestern
Oregon, is the tallest living vascular plant; it may attain
nearly 100 meters (300 feet) in height. Another conifer, the
bristlecone pine (Pinus longaeva) of the White Mountains of
California is the oldest living tree; one is 4900 years of age.
Conifers are found in the colder temperate and sometimes
drier regions of the world, especially in the northern hemi-
sphere. They are sources of timber, paper, resin, turpen-
tine, taxol (used to treat cancer) and other economically
important products.
Pines. More than 100 species of pines exist today, all
native to the northern hemisphere, although the range of
one species does extend a little south of the equator. Pines
and spruces are members of the vast coniferous forests
that lie between the arctic tundra and the temperate de-
ciduous forests and prairies to their south. During the
past century, pines have been extensively planted in the
southern hemisphere.
Pines have tough, needlelike leaves
produced mostly in clusters of two to
five. The leaves, which have a thick cuti-
cle and recessed stomata, represent an
evolutionary adaptation for retarding
water loss. This is important because
many of the trees grow in areas where
the topsoil is frozen for part of the year,
making it difficult for the roots to obtain
water. The leaves and other parts of the
sporophyte have canals into which sur-
rounding cells secrete resin. The resin,
apparently secreted in response to
wounding, deters insect and fungal at-
tacks. The resin of certain pines is har-
vested commercially for its volatile liquid
portion, called turpentine, and for the
solid rosin, which is used on stringed in-
struments. The wood of pines consists
primarily of xylem tissue that lacks some
of the more rigid cell types found in
other trees. Thus it is considered a “soft” rather than a
“hard” wood. The thick bark of pines represents another
adaptation for surviving fires and subzero temperatures.
Some cones actually depend on fire to open, releasing seed
to reforest burnt areas.
746 Part X Plant Form and Function
Nonvascular plants
Seedless vascular plants
Angiosperms
Gymnosperms
FIGURE 37.12
Conifers. Slash pines, Pinus palustris, in Florida, are
representative of the Coniferophyta, the largest phylum of
gymnosperms.
As mentioned earlier, all seed plants
are heterosporous, so the spores give
rise to two types of gametophytes (fig-
ure 37.13). The male gametophytes of
pines develop from pollen grains, which
are produced in male cones that develop
in clusters of 30 to 70, typically at the
tips of the lower branches; there may be
hundreds of such clusters on any single
tree. The male cones generally are 1 to
4 centimeters long and consist of small,
papery scales arranged in a spiral or in
whorls. A pair of microsporangia form
as sacs within each scale. Numerous mi-
crospore mother cells in the microspo-
rangia undergo meiosis, each becoming
four microspores. The microspores de-
velop into four-celled pollen grains with
a pair of air sacs that give them added
buoyancy when released into the air. A
single cluster of male pine cones may
produce more than 1 million pollen
grains.
Female cones typically are produced
on the upper branches of the same tree
that produces male cones. Female
cones are larger than male cones, and
their scales become woody. Two
ovules develop toward the base of each
scale. Each ovule contains a megaspo-
rangium called the nucellus. The nu-
cellus itself is completely surrounded
by a thick layer of cells called the in-
tegument that has a small opening (the
micropyle) toward one end. One of
the layers of the integument later be-
comes the seed coat. A single mega-
spore mother cell within each mega-
sporangium undergoes meiosis,
becoming a row of four megaspores. Three of the mega-
spores break down, but the remaining one, over the better
part of a year, slowly develops into a female gametophyte.
The female gametophyte at maturity may consist of thou-
sands of cells, with two to six archegonia formed at the
micropylar end. Each archegonium contains an egg so
large it can be seen without a microscope.
Female cones usually take two or more seasons to ma-
ture. At first they may be reddish or purplish in color,
but they soon turn green, and during the first spring, the
scales spread apart. While the scales are open, pollen
grains carried by the wind drift down between them,
some catching in sticky fluid oozing out of the mi-
cropyle. As the sticky fluid evaporates, the pollen grains
are slowly drawn down through the micropyle to the top
of the nucellus, and the scales close shortly thereafter.
The archegonia and the rest of the female gametophyte
are not mature until about a year later. While the female
gametophyte is developing, a pollen tube emerges from a
pollen grain at the bottom of the micropyle and slowly
digests its way through the nucellus to the archegonia.
While the pollen tube is growing, one of the pollen
grain’s four cells, the generative cell, divides by mitosis,
with one of the resulting two cells dividing once more.
These last two cells function as sperm. The germinated
pollen grain with its two sperm is the mature male
gametophyte.
About 15 months after pollination, the pollen tube
reaches an archegonium, and discharges its contents into it.
One sperm unites with the egg, forming a zygote. The
other sperm and cells of the pollen grain degenerate. The
zygote develops into an embryo within a seed. After disper-
sal and germination of the seed, the young sporophyte of
the next generation grows into a tree.
Chapter 37 Evolutionary History of Plants 747
Longisection of
seed, showing
embryo
FERTILIZATION
(15 months after
pollination)
Ovulate (seed-bearing)
cone
Megaspore
mother cell
Microspore
mother cell
Microspores
Pollen
Pollen-bearing
cone
Pollen
tube
Pollination
n
2n
Megaspore
MEIOSIS
Sporophyte
Seedling
Pine
seed
Mature seed
cone (2nd year)
Scale
Embryo
Scale
Mitosis
Mitosis
FIGURE 37.13
Life cycle of a typical pine. The male and female gametophytes have been dramatically
reduced in size. Wind generally disperses sperm that is within the male gametophyte
(pollen). Pollen tube growth delivers the sperm to the egg on the female cone. Additional
protection for the embryo is provided by the ovule which develops into the seed coat.
Cycads (Cycadophyta)
Cycads are slow-growing gymnosperms of tropical and
subtropical regions. The sporophytes of most of the 100
known species resemble palm trees (figure 37.14a) with
trunks that can attain heights of 15 meters or more. Unlike
palm trees—which are flowering plants—cycads produce
cones and have a life cycle similar to that of pines. The fe-
male cones, which develop upright among the leaf bases,
are huge in some species and can weigh up to 45 kilograms.
The sperm of cycads, although conveyed to an archego-
nium by a pollen tube, have thousands of spirally arranged
flagella. Several species are facing extinction in the wild and
soon may exist only in botanical gardens.
Gnetophytes (Gnetophyta)
There are three genera and about 70 living species of
Gnetophyta. Gnetophytes are the closest living relatives of
angiosperms and probably share a common ancestor with
that group. They are the only gymnosperms with vessels (a
particularly efficient conducting cell type) in their xylem—
a common feature in angiosperms. The members of the
three genera differ greatly from one another in form. One
of the most bizarre of all plants is Welwitschia, which oc-
curs in the Namib and Mossamedes deserts of southwest-
ern Africa (figure 37.14b). The stem is shaped like a large,
shallow cup that tapers into a taproot below the surface. It
has two strap-shaped, leathery leaves that grow continu-
ously from their base, splitting as they flap in the wind.
The reproductive structures of Welwitschia are conelike,
appear toward the bases of the leaves around the rims of
the stems, and are produced on separate male and female
plants.
More than half of the gnetophyte species are in the
genus Ephedra, which is common in arid regions of the
western United States and Mexico. The plants are shrubby,
with stems that superficially resemble those of horsetails as
they are jointed and have tiny, scalelike leaves at each node.
Male and female reproductive structures may be produced
on the same or different plants. The drug ephedrine,
widely used in the treatment of respiratory problems, was
in the past extracted from Chinese species of Ephedra, but it
has now been largely replaced with synthetic preparations.
Mormon tea is brewed from Ephedra stems in the south-
western United States.
The best known species of Gnetum is a tropical tree, but
most species are vinelike. All species have broad leaves sim-
ilar to those of angiosperms. One Gnetum species is culti-
vated in Java for its tender shoots, which are cooked as a
vegetable.
Ginkgo (Ginkgophyta)
The fossil record indicates that members of the Ginkgo
family were once widely distributed, particularly in the
northern hemisphere; today only one living species, the
maidenhair tree (Ginkgo biloba), remains. The tree, which
sheds its leaves in the fall, was first encountered by Euro-
peans in cultivation in Japan and China; it apparently no
longer exists in the wild (figure 37.14c). The common
name comes from the resemblance of its fan-shaped leaves
to the leaflets of maidenhair ferns. Like the sperm of cy-
cads, those of Ginkgo have flagella. The Ginkgo is
diecious, that is, the male and female reproductive struc-
tures of Ginkgo are produced on separate trees. The fleshy
outer coverings of the seeds of female Ginkgo plants exude
the foul smell of rancid butter caused by butyric and
isobutyric acids. In the Orient, however, the seeds are
considered a delicacy. In Western countries, because of
the seed odor, male plants vegetatively propagated from
shoots are preferred for cultivation. Because it is resistant
to air pollution, Ginkgo is commonly planted along city
streets.
Gymnosperms are mostly cone-bearing seed plants. In
gymnosperms, the ovules are not completely enclosed
by sporophyte tissue at pollination.
748 Part X Plant Form and Function
FIGURE 37.14
Three phyla of
gymnosperms.
(a) An African cycad,
Encephalartos
transvenosus.
(b) Welwitschia
mirabilis, one of the
three genera of
gnetophytes.
(c) Maidenhair tree,
Ginkgo biloba, the only
living representative of
the phylum
Ginkgophyta.
(a) (b) (c)
Angiosperms
The 250,000 known species of flower-
ing plants are called angiosperms be-
cause their ovules, unlike those of
gymnosperms, are enclosed within
diploid tissues at the time of pollina-
tion. The name angiosperm derives
from the Greek words angeion, “ves-
sel,” and sperma, “seed.” The “vessel”
in this instance refers to the carpel,
which is a modified leaf that encapsu-
lates seeds. The carpel develops into
the fruit, a unique angiosperm feature.
While some gymnosperms, including
yew, have fleshlike tissue around their
seeds, it is of a different origin and not
a true fruit.
The origins of the angiosperms
puzzled even Darwin (his “abominable
mystery”). Recently, consensus has
been reached on the most basal, living angiosperm—
Amborella trichopoda (figure 37.15). This has ended the de-
bate between the supporters of magnolias and those of
water lilies as the closest relatives of the original an-
giosperm. Amborella, with small, cream-colored flowers, is
even more primitive than either the magnolias or water
lilies. This small shrub found only on the island of New
Caledonia in the South Pacific is the last remaining species
of the earliest extant lineage of the angiosperms. About 135
million years ago a close relative of Amborella developed
floral parts and branched off from the gymnosperms.
While Amborella is not the original angiosperm, it is suffi-
ciently close that much will be learned from studying its re-
productive biology that will help us understand the early
radiation of the angiosperms.
Flowering plants (phylum Anthophyta) exhibit an almost
infinite variety of shapes, sizes, and textures. They vary, for
example, from the huge Tasmanian Eucalyptus trees, which
have nearly as much mass as the giant redwoods, to the
tiniest duckweeds, which are less than 1 millimeter long. In
addition to the typical flattened green leaves with which
everyone is familiar, flowering plant leaves may be succu-
lent, floating, submerged, cup-shaped, spinelike, scalelike,
feathery, papery, hairy, or insect-trapping, and of almost
any color. Some are so tiny one needs a microscope to ex-
amine them, while others, such as those of the Seychelles
Island palm, can be up to 6 meters long. Their flowers vary
from the simple blossoms of buttercups to the extraordi-
narily complex flowers of some orchids, which may lure
their pollinators with drugs, forcibly attach bags of pollen
to their bodies, or dunk them in fluid they secrete. The
flowers may weigh less than 1 gram and remain functional
for only a few minutes, or they can weigh up to 9 kilograms
and be functional for months. Plants of several families are
parasitic or partially parasitic (for example, dodder, or
mistletoe) on other plants, or mycotrophic (deriving their nu-
trients from fungi that form a mutualism with plant roots).
Others, such as many orchids, are epiphytic (attached to
other plants, with no roots in the ground, and not in any
way parasitic).
The Structure of Flowers
Flowers are considered to be modified stems bearing modi-
fied leaves. Regardless of their size and shape, they all share
certain features (see figure 37.16). Each flower originates as
a primordium that develops into a bud at the end of a stalk
called a pedicel. The pedicel expands slightly at the tip
into a base, the receptacle, to which the remaining flower
parts are attached. The other flower parts typically are at-
tached in circles called whorls. The outermost whorl is
composed of sepals. In most flowers there are three to five
sepals, which are green and somewhat leaflike; they often
function in protecting the immature flower and in some
species may drop off as the flower opens. The next whorl
consists of petals that are often colored and attract pollina-
tors such as insects and birds. The petals, which commonly
number three to five, may be separate, fused together, or
missing altogether in wind-pollinated flowers.
The third whorl consists of stamens, collectively called
the androecium, a term derived from the Greek words an-
dros, “male,” and oikos, “house.” Each stamen consists of a
pollen-bearing anther and a stalk called a filament, which
may be missing in some flowers. The gynoecium, consist-
ing of one or more carpels, is at the center of the flower.
The term gynoecium derives from the Greek words gynos,
which means “female,” and oikos, or “house.” The first
carpel is believed to have been formed from a leaflike struc-
ture with ovules along its margins. The edges of the blade
then rolled inward and fused together, forming a carpel.
Chapter 37 Evolutionary History of Plants 749
Nonvascular plants
Seedless vascular plants
Gymnosperms
Angiosperms
FIGURE 37.15
A flowering plant. Amborella trichopoda.
This plant is believed to be the closest
living relative to the original angiosperm.
Primitive flowers can have several to many separate carpels,
but in most flowers, two to several carpels are fused to-
gether. Such fusion can be seen in an orange sliced in half;
each segment represents one carpel. A carpel has three
major regions (figure 37.16). The ovary is the swollen base,
which contains from one to hundreds of ovules; the ovary
later develops into a fruit. The tip of the pistil is called a
stigma. Most stigmas are sticky or feathery, causing pollen
grains that land on them to adhere. Typically there is a neck
or stalk called a style connecting the stigma and the ovary;
in some flowers, the style may be very short or even miss-
ing. Many flowers have nectar-secreting glands called nec-
taries, often located toward the base of the ovary. Nectar is a
fluid containing sugars, amino acids, and other molecules
used to attract insects, birds, and other animals to flowers.
The Angiosperm Life Cycle
While a flower bud is developing, a single megaspore
mother cell in the ovule undergoes meiosis, producing four
megaspores (figure 37.17). In most flowering plants, three
of the megaspores soon disappear while the nucleus of the
remaining one divides mitotically, and the cell slowly ex-
pands until it becomes many times its original size. While
this expansion is occurring, each of the daughter nuclei di-
vide twice, resulting in eight haploid nuclei arranged in two
groups of four. At the same time, two layers of the ovule,
the integuments, differentiate and become the seed coat of
a seed. The integuments, as they develop, leave a small gap
or pore at one end—the micropyle (see figure 37.16). One
nucleus from each group of four migrates toward the cen-
ter, where they function as polar nuclei. Polar nuclei may
fuse together, forming a single diploid nucleus, or they may
form a single cell with two haploid nuclei. Cell walls also
form around the remaining nuclei. In the group closest to
the micropyle, one cell functions as the egg; the other two
nuclei are called synergids. At the other end, the three
cells are now called antipodals; they have no apparent
function and eventually break down and disappear. The
large sac with eight nuclei in seven cells is called an em-
bryo sac; it constitutes the female gametophyte. Although
it is completely dependent on the sporophyte for nutrition,
it is a multicellular, haploid individual.
While the female gametophyte is developing, a similar
but less complex process takes place in the anthers. Most
anthers have patches of tissue (usually four) that eventu-
ally become chambers lined with nutritive cells. The tis-
sue in each patch is composed of many diploid mi-
crospore mother cells that undergo meiosis more or less
simultaneously, each producing four microspores. The
four microspores at first remain together as a quartet or
tetrad, and the nucleus of each microspore divides once;
in most species the microspores of each quartet then sep-
arate. At the same time, a two-layered wall develops
around each microspore. As the anther matures, the wall
between adjacent pairs of chambers breaks down, leaving
two larger sacs. At this point, the binucleate microspores
have become pollen grains. The outer pollen grain wall
layer often becomes beautifully sculptured, and it con-
tains chemicals that may react with others in a stigma to
signal whether or not development of the male gameto-
phyte should proceed to completion. The pollen grain
has areas called apertures, through which a pollen tube
may later emerge.
Pollination is simply the mechanical transfer of pollen
from its source (an anther) to a receptive area (the stigma of
a flowering plant). Most pollination takes place between
flowers of different plants and is brought about by insects,
wind, water, gravity, bats, and other animals. In as many as
a quarter of all angiosperms, however, a pollen grain may
be deposited directly on the stigma of its own flower, and
self-pollination occurs. Pollination may or may not be fol-
lowed by fertilization, depending on the genetic compatibil-
ity of the pollen grain and the flower on whose stigma it
has landed. (In some species, complex, genetically con-
trolled mechanisms prevent self-fertilization to enhance ge-
netic diversity in the progeny.) If the stigma is receptive,
the pollen grain’s dense cytoplasm absorbs substances from
the stigma and bulges through an aperture. The bulge de-
velops into a pollen tube that responds to chemicals released
by the embryo sac. It follows a diffusion gradient of the
chemicals and grows down through the style and into the
micropyle. The pollen tube usually takes several hours to
two days to reach the micropyle, but in a few instances, it
may take up to a year. One of the pollen grain’s two nuclei,
the generative nucleus, lags behind. This nucleus divides, ei-
750 Part X Plant Form and Function
Stigma
Style
Ovule
Ovary
Pistil
Anther
Filament
Stamen
Petal
Sepal
Receptacle
Pedicel
Megaspore
mother cell
Nucellus
Integuments
Micropyle
Stalk of ovule
(funiculus)
(a) (b)
FIGURE 37.16
Diagram of an angiosperm flower.
(a) The main structures of the flower are
labeled. (b) Details of an ovule. The ovary
as it matures will become a fruit; as the
ovule’s outer layers (integuments) mature,
they will become a seed coat.
ther in the pollen grain or in the pollen tube, producing
two sperm nuclei. Unlike sperm in mosses, ferns, and some
gymnosperms, the sperm of flowering plants have no fla-
gella. At this point, the pollen grain with its tube and sperm
has become a mature male gametophyte.
As the pollen tube enters the embryo sac, it destroys a
synergid in the process and then discharges its contents.
Both sperm are functional, and an event called double
fertilization, unique to angiosperms, follows. One sperm
unites with the egg and forms a zygote, which develops
into an embryo sporophyte plant. The other sperm and
the two polar nuclei unite, forming a triploid primary en-
dosperm nucleus. The primary endosperm nucleus begins
dividing rapidly and repeatedly, becoming triploid en-
dosperm tissue that may soon consist of thousands of
cells. Endosperm tissue can become an extensive part of
the seed in grasses such as corn (see figure 41.7). But in
most flowering plants, it provides nutrients for the embryo
that develops from the zygote; in many species, such as
peas and beans, it disappears completely by the time the
seed is mature. Following double fertilization, the integu-
ments harden and become the seed coat of a seed. The
haploid cells remaining in the embryo sac (antipodals, syn-
ergid, tube nucleus) degenerate. There is some evidence
for a type of double fertilization in gymnosperms believed
to be closely related to the angiosperms. Further studies of
this and of fertilization in Amborella, the most basal, extant
angiosperm, may provide clues to the evolution of this
double fertilization event.
Angiosperms are characterized by ovules that at
pollination are enclosed within an ovary at the base of a
carpel—a structure unique to the phylum; a fruit
develops from the ovary. Evolutionary innovations
including flowers to attract pollinators, fruits to protect
and aid in embryo dispersal, and double fertilization
providing additional nutrients for the embryo all have
contributed to the widespread success of this phylum.
Chapter 37 Evolutionary History of Plants 751
FIGURE 37.17
Life cycle of a typical angiosperm. As in pines, water is no longer required for fertilization. In most species of angiosperms, animals
carry pollen to the carpel. The outer wall of the carpel forms the fruit that entices animals to disperse seed.
Rhizome
n2n
Generative cell
8-nucleate
embryo sac
(megagametophyte ) (n)
Formation of
pollination tube (n)
MEIOSIS
MEIOSIS
Tube nucleus
Tube
nucleus
Sperm
Style
Sperm
Egg
Polar
nuclei
Seed coat
Embryo (2n)
Seed (2n)
Endosperm (3n)
Young
sporophyte
(2n)
Adult
sporophyte
with flowers
(2n)
Anther
Ovary
Stigma
Anther (2n)
Microspore
mother cells (2n)
Microspores (n) Pollen grains
(microgametophytes) (n)
Megaspore (n)
Megaspore
mother cell (2n)
Pollen tube
Ovule
DOUBLE FERTILIZATION
Germination
Egg
Mitosis
Mitosis
752 Part X Plant Form and Function
Chapter 37
Summary Questions Media Resources
37.1 Plants have multicellular haploid and diploid stages in their life cycles.
? Plants evolved from a multicellular, freshwater green
algae 450 million years ago. The evolution of their
conducting tissues, cuticle, stomata, and seeds has
made them progressively less dependent on external
water for reproduction.
? All plants have a haplodiplontic life cycle in which
haploid gametophytes alternate with diploid
sporophytes.
1. Where did the most recent
ancestors of land plants live?
What were they like? What
adaptations were necessary for
the “move” onto land?
2. What does it mean for a plant
to alternate generations?
Distinguish between sporophyte
and gametophyte.
? Three phyla of plants lack well-developed vascular
tissue, are the simplest in structure, and have been
grouped as bryophytes. This grouping does not
reflect a common ancestry or close relationship.
? Sporophytes of mosses, liverworts, and hornworts are
usually nutritionally dependent on the gametophytes,
which are more conspicuous and photosynthetic.
3. Distinguish between male
gametophytes and female
gametophytes. Which specific
haploid spores give rise to each
of these?
4. What reproductive limitations
would a moss tree (if one
existed) face?
37.2 Nonvascular plants are relatively unspecialized, but successful in many terrestrial environments.
? Nine of the 12 plant phyla contain vascular plants,
which have two kinds of well-defined conducting
tissues: xylem, which is specialized to conduct water
and dissolved minerals; and phloem, which is
specialized to conduct the sugars produced by
photosynthesis and plant growth regulators.
? In ferns and other seedless vascular plants, the
sporophyte generation is dominant. The fern
sporophyte has vascular tissue and well-differentiated
roots, stems, and leaves.
5. In what ways are the
gametophytes of seedless plants
different from the gametophytes
of seed plants?
6. Which generation(s) of the
fern are nutritionally
independent?
37.3 Seedless vascular plants have well-developed conducting tissues in their sporophytes.
? Seeds were an important evolutionary advance
providing for a dormant stage in development.
? In gymnosperms, ovules are exposed directly to
pollen at the time of pollination; in angiosperms,
ovules are enclosed within an ovary, and a pollen tube
grows from the stigma to the ovule.
? The pollen of gymnosperms is usually disseminated
by wind. In most angiosperms the pollen is
transported by insects and other animals. Both
flowers and fruits are found only in angiosperms and
may account for the extensive colonization of
terrestrial environments by the flowering plants.
7. What is a seed? Why is the
seed a crucial adaptation to
terrestrial life?
8. What is the principal
difference between
gymnosperms and angiosperms?
9. If all the offspring of a plant
were to develop in a small area,
they would suffer from limited
resources. Compare dispersal
strategies in moss, pine, and
angiosperms.
37.4 Seeds protect and aid in the dispersal of plant embryos.
www.mhhe.com/raven6e www.biocourse.com
? Life Cycles of Plants
? Student Research:
Plant Biodiversity in
New Hampshire
? Book Review: A Rum
Affair by Sabbagh
? Book Review: The
Orchid Thief by
Orlean
? Non-Vascular Plants
? Seedless Vascular
Plants
? Gymnosperms
? Angiosperms
753
38
The Plant Body
Concept Outline
38.1 Meristems elaborate the plant body plan after
germination.
Meristems. Growth occurs in the continually dividing
cells that function like stem cells in animals.
Organization of the Plant Body. The plant body is a
series of iterative units stacked above and below the ground.
Primary and Secondary Growth. Different meristems
allow plants to grow in both height and circumference.
38.2 Plants have three basic tissues, each composed of
several cell types.
Dermal Tissue. This tissue forms the “skin” of the plant
body, protecting it and preventing water loss.
Ground Tissue. Much of a young plant is ground tissue,
which supports the plant body and stores food and water.
Vascular Tissue. Special piping tissues conduct water
and sugars through the plant body.
38.3 Root cells differentiate as they become distanced
from the dividing root apical meristem.
Root Structure. Roots have a durable cap, behind which
primary growth occurs.
Modified Roots. Roots can have specialized functions.
38.4 Stems are the backbone of the shoot,
transporting nutrients and supporting the aerial
plant organs.
Stem Structure. The stem supports the leaves and is
anchored by the roots. Vascular tissues are organized within
the stem in different ways.
Modified Stems. Specialized stems are adapted for
storage and vegetative (asexual) propagation.
38.5 Leaves are adapted to support basic plant
functions.
Leaf External Structure. Leaves have flattened blades
and slender stalks.
Leaf Internal Structure. Leaves contain cells that carry
out photosynthesis, gas exchange, and evaporation.
Modified Leaves. In some plants, leaf development has
been modified to provide for a unique need.
A
lthough the similarities between a cactus, an orchid
plant, and a tree might not be obvious at first sight,
most plants have a basic unity of structure (figure 38.1).
This unity is reflected in how the plants are constructed; in
the way they grow, manufacture, and transport their food;
and in how their development is regulated. This chapter
addresses the question of how a vascular plant is “built.”
We will focus on the diversity of cell, tissue, and organ
types that compose the adult body. The roots and shoots
which give the adult plant its distinct above and below
ground architecture are the final product of a basic body
plan first established during embryogenesis, a process we
will explore in detail in chapter 40.
FIGURE 38.1
All vascular plants share certain characteristics. Vascular plants
such as this tree require an elaborate system of support and fluid
transport to grow this large. Smaller plants have similar (though
simpler) structures. Much of this support system is actually
underground in the form of extensive branching root systems.
754 Part X Plant Form and Function
Meristems
The plant body that develops after germination depends on
the activities of meristematic tissues. Meristematic tissues
are lumps of small cells with dense cytoplasm and propor-
tionately large nuclei that act like stem cells in animals.
That is, one cell divides to give rise to two cells. One re-
mains meristematic, while the other is free to differentiate
and contribute to the plant body. In this way, the popula-
tion of meristem cells is continually renewed. Molecular
genetic evidence supports the hypothesis that stem cells
and meristem cells may also share some common molecular
mechanisms.
Elongation of both root and shoot takes place as a result
of repeated cell divisions and subsequent elongation of the
cells produced by the apical meristems. In some vascular
plants, including shrubs and most trees, lateral meristems
produce an increase in girth.
Apical Meristems
Apical meristems are located at the tips of stems (figure
38.2) and at the tips of roots (figure 38.3), just behind the
root cap. The plant tissues that result from primary
growth are called primary tissues. During periods of
growth, the cells of apical meristems divide and continu-
ally add more cells to the tips of a seedling’s body. Thus,
the seedling lengthens. Primary growth in plants is
brought about by the apical meristems. The elongation of
the root and stem forms what is known as the primary
plant body, which is made up of primary tissues. The pri-
mary plant body comprises the young, soft shoots and
roots of a tree or shrub, or the entire plant body in some
herbaceous plants.
Both root and shoot apical meristems are composed of
delicate cells that need protection. The root apical meri-
stem is protected from the time it emerges by the root cap.
Root cap cells are produced by the root meristem and are
sloughed off and replaced as the root moves through the
soil. A variety of adaptive mechanisms protect shoot apical
meristem during germination (figure 38.4). The epicotyl
or hypocotyl (“stemlike” tissue above or below the cotyle-
dons) may bend as the seedling emerges to minimize the
force on the shoot tip. In the monocots (a late evolving
group of angiosperms) there is often a coleoptile (sheath of
tissue) that forms a protective tube around the emerging
shoot. Later in development, the leaf primordia cover the
shoot apical meristem which is particularly susceptible to
desiccation.
The apical meristem gives rise to three types of embry-
onic tissue systems called primary meristems. Cell divi-
sion continues in these partly differentiated tissues as they
develop into the primary tissues of the plant body. The
38.1 Meristems elaborate the plant body plan after germination.
Young leaf
primordium
Apical meristem
Older leaf
primordium
Lateral bud
primordium
Young leaf
primordium
Apical
meristem
Older leaf
primordium
Lateral bud
primordium
Vascular
tissue
FIGURE 38.2
An apical shoot meristem. This longitudinal section through a
shoot apex in Coleus shows the tip of a stem. Between the young
leaf primordia is the apical meristem.
Epidermis
Phloem
Xylem
Root hair
Cortex
Endodermis
Primary xylem
Primary phloem
Protoderm
Ground meristem
Procambium
Apical meristem
Root cap
FIGURE 38.3
An apical root meristem. This diagram of meristems in the root
shows their relation to the root tip.
three primary meristems are the protoderm, which forms
the epidermis; the procambium, which produces primary
vascular tissues (primary xylem and primary phloem); and
the ground meristem, which differentiates further into
ground tissue, which is composed of parenchyma cells. In
some plants, such as horsetails and corn, intercalary
meristems arise in stem internodes, adding to the inter-
node lengths. If you walk through a corn field (when the
corn is about knee high) on a quiet summer night, you may
hear a soft popping sound. This is caused by the rapid
growth of intercalary meristems. The amount of stem elon-
gation that occurs in a very short time is quite surprising.
Lateral Meristems
Many herbaceous plants exhibit only primary growth, but
others also exhibit secondary growth. Most trees, shrubs,
and some herbs have active lateral meristems, which are
cylinders of meristematic tissue within the stems and roots
(figure 38.5). Although secondary growth increases girth in
many nonwoody plants, its effects are most dramatic in
woody plants which have two lateral meristems. Within the
bark of a woody stem is the cork cambium, a lateral meris-
tem that produces the cork cells of the outer bark. Just be-
neath the bark is the vascular cambium, a lateral meristem
that produces secondary vascular tissue. The vascular cam-
bium forms between the xylem and phloem in vascular
bundles, adding secondary vascular tissue on opposite sides
of the vascular cambium. Secondary xylem is the main com-
ponent of wood. Secondary phloem is very close to the outer
surface of a woody stem. Removing the bark of a tree dam-
ages the phloem and may eventually kill the tree. Tissues
formed from lateral meristems, which comprise most of the
trunk, branches, and older roots of trees and shrubs, are
known as secondary tissues and are collectively called the
secondary plant body.
Meristems are actively dividing, embryonic tissues
responsible for both primary and secondary growth.
Chapter 38 The Plant Body 755
(a) (b)
FIGURE 38.4
Developing seedling. Apical meristems are protected early in
development. (a) In this soybean, a dicot, a bent epicotyl (stem
above the cotyledons), rather than the shoot tip, pushes through
the soil before straightening. (b) In corn, a monocot, a sleeve of
tissue called the coleoptile sheaths the shoot tip until it has made
it to daylight.
Apical meristem
Protoderm
Procambium
Ground
meristem
Shoot
Root
Root cap
Procambium
Ground meristem
Root hairs
Lateral
meristems
Lateral
meristems
Vascular
cambium
Vascular
cambium
Cork
cambium
Cork
cambium
Apical meristem
FIGURE 38.5
Apical and lateral meristems. Apical meristems produce primary
growth, the elongation of the root and stem. In some plants, the
lateral meristems produce an increase in the girth of a plant. This
type of growth is secondary because the meristems were not
directly produced by apical meristems.
Organization of the Plant Body
Coordination of primary and secondary meristematic
growth produces the body of the adult sporophyte plant.
Plant bodies do not have a fixed size. Parts such as leaves,
roots, branches, and flowers all vary in size and number
from plant to plant—even within a species. The develop-
ment of the form and structure of plant parts may be rela-
tively rigidly controlled, but some aspects of leaf, stem, and
root development are quite flexible. As a plant grows, the
number, location, size, and even structure of leaves and
roots are often influenced by the environment.
A vascular plant consists of a root system and a shoot
system (figure 38.6). The root system anchors the plant
and penetrates the soil, from which it absorbs water and
ions crucial to the plant’s nutrition. The shoot system
consists of the stems and their leaves. The stem serves as
a framework for positioning the leaves, the principal sites
of photosynthesis. The arrangement, size, and other fea-
tures of the leaves are of critical importance in the plant’s
production of food. Flowers, other reproductive organs,
and, ultimately, fruits and seeds are also formed on the
shoot (see chapters 40 and 42). The reiterative unit of the
vegetative shoot consists of the internode, node, leaf, and
axillary buds. Axillary buds are apical meristems derived
from the primary apical meristem that allow the plant to
branch or replace the main shoot if it is munched by an
herbivore. A vegetative axillary bud has the capacity to re-
iterate the development of the primary shoot. When the
plant has transited to the reproductive phase of develop-
ment (see chapter 41), these axillaries may produce flow-
ers or floral shoots.
Three basic types of tissues exist in plants: ground tis-
sue, dermal, and vascular tissue. Each of the three basic tis-
sues has its own distinctive, functionally related cell
types. Some of these cell types will be discussed later in
this chapter. In plants limited to primary growth, the
dermal system is composed of the epidermis. This tissue
is one cell thick in most plants, and forms the outer pro-
tective covering of the plant. In young exposed parts of
the plant, the epidermis is covered with a fatty cutin
layer constituting the cuticle; in plants such as the desert
succulents, a layer of wax may be added outside the cuti-
cle. In plants with secondary growth, the bark forms the
outer protective layer and is considered a part of the der-
mal tissue system.
Ground tissue consists primarily of thin-walled
parenchyma cells that are initially (but briefly) more or
less spherical. However, the cells, which have living proto-
plasts, push up against each other shortly after they are
produced and assume other shapes, often ending up with
11 to 17 sides. Parenchyma cells may live for many years;
they function in storage, photosynthesis, and secretion.
Vascular tissue includes two kinds of conducting tis-
sues: (1) xylem, which conducts water and dissolved miner-
als; and (2) phloem, which conducts carbohydrates—
mainly sucrose—used by plants for food. The phloem also
transports hormones, amino acids, and other substances
that are necessary for plant growth. Xylem and phloem dif-
fer in structure as well as in function.
Root and shoot meristems give rise to a plant body
with an extensive underground, branching root
system and aboveground shoot system with reiterative
units of advantageously placed leaves joined at the
node of the plant, internode, and axillary buds.
756 Part X Plant Form and Function
Pith
Node
Shoot
Root
Blade
Leaf
Lateral root
Apical meristem Terminal bud
Petiole
Vein
Primary
growth zone
Secondary
growth zone
(vascular cambium)
Internode
Axillary bud
Vascular system
Primary root
Primary growth zone
Apical meristem
FIGURE 38.6
Diagram of a plant body. Branching in both the root and shoot
system increases the number of apical meristems. A significant
increase in stem/root circumference and the formation of bark can
only occur if there is secondary growth initiated by vascular and
cork cambium (secondary meristems). The lime green areas are
zones of active elongation; secondary growth occurs in the
lavender areas.
Primary and Secondary Growth
Primary and secondary growth play important roles in es-
tablishing the basic body plan of the organism. Here we
will look at how these meristems give rise to highly differ-
entiated tissues that support the growing plant body. In the
earliest vascular plants, the vascular tissues produced by
primary meristems played the same conducting roles as
they do in contemporary vascular plants. There was no dif-
ferentiation of the plant body into stems, leaves, and roots.
The presence of these three kinds of organs is a property of
most modern plants. It reflects increasing specialization in
relation to the demands of a terrestrial existence.
With the evolution of secondary growth, vascular plants
could develop thick trunks and become treelike (figure
38.7). This evolutionary advance in the sporophyte genera-
tion made possible the development of forests and the
domination of the land by plants. As discussed in chapter
37, reproductive constraints would have made secondary
growth and increased height nonadaptive if it had occurred
in the gametophyte generation. Judging from the fossil
record, secondary growth evolved independently in several
groups of vascular plants by the middle of the Devonian
period 380 million years ago.
There were two types of conducting systems in the earli-
est plants—systems that have become characteristic of vas-
cular plants as a group. Sieve-tube members conduct carbo-
hydrates away from areas where they are manufactured or
stored. Vessel members and tracheids are thick-walled cells
that transport water and dissolved minerals up from the
roots. Both kinds of cells are elongated and occur in linked
strands making tubes. Sieve-tube members are characteris-
tic of phloem tissue; vessel members and tracheids are
characteristic of xylem tissue. In primary tissues, which re-
sult from primary growth, these two types of tissue are typ-
ically associated with each other in the same vascular
strands. In secondary growth, the phloem is found on the
periphery, while a very thick xylem core develops more
centrally. You will see that roots and shoots of many vascu-
lar plants have different patterns of vascular tissue and sec-
ondary growth. Keep in mind that water and nutrients
travel between the most distant tip of a redwood root and
the tip of the shoot. For the system to work, these tissues
connect, which they do in the transition zone between the
root and the shoot. In the next section, we will consider the
three tissue systems that are present in all plant organs,
whether the plant has secondary growth or not.
Plants grow from the division of meristematic tissue.
Primary growth results from cell division at the apical
meristem at the tip of the plant, making the shoot
longer. Secondary growth results from cell division at
the lateral meristem in a cylinder encasing the shoot,
and increases the shoot’s girth.
Chapter 38 The Plant Body 757
(a)
Epidermis
Primary
phloem
Primary
xylem
(b)
Primary
phloem
Primary
xylem
Secondary
phloem
Secondary
xylem
Lateral
meristems
(c)
Primary
phloem
Primary
xylem
Secondary
phloem
Secondary
xylem
Annual
growth
layers
FIGURE 38.7
Secondary growth. (a) Before secondary growth begins, primary
tissues continue to elongate as the apical meristems produce
primary growth. (b) As secondary growth begins, the lateral
meristems produce secondary tissues, and the stem’s girth
increases. (c) In this three-year-old stem, the secondary tissues
continue to widen, and the trunk has become thick and woody.
Note that the lateral meristems form cylinders that run axially in
roots and shoots that have them.
Dermal Tissue
Epidermal cells, which originate from
the protoderm, cover all parts of the
primary plant body. This is probably
the earliest tissue system to appear in
embryogenesis. The exposed outer
walls have a cuticle that varies in thick-
ness, depending on the species and en-
vironmental conditions. A number of
types of specialized cells occur in the
epidermis, including guard cells, tri-
chomes, and root hairs.
Guard cells are paired sausage- or
dumbbell-shaped cells flanking a
stoma (plural, stomata), a mouth-
shaped epidermal opening. Guard cells,
unlike other epidermal cells, contain
chloroplasts. Stomata occur in the epi-
dermis of leaves (figure 38.8), and
sometimes on other parts of the plant,
such as stems or fruits. The passage of
oxygen and carbon dioxide, as well as
diffusion of water in vapor form, takes
place almost exclusively through the
stomata. There are between 1000 to
more than 1 million stomata per square
centimeter of leaf surface. In many
plants, stomata are more numerous on
the lower epidermis than on the upper
epidermis of the leaf. Some plants have stomata only on the
lower epidermis, and a few, such as water lilies, have them
only on the upper epidermis.
Guard cell formation is the result of an asymmetrical cell
division just like we saw in the first cell division in an algal
and angiosperm zygote. The patterning of these asymmetri-
cal divisions resulting in stomatal distribution has intrigued
developmental biologists. Research on mutants that get
“confused” about where to position stomata are providing
information on the timing of stomatal initiation and the
kind of intercellular communication that triggers guard cell
formation. For example, the too many mouths mutation may
be caused by a failure of developing stomata to suppress
stomatal formation in neighboring cells (figure 38.9).
The stomata open and shut in response to external fac-
tors such as light, temperature, and availability of water.
During periods of active photosynthesis, the stomata are
open, allowing the free passage of carbon dioxide into and
oxygen out of the leaf. We will consider the mechanism
that governs such movements in chapter 39.
Trichomes are hairlike outgrowths of the epidermis (fig-
ure 38.10). They occur frequently on stems, leaves, and
reproductive organs. A “fuzzy” or “woolly” leaf is covered
with trichomes that can be seen clearly with a microscope
758 Part X Plant Form and Function
38.2 Plants have three basic tissues, each composed of several cell types.
71 μm 71 μm
(a) (b)
FIGURE 38.8
Epidermis of a dicot and monocot leaf (250×). Stomata are evenly distributed over the
epidermis of monocots and dicots, but the patterning is quite different. (a) A pea (dicot) leaf
with a random arrangement of stomata. (b) A corn leaf with stomata evenly spaced in rows.
These photos also show the variety of cell shapes in plants. Some plant cells are boxlike, as
seen in corn (b), while others are irregularly shaped, as seen in peas (a).
FIGURE 38.9
The too many mouths stomatal mutant. This Arabidopsis plant
lacks an essential signal for spacing guard cells.
under low magnification. Trichomes play an important
role in keeping the leaf surface cool and in reducing the
rate of evaporation. Trichomes vary greatly in form in
different kinds of plants; some consist of a single cell,
while others may consist of several cells. Some are glan-
dular, often secreting sticky or toxic substances to deter
herbivory.
Trichome development has been investigated exten-
sively in Arabidopsis. Four genes are needed to specify the
site of trichome formation and initiate it (figure 38.11).
Next, eight genes are necessary for extension growth. Loss
of function of any one of these genes results in a trichome
with a distorted root hair. This is an example of taking a
very simple system and trying to genetically dissect all the
component parts. Understanding the formation of more
complex plant parts is a major challenge.
Root hairs, which are tubular extensions of individual
epidermal cells, occur in a zone just behind the tips of
young, growing roots (see figure 38.3). Because a root hair
is simply an extension and not a separate cell, there is no
crosswall isolating it from the epidermal cell. Root hairs
keep the root in intimate contact with the surrounding soil
particles and greatly increase the root’s surface area and
the efficiency of absorption. As the root grows, the extent
of the root hair zone remains roughly constant as root
hairs at the older end slough off while new ones are pro-
duced at the other end. Most of the absorption of water
and minerals occurs through root hairs, especially in
herbaceous plants. Root hairs should not be confused with
lateral roots which are multicellular and have their origins
deep within the root.
In the case of secondary growth, the cork cambium
(discussed in the section on stems in this chapter) pro-
duces the bark of a tree trunk or root. This replaces the
epidermis which gets stretched and broken with the ra-
dial expansion of the axis. Epidermal cells generally lack
the plasticity of other cells, but in some cases, they can
fuse to the epidermal cells of another organ or organelle
and dedifferentiate.
Some epidermal cells are specialized for protection,
others for absorption. Spacing of these specialized cells
within the epidermis maximizes their function and is an
intriguing developmental puzzle.
Chapter 38 The Plant Body 759
FIGURE 38.10
Trichomes. A covering of trichomes, teardrop-shaped blue
structures above, creates a layer of more humid air near the leaf
surface, enabling the plant to conserve available water supplies.
32 μm
57 μm
FIGURE 38.11
Trichome mutations. Mutants have revealed genes involved in a signal
transduction pathway that regulates the spacing and development of
trichomes. These include (a) DISTORTED1 (DIS1) and (b) DIS2
mutants in which trichomes are swollen and twisted.
(a)
(b)
Ground Tissue
Parenchyma
Parenchyma cells, which have large vacuoles, thin walls,
and an average of 14 sides at maturity, are the most com-
mon type of plant cell. They are the most abundant cells of
primary tissues and may also occur, to a much lesser extent,
in secondary tissues (figure 38.12a). Most parenchyma cells
have only primary walls, which are walls laid down while
the cells are still maturing. Parenchyma are less specialized
than other plant cells, although there are many variations
that do have special functions such as nectar and resin se-
cretion, or storage of latex, proteins, and metabolic wastes.
Parenchyma cells, which have functional nuclei and are
capable of dividing, commonly also store food and water,
and usually remain alive after they mature; in some plants
(for example, cacti), they may live to be over 100 years old.
The majority of cells in fruits such as apples are
parenchyma. Some parenchyma contain chloroplasts, espe-
cially in leaves and in the outer parts of herbaceous stems.
Such photosynthetic parenchyma tissue is called
chlorenchyma.
Collenchyma
Collenchyma cells, like parenchyma cells, have living pro-
toplasts and may live for many years. The cells, which are
usually a little longer than wide, have walls that vary in
thickness (figure 38.12b). Collenchyma cells, which are rel-
atively flexible, provide support for plant organs, allowing
them to bend without breaking. They often form strands or
continuous cylinders beneath the epidermis of stems or leaf
petioles (stalks) and along the veins in leaves. Strands of
collenchyma provide much of the support for stems in
which secondary growth has not taken place. The parts of
celery that we eat (petioles, or leaf stalks), have “strings”
that consist mainly of collenchyma and vascular bundles
(conducting tissues).
Sclerenchyma
Sclerenchyma cells have tough, thick walls; they usually
lack living protoplasts when they are mature. Their sec-
ondary cell walls are often impregnated with lignin, a
highly branched polymer that makes cell walls more rigid.
Cell walls containing lignin are said to be lignified. Lignin
is common in the walls of plant cells that have a supporting
or mechanical function. Some kinds of cells have lignin de-
posited in primary as well as secondary cell walls.
There are two types of sclerenchyma: fibers and sclereids.
Fibers are long, slender cells that are usually grouped to-
gether in strands. Linen, for example, is woven from
strands of sclerenchyma fibers that occur in the phloem of
flax. Sclereids are variable in shape but often branched.
They may occur singly or in groups; they are not elon-
gated, but may have various forms, including that of a star.
The gritty texture of a pear is caused by groups of sclereids
that occur throughout the soft flesh of the fruit (figure
38.12c). Both of these tough, thick-walled cell types serve
to strengthen the tissues in which they occur.
Parenchyma cells are the most common type of plant
cells and have various functions. Collenchyma cells
provide much of the support in young stems and leaves.
Sclerenchyma cells strengthen plant tissues and may be
nonliving at maturity.
760 Part X Plant Form and Function
(a) (b) (c)
FIGURE 38.12
The three types of ground tissue. (a) Parenchyma cells. Only primary cell walls are seen in this cross-section of parenchyma cells from
grass. (b) Collenchyma cells. Thickened side walls are seen in this cross-section of collenchyma cells from a young branch of elderberry
(Sambucus). In other kinds of collenchyma cells, the thickened areas may occur at the corners of the cells or in other kinds of strips. (c)
Sclereids. Clusters of sclereids (“stone cells”), stained red in this preparation, in the pulp of a pear. The surrounding thin-walled cells,
stained light blue, are parenchyma. These sclereid clusters give pears their gritty texture.
Vascular Tissue
Xylem
Xylem, the principal water-conducting tissues of plants,
usually contains a combination of vessels, which are contin-
uous tubes formed from dead, hollow, cylindrical cells (ves-
sel members) arranged end to end, and tracheids, which
are dead cells that taper at the ends and overlap one another
(figure 38.13). In some plants, such as gymnosperms, tra-
cheids are the only water-conducting cells present; water
passes in an unbroken stream through the xylem from the
roots up through the shoot and into the leaves. When the
water reaches the leaves, much of it passes into a film of
water on the outside of the parenchyma cells, and then it
diffuses in the form of water vapor into the intercellular
spaces and out of the leaves into the surrounding air, mainly
through the stomata. This diffusion of water vapor from a
plant is known as transpiration. In addition to conducting
water, dissolved minerals, and inorganic ions such as ni-
trates and phosphates throughout the plant, xylem supplies
support for the plant body.
Primary xylem is derived from the procambium, which
comes from the apical meristem. Secondary xylem is formed
by the vascular cambium, a lateral meristem that develops
later. Wood consists of accumulated secondary xylem.
Vessel members are found almost exclusively in an-
giosperms. In primitive angiosperms, vessel members tend
to resemble fibers and are relatively long. In more ad-
vanced angiosperms, vessel members tend to be shorter and
wider, resembling microscopic, squat coffee cans with both
ends removed. Both vessel members and tracheids have
thick, lignified secondary walls and no living protoplasts at
maturity. Lignin is produced by the cell and secreted to
strengthen the cellulose cell walls before the protoplast
dies, leaving only the cell wall. When the continuous
stream of water in a plant flows through tracheids, it passes
through pits, which are small, mostly rounded-to-elliptical
areas where no secondary wall material has been deposited.
The pits of adjacent cells occur opposite one another. In
contrast, vessel members, which are joined end to end, may
be almost completely open or may have bars or strips of
wall material across the open ends.
Vessels appear to conduct water more efficiently than do
the overlapping strands of tracheids. We know this partly
because vessel members have evolved from tracheids inde-
pendently in several groups of plants, suggesting that they
are favored by natural selection. It is also probable that
some types of fibers have evolved from tracheids, becoming
specialized for strengthening rather than conducting. Some
ancient flowering plants have only tracheids, but virtually
all modern angiosperms have vessels. Plants, with a muta-
tion that prevents the differentiation of xylem, but does not
affect tracheids, wilt soon after germination and are unable
to transport water efficiently.
In addition to conducting cells, xylem typically includes
fibers and parenchyma cells (ground tissue cells). The
parenchyma cells, which are usually produced in horizontal
rows called rays by special ray initials of the vascular cam-
bium, function in lateral conduction and food storage. An
initial is another term for a meristematic cell. It divides to
produce another initial and a cell that differentiates into a
ray cell. In cross-sections of woody stems and roots, the
rays can be seen radiating out from the center of the xylem
like the spokes of a wheel. Fibers are abundant in some
kinds of wood, such as oak (Quercus), and the wood is cor-
respondingly dense and heavy. The arrangements of these
and other kinds of cells in the xylem make it possible to
identify most plant genera and many species from their
wood alone. These fibers are a major component in mod-
ern paper. Earlier paper was made from fibers in phloem.
Chapter 38 The Plant Body 761
Tracheid
Pits
Pores
Vessel
member
Vessel
member
(a) (b)
Tracheids Vessel
FIGURE 38.13
Comparison between vessel
members and tracheids. (a) In
tracheids, the water passes from
cell to cell by means of pits, (b)
while in vessel members, it
moves by way of perforation
plates or between bars of wall
material. In gymnosperm wood,
tracheids both conduct water
and provide support; in most
kinds of angiosperms, vessels
are present in addition to
tracheids, or present
exclusively. These two types of
cells conduct the water, and
fibers provide additional
support. (c) Scanning
micrograph of the wood of red
maple, Acer rubrum (350×). (c)
Phloem
Phloem, which is located toward the outer part of roots
and stems, is the principal food-conducting tissue in vascu-
lar plants. If a plant is girdled (by removing a substantial
strip of bark down to the vascular cambium), the plant
eventually dies from starvation of the roots.
Food conduction in phloem is carried out through two
kinds of elongated cells: sieve cells and sieve-tube mem-
bers (figure 38.14). Seedless vascular plants and gym-
nosperms have only sieve cells; most angiosperms have
sieve-tube members. Both types of cells have clusters of
pores known as sieve areas. Sieve areas are more abun-
dant on the overlapping ends of the cells and connect the
protoplasts of adjoining sieve cells and sieve-tube mem-
bers. Both of these types of cells are living, but most sieve
cells and all sieve-tube members lack a nucleus at maturity.
This type of cell differentiation has parallels to the differ-
entiation of human red blood cells which also lack a nu-
cleus at maturity.
In sieve-tube members, some sieve areas have larger
pores and are called sieve plates. Sieve-tube members
occur end to end, forming longitudinal series called sieve
tubes. Sieve cells are less specialized than sieve-tube mem-
bers, and the pores in all of their sieve areas are roughly of
the same diameter. In an evolutionary sense, sieve-tube
members are more advanced, more specialized, and, pre-
sumably, more efficient.
Each sieve-tube member is associated with an adjacent
specialized parenchyma cell known as a companion cell.
Companion cells apparently carry out some of the meta-
bolic functions that are needed to maintain the associated
sieve-tube member. In angiosperms, a common initial cell
divides asymmetrically to produce a sieve-tube member
cell and its companion cell. Companion cells have all of
the components of normal parenchyma cells, including nu-
clei, and their numerous plasmodesmata (cytoplasmic
connections between adjacent cells) connect their cyto-
plasm with that of the associated sieve-tube members.
Fibers and parenchyma cells are often abundant in
phloem.
Xylem conducts water and dissolved minerals from the
roots to the shoots and the leaves. Phloem carries
organic materials from one part of the plant to another.
762 Part X Plant Form and Function
Sieve
plates
Sieve plate
Sieve-tube
member
Nucleus
Companion
cell
FIGURE 38.14
A sieve-tube member. (a) Looking down into sieve plates in squash phloem reveals the perforations sucrose and hormones move through.
(b) Sieve-tube member cells are stacked with the sieve plates forming the connection. The narrow cell with the nucleus at the left of the
sieve-tube member is a companion cell. This cell nourishes the sieve-tube members, which have plasma membranes, but no nuclei.
(a) (b)
Root Structure
The three tissue systems are found in the three kinds of
vegetative organs in plants: roots, stems, and leaves.
Roots have a simpler pattern of organization and develop-
ment than stems, and we will consider them first. Four
zones or regions are commonly recognized in developing
roots. The zones are called the root cap, the zone of cell
division, the zone of elongation, and the zone of matu-
ration (figure 38.15). In three of the zones, the boundaries
are not clearly defined. When apical initials divide, daugh-
ter cells that end up on the tip end of the root become root
cap cells. Cells that divide in the opposite direction pass
through the three other zones before they finish differenti-
ating. As you consider the different zones, visualize the tip
of the root moving away from the soil surface by growth.
This will counter the static image of a root that diagrams
and photos convey.
The Root Cap
The root cap has no equivalent in stems. It is composed of
two types of cells, the inner columella (they look like
columns) cells and the outer, lateral root cap cells that are
continuously replenished by the root apical meristem. In
some plants with larger roots it is quite obvious. Its most
obvious function is to protect the delicate tissues behind it
as growth extends the root through mostly abrasive soil
particles. Golgi bodies in the outer root cap cells secrete
and release a slimy substance that passes through the cell
walls to the outside. The cells, which have an average life of
less than a week, are constantly being replaced from the in-
side, forming a mucilaginous lubricant that eases the root
through the soil. The slimy mass also provides a medium
for the growth of beneficial nitrogen-fixing bacteria in the
roots of some plants such as legumes.
A new root cap is produced when an existing one is ar-
tificially or accidentally removed from a root. The root
cap also functions in the perception of
gravity. The columella cells are highly
specialized with the endoplasmic retic-
ulum in the periphery and the nucleus
located at either the middle or the top
of the cell. There are no large vacuoles.
Columella cells contain amyloplasts
(plastids with starch grains) that collect
on the sides of cells facing the pull of
gravity. When a potted plant is placed
on its side, the amyloplasts drift or
tumble down to the side nearest the
source of gravity, and the root bends in
that direction. Lasers have been used to
ablate (kill) individual columella cells in
Arabidopsis. It turns out that only two of
the columella cells are essential for
gravity sensing! The precise nature of
the gravitational response is not
known, but some evidence indicates
that calcium ions in the amyloplasts in-
fluence the distribution of growth hor-
mones (auxin in this case) in the cells.
There may be multiple signaling mech-
anisms because bending has been ob-
served in the absence of auxin. A cur-
rent hypothesis is that an electrical
signal moves from the columella cell to
cells in the distal region of the elonga-
tion zone (the region closest to the
zone of cell division).
Chapter 38 The Plant Body 763
38.3 Root cells differentiate as they become distanced from the dividing root
apical meristem.
Zone of
maturation
Zone of
elongation
Zone of
cell division
Epidermis
Ground meristem
Procambium
Protoderm
Endodermis
Ground tissue
Vascular tissue
Quiescent center
Lateral root cap
Columella root cap
Root in
cross-section
Apical meristem
KEY
Dermal tissue
Ground tissue
Vascular tissue
FIGURE 38.15
Root structure. A root tip in corn, Zea mays. This longitudinal section of a root shows
the root cap, apical meristem, procambium, protoderm, epidermis, and ground meristem.
The Zone of Cell Division
The apical meristem is shaped like an inverted, concave dome
of cells and is located in the center of the root tip in the area
protected by the root cap. Most of the activity in this zone of
cell division takes place toward the edges of the dome, where
the cells divide every 12 to 36 hours, often rhythmically,
reaching a peak of division once or twice a day. Most of the
cells are essentially cuboidal with small vacuoles and propor-
tionately large, centrally located nuclei. These rapidly divid-
ing cells are daughter cells of the apical meristem. The quies-
cent center is a group of cells in the center of the root apical
meristem. They divide very infrequently. This makes sense if
you think about a solid ball expanding. The outer surface
would have to increase far more rapidly than the very center.
The apical meristem daughter cells soon subdivide into
the three primary tissue systems previously discussed: proto-
derm, procambium, and ground meristem. Genes have been
identified in the relatively simple root of Arabidopsis that
regulate the patterning of these tissue systems. The pat-
terning of these cells begins in this zone, but it is not until
the cells reach the zone of maturation that the anatomical
and morphological expression of this patterning is fully re-
vealed. WEREWOLF, for example, is required for the pat-
terning of the two root epidermal cell types, those with and
without root hairs (figure 38.16a). The SCARECROW gene
is important in ground cell differentiation (figure 38.16b).
It is necessary for an asymmetric cell division that gives rise
to two cylinders of cells from one. The outer cell layer be-
comes ground tissue and serves a storage function. The
inner cell layer forms the endodermis which regulates the
intercellular flow of water and solutes into the vascular core
of the root. Cells in this region develop according to their
position. If that position changes because of a mistake in
cell division or the ablation of another cell, the cell devel-
ops according to its new position.
The Zone of Elongation
In the zone of elongation, the cells produced by the primary
meristems become several times longer than wide, and
their width also increases slightly. The small vacuoles pre-
sent merge and grow until they occupy 90% or more of the
volume of each cell. No further increase in cell size occurs
above the zone of elongation, and the mature parts of the
root, except for an increase in girth, remain stationary for
the life of the plant.
764 Part X Plant Form and Function
FIGURE 38.16
Tissue-specific gene expression. (a) Epidermal-specific gene expression. The promoter of the WEREWOLF gene of Arabidopsis was
attached to a green fluorescent protein and used to make a transgenic plant. The green fluorescence shows the epidermal cells where the
gene is expressed. The red was used to visually indicate cell boundaries. (b) Ground tissue-specific gene expression. The SCARECROW
gene is needed for an asymmetric cell division allowing for the formation of side-by-side ground tissue and endodermal cells. These two
layers are blue in wild-type, but in the mutant, only one cell layer is blue because the asymmetric cell division does not occur.
(a) (b)
The Zone of Maturation
The cells that have elongated in the zone of elongation be-
come differentiated into specific cell types in the zone of
maturation. The cells of the root surface cylinder mature
into epidermal cells, which have a very thin cuticle. Many of
the epidermal cells each develop a root hair; the protuber-
ance is not separated by a crosswall from the main part of
the cell and the nucleus may move into it. Root hairs,
which can number over 35,000 per square centimeter of
root surface and many billions per plant, greatly increase
the surface area and therefore the absorptive capacity of the
root. The root hairs usually are alive and functional for
only a few days before they are sloughed off at the older
part of the zone of maturation, while new ones are being
produced toward the zone of elongation. Symbiotic bacte-
ria that fix atmospheric nitrogen into a form usable by
legumes enter the plant via root hairs and “instruct” the
plant to create a nodule around it.
Parenchyma cells are produced by the ground meri-
stem immediately to the interior of the epidermis. This
tissue, called the cortex, may be many cells wide and
functions in food storage. The inner boundary of the cor-
tex differentiates into a single-layered cylinder of endo-
dermis (figure 38.17), whose primary walls are impreg-
nated with suberin, a fatty substance that is impervious to
water. The suberin is produced in bands, called Caspar-
ian strips that surround each adjacent endodermal cell
wall perpendicular to the root’s surface (figure 38.18).
This blocks transport between cells. The two surfaces that
are parallel to the root surface are the only way into the
core of the root and the cell membranes control what
passes through.
All the tissues interior to the endodermis are collectively
referred to as the stele. Immediately adjacent and interior
to the endodermis is a cylinder of parenchyma cells known
as the pericycle. Pericycle cells can divide, even after they
mature. They can give rise to lateral (branch) roots or, in di-
cots, to part of the vascular cambium.
Chapter 38 The Plant Body 765
Epidermis
Cortex
Endodermis
Pericycle
Primary phloem
Primary xylem
Pith
FIGURE 38.17
Cross-section of the zone of maturation of a young monocot root. Greenbrier (Smilax), a monocot (100H11003).
Casparian strip
Sectioned
endodermal cells
H
2
O
H
2
O
FIGURE 38.18
Casparian strip. The Casparian strip is a water-proofing band
that protects cells inside the endodermis from flooding.
The water-conducting cells of the primary xylem are dif-
ferentiated as a solid core in the center of young dicot
roots. In a cross-section of a dicot root, the central core of
primary xylem often is somewhat star-shaped, with one or
two to several radiating arms that point toward the pericy-
cle (figure 38.19). In monocot (and a few dicot) roots, the
primary xylem is in discrete vascular bundles arranged in
a ring, which surrounds parenchyma cells, called pith, at
the very center of the root. Primary phloem, composed of
cells involved in food conduction, is differentiated in dis-
crete groups of cells between the arms of the xylem in both
dicot and monocot roots.
In dicots and other plants with secondary growth, part
of the pericycle and the parenchyma cells between the
phloem patches and the xylem arms become the root vascu-
lar cambium, which starts producing secondary xylem to
the inside and secondary phloem to the outside (figure
38.20). Eventually, the secondary tissues acquire the form
of concentric cylinders. The primary phloem, cortex, and
epidermis become crushed and are sloughed off as more
secondary tissues are added. In the pericycle of woody
plants, the cork cambium produces bark which will be dis-
cussed in the section on stems (see figure 38.26). In the
case of secondary growth in dicot roots, everything outside
the stele is lost and replaced with bark.
Root apical meristems produce a root cap at the tip and
root tissue on the opposite side. Cells mature as the
root cap and meristem grows away from them.
Transport systems, external barriers, and a branching
root system develop from the primary root as it
matures.
766 Part X Plant Form and Function
Cortex
Epidermis
Endodermis
Passage cell
Primary xylem
Pericycle
Primary phloem
FIGURE 38.19
Cross-section of the zone of maturation of a young dicot root. (a) Buttercup
(Ranunculus), a dicot (40H11003). (b) The enlargement shows the various tissues present (600H11003).
(a) (b)
Zygote Embryo
Shoot apical
meristem
Root apical
meristem
Cork cambium
Vascular cambium
Leaf primordia
Bud primordia
Shoot elongation
Outer bark
Phloem
Xylem
Inner bark
Wood
Bark
Leaves
Lateral shoots
Cork cambium
Vascular cambium
Pericycle
Phloem
Xylem
Lateral roots
Root elongation
Undifferentiated Stage 1 Stage 2 Stage 3 Stage 4 Stage 5: Fully differentiated
Outer bark
Inner bark
Wood
Bark
FIGURE 38.20
Stages in the
differentiation of
plant tissues.
Modified Roots
Most plants produce either a
taproot system in which there
is a single large root with
smaller branch roots, or a fi-
brous root system in which
there are many smaller roots of
similar diameter. Some plants,
however, have intriguing root
modifications with specific
functions in addition to those of
anchorage and absorption.
Aerial roots. Some plants,
such as epiphytic orchids (or-
chids that are attached to
tree branches and grow un-
connected to the ground
without being parasitic in any
way) have roots that extend
out into the air. Some aerial
roots have an epidermis that
is several cells thick, an adap-
tation to reduce water loss.
These aerial roots may also
be green and photosynthetic,
as in the vanilla orchid. Some
monocots, such as corn, pro-
duce thick roots from the
lower parts of the stem; these
prop roots grow down to the
ground and brace the plants
against wind. Climbing plants such as ivy also produce
roots from their stems; these anchor the stems to tree
trunks or a brick wall. Any root that arises along a stem
or in some place other than the root of the plant is called
an adventitious root. Adventitious root formation in ivy
depends on the developmental stage of the shoot. When
the shoot transitions to the adult phase of development,
it is no longer capable of initiating these roots.
Pneumatophores. Some plants that grow in swamps
and other wet places may produce spongy outgrowths
called pneumatophores from their underwater roots (fig-
ure 38.21a). The pneumatophores commonly extend
several centimeters above water, facilitating the oxygen
supply to the roots beneath.
Contractile roots. The roots from the bulbs of lilies
and of several other plants such as dandelions contract
by spiraling to pull the plant a little deeper into the soil
each year until they reach an area of relatively stable
temperatures. The roots may contract to a third of their
original length as they spiral like a corkscrew due to cel-
lular thickening and constricting.
Parasitic roots. The stems of certain plants that lack
chlorophyll, such as dodder (Cuscuta), produce peglike
roots called haustoria that penetrate the host plants
around which they are twined. The haustoria establish
contact with the conducting tissues of the host and ef-
fectively parasitize their host.
Food storage roots. The xylem of branch roots of
sweet potatoes and similar plants produce at intervals
many extra parenchyma cells that store large quantities
of carbohydrates. Carrots, beets, parsnips, radishes, and
turnips have combinations of stem and root that also
function in food storage. Cross sections of these roots
reveal multiple rings of secondary growth.
Water storage roots. Some members of the pumpkin
family (Cucurbitaceae), especially those that grow in arid
regions, may produce water-storage roots weighing 50
or more kilograms (figure 38.21b).
Buttress roots. Certain species of fig and other tropical
trees produce huge buttress roots toward the base of the
trunk, which provide considerable stability (figure 38.21c).
Some plants have modified roots that carry out
photosynthesis, gather oxygen, parasitize other plants,
store food or water, or support the stem.
Chapter 38 The Plant Body 767
(a) (b)
(c)
FIGURE 38.21
Three types of modified roots. (a)
Pneumatophores (foreground) are
spongy outgrowths from the roots
below. (b) A water storage root
weighing over 25 kilograms (60
pounds). (c) Buttress roots of a
tropical fig tree.
Stem Structure
External Form
The shoot apical meristem initiates stem tissue and inter-
mittently produces bulges (primordia) that will develop
into leaves, other shoots, or even flowers (figure 38.22).
The stem is an axis from which organs grow. Leaves may
be arranged in a spiral around the stem, or they may be in
pairs opposite one another; they also may occur in whorls
(circles) of three or more. Spirals are the most common
and, for reasons still not understood, sequential leaves
tend to be placed 137.5
o
apart. This angle relates to the
golden mean, a mathematical ratio, that is found in nature
(the angle of coiling in some shells, for example), classical
architecture (the Parthenon wall dimensions), and even
modern art (Mondrian). The pattern of leaf arrangement
is called phyllotaxy and may optimize exposure of leaves
to the sun.
The region or area (no structure is
involved) of leaf attachment is called
a node; the area of stem between two
nodes is called an internode. A leaf
usually has a flattened blade and
sometimes a petiole (stalk). When
the petiole is missing, the leaf is then
said to be sessile. Note that the word
sessile as applied to plants has a differ-
ent meaning than it does when ap-
plied to animals (probably obvious, as
plants don’t get up and move
around!); in plants, it means immobile
or attached. The space between a
petiole (or blade) and the stem is
called an axil. An axillary bud is pro-
duced in each axil. This bud is a
product of the primary shoot apical
meristem, which, with its associated
leaf primordia, is called a terminal
bud. Axillary buds frequently de-
velop into branches or may form
meristems that will develop into
flowers. (Refer back to figure 38.6 to review these terms.)
Herbaceous stems do not produce a cork cambium. The
stems are usually green and photosynthetic, with at least
the outer cells of the cortex containing chloroplasts.
Herbaceous stems commonly have stomata, and may have
various types of trichomes (hairs).
Woody stems can persist over a number of years and
develop distinctive markings in addition to the original
organs that form. Terminal buds usually extend the
length of the shoot during the growing season. Some
buds, such as those of geraniums, are unprotected, but
most buds of woody plants have protective winter bud
scales that drop off, leaving tiny bud scars as the buds ex-
pand. Some twigs have tiny scars of a different origin. A
pair of butterfly-like appendages called stipules (part of
the leaf) develop at the base of some leaves. The stipules
can fall off and leave stipule scars. When leaves of decidu-
ous trees drop in the fall, they leave leaf scars with tiny
bundle scars, marking where vascular connections were.
The shapes, sizes, and other features of leaf scars can be
distinctive enough to identify the plants in winter
(figure 38.23).
768 Part X Plant Form and Function
38.4 Stems are the backbone of the shoot, transporting nutrients and
supporting the aerial plant organs.
FIGURE 38.22
A shoot apex
(200×). Scanning
electron
micrograph of the
apical meristem of
wheat (Triticum).
Bundle
scar
Terminal
bud
(a) (b)
Leaf
scar
Petiole
Stipules
Blade
Node
Axillary bud
Terminal
bud scale
scars
Internode
FIGURE 38.23
A woody twig. (a) In winter. (b) In summer.
Internal Form
As in roots, there is an apical meristem at the tip of each
stem, which produces primary tissues that contribute to the
stem’s increases in length. Three primary meristems de-
velop from the apical meristem. The protoderm gives rise to
the epidermis. The ground meristem produces parenchyma
cells. Parenchyma cells in the center of the stem constitute
the pith; parenchyma cells away from the center constitute
the cortex. The procambium produces cylinders of primary
xylem and primary phloem, which are surrounded by ground
tissue.
A strand of xylem and phloem, called a trace, branches
off from the main cylinder of xylem and phloem and enters
the developing leaf, flower, or shoot. These spaces in the
main cylinder of conducting tissues are called gaps. In di-
cots, a vascular cambium develops between the primary
xylem and primary phloem (figure 38.24). In many ways,
this is a connect-the-dots game where the vascular cam-
bium connects the ring of primary vascular bundles. In
monocots, these bundles are scattered throughout the
ground tissue (figure 38.25) and there is no logical way to
connect them that would allow a uniform increase in girth.
This is why monocots do not have secondary growth.
Chapter 38 The Plant Body 769
75 μm
Cortex
Epidermis
Collenchyma
Parenchyma
Primary phloem
Secondary phloem
Vascular cambium
Secondary xylem
Primary xylem
Pith
FIGURE 38.24
Early stage in differentiation of vascular
cambium in the castor bean, Ricirus
(25x). The outer part of the cortex consists
of collenchyma, and the inner part of
parenchyma.
Epidermis
(outer layer)
Pith
Vascular
bundle
Xylem
Phloem
Cortex
Collenchyma
(layers below
epidermis)
Xylem
Ground tissue
Phloem
Vascular bundles
FIGURE 38.25
Stems. Transverse sections of a young stem in (a) a dicot, the common sunflower, Helianthus annuus, in which the vascular bundles are
arranged around the outside of the stem (10×); and (b) a monocot, corn, Zea mays, with the scattered vascular bundles characteristic of the
class (5×).
(a) (b)
The cells of the vascular cambium divide indefinitely,
producing secondary tissues (mainly secondary xylem and
secondary phloem). The production of xylem is extensive in
trees and is called wood. Rings in the stump of a tree re-
veal annual patterns of growth; cell size varies depending
on growth conditions. In woody dicots, a second cam-
bium, the cork cambium, arises in the outer cortex (occa-
sionally in the epidermis or phloem) and produces box-
like cork cells to the outside and also may produce
parenchyma-like phelloderm cells to the inside; the cork
cambium, cork, and phelloderm are collectively referred
to as the periderm (figure 38.26). Cork tissues, whose cells
become impregnated with suberin shortly after they are
formed and then die, constitute the outer bark. The
cork tissue, whose suberin is impervious to moisture, cuts
off water and food to the epidermis, which dies and
sloughs off. In young stems, gas exchange between stem
tissues and the air takes place through stomata, but as the
cork cambium produces cork, it also produces patches of
unsuberized cells beneath the stomata. These unsuber-
ized cells, which permit gas exchange to continue, are
called lenticels (figure 38.27).
The stem results from the dynamic growth of the shoot
apical meristem which initiates stem tissue and organs
including leaves. Shoot apical meristems initiate new
apical meristems at the junction of leaf and stem. These
meristems can form buds which reiterate the growth
pattern of the terminal bud or they can make flowers
directly.
770 Part X Plant Form and Function
Epidermis
Cork
Cork cambium
Phelloderm
Collenchyma
Parenchyma
42 μm
FIGURE 38.26
Section of periderm (50×). An early stage
in the development of periderm in
cottonwood (Populus sp.).
FIGURE 38.27
Lenticels. (a) Lenticels, the
numerous, small, pale, raised
areas shown here on cherry tree
bark (Prunus cerasifera), allow
gas exchange between the
external atmosphere and the
living tissues immediately
beneath the bark of woody
plants. Highly variable in form
in different species, lenticels are
an aid to the identification of
deciduous trees and shrubs in
winter. (b) Transverse section
through a lenticel (extruding
area) in a stem of elderberry,
Sambucus canadensis (30×).
(a)
(b)
Modified Stems
Although most stems grow erect, there are some modifica-
tions that serve special purposes, including that of natural
vegetative propagation. In fact, the widespread artificial vege-
tative propagation of plants, both commercial and private,
frequently involves the cutting of modified stems into seg-
ments, which are then planted and produce new plants. As
you become acquainted with the following modified stems,
keep in mind that stems have leaves at nodes, with internodes
between the nodes, and buds in the axils of the leaves, while
roots have no leaves, nodes, or axillary buds.
Bulbs. Onions, lilies, and tulips have swollen under-
ground stems that are really large buds with adventitious
roots at the base (figure 38.28a). Most of a bulb consists
of fleshy leaves attached to a small, knoblike stem. In
onions, the fleshy leaves are surrounded by papery,
scalelike leaf bases of the long, green aboveground
leaves.
Corms. Crocuses, gladioluses, and other popular gar-
den plants produce corms that superficially resemble
bulbs. Cutting a corm in half, however, reveals no fleshy
leaves. Instead, almost all of a corm consists of stem,
with a few papery, brown nonfunctional leaves on the
outside, and adventitious roots below.
Rhizomes. Perennial grasses, ferns, irises, and many
other plants produce rhizomes, which typically are hori-
zontal stems that grow underground, often close to the
surface (figure 38.28b). Each node has an inconspicuous
scalelike leaf with an axillary bud; much larger photo-
synthetic leaves may be produced at the rhizome tip. Ad-
ventitious roots are produced throughout the length of
the rhizome, mainly on the lower surface.
Runners and stolons. Strawberry plants produce hor-
izontal stems with long internodes, which, unlike rhi-
zomes, usually grow along the surface of the ground.
Several runners may radiate out from a single plant (fig-
ure 38.28c). Some botanists use the term stolon synony-
mously with runner; others reserve the term stolon for a
stem with long internodes that grows underground, as
seen in Irish (white) potato plants. An Irish potato itself,
however, is another type of modified stem—a tuber.
Tubers. In Irish potato plants, carbohydrates may ac-
cumulate at the tips of stolons, which swell, becoming
tubers; the stolons die after the tubers mature (figure
38.28d). The “eyes” of a potato are axillary buds formed
in the axils of scalelike leaves. The scalelike leaves,
which are present when the potato is starting to form,
soon drop off; the tiny ridge adjacent to each “eye” of a
mature potato is a leaf scar.
Tendrils. Many climbing plants, such as grapes and
Boston ivy, produce modified stems knows as tendrils,
which twine around supports and aid in climbing (figure
38.28e). Some tendrils, such as those of peas and pump-
kins, are actually modified leaves or leaflets.
Cladophylls. Cacti and several other plants produce
flattened, photosynthetic stems called cladophylls that re-
semble leaves (figure 38.28f ). In cacti, the real leaves are
modified as spines.
Some plants possess modified stems that serve special
purposes including food storage, support, or vegetative
propagation.
Chapter 38 The Plant Body 771
Fleshy leaves
Knoblike stem
Adventitious
roots
(a) Bulbs (onion)
Photosynthetic
leaf
Scalelike leaf
at each node
Rhizome
(b) Rhizomes (iris)
(c) Runners (strawberry) (d) Tubers (potato)
Runner
Stolen Tuber (swollen
tip of stolen)
Nodes (axillary buds
adjacent to leaf scars)
(e) Tendrils (grape) (f) Cladophylls (prickly pear)
Tendril
Cladophyll
Leaves (modified
as spines)
FIGURE 38.28
Types of modified stems.
Leaf External Structure
Leaves, which are initiated as primordia by the apical meri-
stems (see figure 38.2), are vital to life as we know it. They
are the principal sites of photosynthesis on land. Leaves ex-
pand primarily by cell enlargement and some cell division.
Like our arms and legs, they are determinate structures
which means growth stops at maturity. Because leaves are
crucial to a plant, features such as their arrangement, form,
size, and internal structure are highly significant and can
differ greatly. Different patterns have adaptive value in dif-
ferent environments.
Leaves are really an extension of the shoot apical meri-
stem and stem development. Leaves first emerge as primor-
dia as discussed in the section on stems. At that point, they
are not committed to be leaves. Experiments where very
young leaf primordia in fern and in coleus are isolated and
grown in culture demonstrate this. If the primordia are
young enough, they will form an entire shoot rather than a
leaf. So, positioning the primordia and beginning the initial
cell divisions occurs before those cells are committed to the
leaf developmental pathway.
Leaves fall into two different morphological groups
which may reflect differences in evolutionary origin. A mi-
crophyll is a leaf with one vein that does not leave a gap
when it branches from the vascular cylinder of the stem;
microphylls are mostly small and are associated primarily
with the phylum Lycophyta (see chapter 37). Most plants
have leaves called megaphylls, which have several to many
veins; a megaphyll’s conducting tissue leaves a gap in the
stem’s vascular cylinder as it branches from it.
Most dicot leaves have a flattened blade, and a slender
stalk, the petiole. The flattening of the leaf blade reflects a
shift from radial symmetry to dorsal-ventral (top-bottom)
symmetry. We’re just beginning to understand how this
shift occurs by analyzing mutants like phantastica which
prevents this transition (figure 38.29). In addition, a pair of
stipules may be present at the base of the petiole. The stip-
ules, which may be leaflike or modified as spines (as in the
black locust—Robinia pseudo-acacia) or glands (as in cherry
trees—Prunus cerasifera), vary considerably in size from mi-
croscopic to almost half the size of the leaf blade. Develop-
ment of stipules appears to be independent of development
of the rest of the leaf.
Grasses and other monocot leaves usually lack a petiole
and tend to sheathe the stem toward the base. Veins (a
term used for the vascular bundles in leaves), consisting of
both xylem and phloem, are distributed throughout the leaf
blades. The main veins are parallel in most monocot leaves;
the veins of dicots, on the other hand, form an often intri-
cate network (figure 38.30).
772 Part X Plant Form and Function
38.5 Leaves are adapted to support basic plant functions.
FIGURE 38.29
The phantastica mutant in snapdragon. Snapdragon leaves are
usually flattened with a top and bottom side (plant on left). In the
phantastica mutant (plant on right), the leaf never flattens but
persists as a radially symmetrical bulge.
(a) (b)
FIGURE 38.30
Dicot and monocot leaves. The leaves of dicots, such as this (a)
African violet relative from Sri Lanka, have netted, or reticulate,
veins; (b) those of monocots, like this cabbage palmetto, have
parallel veins. The dicot leaf has been cleared with chemicals and
stained with a red dye to make the veins show up more clearly.
Leaf blades come in a variety of forms from oval to
deeply lobed to having separate leaflets. In simple leaves
(figure 38.31a), such as those of lilacs or birch trees, the
blades are undivided, but simple leaves may have teeth, in-
dentations, or lobes of various sizes, as in the leaves of
maples and oaks. In compound leaves, such as those of
ashes, box elders, and walnuts, the blade is divided into
leaflets. The relationship between the development of
compound and simple leaves is an open question. Two ex-
planations are being debated: (1) a compound leaf is a
highly lobed simple leaf, or (2) a compound leaf utilizes a
shoot development program. There are single mutations
that convert compound leaves to simple leaves which are
being used to address this debate. If the leaflets are
arranged in pairs along a common axis (the axis is called a
rachis—the equivalent of the main central vein, or midrib, in
simple leaves), the leaf is pinnately compound (figure
38.31b). If, however, the leaflets radiate out from a com-
mon point at the blade end of the petiole, the leaf is
palmately compound (figure 38.31c). Palmately com-
pound leaves occur in buckeyes (Aesculus spp.) and Virginia
creeper (Parthenocissus quinquefolia). The leaf blades them-
selves may have similar arrangements of their veins, and are
said to be pinnately or palmately veined.
Leaves, regardless of whether they are simple or com-
pound, may be alternately arranged (alternate leaves usu-
ally spiral around a shoot) or they may be in opposite
pairs. Less often, three or more leaves may be in a whorl, a
circle of leaves at the same level at a node (figure 38.32).
Leaves are the principal sites of photosynthesis. Their
blades may be arranged in a variety of ways. In simple
leaves the blades are undivided, while in compound
leaves the leaf is composed of two or more leaflets.
Chapter 38 The Plant Body 773
FIGURE 38.31
Simple versus compound leaves. (a) A simple leaf, its margin deeply lobed, from the tulip
tree (Liriodendron tulipifera). (b) A pinnately compound leaf, from a mountain ash (Sorbus sp.).
A compound leaf is associated with a single lateral bud, located where the petiole is attached
to the stem. (c) Palmately compound leaves of a Virginia creeper (Parthenocissus quinquefolia).
(c)
(b)
(a)
Alternate (spiral):
Ivy
Opposite:
Periwinkle
Whorled:
Sweet woodruff
FIGURE 38.32
Types of leaf arrangements. The three common types of leaf
arrangements are alternate, opposite, and whorled.
Leaf Internal
Structure
The entire surface of a leaf is covered
by a transparent epidermis, most of
whose cells have no chloroplasts.
The epidermis itself has a waxy cuticle
of variable thickness, and may have
different types of glands and tri-
chomes (hairs) present. The lower
epidermis (and occasionally the
upper epidermis) of most leaves con-
tains numerous slit-like or mouth-
shaped stomata (figure 38.33). Stom-
ata, as discussed earlier, are flanked
by guard cells and function in gas ex-
change and regulation of water
movement through the plant.
The tissue between the upper and
lower epidermis is called mesophyll. Mesophyll is inter-
spersed with veins (vascular bundles) of various sizes. In
most dicot leaves, there are two distinct types of meso-
phyll. Closest to the upper epidermis are one to several
(usually two) rows of tightly packed, barrel-shaped to
cylindrical chlorenchyma cells (parenchyma with chloro-
plasts) that constitute the palisade mesophyll (figure
38.34). Some plants, including species of Eucalyptus, have
leaves that hang down, rather than extending horizontally.
They have palisade parenchyma on both sides of the leaf,
and there is, in effect, no upper side. In nearly all leaves
there are loosely arranged spongy mesophyll cells be-
tween the palisade mesophyll and the lower epidermis,
with many air spaces throughout the tissue. The intercon-
nected intercellular spaces, along with the stomata, func-
tion in gas exchange and the passage of water vapor from
774 Part X Plant Form and Function
Epidermal cell
Guard cell
Stoma
Thickened
inner wall of
guard cell
Stoma
Epidermal cell
Nucleus
Chloroplast
Guard
cell
FIGURE 38.33
A stoma. (a) Surface view. (b) View in cross-section.
Upper
epidermis
Palisade
mesophyll
Spongy
mesophyll
Lower
epidermis
Cuticle
Guard cell Stoma Vein
Guard cell
Stoma
Vein
FIGURE 38.34
A leaf in cross-section. Transection of a leaf showing the arrangement of palisade and spongy mesophyll, a vascular bundle or vein, and
the epidermis with paired guard cells flanking the stoma.
(a) (b)
the leaves. The mesophyll of monocot leaves is not differ-
entiated into palisade and spongy layers and there is often
little distinction between the upper and lower epidermis.
This anatomical difference often correlates with a modi-
fied photosynthetic pathway that maximizes the amount of
CO
2
relative to O
2
to reduce energy loss through pho-
torespiration (refer to chapter 10). Leaf anatomy directly
relates to its juggling act to balance water loss, gas ex-
change, and transport of photosynthetic products to the
rest of the plant.
Leaves are basically flattened bags of epidermis
containing vascular tissue and tightly packed palisade
mesophyll rich in chloroplasts and loosely packed
spongy mesophyll with many interconnected air spaces
that function in gas and water vapor exchange.
Modified Leaves
As plants colonized a wide variety of environments, from
deserts to lakes to tropical rain forests, modifications of
plant organs that would adapt the plants to their specific
habitats arose. Leaves, in particular, have evolved some re-
markable adaptations. A brief discussion of a few of these
modifications follows.
Floral leaves (bracts). Poinsettias and dogwoods have
relatively inconspicuous, small, greenish-yellow flowers.
However, both plants produce large modified leaves,
called bracts (mostly colored red in poinsettias and
white or pink in dogwoods). These bracts surround the
true flowers and perform the same function as showy
petals (figure 38.35). It should be noted, however, that
bracts can also be quite small and not as conspicuous as
those of the examples mentioned.
Spines. The leaves of many cacti, barberries, and
other plants are modified as spines (see figure 38.28f ).
In the case of cacti, the reduction of leaf surface reduces
water loss and also may deter predators. Spines should
not be confused with thorns, such as those on honey lo-
cust (Gleditsia triacanthos), which are modified stems, or
with the prickles on raspberries and rose bushes, which
are simply outgrowths from the epidermis or the cortex
just beneath it.
Reproductive leaves. Several plants, notably Kalan-
cho?, produce tiny but complete plantlets along their
margins. Each plantlet, when separated from the leaf, is
capable of growing independently into a full-sized plant.
The walking fern (Asplenium rhizophyllum) produces new
plantlets at the tips of its fronds. While leaf tissue iso-
lated from many species will regenerate a whole plant,
this in vivo regeneration is unique among just a few
species.
Window leaves. Several genera of plants growing in
arid regions produce succulent, cone-shaped leaves with
transparent tips. The leaves often become mostly buried
in sand blown by the wind, but the transparent tips,
which have a thick epidermis and cuticle, admit light to
the hollow interiors. This allows photosynthesis to take
place beneath the surface of the ground.
Shade leaves. Leaves produced where they receive
significant amounts of shade tend to be larger in surface
area, but thinner and with less mesophyll than leaves on
the same tree receiving more direct light. This plasticity
in development is remarkable, as both types of leaves on
the plant have exactly the same genes. Environmental
signals can have a major effect on development.
Insectivorous leaves. Almost 200 species of flowering
plants are known to have leaves that trap insects, with
some digesting their soft parts. Plants with insectivorous
leaves often grow in acid swamps deficient in needed el-
ements, or containing elements in forms not readily
available to the plants; this inhibits the plants’ capacities
to maintain metabolic processes sufficient to meet their
growth requirements. Their needs are, however, met by
the supplementary absorption of nutrients from the ani-
mal kingdom.
Pitcher plants (for example, Sarracenia, Darlingtonia,
Nepenthes) have cone-shaped leaves in which rainwater
can accumulate. The insides of the leaves are very
smooth, but there are stiff, downward-pointing hairs at
the rim. An insect falling into such a leaf finds it very
difficult to escape and eventually drowns. The nutrients
released when bacteria, and in most species digestive en-
zymes, decompose the insect bodies are absorbed into
the leaf. Other plants, such as sundews (Drosera), have
glands that secrete sticky mucilage that trap insects,
which are then digested by enzymes. The Venus flytrap
(Dionaea muscipula) produces leaves that look hinged at
the midrib. When tiny trigger hairs on the leaf blade are
stimulated by a moving insect, the two halves of the leaf
snap shut, and digestive enzymes break down the soft
parts of the trapped insect into nutrients that can be ab-
sorbed through the leaf surface. Nitrogen is the most
common nutrient needed. Curiously, the Venus flytrap
will not survive in a nitrogen-rich environment, perhaps
a trade-off made in the intricate evolutionary process
that resulted in its ability to capture and digest insects.
The leaves of plants exhibit a variety of adaptations,
including spines, vegetative reproduction, and even
leaves that are carnivorous.
Chapter 38 The Plant Body 775
FIGURE 38.35
Modified leaves. In this dogwood “flower,” the white-colored
bracts (modified leaves) surround the several true flowers without
petals in the center.
776 Part X Plant Form and Function
Chapter 38
Summary Questions Media Resources
38.1 Meristems elaborate the plant body plan after germination.
? A plant body is basically an axis that includes two
parts: root and shoot—with associated leaves. There
are four basic types of tissues in plants: meristems,
ground tissue, epidermis, and vascular tissue.
1. What are the three major
tissue systems in plants? What
are their functions?
? Ground tissue supports the plant and stores food and
water.
? Epidermis forms an outer protective covering for the
plant.
? Vascular tissue conducts water, carbohydrates, and
dissolved minerals to different parts of the plant.
Xylem conducts water and minerals from the roots to
shoots and leaves, and phloem conducts food
molecules from sources to all parts of the plant.
2. What is the function of
xylem? How do primary and
secondary xylem differ in origin?
What are the two types of
conducting cells within xylem?
3. What is the function of
phloem? How do the two types
of conducting cells in phloem
differ?
38.2 Plants have three basic tissues, each composed of several cell types.
? Roots have four growth zones: the root cap, zone of
cell division, zone of elongation, and zone of
maturation.
? Some plants have modified roots, adapted for
photosynthesis, food or water storage, structural
support, or parasitism.
4. Compare monocot and dicot
roots. How does the
arrangement of the tissues
differ?
5. How are lateral branches of
roots formed?
38.3 Root cells differentiate as they become distanced from the dividing root apical meristem.
? Plants branch by means of buds derived from the
primary apical meristem. They are found in the
junction between the leaf and the stem.
? The vascular cambium is a cylinder of dividing cells
found in both roots and shoots. As a result of their
activity, the girth of a plant increases.
6. What types of cells are
produced when the vascular
cambium divides outwardly,
inwardly, or laterally?
7. Why don’t monocots have
secondary growth?
38.4 Stems are the backbone of the shoot, transporting nutrients and supporting the aerial plant organs.
? Leaves emerge as bulges on the meristem in a variety
of patterns, but most form a spiral around the stem.
The bulge lengthens and loses its radial symmetry as
it flattens.
? Photosynthesis occurs in the ground tissue system
which is called mesophyll in the leaf. Vascular tissue
forms the venation patterns in the leaves, serving as
the endpoint for water conduction and often the
starting point for the transport of photosynthetically
produced sugars.
8. How do simple and
compound leaves differ from
each other? Name and describe
the three common types of leaf
growth patterns.
38.5 Leaves are adapted to support basic plant functions.
www.mhhe.com/raven6e www.biocourse.com
? Art Activity: Plant
Body Organization
? Art Activity: Stem Tip
Structure
? Art Activity: Primary
Meristem Structure
? Characteristics of
Plants
? Meristems
? Cambia
? Art Activity: Dicot
Root Structure
? Roots
? Effect of Water on
Leaves
? Girth Increase in
Woody Dicots
? Vascular System of
Plants
? Art Activity: Dicot
Stem Structure
? Art Activity:
Secondary Growth
? Art Activity:
Herbaceous Dicot
Stem Anatomy
? Activity: Cambium
? Stems
? Art Activity: Plant
Anatomy
? Art Activity: Leaf
Structure
? Leaves
? Activity: Vascular
Tissue
? Ground Tissue
? Dermal Tissue
? Vascular Tissue
? Student Research:
Leaf Structure
Wetness
777
39
Nutrition and Transport
in Plants
Concept Outline
39.1 Plants require a variety of nutrients in addition to
the direct products of photosynthesis.
Plant Nutrients. Plants require a few macronutrients in
large amounts and several micronutrients in trace amounts.
Soil. Plant growth is significantly influenced by the
nature of the soil.
39.2 Some plants have novel strategies for obtaining
nutrients.
Nutritional Adaptations. Venus flytraps and other
carnivorous plants lure and capture insects and then digest
them to obtain energy and nutrients. Some plants entice
bacteria to produce organic nitrogen for them. These
bacteria may be free-living or form a symbiotic relationship
with a host plant. About 90% of all vascular plants rely on
fungal associations to gather essential nutrients, especially
phosphorus.
39.3 Water and minerals move upward through the
xylem.
Overview of Water and Mineral Movement through
Plants. The bulk movement of water and dissolved
minerals is the result of movement between cells, across cell
membranes, and through tubes of xylem.
Water and Mineral Absorption. Water and minerals
enter the plant through the roots.
Water and Mineral Movement. A combination of the
properties of water, structure of xylem, and transpiration of
water through the leaves results in the passive movement of
water to incredible heights. Water leaves the plant through
openings in the leaves called stomata. Too much water is
harmful to a plant, although many plants have adaptations
that make them tolerant of flooding.
39.4 Dissolved sugars and hormones are transported
in the phloem.
Phloem Transport Is Bidirectional. Sucrose and
hormones can move from shoot to root or root to shoot in
the phloem. Phloem transport requires energy to load and
unload sieve tubes.
V
ast energy inputs are required for the ongoing con-
struction of a plant such as described in chapter 38. In
this chapter, we address two major questions: (1) what in-
puts, besides energy from the sun, does a plant need to sur-
vive? and (2) how do all parts of the complex plant body
share the essentials of life? Plants, like animals, need various
nutrients to remain alive and healthy. Lack of an important
nutrient may slow a plant’s growth or make the plant more
susceptible to disease or even death. Plants acquire these
nutrients through photosynthesis and from the soil,
although some take a more direct approach (figure 39.1).
Carbohydrates produced in leaves must be carried through-
out the plant, and minerals and water absorbed from the
ground must be transported up to the leaves and other parts
of the plant. As discussed in chapter 38, these two types of
transport take place in specialized tissues, xylem and
phloem.
FIGURE 39.1
A carnivorous plant. Most plants absorb water and essential
nutrients from the soil, but carnivorous plants are able to obtain
some nutrients directly from small animals.
(essential for amino acids), potassium, calcium, phospho-
rus, magnesium (the center of the chlorophyll molecule),
and sulfur. Each of these nutrients approaches or, as in the
case with carbon, may greatly exceed 1% of the dry weight
of a healthy plant. The seven micronutrient elements—
iron, chlorine, copper, manganese, zinc, molybdenum, and
boron—constitute from less than one to several hundred
parts per million in most plants (figure 39.2). The
macronutrients were generally discovered in the last cen-
tury, but the micronutrients have been detected much
more recently as technology developed to identify and
work with such small quantities.
Nutritional requirements are assessed in hydroponic
cultures; the plants roots are suspended in aerated water
containing nutrients. The solutions contain all the neces-
sary nutrients in the right proportions but with certain
known or suspected nutrients left out. The plants are then
778 Part X Plant Form and Function
Plant Nutrients
The major source of plant nutrition is the fixation of at-
mospheric CO
2
into simple sugar using the energy of the
sun. CO
2
enters through the stomata. O
2
is a product of
photosynthesis and atmospheric component that also
moves through the stomata. It is used in cellular respira-
tion to release energy from the chemical bonds in the
sugar to support growth and maintenance in the plant.
However, CO
2
and light energy are not sufficient for the
synthesis of all the molecules a plant needs. Plants require
a number of inorganic nutrients (table 39.1). Some of
these are macronutrients, which the plants need in rela-
tively large amounts, and others are micronutrients, which
are required in trace amounts. There are nine macronutri-
ents: carbon, hydrogen, and oxygen—the three elements
found in all organic compounds—as well as nitrogen
39.1 Plants require a variety of nutrients in addition to the direct products
of photosynthesis.
Table 39.1 Essential Nutrients in Plants
Principal Form Approximate
in which Element Percent of
Elements Is Absorbed Dry Weight Examples of Important Functions
MACRONUTRIENTS
Carbon (CO
2
) 44 Major component of organic molecules
Oxygen (O
2
, H
2
O) 44 Major component of organic molecules
Hydrogen (H
2
O) 6 Major component of organic molecules
Nitrogen (NO
3
–
, NH
4
+
) 1–4 Component of amino acids, proteins, nucleotides, nucleic acids,
chlorophyll, coenzymes, enzymes
Potassium (K
+
) 0.5–6 Protein synthesis, operation of stomata
Calcium (Ca
++
) 0.2–3.5 Component of cell walls, maintenance of membrane structure and
permeability, activates some enzymes
Magnesium (Mg
++
) 0.1–0.8 Component of chlorophyll molecule, activates many enzymes
Phosphorus (H
2
PO
4
–
, HPO
4
=
) 0.1–0.8 Component of ADP and ATP, nucleic acids, phospholipids, several
coenzymes
Sulfur (SO
4
=
) 0.05–1 Components of some amino acids and proteins, coenzyme A
MICRONUTRIENTS (CONCENTRATIONS IN PPM)
Chlorine (Cl
–
) 100–10,000 Osmosis and ionic balance
Iron (Fe
++
, Fe
+++
) 25–300 Chlorophyll synthesis, cytochromes, nitrogenase
Manganese (Mn
++
) 15–800 Activator of certain enzymes
Zinc (Zn
++
) 15–100 Activator of many enzymes, active in formation of chlorophyll
Boron (BO
3
–
or B
4
O
7
=
) 5–75 Possibly involved in carbohydrate transport, nucleic acid synthesis
Copper (Cu
++
) 4–30 Activator or component of certain enzymes
Molybdenum (MoO
4
=
) 0.1–5 Nitrogen fixation, nitrate reduction
allowed to grow and are studied for the presence of abnor-
mal symptoms that might indicate a need for the missing
element (figure 39.3). However, the water or vessels used
often contain enough micronutrients to allow the plants to
grow normally, even though these substances were not
added deliberately to the solutions. To give an idea of how
small the quantities of micronutrients may be, the standard
dose of molybdenum added to seriously deficient soils in
Australia amounts to about 34 grams (about one handful)
per hectare, once every 10 years! Most plants grow satis-
factorily in hydroponic culture, and the method, although
expensive, is occasionally practical for commercial pur-
poses. Analytical chemistry has made it much easier to take
plant material and test for levels of different molecules.
One application has been the investigation of elevated lev-
els of CO
2
(a result of global warming) on plant growth.
With increasing levels of CO
2
, the leaves of some plants
increase in size, but the amount of nitrogen decreases rela-
tive to carbon. This decreases the nutritional value of the
leaves to herbivores.
The plant macronutrients carbon, oxygen, and
hydrogen constitute about 94% of a plant’s dry weight;
the other macronutrients—nitrogen, potassium,
calcium, phosphorus, magnesium, and sulfur—each
approach or exceed 1% of a plant’s dry weight.
Chapter 39 Nutrition and Transport in Plants 779
(a) (b)
(c) (d)
FIGURE 39.2
Mineral deficiencies in
plants. (a) Leaves of a
healthy Marglobe tomato
(Lycopersicon esculentum)
plant. (b) Chlorine-
deficient plant with
necrotic leaves (leaves
with patches of dead
tissue). (c) Copper-
deficient plant with blue-
green, curled leaves.
(d) Zinc-deficient plant
with small, necrotic
leaves. (f) Manganese-
deficient plant with
chlorosis (yellowing)
between the veins. The
agricultural implications
of deficiencies such as
these are obvious; a
trained observer can
determine the nutrient
deficiencies that are
affecting a plant simply
by inspecting it.
Complete
nutrient
solution
Solution lacking
one suspected
essential nutrient
Suspected
nutrient is
essential
Abnormal
growth
Normal
growth
Suspected
nutrient is
not essential
T
r
a
ns
pla
n
t
Monitor
growth
FIGURE 39.3
Identifying nutritional requirements of plants. A seedling is
first grown in a complete nutrient solution. The seedling is then
transplanted to solution that lacks one suspected essential nutrient.
The growth of the seedling is then studied for the presence of
abnormal symptoms, such as discolored leaves and stunted growth.
If the seedling’s growth is normal, the nutrient that was left out
may not be essential; if the seedling’s growth is abnormal, the
lacking nutrient is essential for growth.
Soil
Plant growth is affected by soil composition. Soil is the
highly weathered outer layer of the earth’s crust. It is com-
posed of a mixture of ingredients, which may include sand,
rocks of various sizes, clay, silt, humus, and various other
forms of mineral and organic matter; pore spaces containing
water and air occur between the particles. The mineral frac-
tion of soils varies according to the composition of the rocks.
The crust includes about 92 naturally occurring elements;
table 2.1 in chapter 2 lists the most common of these ele-
ments and their percentage of the earth’s crust by weight.
Most elements are combined as inorganic compounds called
minerals; most rocks consist of several different minerals.
The soil is also full of microorganisms that break down and
recycle organic debris. About 5 metric tons of carbon is tied
up in the organisms that are present in the soil under a
hectare (0.06 mile
2
) of wheat land in England, an amount
that approximately equals the weight of 100 sheep!
Most roots are found in topsoil (figure 39.4), which is a
mixture of mineral particles of varying size (most less than
2 mm thick), living organisms, and humus. Humus consists
of partly decayed organic material. When topsoil is lost
because of erosion or poor landscaping, both the water-
holding capacity and the nutrient relationships of the soil
are adversely affected.
About half of the total soil volume is occupied by spaces
or pores, which may be filled with air or water, depending
on moisture conditions. Some of the soil water, because of
its properties described below, is unavailable to plants. Due
to gravity, some of the water that reaches a given soil will
drain through it immediately. Another fraction of the water
is held in small soil pores, which are generally less than
about 50 micrometers in diameter. This water is readily
available to plants. When it is depleted through evapora-
tion or root uptake, the plant will wilt and eventually die
unless more water is added to the soil.
Cultivation
In natural communities, nutrients are recycled and made
available to organisms on a continuous basis. When these
communities are replaced by cultivated crops, the situation
changes drastically: the soil is much more exposed to ero-
sion and the loss of nutrients. For this reason, cultivated
crops and garden plants usually must be supplied with addi-
tional mineral nutrients.
One solution to this is crop rotation. For example, a
farmer might grow corn in a field one year and soybeans the
next year. Both crops remove nutrients from the soil, but the
plants have different nutritional requirements, and therefore
the soil does not lose the same nutrients two years in a row.
Soybean plants even add nitrogen compounds to the soil, re-
leased by nitrogen-fixing bacteria growing in nodules on their
roots. Sometimes farmers allow a field to lie fallow—that is,
they do not grow a crop in the field for a year or two. This al-
lows natural processes to rebuild the field’s store of nutrients.
Other farming practices that help maintain soil fertility
involve plowing under plant material left in fields. You can
do the same thing in a lawn or garden by leaving grass clip-
pings and dead leaves. Decomposers in the soil do the rest,
turning the plant material into humus.
Fertilizers are also used to replace nutrients lost in culti-
vated fields. The most important mineral nutrients that need
to be added to soils are nitrogen (N), phosphorus (P), and
potassium (K). All of these elements are needed in large
quantities (see table 39.1) and are the most likely to become
deficient in the soil. Both chemical and organic fertilizers are
often added in large quantities and can be significant sources
of pollution in certain situations (see chapter 30). Organic
fertilizers were widely used long before chemical fertilizers
were available. Substances such as manure or the remains of
dead animals have traditionally been applied to crops, and
plants are often plowed under to increase the soil’s fertility.
There is no basis for believing that organic fertilizers supply
any element to plants that inorganic fertilizers cannot pro-
vide and they can. However, organic fertilizers build up the
humus content of the soil, which often enhances its water-
and nutrient-retaining properties. For this reason, nutrient
availability to plants at different times of the year may be im-
proved, under certain circumstances, with organic fertilizers.
Soils contain organic matter and various minerals and
nutrients. Farming practices like crop rotation, plowing
crops under, and fertilization are often necessary to
maintain soil fertility.
780 Part X Plant Form and Function
A Topsoil
B Subsoil
C Weathering
bedrock
FIGURE 39.4
Most roots occur in
topsoil. The uppermost
layer in soil is called
topsoil, and it contains
organic matter, such as
roots, small animals, and
humus, and mineral
particles of various sizes.
Subsoil lies underneath
the topsoil and contains
larger mineral particles
and relatively little
organic matter. Beneath
the subsoil are layers of
bedrock, the raw
material from which soil
is formed over time and
weathering.
Nutritional
Adaptations
Carnivorous Plants
Some plants are able to obtain nitro-
gen directly from other organisms,
just as animals do. These carnivorous
plants often grow in acidic soils, such
as bogs that lack organic nitrogen. By
capturing and digesting small animals
directly, such plants obtain adequate
nitrogen supplies and thus are able to
grow in these seemingly unfavorable
environments. Carnivorous plants
have modified leaves adapted to lure
and trap insects and other small ani-
mals (figure 39.5). The plants digest
their prey with enzymes secreted from
various types of glands.
The Venus flytrap (Dionaea mus-
cipula), which grows in the bogs of
coastal North and South Carolina,
has three sensitive hairs on each side
of each leaf, which, when touched,
trigger the two halves of the leaf to
snap together (see figure 39.1). Once
the Venus flytrap enfolds a prey item
within a leaf, enzymes secreted from
the leaf surfaces digest the prey.
These flytraps actually shut and open
by a growth mechanism. They have a
limited number of times they can
open and close as a result. In the sun-
dews, the glandular trichomes secrete
both sticky mucilage, which traps
small animals, and digestive enzymes.
Unlike Venus flytraps they do not
close rapidly and it is possible that
the two share a common ancestor.
Pitcher plants attract insects by
the bright, flowerlike colors within
their pitcher-shaped leaves and per-
haps also by sugar-rich secretions.
Once inside the pitchers, insects slide
down into the cavity of the leaf,
which is filled with water and diges-
tive enzymes.
Bladderworts, Utricularia, are
aquatic. They sweep small animals
into their bladderlike leaves by the
rapid action of a springlike trapdoor,
and then they digest these animals.
Nitrogen-Fixing Bacteria
Plants need ammonia (NH
3
) to build
amino acids, but most of the nitrogen
is in the atmosphere in the form of
N
2
. Plants lack the biochemical
pathways (including the enzyme ni-
trogenase) necessary to convert
gaseous nitrogen to ammonia, but
some bacteria have this capacity.
Some of these bacteria live in close
association with the roots of plants.
Others go through an intricate dance
and end up being housed in plant tis-
sues created especially for them called
nodules (figure 39.6). Only legumes
are capable of forming root nodules
and there is a very specific recogni-
tion required by a bacteria species
and its host. Hosting these bacteria
costs the plant in terms of energy, but
is well worth it when there is little
ammonia in the soil. An energy con-
servation mechanism has evolved in
the legumes so that the root hairs will
not respond to bacterial signals when
nitrogen levels are high.
Mycorrhizae
While symbiotic relationships with
nitrogen-fixing bacteria are rare, sym-
biotic associations with mycorrhizal
fungi are found in about 90% of the
vascular plants. These fungi have
been described in detail in chapter 36.
In terms of plant nutrition, it is im-
portant to recognize the significant
role these organisms play in enhanc-
ing phosphorus transfer to the plant.
The uptake of some of the micronu-
trients is also enhanced. Functionally,
the mycorrhizae extend the surface
area of nutrient uptake substantially
Carnivorous plants obtain
nutrients, especially nitrogen,
directly by capturing and
digesting insects and other
organisms. Nitrogen can also be
obtained from bacteria living in
close association with the roots.
Fungi help plants obtain
phosphorus and other nutrients
from the soil.
Chapter 39 Nutrition and Transport in Plants 781
39.2 Some plants have novel strategies for obtaining nutrients.
FIGURE 39.5
A carnivorous plant. A tropical Asian
pitcher plant, Nepenthes. Insects enter the
pitchers and are trapped and digested.
Complex communities of invertebrate
animals and protists inhabit the pitchers.
FIGURE 39.6
Nitrogen-fixing nodule. A root hair of
alfalfa is invaded by Rhizobium, a bacterium
(yellow structures) that fixes nitrogen.
Through a series of exchanges of chemical
signals, the plant cells divide to create a
nodule for the bacteria which differentiate
and begin producing ammonia.
Overview of Water and Mineral
Movement through Plants
Local Changes Result in the Long-Distance,
Upward Movement of Water
Most of the nutrients and water discussed above enter the
plant through the roots and move upward in the xylem. It
is not unusual for a large tree to have leaves more than 10
stories off the ground (figure 39.7). Did you ever wonder
how water gets from the roots to the top of a tree that
high? Water moves through the spaces between the proto-
plasts of cells, through plasmodesmata (membrane connec-
tions between cells), through cell membranes and through
the continuous tubing system in the xylem. We know that
there are interconnected, water-conducting xylem elements
extending throughout a plant. We also know that water
first enters the roots and then moves to the xylem. After
that, however, water rises through the xylem because of a
combination of factors and some exits through the stomata
in the leaves.
While most of our focus will be on the mechanics of
water transport through xylem, the movement of water at
the cellular level plays a significant role in bulk water
transport in the plant as well, although over much shorter
distances. You know that the Casparian strip in the root
forces water to move through cells. In the case of
parenchyma cells it turns out that most water also moves
across membranes rather than in the intercellular spaces.
For a long time, it was believed that water moved across
cell membranes only by osmosis through the lipid bilayer.
We now know that osmosis is enhanced by water channels
called aquaporins. These transport channels are found in
both plants and animals. In plants they exist in vacuole and
plasma membranes. There at least 30 different genes cod-
ing for aquaporin-like proteins in Arabidopsis. Some aqua-
porins only appear or open during drought stress. Aqua-
porins allow for faster water movement between cells than
osmosis. They are important not only in maintaining
water balance within a cell, but in getting water between
many plant cells and the xylem. The greatest distances
traveled by water molecules and dissolved minerals are in
the xylem.
Once water enters the xylem, it can move upward
100 m in the redwoods. Some “pushing” from the pres-
sure of water entering the roots is involved. However,
most of the force is “pulling” caused by water evaporating
(transpiration) through the stomata on the leaves and
other plant surfaces. This works because water molecules
stick to themselves with hydrogen bonds (cohesion) and
to the walls of the tracheid or xylem vessel (adhesion).
The result is an unusually stable column of liquid reach-
ing great heights.
782 Part X Plant Form and Function
39.3 Water and minerals move upward through the xylem.
FIGURE 39.7
How does water get to the top of this tree? We would expect
gravity to make such a tall column of water too heavy to be
maintained by capillary action. What pulls the water up?
Water Potential
Plant biologists often discuss the forces that act on water
within a plant in terms of potentials. The turgor pressure,
which is a physical pressure that results as water enters
the cell vacuoles, is referred to as pressure potential.
Water coming through a garden hose is an example of
physical pressure. There is also a potential caused by an
uneven distribution of a solute on either side of a mem-
brane, which will result in osmosis (movement of water
to the side with the greater concentration of solute). By
applying pressure (on the side that has the greater con-
centration of solute), it is possible to prevent osmosis
from taking place. The smallest amount of pressure
needed to stop osmosis is referred to as the solute (or
osmotic) potential of the solution. Water will enter a
cell osmotically until it is stopped by the pressure poten-
tial caused by the cell wall. The water potential of a
plant cell is, in essence, the combination of its pressure
potential and solute potential; it represents the total po-
tential energy of the water in a plant. If two adjacent cells
have different water potentials, water will move from the
cell with the higher water potential to the cell with the
lower water potential. Water in a plant moves along a
gradient between the relatively high water potential in
the soil to successively lower water potentials in the
roots, stems, leaves, and atmosphere.
Water potential in a plant regulates movement of
water. At the roots there is a positive water potential (ex-
cept in the case of severe drought). On the surface of
leaves and other organs, water loss called transpiration
creates a negative pressure. It depends on its osmotic ab-
sorption by the roots and the negative pressures created
by water loss from the leaves and other plant surfaces (fig-
ure 39.8). The negative pressure generated by transpira-
tion is largely responsible for the upward movement of
water in xylem.
Aquaporins enhance water transport at the cellular
level, which ultimately affects bulk water transport.
The loss of water from the leaf surface, called
transpiration, literally pulls water up the stem from the
roots which have the greater water potential. This
works because of the strong cohesive forces between
molecules of water that allow them to stay “stuck”
together in a liquid column and adhesion to walls of
tracheids and vessels.
Chapter 39 Nutrition and Transport in Plants 783
H
2
O
H
2
O
H
2
O
H
2
O
H
2
O
and
minerals
H
2
O
H
2
O
and
minerals
and
minerals
H
2
O
Carbohydrates
Carbohydrates
Phloem
Xylem
Xylem
Spongy
mesophyll
layer
Stoma
Water exits the plant
through stomata in leaves.
The water potential
of air is low.
Water enters the
plant through the roots.
The water potential of
soil is high.
Water and minerals
pass up through
xylem along a
gradient of
successively lower
water potentials.
Water and carbohydrates
travel to all parts of the plant.
FIGURE 39.8
Water movement through a plant. This diagram illustrates the path of water and inorganic materials as they move into, through, and
out of the plant body.
Water and Mineral Absorption
Most of the water absorbed by the plant comes in through
root hairs, which collectively have an enormous surface
area (figure 39.8). Root hairs are almost always turgid be-
cause their solute potential is greater than that of the sur-
rounding soil due to mineral ions being actively pumped
into the cells. Because the mineral ion concentration in
the soil water is usually much lower than it is in the plant,
an expenditure of energy (supplied by ATP) is required
for the accumulation of such ions in root cells. The
plasma membranes of root hair cells contain a variety of
protein transport channels, through which proton pumps
(see page 120) transport specific ions against even large
concentration gradients. Once in the roots, the ions,
which are plant nutrients, are transported via the xylem
throughout the plant.
The ions may follow the cell walls and the spaces
between them or more often go directly through the
plasma membranes and the protoplasm of adjacent cells
(figure 39.9). When mineral ions pass between the cell
walls, they do so nonselectively. Eventually, on their jour-
ney inward, they reach the endodermis and any further
passage through the cell walls is blocked by the Casparian
strips. Water and ions must pass through the plasma mem-
branes and protoplasts of the endodermal cells to reach the
xylem. However, transport through the cells of the endo-
dermis is selective. The endodermis, with its unique struc-
ture, along with the cortex and epidermis, controls which
ions reach the xylem.
Transpiration from the leaves (figure 39.10), which cre-
ates a pull on the water columns, indirectly plays a role in
helping water, with its dissolved ions, enter the root cells.
However, at night, when the relative humidity may ap-
proach 100%, there may be no transpiration. Under these
circumstances, the negative pressure component of water
potential becomes small or nonexistent.
Active transport of ions into the roots still continues to
take place under these circumstances. This results in an in-
creasingly high ion concentration with the cells, which
causes more water to enter the root hair cells by osmosis.
In terms of water potential, we say that active transport in-
creases the solute potential of the roots. The result is
movement of water into the plant and up the xylem
columns despite the absence of transpiration. This phe-
nomenon is called root pressure, which in reality is an os-
motic phenomenon.
784 Part X Plant Form and Function
Xylem
Phloem
Pericycle
Vascular
cylinder
Endodermis
Cortex
Endodermal
cells
Casparia
strip
Cell wall
Water
and
solutes
Root
hair
Casparian
strips
Water and
solutes
Water
and
solutes
FIGURE 39.9
The pathways of mineral transport in roots. Minerals are absorbed at the surface of the root, mainly by the root hairs. In passing
through the cortex, they must either follow the cell walls and the spaces between them or go directly through the plasma membranes and
the protoplasts of the cells, passing from one cell to the next by way of the plasmodesmata. When they reach the endodermis, however,
their further passage through the cell walls is blocked by the Casparian strips, and they must pass through the membrane and protoplast of
an endodermal cell before they can reach the xylem.
Under certain circumstances, root pressure is so strong
that water will ooze out of a cut plant stem for hours or
even days. When root pressure is very high, it can force
water up to the leaves, where it may be lost in a liquid
form through a process known as guttation (figure 39.11).
Guttation does not take place through the stomata, but in-
stead occurs through special groups of cells located near
the ends of small veins that function only in this process.
Root pressure is never sufficient to push water up great
distances.
Water enters the plant by osmosis. Transport of
minerals (ions) across the endodermis is selective. Root
pressure, which often occurs at night, is caused by the
continued, active accumulation of ions in the roots at
times when transpiration from the leaves is very low or
absent.
Chapter 39 Nutrition and Transport in Plants 785
Water exits
plant through
stomata
Water moves
up plant through
xylem
Water enters
plant through
roots
Upper epidermis
Palisade mesophyll
Vascular bundle
Spongy mesophyll
Intercellular space (100% humidity)
Epidermis
Stoma
Water molecule
FIGURE 39.10
Transpiration. Water evaporating from the leaves through the stomata causes the movement of water upward in the xylem and the
entrance of water through the roots.
FIGURE 39.11
Guttation. In herbaceous plants, water passes through specialized
groups of cells at the edges of the leaves; it is visible here as small
droplets around the edge of the leaf in this strawberry plant
(Fragaria ananassa).
Water and Mineral Movement
Water and Mineral Movement through the Xylem
It is clear that root pressure is insufficient to push water
to the top of a tall tree, although it can help. So, what
does work? Otto Renner proposed the solution in Ger-
many in 1911. Passage of air across leaf surfaces results in
loss of water by evaporation, creating a pull at the open
upper end of the “tube.” Evaporation from the leaves pro-
duces a tension on the entire water column that extends
all the way down to the roots. Water has an inherent ten-
sile strength that arises from the cohesion of its mole-
cules, their tendency to form hydrogen bonds with one
another. The tensile strength of a column of water varies
inversely with the diameter of the column; that is, the
smaller the diameter of the column, the greater the tensile
strength. Because plants have transporting vessels of very
narrow diameter, the cohesive forces in them are strong.
The water molecules also adhere to the sides of the tra-
cheid or xylem vessels, further stabilizing the long column
of water.
The water column would fail if air bubbles were in-
serted (visualize a tower of blocks and then pull one out in
the middle). Anatomical adaptations decrease the proba-
bility of this. Individual tracheids and vessel members are
connected by one of more pits (cavities) in their walls. Air
bubbles are generally larger than the openings, so they
cannot pass through them. Furthermore, the cohesive
force of water is so great that the bubbles are forced into
rigid spheres that have no plasticity and therefore cannot
squeeze through the openings. Deformed cells or freezing
can cause small bubbles of air to form within xylem cells.
Any bubbles that do form are limited to the xylem ele-
ments where they originate, and water may continue to
rise in parallel columns. This is more likely to occur with
seasonal temperature changes. As a result, most of the ac-
tive xylem in woody plants occurs peripherally, toward the
vascular cambium.
Most minerals the plant needs enter the root through
active transport. Ultimately, they are removed from the
roots and relocated through the xylem to other metaboli-
cally active parts of the plant. Phosphorus, potassium, ni-
trogen, and sometimes iron may be abundant in the
xylem during certain seasons. In many plants, such a pat-
tern of ionic concentration helps to conserve these essen-
tial nutrients, which may move from mature deciduous
parts such as leaves and twigs to areas of active growth.
Keep in mind that minerals that are relocated via the
xylem must move with the generally upward flow
through the xylem. Not all minerals can re-enter the
xylem conduit. Calcium, an essential nutrient, cannot be
transported elsewhere once it has been deposited in plant
parts.
Transpiration of Water from Leaves
More than 90% of the water taken in by the roots of a
plant is ultimately lost to the atmosphere through transpi-
ration from the leaves. Water moves into the pockets of air
in the leaf from the moist surfaces of the walls of the meso-
phyll cells. As you saw in chapter 38, these intercellular
spaces are in contact with the air outside of the leaf by way
of the stomata. Water that evaporates from the surfaces of
the mesophyll cells leaves the stomata as vapor. This water
is continuously replenished from the tips of the veinlets in
the leaves.
Water is essential for plant metabolism, but is continu-
ously being lost to the atmosphere through the stomata.
Photosynthesis requires a supply of CO
2
entering the
stomata from the atmosphere. This results in two some-
what conflicting requirements: the need to minimize the
loss of water to the atmosphere and the need to admit car-
bon dioxide. Structural features such as stomata and the cu-
ticle have evolved in response to one or both of these re-
quirements.
The rate of transpiration depends on weather condi-
tions like humidity and the time of day. After the sun sets,
transpiration from the leaves decreases. The sun is the ul-
timate source of potential energy for water movement.
The water potential that is responsible for water move-
ment is largely the product of negative pressure generated
by transpiration, which is driven by the warming effects of
sunlight.
The Regulation of Transpiration Rate. On a short-
term basis, closing the stomata can control water loss.
This occurs in many plants when they are subjected to
water stress. However, the stomata must be open at least
part of the time so that CO
2
can enter. As CO
2
enters the
intercellular spaces, it dissolves in water before entering
the plant’s cells. The gas dissolves mainly in water on the
walls of the intercellular spaces below the stomata. The
continuous stream of water that reaches the leaves from
the roots keeps these walls moist. A plant must respond
both to the need to conserve water and to the need to
admit CO
2.
Stomata open and close because of changes in the turgor
pressure of their guard cells. The sausage- or dumbbell-
shaped guard cells stand out from other epidermal cells not
only because of their shape, but also because they are the
only epidermal cells containing chloroplasts. Their distinc-
tive wall construction, which is thicker on the inside and
thinner elsewhere, results in a bulging out and bowing
when they become turgid. You can make a model of this
for yourself by taking two elongated balloons, tying the
closed ends together, and inflating both balloons slightly.
When you hold the two open ends together, there should
be very little space between the two balloons. Now place
786 Part X Plant Form and Function
duct tape on the inside edge of both balloons and inflate
each one a bit more. Hold the open ends together. You
should now be holding a doughnut-shaped pair of “guard
cells” with a “stoma” in the middle. Real guard cells rely on
the influx and efflux of water, rather than air, to open and
shut.
Loss of turgor in guard cells causes the uptake of potas-
sium (K
+
) ions through ATP-powered ion transport chan-
nels in their plasma membranes. This creates a solute po-
tential within the guard cells that causes water to enter
osmotically. As a result, these cells accumulate water and
become turgid, opening the stomata (figure 39.12a). Keep-
ing the stomata open requires a constant expenditure of
ATP, and the guard cells remain turgid only as long as ions
are pumped into the cells. When stomata close, sucrose,
rather than K
+
, leaves the cell through sucrose transporters.
Water then leaves the guard cells, which lose turgor, and
the stomata close (figure 39.12b). Why closing depends on
sucrose transport out of the cell and opening on K
+
uptake
is an open question. Experimental evidence is consistent
with several pathways regulating stomatal opening and
closing.
Photosynthesis in the guard cells apparently provides an
immediate source of ATP, which drives the active transport
of K
+
by way of a specific K
+
channel; this K
+
channel has
now been isolated and studied. In some species, Cl
–
accom-
panies the K
+
in and out of the guard cells, thus maintain-
ing electrical neutrality. In most species, both Cl
–
and
malate
2-
move in the opposite direction of K
+
.
When a whole plant wilts because there is insufficient
water available, the guard cells may also lose turgor, and as
a result, the stomata may close. The guard cells of many
plant species regularly become turgid in the morning, when
photosynthesis occurs, and lose turgor in the evening, re-
gardless of the availability of water. When they are turgid,
the stomata open, and CO
2
enters freely; when they are
flaccid, CO
2
is largely excluded, but water loss is also re-
tarded.
Abscisic acid, a plant hormone discussed in chapter 41,
plays a primary role in allowing K
+
to pass rapidly out of
guard cells, causing the stomata to close in response to
drought. This hormone is released from chloroplasts and
produced in leaves. It binds to specific receptor sites in the
plasma membranes of guard cells. Plants likely control the
duration of stomatal opening through the integration of
several stimuli, including blue light. In the next chapter, we
will explore the interactions between the environment and
the plant in more detail.
Chapter 39 Nutrition and Transport in Plants 787
Chloroplasts Epidermal cell NucleusGuard cell
Thickened inner
cell wall (rigid)
Stoma open Stoma closed
H
2
O
H
2
O
H
2
OH
2
OH
2 2
O
H
2
O
H
2
O
H
2
O
H
2
O
H
2
O
H
2
O
Solute potential is high;
water moves into guard
cells
Solute potential is low;
water moves out of
guard cells
Chloroplasts
(a) (b)
FIGURE 39.12
How a stoma opens and closes. (a) When potassium ions from surrounding cells are pumped into guard cells, the guard cell turgor
pressure increases as water enters by osmosis. The increased turgor pressure causes the guard cells to bulge, with the thick walls on the
inner side of each guard cell bowing outward, thereby opening the stoma. (b) When the potassium ions leave the guard cells and their
solute potential becomes low, they lose water and turgor, and the stoma closes.
Other Factors Regulating Transpiration. Factors such
as CO
2
concentration, light, and temperature can also af-
fect stomatal opening. When CO
2
concentrations are high,
guard cells of many plant species lose turgor, and their
stomata close. Additional CO
2
is not needed at such times,
and water is conserved when the guard cells are closed. The
stomata also close when the temperature exceeds 30° to
34°C when transpiration would increase substantially. In
the dark, stomata will open at low concentrations of CO
2
.
In chapter 10, we mentioned CAM photosynthesis, which
occurs in some succulent like cacti. In this process, CO
2
is
taken in at night and fixed during the day. CAM photosyn-
thesis conserves water in dry environments where succulent
plants grow.
Many mechanisms to regulate the rate of water loss have
evolved in plants. One involves dormancy during dry times
of the year; another involves loss of leaves. Deciduous
plants are common in areas that periodically experience a
severe drought. Plants are often deciduous in regions with
severe winters, when water is locked up in ice and snow and
thus unavailable to them. In a general sense, annual plants
conserve water when conditions are unfavorable, simply by
going into dormancy as seeds.
Thick, hard leaves, often with relatively few stomata—
and frequently with stomata only on the lower side of the
leaf—lose water far more slowly than large, pliable leaves
with abundant stomata. Temperatures are significantly re-
duced in leaves covered with masses of woolly-looking tri-
chomes. These trichomes also increase humidity at the leaf
surface. Plants in arid or semiarid habitats often have their
stomata in crypts or pits in the leaf surface. Within these
depressions the water vapor content of the air may be high,
reducing the rate of water loss.
Plant Responses to Flooding
Plants can also receive too much water, and ultimately
“drown.” Flooding rapidly depletes available oxygen in the
soil and interferes with the transport of minerals and carbo-
hydrates in the roots. Abnormal growth often results. Hor-
mone levels change in flooded plants—ethylene (the only
hormone that is a gas) increases, while gibberellins and cy-
tokinins usually decrease. Hormonal changes contribute to
the abnormal growth patterns.
Oxygen-deprivation is among the most significant prob-
lems. Standing water has much less oxygen than moving
water. Generally, standing water flooding is more harmful
to a plant (riptides excluded). Flooding that occurs when a
plant is dormant is much less harmful than flooding when it
is growing actively.
Physical changes that occur in the roots as a result of
oxygen deprivation may halt the flow of water through the
plant. Paradoxically, even though the roots of a plant may
be standing in water, its leaves may be drying out. One
adaptive solution is that stomata of flooded plants often
close to maintain leaf turgor.
Adapting to Life in Fresh Water. Algal ancestors of
plants adapted to a freshwater environment from a salt-
water environment before the “move” onto land. This in-
volved a major change in controlling salt balance. Since
that time, many have “moved” back into fresh water and
grow in places that are often or always flooded naturally;
they have adapted to these conditions during the course of
their evolution (figure 39.13). One of the most frequent
adaptations among such plants is the formation of
aerenchyma, loose parenchymal tissue with large air
spaces in it (figure 39.14). Aerenchyma is very prominent in
water lilies and many other aquatic plants. Oxygen may be
transported from the parts of the plant above water to
those below by way of passages in the aerenchyma. This
supply of oxygen allows oxidative respiration to take place
even in the submerged portions of the plant.
Some plants normally form aerenchyma, whereas others,
subject to periodic flooding, can form it when necessary. In
corn, ethylene, which becomes abundant under the anaero-
bic conditions of flooding, induces aerenchyma formation.
Plants also respond to flooded conditions by forming larger
lenticels (which facilitate gas exchange) and additional ad-
ventitious roots.
788 Part X Plant Form and Function
FIGURE 39.13
Adaptation to flooded conditions. The “knees” of the bald
cypress (Taxodium) form whenever it grows in wet conditions,
increasing its ability to take in oxygen.
Adapting to Life in Salt Water. Plants such as man-
groves that are normally flooded with salt water must not
only provide a supply of oxygen for their submerged parts,
but also control their salt balance. The salt must be ex-
cluded, actively secreted, or diluted as it enters. The arch-
ing silt roots of mangroves are connected to long, spongy,
air-filled roots that emerge above the mud. These roots,
called pneumatophores (see chapter 38), have large
lenticels on their above-water portions through which oxy-
gen enters; it is then transported to the submerged roots
(figure 39.15). In addition, the succulent leaves of man-
groves contain large quantities of water, which dilute the
salt that reaches them. Many plants that grow in such con-
ditions also secrete large quantities of salt.
Transpiration from leaves pulls water and minerals up
the xylem. This works because of the physical
properties of water and the narrow diameters of the
conducting tubes. Stomata open when their guard cells
become turgid. Opening and closing of stomata is
osmotically regulated. Biochemical, anatomical, and
morphological adaptations have evolved to reduce water
loss through transpiration. Plants are harmed by excess
water. However, plants can survive flooded conditions,
and even thrive in them, if they can deliver oxygen to
their submerged parts.
Chapter 39 Nutrition and Transport in Plants 789
O
2,
CO
2
Gas exchange
Gas exchange
Stoma
Upper epidermis
of leafVein
Lower epidermis
of leaf
Air spaces Aerenchyma
(a) (b)
FIGURE 39.14
Aerenchyma tissue. Gas exchange in aquatic plants. (a) Water lilies float on the surface of ponds where oxygen is collected and
transported to submerged portions of the plant. (b) Large air spaces in the leaves add buoyancy. The specialized parenchyma tissue that
forms these open spaces is called aerenchyma. Gas exchange occurs through stomata found only on the upper surface of the leaf.
Stilt roots
Pneumatophores
Section of
pneumatophore
Lenticel
O
2
O
2
transported
to submerged
portions of plants
FIGURE 39.15
How mangroves get oxygen to their submerged part.
Mangrove plants grow in areas that are commonly flooded, and
much of each plant is usually submerged. However, modified
roots called pneumatophores supply the submerged portions of
the plant with oxygen because these roots emerge above the water
and have large lenticels. Oxygen can enter the roots through the
lenticels, pass into the abundant aerenchyma, and move to the rest
of the plant.
Phloem Transport Is Bidirectional
Most carbohydrates manufactured in leaves and other
green parts are distributed through the phloem to the rest
of the plant. This process, known as translocation, is re-
sponsible for the availability of suitable carbohydrate
building blocks in roots and other actively growing re-
gions of the plant. Carbohydrates concentrated in storage
organs such as tubers, often in the form of starch, are also
converted into transportable molecules, such as sucrose,
and moved through the phloem. The pathway that sugars
and other substances travel within the plant has been
demonstrated precisely by using radioactive tracers, de-
spite the fact that living phloem is delicate and the process
of transport within it is easily disturbed. Radioactive car-
bon dioxide (
14
CO
2
) gets incorporated into glucose as a
result of photosynthesis. The glucose is used to make su-
crose, which is transported in the phloem. Such studies
have shown that sucrose moves both up and down in the
phloem.
Aphids, a group of insects that extract plant sap for food,
have been valuable tools in understanding translocation.
Aphids thrust their stylets (piercing mouthparts) into
phloem cells of leaves and stems to obtain abundant sugars
there. When a feeding aphid is removed by cutting its
stylet, the liquid from the phloem continues to flow
through the detached mouthpart and is thus available in
pure form for analysis (figure 39.16). The liquid in the
phloem contains 10 to 25% dry matter, almost all of which
is sucrose. Using aphids to obtain the critical samples and
radioactive tracers to mark them, it has been demonstrated
that movement of substances in phloem can be remarkably
fast; rates of 50 to 100 centimeters per hour have been
measured.
While the primary focus of this chapter is on nutrient
and water transport, it is important to note that phloem
also transports plant hormones. As we will explore in the
next chapter, environmental signals can result in the rapid
translocation of hormones in the plant.
Energy Requirements for Phloem Transport
The most widely accepted model of how carbohydrates in
solution move through the phloem has been called the
mass-flow hypothesis, pressure flow hypothesis, or
bulk flow hypothesis. Experimental evidence supports
much of this model. Dissolved carbohydrates flow from a
source and are released at a sink where they are utilized.
Carbohydrate sources include photosynthetic tissues, such
as the mesophyll of leaves, and food-storage tissues, such as
the cortex of roots. Sinks occur primarily at the growing
tips of roots and stems and in developing fruits.
In a process known as phloem loading, carbohydrates
(mostly sucrose) enter the sieve tubes in the smallest
veinlets at the source. This is an energy-requiring step, as
active transport is needed. Companion cells and
parenchyma cells adjacent to the sieve tubes provide the
ATP energy to drive this transport. Then, because of the
790 Part X Plant Form and Function
39.4 Dissolved sugars and hormones are transported in the phloem.
FIGURE 39.16
Feeding on phloem. (a) Aphids, like this individual of
Macrosiphon rosae shown here on the edge of a rose leaf, feed on
the food-rich contents of the phloem, which they extract through
their piercing mouthparts (b), called stylets. When an aphid is
separated from its stylet and the cut stylet is left in the plant, the
phloem fluid oozes out of it and can then be collected and
analyzed.
(a)
(a)
difference between the water potentials in the sieve tubes
and in the nearby xylem cells, water flows into the sieve
tubes by osmosis. Turgor pressure in the sieve tubes in-
creases. The increased turgor pressure drives the fluid
throughout the plant’s system of sieve tubes. At the sink,
carbohydrates are actively removed. Water moves from
the sieve tubes by osmosis and the turgor pressure there
drops, causing a mass flow from the higher pressure at
the source to the lower pressure sink (figure 39.17). Most
of the water at the sink diffuses then back into the xylem,
where it may either be recirculated or lost through
transpiration.
Transport of sucrose and other carbohydrates through
sieve tubes does not require energy. The loading and
unloading of these substances from the sieve tubes
does.
Chapter 39 Nutrition and Transport in Plants 791
Xylem
Phloem
Mesophyll
Shoot
(sink)
Root
(sink)
Leaf
(source)
Sieve
tubes
Leaf cells Sucrose
Active transport of sucrose
out of sieve tube cells
into root and other growth
areas (sink)
Transport of water into
sieve tube cells by
osmosis
Active transport of sucrose
from leaf cells into sieve
tube cells (source)
KEY:
Transport of water
in xylem
Transport of sucrose
and water in phloem
KEY:
Water
FIGURE 39.17
Diagram of mass flow. In this diagram, red dots represent sucrose molecules, and blue dots symbolize water molecules. Moving from the
mesophyll cells of a leaf or another part of the plant into the conducting cells of the phloem, the sucrose molecules are then transported to
other parts of the plant by mass flow and unloaded where they are required.
792 Part X Plant Form and Function
Chapter 39
Summary Questions Media Resources
39.1 Plants require a variety of nutrients in addition to the direct products of photosynthesis.
? Plants require a few macronutrients in large amounts
and several micronutrients in trace amounts. Most of
these are obtained from the soil through the roots.
? Plant growth is significantly influenced by the nature
of the soil. Soils vary in terms of nutrient composi-
tion and water-holding capacity.
1. What is the difference
between a macronutrient and a
micronutrient? Explain how a
plant would utilize each of the
macronutrients.
? Some plants entice bacteria to produce organic
nitrogen for them. These bacteria may be free-living
or form a symbiotic relationship with a host plant.
? About 90% of all vascular plants rely on fungal
associations to gather essential nutrients.
2. The atmosphere is full of
nitrogen yet it is inaccessible to
most plants. Why is that? What
solution has evolved in legumes?
39.2 Some plants have novel strategies for obtaining nutrients.
? Water and minerals enter the plant through the
roots. Energy is required for active transport.
? The bulk movement of water and minerals is the re-
sult of movement between cells, across cell mem-
branes, and through tubes of xylem. Aquaporins are
water channels that enhance osmosis.
? A combination of the properties of water, structure of
xylem, and transpiration of water through the leaves
results in the passive movement of water to incredible
heights. The ultimate energy source for pulling water
through xylem vessels and tracheids is the sun.
? Water leaves the plant through openings in the leaves
called stomata. Stomata open when their guard cells
are turgid and bulge, causing the thickened inner
walls of these cells to bow away from the opening.
? Plants can tolerate long submersion in water, if they
can deliver oxygen to their submerged tissues.
3. What is pressure potential?
How does it differ from solute
potential? How do these
pressures cause water to rise in a
plant?
4. What proportion of water
that enters a plant leaves it via
transpiration?
5. Why are root hairs usually
turgid? Does the accumulation
of minerals within a plant root
require the expenditure of
energy? Why or why not?
6. Under what environmental
condition is water transport
through the xylem reduced to
near zero? How much
transpiration occurs under these
circumstances?
7. Does stomatal control require
energy? Explain.
39.3 Water and minerals move upward through the xylem.
? Sucrose and hormones can move up and down in the
phloem between sources and sinks.
? The movement of water containing dissolved sucrose
and other substances in the phloem requires energy.
Sucrose is loaded into the phloem near sites of syn-
thesis, or sources, using energy supplied by the com-
panion cells or other nearby parenchyma cells.
8. What is translocation? What
is the driving force behind
translocation?
9. Describe the movement of
carbohydrates through a plant,
beginning with the source and
ending with the sink. Is this
process active or passive?
39.4 Dissolved sugars and hormones are transported in the phloem.
www.mhhe.com/raven6e www.biocourse.com
? Nutrients
? Soil
? Activity: Water
Movement
? Uptake by Roots
? Water Movement
? Student Research:
Heavy Metal Uptake
793
The Control of Patterning in Plant
Root Development
Did you ever think of how a root grows? Down in the dark,
with gravity its only cue, the very tip of the root elongates,
periodically forming a node from which root branches will
extend. How does the root determine the position of its
branches, and the spacing between them? The serial organi-
zation of the root’s branches is controlled by events that
happen on a microscopic scale out at the very tip of the root,
the so-called root apex. There, within a space of a millime-
ter or less, molecular events occur that orchestrate how the
root will grow and what it will be like.
The problem of understanding how a plant’s root apex
controls the way a root develops is one example of a much
larger issue, perhaps the most challenging research problem
in modern botany: What mechanism mediates central pat-
tern formation in the plant kingdom? Almost nothing was
known of these mechanisms a decade ago, but intensive re-
search is now rapidly painting in the blank canvas.
Much of the most exciting research on plant pattern for-
mation is being performed on a small weedy relative of the
mustard plant, the wall cress Arabidopsis thaliana (see photo
above). With individual plants no taller than your thumb
that grow quickly in laboratory test-tubes, Arabidopsis is an
ideal model for studying plant development. Its genome,
about the size of the fruit fly Drosophila, has been completely
sequenced, greatly aiding research into the molecular events
underlying pattern formation.
To gain some insight into the sort of research being
done, we will focus on work being done by John Schiefel-
bein and colleagues at the University of Michigan. Schiefel-
bein has focused on one sharply defined aspect of plant root
pattern formation in Arabidopsis, the formation of root hairs
on the epidermis, the root’s outer layer of cells. These root
hairs constitute the principal absorbing surface of the root,
and their position is under tight central control.
In a nutshell, the problem of properly positioning root
hairs is one of balancing cell production and cell differentia-
tion. Cells in the growth zone beneath the surface of the
root—a sheath called a meristem—are constantly dividing.
The cells that are produced by the meristem go on to differ-
entiate into two kinds of cells: trichoblasts which form hair-
bearing epidermal cells, and atrichoblasts which form hair-
less epidermal cells. The positioning of trichoblasts among
atrichoblasts determines the pattern of root hairs on the de-
veloping root.
When researchers looked very carefully at the dividing
root meristem, they found that the initial cells determined
to be trichoblasts and atrichoblasts alternate with one an-
other in a ring of 16 cells around the circumference of the
root. As the cells divide, more and more cells are added,
forming columns of cells extending out in 16 files. As the
files extend farther and farther out, occasional side-ways di-
visions fill in the gaps that develop, forming new files.
Maintaining this simple architecture requires that the
root maintain a tight control of the plane and rate to cell di-
vision. Because this rate is different for the two cell types,
the root must also control the rate at which the cell types
differentiate. Schiefelbein set out to learn how the root apex
coordinates these two processes.
To get a handle on the process, Schiefelbein seized on a
recently characterized root pattern mutant called transparent
testa glabra(TTG). This mutant changes the pattern of root
hairs in Arabidopsis,and it has been proposed that it controls
whether a cell becomes a trichoblast or an atrichoblast. But
does it control the rate and orientation in the root meristem
epidermis?
To answer this question, Schiefelbein’s team used clonal
analysis to microscopically identify individual cell types in
the root epidermis, and set out to see if they indeed divide at
different rates, and if the TTG mutation affects these rates
differently. If so, there must be a link between cell differen-
tiation and the control of cell division in plants.
Part
XI
Plant Growth and
Reproduction
Arabidopsis thaliana. An important plant for studying root devel-
opment because it offers a simple pattern of cellular organization
in the root.
Real People Doing Real Science
The Experiment
Two developmental mutants of A. thaliana were compared
to investigate whether the control of cell differentiation and
the rate of cell division were linked. One, TTG, alters early
events in root epidermal cell differentiation, while the other,
glabra2 (gl2) acts later.
The investigators first set out to map the surface of the
roots of each mutant type, as well as those of nonmutant
wild type. To avoid confusion in studying files of cells, it is
necessary to clearly identify the starting point of each file of
cells. To do this, roots were selected that contained clones
of trichoblast and atrichoblast produced by longitudinal cell
divisions perpendicular to the surface of the root. Called
longitudinal anticlinal cell divisions, these clones are rare
but easily recognized when stained with propidium iodide.
Careful mapping of individual cells with a confocal micro-
scope allowed investigators to determine the number and
location of trichoblast and atrichoblast cells present in the
epidermal tissue of each clone.
The Results
The researchers made two important observations based on
their visual identification of individual trichoblast and atri-
choblast cells in the various plant types examined.
1. The two cell types are produced at different rates.Among plants
that had been cultured for up to six cell divisions, they ob-
served a significant difference in the ratio of trichoblast (T)
versus atrichoblast (A) cells following two or more cell divi-
sions. In their study you can readily see that the TTGmutant
produces a significantly lower ratio of T cells to A cells com-
pared to the wild-type plants or gl2 mutants (see graph a
above). This strongly suggests that TTG is involved in con-
trolling the rate of cell division in the T cell file.
2. TTG controls the rate of longitudinal cell division. The re-
search team went on to examine longitudinal cell divisions
that fill in the gaps as cell division causes files of cells to ex-
tend outward from the meristem. The researchers set out to
determine the probability of such longitudinal anticlinal cell
division occurring in the three types of plants shown in graph
a. The more rapidly cell files are produced, the more often
longitudinal divisions would be required to fill in gaps be-
tween files. For proper root hair position to be maintained,
the rate of this longitudinal division would have to be tightly
coordinated with the rate of vertical division within the file.
The investigators found that longitudinal cell division, al-
ways rare, was usually seen, when it did occur, in T cell files.
Did the TTG mutation affect this process as well as file-ex-
tending cell divisions? This was determined by examining
the ratio of the probability of longitudinal anticlinal divi-
sions in T cells versus A cells (pLT/pLA).
Researchers compared the ratio in wild-type plants with
that in the two mutants, TTG and gl2. Did the TTG muta-
tion alter longitudinal division? Yes! Their results indicate
at least a 60% reduction in the pLT/pLA ratio of the TTG
mutant compared to wild type and gl2 plants (see graph b
above). The percent of clones in the A file of the TTGmu-
tants exhibiting this type of cell division was twice that seen
in the wild-type or gl2mutants.
This observation directly supports the hypothesis that
the TTGgene is not only required for cell division in the T
cell file, but also controls longitudinal cell divisions which
are characteristically more frequent in trichoblasts.
The research team concluded from these studies that
TTG is probably the earliest point of control of root epi-
dermis cell fate specification, and that this control most
likely acts by negatively controlling trichoblast cell fate.
Number of cell divisions
Ratio of
T
cells/A
cells
Ratio of probability of longitudinal
division in T vesus A cells (pLT/pLA)
1.6
1.5
1.4
1.3
1.2
1.1
054321
Wild-type
6
0
20
Wild-type gl2 mutant TTG mutant
5
10
15
(b)(a)
gl2 mutant
TTG mutant
Comparing the differentiation and cell division of trichoblast (T) cells versus atrichoblast (A) cells in root epidermis.(a) As cell di-
visions proceeded, T cells and A cells were identified in the root epidermis of wild-type plants and two mutants, gl2and TTG. Comparing
the ratio of T cells to A cells, there is an increase in the number of A cells compared to T cells in the TTG mutant. (b) The rate of cell di-
vision was also examined by comparing the ratio of probabilities of longitudinal anticlinal cell division in T cells and A cells among the
wild-type and mutant plants. This ratio was lowest in TTG mutants, indicating that this mutation affects cell division.
To explore this experiment further, go to the Vir-
tual Lab at www.mhhe.com/raven6/vlab11.mhtml
795
40
Early Plant Development
Concept Outline
40.1 Plant embryo development establishes a basic
body plan.
Establishing the Root-Shoot Axis. Asymmetric cell
division starts patterning the embryo. Early in
embryogenesis the root-shoot axis is established.
Establishing Three Tissue Systems. Three tissue
systems are established without any cell movement. While
the embryo is still a round ball, the root-shoot axis is
established. The shape of the plant is determined by planes
of cell division and direction of cell elongation. Nutrients
are used during embryogenesis, but proteins, lipids, and
carbohydrates are also set aside to support the plant during
germination before it becomes photosynthetic.
40.2 Seed formation protects the dormant embryo
from water loss.
How Seeds Form. Seeds allow plants to survive
unfavorable conditions and invade new habitats.
40.3 Fruit formation enhances the dispersal of seeds.
How Fruits Form. Seed-containing fruits are carried
far by animals, wind and water, allowing angiosperms
to colonize large areas.
40.4 Germination initiates post-seed development.
Mechanisms of Germination. External signals including
water, light, abrasion, and temperature can trigger
germination. Rupturing the seed coat and adequate oxygen
are essential.Stored reserves in the endosperm or
cotyledon are made available to the embryo during
germination.
I
n chapter 37 we emphasized evolutionary changes in re-
production and physiology that gave rise to the highly
successful flowering plants (angiosperms). Chapters 38 and
39 explored the morphological and anatomical develop-
ment of the angiosperm sporophyte, where most of these
innovations occurred. In the next few chapters, we continue
our focus on the sporophyte generation of the an-
giosperms. In many cases, we will use the model plant Ara-
bidopsis, a weedy member of the mustard family. Its very
small genome has allowed plant biologists to study how
genes regulate plant growth and development. In this chap-
ter, we will follow the development of the embryo through
seed germination (figure 40.1). The next few chapters will
also continue to emphasize the roles of gene expression,
hormones, and environmental signals in regulating plant
development and function.
FIGURE 40.1
This plant has recently emerged from its seed.It is extending
its shoot and leaves up into the air, toward light.
These tissue systems are organized radially around the
root-shoot axis.
While the embryo is developing, two other critical
events are occurring. A food supply is established that will
support the embryo during germination while it gains pho-
tosynthetic capacity. This starts with the second fertiliza-
tion event that produces endosperm in angiosperms. Sec-
ondly, ovule tissue (from the parental sporophyte)
differentiates to form a hard, protective covering around
the embryo. The seed (ovule containing the embryo) then
enters a dormant phase, signaling the end of embryogene-
sis. Environmental signals (for example, water, tempera-
ture, and light) can break dormancy and trigger a cascade
of internal events resulting in germination.
Early Cell Division and Patterning
The first division of the fertilized egg in a flowering plant
is asymmetric and generates cells with two different fates
(figure 40.2). One daughter cell is small, with dense cyto-
plasm. That cell, which will become the embryo, begins to
divide repeatedly in different planes, forming a ball of cells.
The other daughter cell divides repeatedly, forming an
elongated structure called a suspensor, which links the em-
bryo to the nutrient tissue of the seed. The suspensor also
796 Part XI Plant Growth and Reproduction
Establishing the Root-Shoot Axis
In plants, three-dimensional shape and form arises by regu-
lating the amount and pattern of cell division. Even the
very first cell division is asymmetric resulting in two differ-
ent cell types. Early in embryo development most cells can
give rise to a wide range of cell and organ types, including
leaves. As development proceeds, the cells with multiple
potentials are restricted to regions called meristems. Many
meristems are established by the time embryogenesis ends
and the seed becomes dormant. Apical meristems will con-
tinue adding cells to the growing root and shoot tips after
germination. These generate the large numbers of cells
needed to form leaves, flowers, and all other components of
the mature plant. Apical meristem cells of corn, for exam-
ple, divide every 12 hours, producing half a million cells a
day in an actively growing corn plant. Lateral meristems
can cause an increase in the girth of some plants, while in-
tercalary meristems within the stems allow for elongation.
In addition to developing the root-shoot axis in em-
bryogenesis, cell differentiation occurs and three basic tis-
sue systems are established. These are the dermal,
ground, and vascular tissue systems and they are radially
patterned. These tissue systems contain various cell types
that can be highly differentiated for specific functions.
40.1 Plant embryo development establishes a basic body plan.
Polar
nuclei
Egg
Micropyle
Sperm
Pollen
tube
Triploid endosperm
mother cell
Zygote
Endosperm
Suspensor
Basal cell
Cotyledon
Procambium
Ground
meristem
Protoderm
Root apex (radicle)
Proembryo
Hypocotyl
Root apical
meristem
Cotyledons
Shoot apical
meristem
Endosperm
Cotyledons
FIGURE 40.2
Stages of development in an angiosperm embryo.The very first cell division is asymmetric. Differentiation begins almost immediately
after fertilization.
provides a route for nutrients to reach the developing em-
bryo. The root-shoot axis also forms at this time. Cells
near the suspensor are destined to form a root, while those
at the other end of the axis ultimately become a shoot.
Investigating the asymmetry of the first cell division is
difficult because the fertilized egg is embedded within the
gametophyte, which is surrounded by sporophyte tissue
(ovule and carpel tissue). One approach has been to use the
brown algae Fucus as a model system. Although there is a
huge evolutionary difference between brown algae and the
angiosperms, there are similarities in early embryogenesis
which may have ancient origins. The egg is released prior
to fertilization so there are no extra tissues surrounding the
zygote (fertilized egg). One side of the zygote begins to
bulge establishing the vertical axis. Cell division occurs and
the original bulge becomes the smaller of the two cells. It
develops into a rhizoid that anchors the alga and the other
cell develops into the main body, or thallus, of the sporo-
phyte. This axis is first established by the point of sperm
entry, but can be changed by environmental signals, espe-
cially light and gravity which ensure that the rhizoid is
down and the thallus is up. Internal gradients are estab-
lished that specify where the rhizoid will form in response
to environmental signals (figure 40.3). The ability to “re-
member” where the rhizoid will form depends on the cell
wall. Enzymatic removal of the cell wall in Fucuscells spec-
ified to form either rhizoids or plant body, resulted in cells
that could give rise to both. Cell walls contain a wide vari-
ety of carbohydrates and proteins attached to the wall’s
structural fibers. Attempting to pin down the identities of
these suspected developmental signals is an area of active
research.
Another approach to investigating the initial asymmetry
in embryos has been to study mutants with abnormal sus-
pensors. By understanding what is going wrong, it is often
possible to infer normal developmental mechanisms. For
example, the suspensor mutant in Arabidopsis has aberrant
development in the embryo followed by embryo-like devel-
opment of the suspensor (figure 40.4). From this, one can
conclude that the embryo normally prevents the suspensor
from developing into a second embryo.
Early in embryogenesis the root-shoot axis is
established.
Chapter 40 Early Plant Development 797
Light
Fertilized
egg
Bulge
Rhizoid
Rhizoid
cell
ThallusThallus
cell
Young alga
Adult alga
First cell
division
(asymmetric)
FIGURE 40.3
Asymmetric cell division in a Fucus zygote.
An unequal distribution of material in the
fertilized egg leads to a bulge where the first
cell division will occur. This division results in
a smaller cell that will go on to divide and
produce the rhizoid that anchors the plant; the
larger cell divides to form the thallus or main
plant body. The point of sperm entry
determines where the smaller rhizoid cell will
form, but light and gravity can modify this to
ensure that the rhizoid will point downward
where it can anchor this brown alga. Calcium-
mediated currents set up an internal gradient of
charged molecules which lead to a weakening
of the cell wall where the rhizoid will form.
The fate of the two resulting cells is held in
memory by cell wall components.
Embryo
proper
Suspensor
FIGURE 40.4
The embryo suppresses development of the suspensor as a
second embryo. This suspensor mutant of Arabidopsishas a defect
appear in embryo development followed by embryo-like
development of the suspensor.
Establishing Three Tissue Systems
Three basic tissues differentiate while the plant embryo is
still a ball of cells, the globular stage (figure 40.5), but no
cell movements are involved. The protoderm consists of the
outermost cells in a plant embryo and will become dermal
tissue. These cells almost always divide with their cell plate
perpendicular to the surface. This perpetuates a single
outer layer of cells. Dermal tissue produces cells that pro-
tect the plant from desiccation, including the stomata that
open and close to facilitate gas exchange and minimize
water loss. The bulk of the embryonic interior consists of
ground tissuecells that eventually function in food and water
storage. Lastly, procambium at the core of the embryo is
destined to form the future vascular tissue responsible for
water and nutrient transport.
Root and Shoot Formation
The root-shoot axis is established during the globular stage
of development. The shoot apical meristem will later give
rise to leaves and eventually reproductive structures.
While both the shoot and root meristems are apical meris-
tems, their formation is controlled independently. This
conclusion is supported by mutant analysis in Arabidopsis
where the shootmeristemless (stm) mutant fails to produce a
viable shoot, but does produce a root (figure 40.6). Simi-
larly, root meristem–specific genes have been identified.
For example, monopterous mutants of Arabidopsis lack
roots. The hormone auxin may play a role in root-shoot
axis formation. Auxin is one of six classes of hormones that
regulate plant development and function that we will ex-
plore in more detail later in this unit.
As you study the development of roots and shoots after
germination, you will notice that many of the same patterns
of tissue differentiation seen in the embryo are reiterated in
the apical meristems. Remember that there are also many
events discussed earlier in this chapter that are unique to
embryogenesis. For example, the LEAFY COTYLEDON
798 Part XI Plant Growth and Reproduction
Wild type stm mutant
FIGURE 40.6
Shoot-specific genes specify formation of the shoot apical
meristem. The shootmeristemless mutant of Arabidopsishas a
normal root meristem, but fails to produce a shoot meristem.
(a) (b) (c) (d)
FIGURE 40.5
Early developmental stages of Arabidopsis thaliana.(a) Early cell division has produced the embryo and suspensor. (b) Globular stage.
(c,d) Heart-shaped stage.
gene in Arabidopsisis active in early and late embryo devel-
opment and may be responsible for maintaining an embry-
onic environment. It is possible to turn this gene on later
in development using recombinant DNA techniques (see
chapter 43). In that case, embryos can form on leaves!
Morphogenesis
The globular stage gives rise to a heart-shaped embryo in
one group of angiosperms (the dicots, see figure 40.5) and a
ball with a bulge on a single side in another group (the
monocots). The bulges are cotyledons (“first leaves”) and
are produced by the embryonic cells, not the shoot apical
meristem that begins forming during the globular stage.
This process, called morphogenesis (generation of form), re-
sults from changes in planes and rates of cell division. Be-
cause plant cells cannot move, the form of a plant body is
largely determined by the plane in which cells divide and
by controlled changes in cell shape as they expand osmoti-
cally after they form. Both microtubules and actin play a
role in establishing the position of the cell plate which de-
termines the direction of division. Plant growth-regulators
and other factors influence the orientation of bundles of
microtubules on the interior of the plasma membrane.
These microtubules also guide cellulose deposition as the
cell wall forms around the outside of a new cell, determin-
ing its final shape. For example, if you start with a box and
reinforce four of the six sides more heavily with cellulose,
the cell will expand and grow in the direction of the two
sides with less reinforcement. Much is being learned at the
cell biological level about morphogenesis from mutants
that divide, but cannot control their plane of cell division
or the direction of cell expansion.
Food Storage
Throughout embryogenesis there is the production of
starch, lipids, and proteins. The seed storage proteins are
so abundant that the genes coding for them were the first
cloning targets for plant molecular biologists. As noted in
chapter 37, the evolutionary trend in the plants has been
toward increased protection of the embryo. One way this
is accomplished is through parental sporophyte input
transferred by the suspensor in angiosperms (in gym-
nosperms the suspensor serves only to push the embryo
closer to the gametophytic nutrient source produced by
multiple nuclear divisions without cell division). This hap-
pens concurrently with the development of the endosperm
(present only in angiosperms, although double fertilization
has been observed in the gymnosperm Ephedra) which may
be extensive or minimal. Endosperm in coconut is the
“milk” and is in liquid form. In corn the endosperm is
solid and in popping corn expands with heat to form the
edible part of popcorn. In peas and beans, the endosperm
is used up during embryo development and nutrients are
stored in thick, fleshy cotyledons (figure 40.7). The pho-
tosynthetic machinery is built in response to light. So, it is
critical that seeds have stored nutrients to aid in germina-
tion until the growing sporophyte can photosynthesize. A
seed buried too deeply will use up all its reserves before
reaching the surface and sunlight.
After the root-shoot axis is established, a radial, three-
tissue system, and a stored food supply, are formed
through controlled cell division and expansion.
Chapter 40 Early Plant Development 799
Embryo
Cotyledon
Bean
FIGURE 40.7
Endosperm in corn and bean.The corn
kernel has endosperm that is still present
at maturity, while the endosperm in the
bean has disappeared; the bean embryo’s
cotyledons take over food storage
functions.
Endosperm
Embryo
Corn
(a)
(b)
How Seeds Form
A protective seed coat forms from the outer layers of ovule
cells, and the embryo within is now either surrounded by
nutritive tissue or has amassed stored food in its cotyle-
dons. The resulting package, known as a seed,is resistant to
drought and other unfavorable conditions; in its dormant
state, it is a vehicle for dispersing the embryo to distant
sites and allows a plant embryo to survive in environments
that might kill a mature plant. In some embryos, the
cotyledons are bent over to fit within the constraints of the
hardening ovule wall with the inward cotyledon being
slightly smaller for efficient packing (figure 40.8). Remem-
ber that the ovule wall is actually tissue from the previous
sporophyte generation.
Adaptive Importance of Seeds
Early in the development of an angiosperm embryo, a pro-
foundly significant event occurs: the embryo stops develop-
ing. In many plants, development of the embryo is arrested
soon after the meristems and cotyledons differentiate. The
integuments—the outer cell layers of the ovule—develop
into a relatively impermeable seed coat, which encloses the
seed with its dormant embryo and stored food. Seeds are
important adaptively in at least four ways:
1. They maintain dormancy under unfavorable condi-
tions and postpone development until better condi-
tions arise. If conditions are marginal, a plant can “af-
ford” to have some seeds germinate, because others
will remain dormant.
2. The seed affords maximum protection to the young
plant at its most vulnerable stage of development.
3. The seed contains stored food that permits develop-
ment of a young plant prior to the availability of an
adequate food supply from photosynthetic activity.
4. Perhaps most important, the dispersal of seeds facili-
tates the migration and dispersal of plant genotypes
into new habitats.
Once a seed coat forms, most of the embryo’s metabolic
activities cease. A mature seed contains only about 5% to
20% water. Under these conditions, the seed and the
young plant within it are very stable; it is primarily the pro-
gressive and severe desiccation of the embryo and the asso-
ciated reduction in metabolic activity that are responsible
for its arrested growth. Germination cannot take place
until water and oxygen reach the embryo, a process that
sometimes involves cracking the seed coat through abrasion
or alternate freezing and thawing. Seeds of some plants
have been known to remain viable for hundreds and, in rare
instances, thousands of years (figure 40.9).
800 Part XI Plant Growth and Reproduction
40.2 Seed formation protects the dormant embryo from water loss.
Procambium
Root apical
meristem
Root cap
Cotyledons
Seed coat
Shoot apical
meristem
FIGURE 40.8
A mature angiosperm embryo.Note that two cotyledons have grown in a bent shape to accommodate the tight confines of the seed. In
some embryos, the shoot apical meristem will have already initiated a few leaf primordia as well.
Specific adaptations often help ensure that the plant will
germinate only under appropriate conditions. Sometimes,
seeds lie within tough cones that do not open until they are
exposed to the heat of a fire (figure 40.10). This causes the
plant to germinate in an open, fire-cleared habitat; nutri-
ents will be relatively abundant, having been released from
plants burned in the fire. Seeds of other plants will germi-
nate only when inhibitory chemicals have been leached
from their seed coats, thus guaranteeing their germination
when sufficient water is available. Still other plants will ger-
minate only after they pass through the intestines of birds
or mammals or are regurgitated by them, which both weak-
ens the seed coats and ensures the dispersal of the plants in-
volved. Sometimes seeds of plants thought to be extinct in a
particular area may germinate under unique or improved
environmental circumstances, and the plants may then
reappear.
Seed dormancy is an important evolutionary factor in
plants, ensuring their survival in unfavorable conditions
and allowing them to germinate when the chances of
survival for the young plants are the greatest.
Chapter 40 Early Plant Development 801
FIGURE 40.9
Seeds can remain dormant for long periods.This seedling was
grown from a lotus seed recovered from the mud of a dry lake bed
in Manchuria, northern China. The radiocarbon age of this seed
indicates that it was formed around A.D. 1515. The coin is
included in the photo to give some idea of the size.
(a)
(b) (c)
FIGURE 40.10
Fire induces seed germination in some pines. (a) Fire will
destroy these adult jack pines, but stimulate growth of the next
generation. (b) Cones of a jack pine are tightly sealed and cannot
release the seeds protected by the scales. (c) High temperatures
lead to the release of the seeds.
How Fruits Form
Paralleling the evolution of angiosperm flowers, and nearly
as spectacular, has been the evolution of their fruits, which
are defined simply as mature ovaries (carpels). During seed
formation, the flower ovary begins to develop into fruit.
Fruits form in many ways and exhibit a wide array of spe-
cializations in relation to their dispersal. The differences
among some of the fruit types seen today are shown in fig-
ure 40.11. Three layers of ovary wall can have distinct fates
which accounts for the diversity of fruit types from fleshy
to dry and hard. An array of mechanisms allow for the re-
lease of the seed(s) within the fruits. Developmentally,
fruits are fascinating organs that contain three generations
802 Part XI Plant Growth and Reproduction
40.3 Fruit formation enhances the dispersal of seeds.
Follicles
Split along one carpel edge only; milkweed,
larkspur.
Legumes
Split along two carpel edges with seeds at-
tached to carpel edges; peas, beans.
Samaras
Not split and with a wing formed from the
outer tissues; maples, elms, ashes.
Drupes
Single seed enclosed in a hard pit; peaches,
plums, cherries.
True berries
More than one seed and a thin skin; blue-
berries, tomatoes, grapes, peppers.
Hesperidiums
More than one seed and a leathery skin; or-
anges, lemons, limes.
FIGURE 40.11
Examples of some kinds of fruits.Distinguishing
features of each of these fruit types are listed below
each photo. Follicles, legumes, and samaras are
examples of dry fruits. Drupes, true berries, and
hesperidiums are simple fleshy fruits; they develop
from a flower with a single pistil. Aggregate and
multiple fruits are compound fleshy fruits; they develop
from flowers with more than one ovary or from more
than one flower.
Aggregate fruits
Derived from many ovaries of a single flower;
strawberries, blackberries.
Multiple fruits
Develop from a cluster of flowers; mulberries, pineapples.
in one package. The fruit and seed
coat are from the prior sporophyte
generation. Within the seed are rem-
nants of the gametophyte generation
that produced the egg that was fertil-
ized to give rise to the next sporophyte
generation, the embryo.
The Dispersal of Fruits
Aside from the many ways fruits can
form, they also exhibit a wide array of
specialized dispersal methods. Fruits
with fleshy coverings, often shiny
black or bright blue or red, normally
are dispersed by birds or other verte-
brates (figure 40.12a). Like red flow-
ers, red fruits signal an abundant food
supply. By feeding on these fruits,
birds and other animals may carry
seeds from place to place and thus
transfer plants from one suitable habi-
tat to another.
Fruits with hooked spines, like those of burgrass (figure
40.12b), are typical of several genera of plants that occur
in the northern deciduous woods. Such fruits are often
disseminated by mammals, including humans. Squirrels
and similar mammals disperse and bury fruits such as
acorns and other nuts. Other fruits, such as those of
maples, elms, and ashes, have wings which aid in their dis-
tribution by the wind. The dandelion provides another fa-
miliar example of a fruit type that is dispersed by wind
(figure 40.13), and the dispersal of seeds from plants such
as milkweeds, willows, and cottonwoods is similar. Or-
chids have minute, dustlike seeds, which are likewise
blown away by the wind.
Coconuts and other plants that characteristically occur
on or near beaches are regularly spread throughout a re-
gion by water (figure 40.14). This sort of dispersal is es-
pecially important in the colonization of distant island
groups, such as the Hawaiian Islands. It has been calcu-
lated that seeds of about 175 original angiosperms, nearly
a third from North America, must have reached Hawaii
to have evolved into the roughly 970 species found there
today. Some of these seeds blew through the air, others
were transported on the feathers or in the guts of birds,
and still others drifted across the Pacific. Although the
distances are rarely as great as the distance between
Hawaii and the mainland, dispersal is just as important
for mainland plant species that have discontinuous habi-
tats, such as mountaintops, marshes, or north-facing
cliffs.
Fruits, which are characteristic of angiosperms, are
extremely diverse. The evolution of specialized
structures allows fruits to be dispersed by animals,
wind, and water.
Chapter 40 Early Plant Development 803
(a) (b)
FIGURE 40.12
Animal-dispersed fruits.(a) The bright red berries of this honeysuckle, Lonicera hispidula,
are highly attractive to birds, just as are red flowers. After eating the fruits, birds may carry
the seeds they contain for great distances either internally or, because of their sticky pulp,
stuck to their feet or other body parts. (b) The spiny fruits of this burgrass, Cenchrus incertus,
adhere readily to any passing animal, as you will know if you have ever stepped on them.
FIGURE 40.13
Wind-dispersed
fruits.False
dandelion,
Pyrrhopappus
carolinanus.The
“parachutes”
disperse the fruits
of both false and
true dandelions
widely in the
wind, much to the
gardener’s
despair.
FIGURE 40.14
A water-dispersed fruit.This fruit of the coconut, Cocos nucifers,
is sprouting on a sandy beach. Coconuts, one of the most useful
plants for humans in the tropics, have become established on
even the most distant islands by drifting on the waves.
When conditions are satisfactory, the embryo emerges
from its desiccated state, utilizes food reserves, and re-
sumes growth. As the sporophyte pushes through the
seed coat it orients with the environment so the root
grows down and the shoot grows up. New growth comes
from delicate meristems that are protected from environ-
mental rigors. The shoot becomes photosynthetic and
the post-embryonic phase of growth and development is
underway.
Mechanisms of Germination
Germination is the first step in the development of the
plant outside of its seed coat. Germination occurs when
a seed absorbs water and its metabolism resumes. The
amount of water a seed can absorb is phenomenal and
creates a force strong enough to break the seed coat. At
this point, it is important that oxygen be available to the
developing embryo because plants, like animals, require
oxygen for cellular respiration. Few plants produce seeds
that germinate successfully under water, although some,
such as rice, have evolved a tolerance to anaerobic
conditions.
A dormant seed, although it may have imbibed a full
supply of water and may be respiring, synthesizing proteins
and RNA, and apparently carrying on normal metabolism,
may nonetheless fail to germinate without an additional
signal from the environment. This signal may be light of
the correct wavelengths and intensity, a series of cold days,
or simply the passage of time at temperatures appropriate
for germination. Seeds of many plants will not germinate
unless they have been stratified—held for periods of time
at low temperatures. This phenomenon prevents seeds of
plants that grow in cold areas from germinating until they
have passed the winter, thus protecting their seedlings from
cold conditions.
Germination can occur over a wide temperature
range (5°to 30°C), although certain species and specific
habitats may have relatively narrow optimum ranges.
Some seeds will not germinate even under the best con-
ditions. In some species, a significant fraction of a sea-
son’s seeds remain dormant, providing a gene pool of
great evolutionary significance to the future plant
population.
The Utilization of Reserves
Germination occurs when all internal and external re-
quirements are met. Germination and early seedling
growth require the utilization of metabolic reserves; these
reserves are stored in the starch grains of amyloplasts
(colorless plastids that store starch) and protein bodies.
Fats and oils also are important food reserves in some
kinds of seeds. They can readily be digested during ger-
mination, producing glycerol and fatty acids, which yield
energy through cellular respiration; they can also be con-
verted to glucose. Depending on the kind of plant, any of
these reserves may be stored in the embryo itself or in the
endosperm.
In the kernels of cereal grains, the single cotyledon is
modified into a relatively massive structure called the
scutellum (figure 40.15), from the Latin word meaning
“shield.” The abundant food stored in the scutellum is used
up first because these plants do not need to use the en-
dosperm during germination. Later, while the seedling is
becoming established, the scutellum serves as a nutrient
conduit from the endosperm to the embryo. This is one of
the best examples of how plant growth regulators modulate
development in plants (40.16). The embryo produces gib-
berellic acid which signals the outer layer of the endosperm
called the aleurone to produce α-amylase. This enzyme is
responsible for breaking the starch in the endosperm down
into sugars that are passed by the scutellum to the embryo.
Abscisic acid, another plant growth regulator, which is im-
portant in establishing dormancy, can inhibit this process.
Abscisic acid levels may be reduced further when a seed ab-
sorbs water.
The emergence of the embryonic root and shoot from
the seed during germination varies widely from species
to species. In most plants, the root emerges before the
shoot appears and anchors the young seedling in the soil
(see figure 40.15). In plants such as peas and corn, the
cotyledons may be held below ground; in other plants,
such as beans, radishes, and sunflowers, the cotyledons
are held above ground. The cotyledons may or may not
become green and contribute to the nutrition of the
seedling as it becomes established. The period from the
germination of the seed to the establishment of the
young plant is a very critical one for the plant’s survival;
the seedling is unusually susceptible to disease and
drought during this period.
During germination and early seedling establishment,
the utilization of food reserves stored in the embryo or
the endosperm is mediated by hormones, which, in
some cases, are gibberellins.
804 Part XI Plant Growth and Reproduction
40.4 Germination initiates post-seed development.
Chapter 40 Early Plant Development 805
FIGURE 40.16
Hormonal regulation of seedling
growth.The germinating barley embryos
utilize the starch stored in the endosperm
by releasing the hormone gibberellic acid
(GA) that triggers the outer layers of the
endosperm (aleurone layers) to produce
the starch-digesting enzyme α-amylase.
The α-amylase breaks down starch into
sugar which moves through the scutellum
(cotyledon) into the embryo where it
provides energy for growth. A second
hormone, abscisic acid (ABA), is important
in establishing dormancy and becomes
diluted as seeds imbibe water. When
there is excess ABA, the GA-triggered
production of alpha-amylase is inhibited.
Hypocotyl
Epicotyl
Secondary
roots
Primary
roots
Seed
coat
Cotyledon
Withered
cotyledonsHypocotyl
Plumule
Scutellum
Primary
root
Adventitious
root
Radicle
Coleorhiza
First
leaf
Coleoptile
FIGURE 40.15
Shoot development.The stages shown are for a dicot, the common bean, (a) Phaseolus vulgaris,and a monocot, corn, (b) Zea mays.
Aleurone
Endosperm
Scutellum
Embryo
Gibberellic
acid
H9251-amylase
Sugars
Starch
(a) (b)
806 Part XI Plant Growth and Reproduction
Chapter 40
Summary Questions Media Resources
40.1 Plant embryo development establishes a basic body plan.
? Plant shape is determined by the direction of cell
division and expansion.
? Three tissue systems form radially through regulated
cell division and differentiation.
? Shoot and root apical meristems are established to
continuously produce new tissues, which then differ-
entiate into body parts.
? Carbohydrates, lipids, and proteins are stored for ger-
mination in the endosperm or cotyledons.
1.The pattern of cell division
regulates the shape of an
embryo. Describe the cell
division pattern that results in
the single, outer layer of
protoderm in the globular stage
embryo.
2.What evidence supports the
claim that the shoot meristem is
genetically specified separately
from the root?
? The ovule wall (integuments) around the embryo
hardens to protect the embryo as embryogenesis
ends.
? Seed formation allows the embryo to enter a dormant
state and continue growth under more optimal condi-
tions.
3.Why are seeds adaptively
important? Why may a seed
showing proper respiration and
synthesis of proteins and nucleic
acids and all other normal
metabolic activities still fail to
germinate?
40.2 Seed formation protects the dormant embryo from water loss.
? Fruits are an angiosperm innovation that develop
from the ovary wall (a modified leaf) that surrounds
the ovule(s).
? Fruits are highly diverse in terms of their dispersal
mechanisms, often displaying wings, barbs, or other
structures that aid in their transport from place to
place. Fruit dispersal methods are especially impor-
tant in the colonization of islands or other distant
patches of suitable habitat.
4.Why is it advantageous for a
plant to produce fruit? How
does the genotype of the fruit
compare with the genotype of
the embryo? How does the
genotype of the seed wall
compare with the fruit wall?
40.3 Fruit formation enhances seed dispersal.
? In a seed, the embryo with its food supply is encased
within a sometimes rigid, relatively impermeable seed
coat that may need to be abraded before germination
can occur. Weather or passage through an animal’s
digestive tract may be necessary for germination to
begin.
? When temperature, light, and water conditions are
appropriate, germination can begin. In some cases, a
period of chilling is required prior to germination.
This adaptation protects seeds from germinating dur-
ing the cold season.
? At germination, the mobilization of the food reserves
is critical. Hormones control this process.
5.Explain how the embryo
signals the endosperm to obtain
sugars for growth during
germination.
6.Why does the root (actually
the radicle) of the embryo
emerge first?
40.4 Germination initiates post-seed development.
www.mhhe.com/raven6e www.biocourse.com
? Art Activity: Corn
Grain Structure
? Art Activity: Garden
Bean Seed Structure
? Embryos and Seeds
? Activity: Fruits
? Fruits
? Germination
807
41
How Plants Grow in
Response to Their
Environment
Concept Outline
41.1 Plant growth is often guided by environmental
cues.
Tropisms. Plant growth is often influenced by light,
gravity, and contact with other plants and animals.
Dormancy. The ability to cease growth allows plants to
wait out the bad times.
41.2 The hormones that guide growth are keyed to
the environment.
Plant Hormones. Hormones are grouped into seven
classes.
Auxin. Auxin is involved in the elongation of stems.
Cytokinins. Cytokinins stimulate cell division.
Gibberellins. Gibberellins control stem elongation.
Brassinosteroids and Oligosaccharins. There are
several recent additions to the plant hormone family.
Ethylene. Ethylene controls leaves and flower abscision.
Abscisic Acid. Abscisic acid suppresses growth of buds
and promotes leaf senescence.
41.3 The environment influences flowering.
Plants Undergo Metamorphosis. The transition of a
shoot meristem from vegetative to adult is called phase
change.
Pathways Leading to Flower Production. Photoperiod
is regulated in complex ways.
Identity Genes and the Formation of Floral Meristems
and Floral Organs. Floral meristem identity genes
activate floral organ identity genes.
41.4 Many short-term responses to the environment
do not require growth.
Turgor Movement. Changes in the water pressure
within plant cells result in quick and reversible plant
movements.
Plant Defense Responses. In addition to generalized
defense mechanisms, some plants have highly evolved
recognition mechanisms for specific pathogens.
A
ll organisms sense and interact with their environ-
ment. This is particularly true of plants. Plant survival
and growth is critically influenced by abiotic factors includ-
ing water, wind, and light. In this chapter, we will explore
how a plant senses such factors, and transduces these sig-
nals to elicit an optimal physiological, growth, or develop-
mental response. Hormones play an important role in the
internal signaling that brings about environmental re-
sponses, and is keyed in many ways to the environment.
The effect of the local environment on plant growth also
accounts for much of the variation in adult form within a
species (figure 41.1). Precisely regulated responses to the
environment not only allow a plant to survive from day to
day but also determine when a flowering plant will produce
a flower. The entire process of constructing a flower in
turn sets the stage for intricate reproductive strategies that
will be discussed in the next chapter.
FIGURE 41.1
Plant growth is affected by environmental cues.The branches
of this fallen tree are growing straight up in response to gravity
and light.
given biological reaction that is affected by phytochrome
will occur. When most of the P
fr
has been replaced by P
r
,
the reaction will not occur (figure 41.3). While we refer
to phytochrome as a single molecule here, it is important
to note that several different phytochromes have now
been identified that appear to have specific biological
functions.
Phytochrome is a light receptor, but it does not act di-
rectly to bring about reactions to light. The existence of
phytochrome was conclusively demonstrated in 1959 by
Harry A. Borthwick and his collaborators at the U.S. De-
partment of Agriculture Research Center at Beltsville,
Maryland. It has since been shown that the molecule con-
sists of two parts: a smaller one that is sensitive to light
and a larger portion that is a protein. The protein compo-
nent initiates a signal transduction leading to a particular
tropism. The phytochrome pigment is blue, and its light-
sensitive portion is similar in structure to the phycobilins
that occur in cyanobacteria and red algae. Phytochrome is
present in all groups of plants and in a few genera of
green algae, but not in bacteria, fungi, or protists (other
than the few green algae). It is likely that phytochrome
systems for measuring light evolved among the green
algae and were present in the common ancestor of the
plants.
Phytochrome is involved in many plant growth re-
sponses. For example, seed germination is inhibited by far-
red light and stimulated by red light in many plants. Be-
cause chlorophyll absorbs red light strongly but does not
absorb far-red light, light passing through green leaves in-
hibits seed germination. Consequently, seeds on the
808 Part XI Plant Growth and Reproduction
Tropisms
Growth patterns in plants are often guided by environ-
mental signals. Tropisms (from trope, the Greek word for
“turn”) are positive or negative growth responses of plants
to external stimuli that usually come from one direction.
Some responses occur independently of the direction of
the stimuli and are referred to as nastic movements. For
example, a tendril of a pea plant will always coil in one di-
rection when touched. Tropisms, on the other hand, are
directional and offer significant compensation for the
plant’s inability to get up and walk away from unfavorable
environmental conditions. Tropisms contribute the vari-
ety of branching patterns we see within a species. Here we
will consider three major classes of plant tropisms: pho-
totropism, gravitropism, and thigmotropism. Tropisms
are particularly intriguing because they challenge us to
connect environmental signals with cellular perception of
the signal, transduction into biochemical pathways, and
ultimately an altered growth response.
Phototropism
Phototropic responses involve the bending of growing
stems and other plant parts toward sources of light (figure
41.2). In general, stems are positively phototropic, growing
toward a light source, while most roots do not respond to
light or, in exceptional cases, exhibit only a weak negative
phototropic response. The phototropic reactions of stems
are clearly of adaptive value, giving plants greater exposure
to available light. They are also important in determining
the development of plant organs and, therefore, the ap-
pearance of the plant. Individual leaves may display pho-
totropic responses. The position of leaves is important to
the photosynthetic efficiency of the plant. A plant hormone
called auxin (discussed later in this chapter) is probably in-
volved in most, if not all, of the phototropic growth re-
sponses of plants.
The first step in a phototropic response is perceiving
the light. Photoreceptors perceive different wavelengths
of light with blue and red being the most common. Blue
light receptors are being characterized and we are begin-
ning to understand how plants “see blue.” Much more is
known about “seeing red” and translating that perception
into a signal transduction pathway leading to an altered
growth response. Plants contain a pigment, phy-
tochrome, which exists in two interconvertible forms, P
r
and P
fr
. In the first form, phytochrome absorbs red light;
in the second, it absorbs far-red light. When a molecule
of P
r
absorbs a photon of red light (660 nm), it is instantly
converted into a molecule of P
fr
, and when a molecule of
P
fr
absorbs a photon of far-red light (730 nm), it is in-
stantly converted to P
r
. P
fr
is biologically active and P
r
is
biologically inactive. In other words, when P
fr
is present, a
41.1 Plant growth is often guided by environmental cues.
FIGURE 41.2
Phototropism. Impatiensplant growing toward light.
ground under deciduous plants that lose their leaves in
winter are more apt to germinate in the spring after the
leaves have decomposed and the seedlings are exposed to
direct sunlight. This greatly improves the chances the
seedlings will become established.
A second example of these relationships is the elonga-
tion of the shoot in an etiolated seedling (one that is pale
and slender from having been kept in the dark). Such plants
become normal when exposed to light, especially red light,
but the effects of such exposure are canceled by far-red
light. This indicates a relationship similar to that observed
in seed germination. There appears to be a link between
phytochrome light perception and brassinosteroids in the
etiolation response. Etiolation is an energy conservation
strategy to help plants growing in the dark reach the light
before they die. They don't green-up until there is light,
and they divert energy to growing as tall as possible
through internode elongation. The de-etiolated (det2) Ara-
bidopsis mutant has a poor etiolation response. It does not
have elongated internodes and greens up a bit in the dark.
It turns out that det2 mutants are defective in an enzyme
necessary for brassinosteroid biosynthesis. Researchers sus-
pect that brassinosteroids play a role in how plants respond
to light through phytochrome. Thus, because det2 mutants
lack brassinosteroids, they do not respond to light, or lack
of light, as normal plants do, and the det2 mutants grow
normally in the dark.
Red and far-red light also are used as signals for plant
spacing. The closer plants are together, the more likely
they are to grow tall and try to outcompete others for the
sunshine. Plants somehow measure the amount of far-red
light being bounced back to them from neighboring trees.
If their perception is messed up by putting a collar around
the stem with a solution that blocks light absorption, the
elongation response is no longer seen.
Gravitropism
When a potted plant is tipped over, the shoot bends and
grows upward (figure 41.4). The same thing happens when
a storm pushes over plants in a field. These are examples of
gravitropism, the response of a plant to the gravitational
field of the earth. We saw in chapter 40 that brown algae
orient their first cell division so the rhizoid grows down-
ward. Rhizoids also develop away from a unilateral light
source. Separating out phototropic effects is important in
the study of gravitropisms.
Gravitropic responses are present at germination when
the root grows down and the shoot grows up. Why does a
shoot have a negative gravitropic response (growth away
from gravity), while a root has a positive gravitropic re-
sponse? The opportunity to experiment on the space
shuttle in a gravity-free environment has accelerated re-
search in this area. Auxins play a primary role in gravit-
ropic responses, but they may not be the only way gravita-
tional information is sent through the plant. When John
Glenn made his second trip into space, he was accompa-
nied by an experiment designed to test the role of gravity
and electrical signaling in root bending. Analysis of grav-
itropic mutants is also adding to our understanding of
gravitropism. There are four steps that lead to a gravit-
ropic response:
1. Gravity is perceived by the cell
2. Signals form in the cell that perceives gravity
3. The signal is transduced intra- and intercellularly
4. Differential cell elongation occurs between cells in
the “up” and “down” sides of the root or shoot.
Chapter 41 How Plants Grow in Response to Their Environment 809
Far-red light
(730 nm)
Biological response
is blocked
P
d
Red light
(660 nm)
Phytochrome
P
r
Long period
of darkness
Destruction
Synthesis
Phytochrome
P
fr
Precursor
P
p
FIGURE 41.3
How phytochrome works.Phytochrome is synthesized in the P
r
form from amino acids, designated P
p
for phytochrome precursor.
When exposed to red light, P
r
changes to P
fr
, which is the active
form that elicits a response in plants. P
fr
is converted to P
r
when
exposed to far-red light, and it also converts to P
r
or is destroyed
in darkness. The destruction product is designated P
d
.
FIGURE 41.4
Plant response to
gravity. This plant
(Zebrina pendula) was
placed horizontally and
allowed to grow for
7 days. Note the
negative gravitational
response of the shoot.
One of the first steps in perceiving gravity is that amy-
loplasts, plastids that contain starch, sink toward the gravi-
tational field. These may interact with the cytoskeleton,
but the net effect is that auxin becomes more concentrated
on the lower side of the stem axis than on the upper side.
The increased auxin concentration on the lower side in
stems causes the cells in that area to grow more than the
cells on the upper side. The result is a bending upward of
the stem against the force of gravity—in other words there
is a negative gravitropic response. Such differences in hor-
mone concentration have not been as well documented in
roots. Nevertheless, the upper sides of roots oriented hori-
zontally grow more rapidly than the lower sides, causing
the root ultimately to grow downward; this phenomenon
is known as positive gravitropism. In shoots, the gravity-
sensing cells are in the endoderm. Mutants like scarecrow
and short root in Arabidopsis that lack normal endodermal
development fail to have a normal gravitropic response.
These endodermal cells are the sites of the amyloplasts in
the stems.
In roots, the gravity-sensing cells are located in the
root cap and the cells that actually do the asymmetric
growth are in the distal elongation zone which is closest
to the root cap. How the information gets transferred
over this distance is an intriguing problem. Auxin may be
involved, but when auxin transport is suppressed, there is
still a gravitropic response in the distal elongation zone.
Some type of electrical signaling involving membrane po-
larization has been hypothesized and this was tested
aboard the space shuttle. So far the verdict is still not in
on the exact mechanism.
It may surprise you to learn that in tropical rain forests,
roots of some plants may grow up the stems of neighboring
plants, instead of exhibiting the normal positive gravitropic
responses typical of other roots. The rainwater dissolves
nutrients, both while passing through the lush upper
canopy of the forest, and also subsequently as it trickles
down tree trunks. Such water functions as a more reliable
source of nutrients for the roots than the nutrient-poor
soils in which the plants are anchored. Explaining this in
terms of current hypotheses is a challenge. It has been pro-
posed that roots are more sensitive to auxin than shoots
and that auxin may actually inhibit growth on the lower
side of a root, resulting in a positive gravitropic response.
Perhaps in these tropical plants, the sensitivity to auxin in
roots is reduced.
Thigmotropism
Thigmotropism is a name derived from the Greek root
thigma, meaning “touch.” A thigmotropism is a response of
a plant or plant part to contact with the touch of an object,
animal, plant, or even the wind. (figure 41.5). When a ten-
dril makes contact with an object, specialized epidermal
cells, whose action is not clearly understood, perceive the
contact and promote uneven growth, causing the tendril to
curl around the object, sometimes within as little as 3 to 10
minutes. Both auxin and ethylene appear to be involved in
tendril movements, and they can induce coiling in the ab-
sence of any contact stimulus. In other plants, such as
clematis, bindweed, and dodder, leaf petioles or unmodified
stems twine around other stems or solid objects.
Again, Arabidopsis is proving valuable as a model system.
A gene has been identified that is expressed in 100-fold
higher levels 10 to 30 minutes after touch. Given the value
of a molecular genetics approach in dissecting the pathways
leading from an environmental signal to a growth response,
this gene provides a promising first step in understanding
how plants respond to touch.
Other Tropisms
The tropisms just discussed are among the best known, but
others have been recognized. They include electrotropism
(responses to electricity); chemotropism (response to chemi-
cals); traumotropism (response to wounding which we dis-
cuss on page 834); thermotropism (response to temperature);
aerotropism (response to oxygen); skototropism (response to
dark); and geomagnetotropism (response to magnetic fields).
Roots will often follow a diffusion gradient of water com-
ing from a cracked pipe and enter the crack. Some call such
growth movement hydrotropism, but there is disagreement
whether responses to water and several other “stimuli” are
true tropisms.
While plants can’t move away or toward optimal
conditions, they can grow. Phototropisms are growth
responses of plants to a unidirectional source of light.
Gravitropism, the response of a plant to gravity,
generally causes shoots to grow up (negative
gravitropism) and roots to grow down (positive
gravitropism). Thigmotropisms are growth responses of
plants to contact.
810 Part XI Plant Growth and Reproduction
FIGURE 41.5
Thigmotropism.
The thigmotropic
response of these
twining stems causes
them to coil around
the object with which
they have come in
contact.
Dormancy
Sometimes modifying the direction of growth is not
enough to protect a plant from harsh conditions. The abil-
ity to cease growth and go into a dormant stage provides a
survival advantage. The extreme example is seed dormancy,
but there are intermediate approaches to waiting out the
bad times as well. Environmental signals both initiate and
end dormant phases in the life of a plant.
In temperate regions, we generally associate dormancy
with winter, when freezing temperatures and the accompa-
nying unavailability of water make it impossible for plants
to grow. During this season, buds of deciduous trees and
shrubs remain dormant, and apical meristems remain well
protected inside enfolding scales. Perennial herbs spend the
winter underground as stout stems or roots packed with
stored food. Many other kinds of plants, including most an-
nuals, pass the winter as seeds.
In some seasonally dry climates, seed dormancy occurs
primarily during the dry season, often the summer. Rain-
falls trigger germination when conditions for survival are
more favorable. Annual plants occur frequently in areas of
seasonal drought. Seeds are ideal for allowing annual plants
to bypass the dry season, when there is insufficient water
for growth. When it rains, they can germinate and the
plants can grow rapidly, having adapted to the relatively
short periods when water is available. Chapter 40 covered
some of the mechanisms involved in breaking seed dor-
mancy and allowing germination under favorable circum-
stances. These include the leaching from the seed coats of
chemicals that inhibit germination, or mechanically crack-
ing the seed coats, a procedure that is particularly suitable
for promoting growth in seasonally dry areas. Whenever
rains occur, they will leach out the chemicals from the seed
coats, and the hard coats of other seeds may be cracked
when they are being washed down along temporarily
flooded arroyos (figure 41.6).
Seeds may remain dormant for a surprisingly long time.
Many legumes (plants of the pea and bean family,
Fabaceae) have tough seeds that are virtually impermeable
to water and oxygen. These seeds often last decades and
even longer without special care; they will eventually ger-
minate when their seed coats have been cracked and water
is available. Seeds that are thousands of years old have been
successfully germinated!
A period of cold is necessary before some kinds of
seeds will germinate, as we mentioned in chapter 40. The
seeds of other plants will germinate only when adequate
water is available and the temperatures are relatively high.
For this reason, certain weeds germinate and grow in the
cooler part of the year and others in the warmer part of
the year. Similarly, a period of cold is needed before the
buds of some trees and shrubs will break dormancy and
develop normally. For this reason, many plants that nor-
mally grow in temperate regions do not thrive in warmer
regions near the equator, because even at high elevations
in the tropics it still does not get cold enough, and the
day-length relationships are different from those of tem-
perate regions.
Mature plants may become dormant in dry or cold
seasons that are unfavorable for growth. Dormant
plants usually lose their leaves and drought-resistant
winter buds are produced. Long unfavorable periods
may be bypassed through the production of seeds,
which themselves can remain dormant for long
periods.
Chapter 41 How Plants Grow in Response to Their Environment 811
FIGURE 41.6
Palo verde (Cercidium floridum).This desert tree (a) has tough
seeds (b) that germinate only after they are cracked.
(a)
(b)
Plant Hormones
While initial responses of plants to environmental signals
may rely primarily on electrical signaling, longer-term re-
sponses that alter morphology rely on complex physiologi-
cal networks. Many internal signaling pathways involve
plant hormones, which are the focus of this section. Hor-
mones are involved in responses to the environment, as
well as internally regulated development (examples of
which you saw in chapter 40).
Hormones are chemical substances produced in small,
often minute, quantities in one part of an organism and
then transported to another part, where they bring about
physiological or developmental responses. The activity of
hormones results from their capacity to stimulate certain
physiological processes and to inhibit others (figure
41.7). How they act in a particular instance is influenced
both by the hormone and the tissue that receives the
message.
In animals, hormones are usually produced at definite
sites, usually organs. In plants, hormones are not produced
in specialized tissues but, instead, in tissues that also carry
out other, usually more obvious, functions. There are seven
major kinds of plant hormones: auxin, cytokinins, gib-
berellins, brassinosteroids, oligosaccharins, ethylene, and
abscisic acid (table 41.1). Current research is focused on the
biosynthesis of hormones and on characterizing the hor-
mone receptors that trigger signal transduction pathways.
Much of the molecular basis of hormone function remains
enigmatic.
Because hormones are involved in so many aspects of
plant function and development, we have chosen to inte-
grate examples of hormone activity with specific aspects of
plant biology throughout the text. In this section, our goal
is to give you a brief overview of these hormones. Use this
section as a reference when specific hormones are discussed
in the next few chapters.
There are seven major kinds of plant hormones: auxin,
cytokinins, gibberellins, brassinosteroids,
oligosaccharins, ethylene, and abscisic acid.
812 Part XI Plant Growth and Reproduction
41.2 The hormones that guide growth are keyed to the environment.
FIGURE 41.7
Effects of plant hormones. Plant hormones, often
acting together, influence many aspects of plant
growth and development, including (a) leaf
abscission and (b) the formation of mature fruit.
(a)
(b)
Chapter 41 How Plants Grow in Response to Their Environment 813
Table 41.1 Functions of the Major Plant Hormones
Where Produced
Hormone Major Functions or Found in Plant
Auxin (IAA) Promotion of stem elongation Apical meristems; other
and growth; formation of immature parts of plants
adventitious roots; inhibition of
leaf abscission; promotion of cell
division (with cytokinins);
inducement of ethylene production;
promotion of lateral bud dormancy
Cytokinins Stimulation of cell division, Root apical meristems;
but only in the presence of immature fruits
auxin; promotion of chloroplast
development; delay of leaf aging;
promotion of bud formation
Gibberellins Promotion of stem elongation; Roots and shoot tips; young
stimulation of enzyme production leaves; seeds
in germinating seeds
Brassinosteroids Overlapping functions with Pollen, immature seeds,
auxins and gibberellins shoots, leaves
Oligosaccharins Pathogen defense, possibly Cell walls
reproductive developmentz
Ethylene Control of leaf, flower, and Roots, shoot apical meristems;
fruit abscission; promotion leaf nodes; aging flowers;
of fruit ripening ripening fruits
Abscisic acid Inhibition of bud growth; Leaves, fruits, root caps, seeds
control of stomatal closure;
some control of seed dormancy;
inhibition of effects of other
hormones
CH
2
NH
NN
N
H
N
CH
2
COOH
OH
CH
2
COOHCH
3
O
HO
C O
H
O
HO
HO
O
O
OH
OH
N
O
OH
OHHO
HO
O
O
OHHO
O
OH
OHHO
O
O
OHHO
O
OH
OHHO
OH
OH
HO OH
HO
OH
OH HO
OH
O
O
O
O
O
O
CH
3
CH
3
OH
CH
3
CH
3
COOH
O
CCC
H
H
H
H
Auxin
More than a century ago, an organic substance known as
auxin became the first plant hormone to be discovered.
Auxin increases the plasticity of plant cell walls and is in-
volved in elongation of stems. Cells can enlarge in re-
sponse to changes in turgor pressure when their cell walls
have enhanced plasticity from auxin action. The discov-
ery of auxin and its role in plant growth is an elegant ex-
ample of thoughtful experimental design. The historical
story is recounted here for that reason. Recent efforts
have uncovered an auxin receptor. Transport mecha-
nisms are also being unraveled. As with all the classes of
hormones, we are just beginning to understand, at a cel-
lular and molecular level, how hormones regulate growth
and development.
Discovery of Auxin
In his later years, the great evolutionist, Charles Darwin,
became increasingly devoted to the study of plants. In
1881, he and his son Francis published a book called The
Power of Movement of Plants. In this book, the Darwins re-
ported their systematic experiments on the response of
growing plants to light—responses that came to be
known as phototropisms. They used germinating oat
(Avena sativa) and canary grass (Phalaris canariensis)
seedlings in their experiments and made many observa-
tions in this field.
Charles and Francis Darwin knew that if light came pri-
marily from one direction, the seedlings would bend
strongly toward it. If they covered the tip of the shoot with
a thin glass tube, the shoot would bend as if it were not
covered. However, if they used a metal foil cap to exclude
light from the plant tip, the shoot would not bend (figure
41.8). They also found that using an opaque collar to ex-
clude light from the stem below the tip did not keep the
area above the collar from bending.
In explaining these unexpected findings, the Darwins
hypothesized that when the shoots were illuminated from
one side, they bent toward the light in response to an “in-
fluence” that was transmitted downward from its source at
the tip of the shoot. For some 30 years, the Darwins’ per-
ceptive experiments remained the sole source of informa-
tion about this interesting phenomenon. Then Danish
plant physiologist Peter Boysen-Jensen and the Hungarian
plant physiologist Arpad Paal independently demonstrated
that the substance that caused the shoots to bend was a
chemical. They showed that if the tip of a germinating
grass seedling was cut off and then replaced with a small
block of agar separating it from the rest of the seedling, the
seedling would grow as if there had been no change. Some-
thing evidently was passing from the tip of the seedling
through the agar into the region where the bending oc-
curred. On the basis of these observations under conditions
of uniform illumination or of darkness, Paal suggested that
814 Part XI Plant Growth and Reproduction
(a)
1
2
3
4
Light
Lightproof cap
Transparent cap
Lightproof collar
(b)
FIGURE 41.8
The Darwins’ experiment.(a) Young grass seedlings normally
bend toward the light. (b) The bending (1) did not occur when the
tip of a seedling was covered with a lightproof cap (2), but did
occur when it was covered with a transparent one (3). When a
collar was placed below the tip (4), the characteristic light
response took place. From these experiments, the Darwins
concluded that, in response to light, an “influence” that caused
bending was transmitted from the tip of the seedling to the area
below, where bending normally occurs.
an unknown substance continually moves down from the
tips of grass seedlings and promotes growth on all sides.
Such a light pattern would not, of course, cause the shoot
to bend.
Then, in 1926, Dutch plant physiologist Frits Went car-
ried Paal’s experiments an important step further. Went
cut off the tips of oat seedlings that had been illuminated
normally and set these tips on agar. He then took oat
seedlings that had been grown in the dark and cut off their
tips in a similar way. Finally, Went cut tiny blocks from the
agar on which the tips of the light-grown seedlings had
been placed and placed them off-center on the tops of the
decapitated dark-grown seedlings (figure 41.9). Even
though these seedlings had not been exposed to the light
themselves, they bent away from the side on which the agar
blocks were placed.
Went then put blocks of pure agar on the decapitated
stem tips and noted either no effect or a slight bending to-
ward the side where the agar blocks were placed. Finally,
Went cut sections out of the lower portions of the light-
grown seedlings to see whether the active principle was
present in them. He placed these sections on the tips of de-
capitated, dark-green oat seedlings and again observed no
effect.
As a result of his experiments, Went was able to show
that the substance that had diffused into the agar from the
tips of light-grown oat seedlings could make seedlings
curve when they otherwise would have remained straight.
He also showed that this chemical messenger caused the
cells on the side of the seedling into which it flowed to
grow more than those on the opposite side (figure 41.10).
In other words, it enhanced rather than retarded cell elon-
gation. He named the substance that he had discovered
auxin, from the Greek word auxein, which means “to in-
crease.”
Went’s experiments provided a basis for understanding
the responses that the Darwins had obtained some 45 years
earlier. The oat seedlings bent toward the light because of
differences in the auxin concentrations on the two sides of
the shoot. The side of the shoot that was in the shade had
more auxin, and its cells therefore elongated more than
those on the lighted side, bending the plant toward the
light.
Chapter 41 How Plants Grow in Response to Their Environment 815
Auxin in tip
of seedling
Agar
Auxin diffuses into
agar block
Auxin
1 2 3
FIGURE 41.9
Frits Went’s experiment.(1) Went
removed the tips of oat seedlings and
put them in agar, an inert, gelatinous
substance. (2) Blocks of agar were then
placed off-center on the ends of other
oat seedlings from which the tips had
been removed. (3) The seedlings bent
away from the side on which the agar
block was placed. Went concluded that
the substance that he named auxin
promoted the elongation of the cells
and that it accumulated on the side of
an oat seedling away from the light.
Light
Lighted side
of seedling
Shaded side
of seedling
FIGURE 41.10
Auxin causes cells on the dark side to elongate.Went
determined that a substance called auxin enhanced cell elongation.
Plant cells that are in the shade have more auxin and grow faster
than cells on the lighted side, causing the plant to bend toward
light. Further experiments showed exactly why there is more auxin
on the shaded side of a plant.
The Effects of Auxins
Auxin acts to adapt the plant to its environment in a highly
advantageous way. It promotes growth and elongation and
facilitates the plant’s response to its environment. Environ-
mental signals directly influence the distribution of auxin in
the plant. How does the environment—specifically, light—
exert this influence? Theoretically, it might destroy
the auxin, decrease the cells’ sensitivity to auxin, or cause
the auxin molecules to migrate away from the light into the
shaded portion of the shoot. This last possibility has proved
to be the case.
In a simple but effective experiment, Winslow Briggs in-
serted a thin sheet of mica vertically between the half of the
shoot oriented toward the light and the half of the shoot
oriented away from it (figure 41.11). He found that light
from one side does not cause a shoot with such a barrier to
bend. When Briggs examined the illuminated plant, he
found equal auxin levels on both the light and dark sides of
the barrier. He concluded that a normal plant’s response to
light from one direction involves auxin migrating from the
light side to the dark side, and that the mica barrier pre-
vented a response by blocking the migration of auxin.
The effects of auxin are numerous and varied. Auxin
promotes the activity of the vascular cambium and the vas-
cular tissues. Also, auxins are present in pollen in large
quantities and play a key role in the development of fruits.
Synthetic auxins are used commercially for the same pur-
pose. Fruits will normally not develop if fertilization has
not occurred and seeds are not present, but frequently they
will if auxins are applied. Pollination may trigger auxin re-
lease in some species leading to fruit development occur-
ring even before fertilization.
How Auxin Works
In spite of this long history of research on auxin, its molec-
ular basis of action has been an enigma. The chemical
structure of IAA resembles that of the amino acid trypto-
phan, from which it is probably synthesized by plants (fig-
ure 41.12). Unlike animal hormones, a specific signal is not
sent to specific cells, eliciting a predictable response. There
are most likely multiple auxin perception sites. Auxin is also
unique among the plant hormones in that it is transported
toward the base of the plant. Two families of genes have
been identified in Arabidopsis that are involved in auxin
transport. For example, one protein is involved in the top
to bottom transport of auxin; while two other proteins
function in the root tip to regulate the growth response to
gravity. We are still a ways from linking the measurable
and observable effects of auxin to events that transpire after
it travels to a site and binds to a receptor.
816 Part XI Plant Growth and Reproduction
Light
Light
Auxin in
seedling tip
(a) (b) (c)
24H11543
31H11543
12H11543
(d)
(a) IAA (Indoleacetic acid)
CH
2
CH
NH
2
COOH
(b) Tryptophan
CH
2
O COOH
CH
2
COOH
(c) Dichlorophenoxyacetic acid
(2,4-D)
Cl Cl
N
H
N
H
FIGURE 41.12
Auxins.(a) Indoleacetic acid (IAA), the
principal naturally occurring auxin. (b)
Tryptophan, the amino acid from which
plants probably synthesize IAA. (c)
Dichlorophenoxyacetic acid (2,4-D), a
synthetic auxin, is a widely used herbicide.
FIGURE 41.11
Phototropism and auxin: the Winslow Briggs experiments.The basic design of these
experiments was to place the tip of an oat seedling on an agar block, apply light from one
side, and observe the degree of curvature produced when the agar blocks were later placed
on the decapitated seedlings. However, Briggs inserted a barrier in various places and
noted how this affected the location of auxin. A comparison of (a) and (b) with similar
experiments performed in the dark showed that auxin production does not depend on
light; all produced approximately 24? of curvature. If a barrier was inserted in the agar
block (d), light caused the displacement of the auxin away from the light. Finally,
experiment (c) showed that it was displacement that had occurred, and not different rates
of auxin production on the dark and light sides, because when displacement was prevented
with a barrier, both sides of the agar block produced about 24? of curvature.
One of the downstream effects of auxin is an increase in
plasticity of the plant cell wall. This will only work on
young cell walls without extensive secondary cell wall for-
mation. A more plastic wall will stretch more while its pro-
toplast is swelling during active cell growth. The acid
growth hypothesis provides a model linking auxin to cell
wall expansion (figure 41.13). Auxin causes responsive cells
to release hydrogen ions into the cell wall. This decreases
the pH which activates enzymes that can break bonds be-
tween cell wall fibers. Remember that different enzymes op-
erate optimally at different pHs. This hypothesis has been
experimentally supported in several ways. Buffers that pre-
vent cell wall acidification block cell expansion. Other com-
pounds that release hydrogen ions from the cell can also
cause cell expansion. The movement of hydrogen ions has
been observed in response to auxin treatment. This hypoth-
esis explains the rapid growth response. There are also de-
layed responses which most likely involve auxin-stimulated
gene expression.
Synthetic Auxins. Synthetic auxins such as NAA (naph-
thalene acetic acid) and IBA (indolebutyric acid) have many
uses in agriculture and horticulture. One of their most im-
portant uses is based on their prevention of abscission, the
process that causes a leaf or other organ to fall from a
plant. Synthetic auxins are used to prevent fruit drop in ap-
ples before they are ripe and to hold berries on holly that is
being prepared for shipping. Synthetic auxins are also used
to promote flowering and fruiting in pineapples and to in-
duce the formation of roots in cuttings.
Synthetic auxins are routinely used to control weeds.
When used as herbicides, they are applied in higher con-
centrations than IAA would normally occur in plants. One
of the most important synthetic auxin herbicides is 2,4-
dichlorophenoxyacetic acid, usually known as 2,4-D (see
figure 41.12c). It kills weeds in grass lawns by selectively
eliminating broad-leaved dicots. The stems of the dicot
weeds cease all axial growth.
The herbicide 2,4,5-trichlorophenoxyacetic acid, better
known as 2,4,5-T, is closely related to 2,4-D. 2,4,5-T was
widely used as a broad-spectrum herbicide to kill weeds and
seedlings of woody plants. It became notorious during the
Vietnam War as a component of a jungle defoliant known
as Agent Orange and was banned in 1979 for most uses in
the United States. When 2,4,5-T is manufactured, it is un-
avoidably contaminated with minute amounts of dioxin.
Dioxin, in doses as low as a few parts per billion, has pro-
duced liver and lung diseases, leukemia, miscarriages, birth
defects, and even death in laboratory animals. Vietnam vet-
erans and children of Vietnam veterans exposed to Agent
Orange have been among the victims.
Auxin is synthesized in apical meristems of shoots. It
causes young stems to bend toward light when it
migrates toward the darker side, where it makes young
cell walls more plastic and thereby promotes cell
elongation. By interacting with other hormones, auxin
can promote an increase in girth and is involved in
growth responses to gravity and fruit ripening.
Chapter 41 How Plants Grow in Response to Their Environment 817
H
+
H
+
H
+
H
+
H
+
H
+
Turgor
Cytoplasm
Auxin
Cellulose fiber in cell wall
Enzyme (inactive)
Cross bridge
Active
enzyme
1. Auxin causes cells to pump
hydrogen ions into the cell wall.
2. pH in the cell wall decreases,
activating enzymes that break
cross-bridges between cellulose
fibers in the cell wall.
3. Cellulose fibers loosen and
allow the cell to expand as turgor
pressure inside the cell pushes
against the cell wall.
FIGURE 41.13
Acid growth
hypothesis.Auxin
stimulates the release of
hydrogen ions from the
target cells which alters
the pH of the cell wall.
This optimizes the
activity of enzymes
which break bonds in
the cell wall, allowing
them to expand.
Cytokinins
Cytokinins comprise another group of naturally occurring
growth hormones in plants. Studies by Gottlieb Haber-
landt of Austria around 1913 demonstrated the existence of
an unknown chemical in various tissues of vascular plants
that, in cut potato tubers, would cause parenchyma cells to
become meristematic, and would induce the differentiation
of a cork cambium. The role of cytokinins, active compo-
nents of coconut milk, in promoting the differentiation of
organs in masses of plant tissue growing in culture later led
to their discovery. Subsequent studies have focused on the
role cytokinins play in the differentiation of tissues from
callus.
A cytokinin is a plant hormone that, in combination
with auxin, stimulates cell division and differentiation in
plants. Most cytokinins are produced in the root apical
meristems and transported throughout the plant. Develop-
ing fruits are also important sites of cytokinin synthesis. In
mosses, cytokinins cause the formation of vegetative buds
on the gametophyte. In all plants, cytokinins, working with
other hormones, seem to regulate growth patterns.
All naturally occurring cytokinins are purines that ap-
pear to be derivatives of, or have molecule side chains simi-
lar to, those of adenine (figure 41.14). Other chemically di-
verse molecules, not known to occur naturally, have effects
similar to those of cytokinins. Cytokinins promote growth
of lateral buds into branches (figure 41.15); though, along
with auxin and ethylene, they also play a role in apical
dominance (the suppression of lateral bud growth). Con-
818 Part XI Plant Growth and Reproduction
Adenine
Kinetin
CH
2
O
6-Benzylamino purine (BAP)
CH
2
NH
NN
N
H
N
NH
2
NN
N
H
N
NH
NN
N
H
N
FIGURE 41.14
Some cytokinins.Two commonly used synthetic cytokinins:
kinetin and 6-benzylamino purine. Note their resemblance to the
purine base adenine.
Lateral buds
Lateral branches
Apical bud
removed
Apical bud
removed
Auxin
Lateral
buds
FIGURE 41.15
Cytokinins stimulate lateral
bud growth.(a) When the
apical meristem of a plant is
intact, auxin from the apical
bud will inhibit the growth of
lateral buds. (b) When the
apical bud is removed,
cytokinins are able to produce
the growth of lateral buds
into branches. (c) When the
apical bud is removed and
auxin is added to the cut
surface, axillary bud
outgrowth is suppressed.
(a) (b) (c)
versely, cytokinins inhibit formation of lateral roots, while
auxins promote their formation. As a consequence of these
relationships, the balance between cytokinins and auxin,
along with other factors, determines the appearance of a
mature plant. In addition, the application of cytokinins to
leaves detached from a plant retards their yellowing. They
function as anti-aging hormones.
The action of cytokinins, like that of other hormones,
has been studied in terms of its effects on growth and dif-
ferentiation of masses of tissue growing in defined media.
Plant tissue can form shoots, roots, or an undifferentiated
mass of tissues, depending on the relative amounts of auxin
and cytokinin (figure 41.16). In the early cell-growth exper-
iments coconut “milk” was an essential factor. Eventually,
it was discovered that coconut “milk” is not only rich in
amino acids and other reduced nitrogen compounds re-
quired for growth, but it also contains cytokinins. Cy-
tokinins seem to be essential for mitosis and cell division.
They apparently promote the synthesis or activation of
proteins that are specifically required for mitosis.
Cytokinins have also been used against plants by
pathogens. The bacteria Agrobacterium, for example, intro-
duces genes into the plant genome that increase the rate of
cytokinin, as well as auxin, production. This causes massive
cell division and the formation of a tumor called crown gall
(figure 41.17). How these hormone biosynthesis genes
ended up in a bacterium is an intriguing evolutionary ques-
tion. Coevolution does not always work to the plant’s
advantage.
Cytokinins are plant hormones that, in combination
with auxin, stimulate cell division and, along with a
number of other factors, determine the course of
differentiation. In contrast to auxins, cytokinins are
purines that are related to or derived from adenine.
Chapter 41 How Plants Grow in Response to Their Environment 819
Auxin:
Cytokinin:
High
HighLow
Low Intermediate
Intermediate
FIGURE 41.16
Relative amounts of cytokinins and auxin affect organ
regeneration in culture.In the case of tobacco, (a) high auxin to
cytokinin ratios favor root development; (b) high cytokinin to
auxin ratios favor shoot development; and (c) intermediate
concentrations result in the formation of undifferentiated cells.
These developmental responses to cytokinin/auxin ratios in
culture are species specific.
FIGURE 41.17
Crown gall tumor. Sometimes cytokinins can be used against the
plant by a pathogen. In this case Agrobacterium tumefaciens(a
bacteria) has incorporated a piece of its DNA into the plant
genome. This DNA contains genes coding for enzymes necessary
for cytokinin and auxin biosynthesis. The increased levels of these
hormones in the plant cause massive cell division and the
formation of a tumor.
Gibberellins
Gibberellins are named after the fungus Gibberella fu-
jikuroi, which causes rice plants, on which it is parasitic, to
grow abnormally tall. Japanese plant pathologist Eiichi
Kurosawa investigated Bakane (“foolish seedling”) disease
in the 1920s. He grew Gibberella in culture and obtained a
substance that, when applied to rice plants, produced
bakane. This substance was isolated and the structural for-
mula identified by Japanese chemists in 1939. British
chemists reconfirmed the formula in 1954. Although such
chemicals were first thought to be only a curiosity, they
have since turned out to belong to a large class of more
than 100 naturally occurring plant hormones called gib-
berellins. All are acidic and are usually abbreviated to GA
(for gibberellic acid), with a different subscript (GA
1
, GA
2
,
and so forth) to distinguish each one. While gibberellins
function endogenously as hormones, they also function as
pheromones in ferns. In ferns gibberellin-like compounds
released from one gametophyte can trigger the develop-
ment of male reproductive structures on a neighboring ga-
metophyte.
Gibberellins, which are synthesized in the apical por-
tions of stems and roots, have important effects on stem
elongation. The elongation effect is enhanced if auxin is
also present. The application of gibberellins to dwarf mu-
tants is known to restore the normal growth and develop-
ment in many plants (figure 41.18). Some dwarf mutants
produce insufficient amounts of gibberellin; while others
lack the ability to perceive gibberellin. The large number
of gibberellins are all part of a complex biosynthetic path-
way that has been unraveled using gibberellin-deficient
mutants in maize (corn). While many of these gibberellins
are intermediate forms in the production of GA
1
, recent
work shows that different forms may have specific biologi-
cal roles.
In chapter 41, we noted the role gibberellins stimulate
the production of H11008-amylase and other hydrolytic en-
zymes needed for utilization of food resources during ger-
mination and establishment of cereal seedlings. How are
the genes encoding these enzymes transcribed? Experi-
mental studies in the aleurone layer surrounding the en-
dosperms of cereal grains have shown that transcription
occurs when the gibberellins initiate a burst of messenger
RNA (mRNA) and protein synthesis. GA somehow en-
hances DNA binding proteins, which in turn allow DNA
transcription of a gene. Synthesis of DNA does not seem
to occur during the early stages of seed germination but
becomes important when the radicle has grown through
the seed coats.
Gibberellins also affect a number of other aspects of
plant growth and development. These hormones also has-
ten seed germination, apparently because they can substi-
tute for the effects of cold or light requirements in this
process. Gibberellins are used commercially to space grape
flowers by extending internode length so the fruits have
more room to grow (figure 41.19).
Gibberellins are an important class of plant hormones
that are produced in the apical regions of shoots and
roots. They play the major role in controlling stem
elongation for most plants, acting in concert with auxin
and other hormones.
820 Part XI Plant Growth and Reproduction
FIGURE 41.18
Effects of gibberellins.This rapid cycling member of the
mustard family plant (Brassica rapa)will “bolt” and flower because
of increased gibberellin levels. Mutants such as the rosette mutant
shown here (left) are defective in producing gibberellins. They can
be rescued by applying gibberellins. Other mutants have been
identified that are defective in perceiving gibberellins and they
will not respond to gibberellin applications.
FIGURE 41.19
Applications of gibberellins increase the space between
grapes. Larger grapes develop because there is more room
between individual grapes.
Brassinosteroids and
Oligosaccharins
Brassinosteroids
Although we’ve known about brassinosteroids for 30 years,
it is only recently that they have claimed their place as a
class of plant hormones. They were first discovered in Bras-
sica pollen, hence the name. Their historical absence in dis-
cussions of hormones may be partially due to their func-
tional overlap with other plant hormones, especially auxins
and gibberellins. Additive effects among these three classes
have been reported. The application of molecular genetics
to the study of brassinosteroids has led to tremendous ad-
vances in our understanding of how they are made and, to
some extent, how they function in signal transduction path-
ways. What is particularly intriguing about brassinosteroids
are similarities to animal steroid hormones (figure 41.20).
One of the genes coding for an enzyme in the brassinos-
teroid biosynthetic pathway has significant similarity to an
enzyme used in the synthesis of testosterone and related
steroids. Brassinosteroids have been identified in algae and
appear to be quite ubiquitous among the plants. It is plausi-
ble that their evolutionary origin predated the plant-animal
split.
Brassinosteroids have a broad spectrum of physiological
effects—elongation, cell division, bending of stems, vascu-
lar tissue development, delayed senescence, membrane po-
larization, and reproductive development. Environmental
signals can trigger brassinosteroid actions. Mutants have
been identified that alter the response to brassinosteroid,
but signal transduction pathways remain to be uncovered.
From an evolutionary perspective, it will be quite interest-
ing to see how these pathways compare with animal steroid
signal transduction pathways.
Oligosaccharins
In addition to cellulose, plant cell walls are composed of
numerous complex carbohydrates called oligosaccharides.
There is some evidence that these cell wall components
function as signaling molecules as well as structural wall
components. Oligosaccharides that are proposed to have a
hormonelike function are called oligosaccharins. Oligosac-
charins can be released from the cell wall by enzymes se-
creted by pathogens. These carbohydrates are believed to
signal defense responses, such as the hypersensitive re-
sponse discussed later in this chapter. Another oligosaccha-
rin has been shown to inhibit auxin-stimulated elongation
of pea stems. These molecules are active at concentrations
one to two orders of magnitude less than the traditional
plant hormones. You have seen how auxin and cytokinin
ratios can affect organogenesis in culture. Oligosaccharins
also affect the phenotype of regenerated tobacco tissue, in-
hibiting root formation and stimulating flower production
in tissues that are competent to regenerate flowers. How
the culture results translate to in vivosystems is an open
question. The structural biochemistry of oligosaccharins
makes them particularly challenging molecules to study.
How they interface with cells and initiate signal transduc-
tion pathways is an open question.
Brassinosteroids are structurally similar to animal
steroid hormones. They have many effects on plant
growth and development that parallel those of auxins
and gibberellins. Oligosaccharins are complex
carbohydrates that are released from cell walls and
appear to regulate both pathogen responses and growth
and development in some plants.
Chapter 41 How Plants Grow in Response to Their Environment 821
HO
HO
O
O
OH
OH
O
OH
O
CH
2
OH
CH
3
CH
3
HO
C
O
OH
CH
3
CH
3
HO
OH
CH
3
Estradiol
Testosterone
Cortisol
Brassinolide
FIGURE 41.20
Brassinosteroids, such as brassinolide, have structural
similarities to animal steroid hormones.
Ethylene
Long before its role as a plant hormone
was appreciated, the simple, gaseous
hydrocarbon ethylene (H
2
CH33527CH
2
) was
known to defoliate plants when it
leaked from gaslights in streetlamps.
Ethylene is, however, a natural product
of plant metabolism that, in minute
amounts, interacts with other plant
hormones. When auxin is transported
down from the apical meristem of the
stem, it stimulates the production of
ethylene in the tissues around the lat-
eral buds and thus retards their growth.
Ethylene also suppresses stem and root
elongation, probably in a similar way.
An ethylene receptor has been identi-
fied and characterized. It appears to
have evolved early in the evolution of
photosynthetic organisms, sharing fea-
tures with environmental-sensing pro-
teins identified in bacteria.
Ethylene plays a major role in fruit
ripening. At first, auxin, which is pro-
duced in significant amounts in polli-
nated flowers and developing fruits,
stimulates ethylene production; this, in
turn, hastens fruit ripening. Complex
carbohydrates are broken down into
simple sugars, chlorophylls are broken
down, cell walls become soft, and the
volatile compounds associated with fla-
vor and scent in ripe fruits are produced.
One of the first observations that led
to the recognition of ethylene as a plant
hormone was the premature ripening in
bananas produced by gases coming from oranges. Such rela-
tionships have led to major commercial uses of ethylene.
For example, tomatoes are often picked green and artifi-
cially ripened later by the application of ethylene. Ethylene
is widely used to speed the ripening of lemons and oranges
as well. Carbon dioxide has the opposite effect of arresting
ripening. Fruits are often shipped in an atmosphere of car-
bon dioxide. A biotechnology solution has also been devel-
oped (figure 41.21). One of the genes necessary for ethylene
biosynthesis has been cloned, and its antisense copy has
been inserted into the tomato genome. The antisense copy
of the gene is a nucleotide sequence that is complementary
to the sense copy of the gene. In this transgenic plant, both
the sense and antisense sequences for the ethylene biosyn-
thesis gene are transcribed. The sense and antisense mRNA
sequences then pair with each other. This blocks transla-
tion, which requires single-stranded RNA; ethylene is not
synthesized, and the transgenic tomatoes do not ripen.
Sturdy green tomatoes can be shipped without ripening and
rotting. Exposing these tomatoes to ethylene later will allow
them to ripen.
Studies have shown that ethylene plays an important eco-
logical role. Ethylene production increases rapidly when a
plant is exposed to ozone and other toxic chemicals, temper-
ature extremes, drought, attack by pathogens or herbivores,
and other stresses. The increased production of ethylene
that occurs can accelerate the loss of leaves or fruits that have
been damaged by these stresses. Some of the damage associ-
ated with exposure to ozone is due to the ethylene produced
by the plants. The production of ethylene by plants attacked
by herbivores or infected with diseases may be a signal to ac-
tivate the defense mechanisms of the plants. This may in-
clude the production of molecules toxic to the pests.
Ethylene, a simple gaseous hydrocarbon, is a naturally
occurring plant hormone. Among its numerous effects is
the stimulation of ripening in fruit. Ethylene production
is also elevated in response to environmental stress.
822 Part XI Plant Growth and Reproduction
Enzyme for
ethylene
biosynthesis
Gene for ethylene
biosynthesis enzyme
Transcription
mRNA
Translation
T
omatoes
T
omatoes
Antisense copy of gene
Transcription
Sense mRNA Antisense mRNA
Hybridization
No translation
and
no ethylene synthesis
Ethylene synthesis
Ethylene
Wild type
tomatoes
Transgenic
tomatoes
DNA
DNA
FIGURE 41.21
Genetic regulation of fruit ripening. An antisense copy of the gene for ethylene
biosynthesis prevents the formation of ethylene and subsequent ripening of transgenic fruit.
The antisense strand is complementary to the sequence for the ethylene biosynthesis gene.
After transcription, the antisense mRNA pairs with the sense mRNA, and the double-
stranded mRNA cannot be translated into a functional protein. Ethylene is not produced,
and the fruit does not ripen. The fruit is sturdier for shipping in its unripened form and can
be ripened later with exposure to ethylene. Thus, while wild-type tomatoes may already be
rotten and damaged by the time they reach stores, transgenic tomatoes stay fresh longer.
Abscisic Acid
Abscisic acid, a naturally occurring plant hormone, ap-
pears to be synthesized mainly in mature green leaves,
fruits, and root caps. The hormone earned its name be-
cause applications of it appear to stimulate leaf senescence
(aging) and abscission, but there is little evidence that it
plays an important role in this process. In fact, it is believed
that abscisic acid may cause ethylene synthesis, and that it
is actually the ethylene that promotes senescence and absci-
sion. When abscisic acid is applied to a green leaf, the areas
of contact turn yellow. Thus, abscisic acid has the exact op-
posite effect on a leaf from that of the cytokinins; a yellow-
ing leaf will remain green in an area where cytokinins are
applied.
Abscisic acid probably induces the formation of winter
buds—dormant buds that remain through the winter—by
suppressing growth. The conversion of leaf primordia into
bud scales follows (figure 41.22a). Like ethylene, it may
also suppress growth of dormant lateral buds. It appears
that abscisic acid, by suppressing growth and elongation of
buds, can counteract some of the effects of gibberellins
(which stimulate growth and elongation of buds); it also
promotes senescence by counteracting auxin (which tends
to retard senescence). Abscisic acid plays a role in seed dor-
mancy and is antagonistic to gibberellins during germina-
tion. It is also important in controlling the opening and
closing of stomata (figure 41.22b).
Abscisic acid occurs in all groups of plants and appar-
ently has been functioning as a growth-regulating sub-
stance since early in the evolution of the plant kingdom.
Relatively little is known about the exact nature of its phys-
iological and biochemical effects. These effects are very
rapid—often taking place within a minute or two—and
therefore they must be at least partly independent of gene
expression. Some longer-term effects of abscisic acid in-
volve the regulation of gene expression, but the way this
occurs is poorly understood. Abscisic acid levels become
greatly elevated when the plant is subject to stress, espe-
cially drought. Like other plant hormones, abscisic acid
probably will prove to have valuable commercial applica-
tions when its mode of action is better understood. It is a
particularly strong candidate for understanding desiccation
tolerance.
Abscisic acid, produced chiefly in mature green leaves
and in fruits, suppresses growth of buds and promotes
leaf senescence. It also plays an important role in
controlling the opening and closing of stomata. Abscisic
acid may be critical in ensuring survival under
environmental stress, especially water stresses.
Chapter 41 How Plants Grow in Response to Their Environment 823
FIGURE 41.22
Effects of abscisic acid.(a) Abscisic acid plays a role in the
formation of these winter buds of an American basswood. These
buds will remain dormant for the winter, and bud scales—
modified leaves—will protect the buds from desiccation.
(b) Abscisic acid also affects the closing of stomata by influencing
the movement of potassium ions out of guard cells.
(a)
(b)
Plants Undergo
Metamorphosis
Overview of Initiating Flowering
Carefully regulated processes deter-
mine when and where flowers will
form. Plants must often gain compe-
tence to respond to internal or exter-
nal signals regulating flowering. Once
plants are competent to reproduce, a
combination of factors including light,
temperature, and both promotive and
inhibitory internal signals determine
when a flower is produced (figure
41.23). These signals turn on genes
that specify where the floral organs,
sepals, petals, stamens, and carpels will
form. Once cells have instructions to become a specific flo-
ral organ, yet another developmental cascade leads to the
three-dimensional construction of flower parts.
Phase Change
Plants go through developmental changes leading to re-
productive maturity just like many animals. This shift
from juvenile to adult development is seen in the meta-
morphosis of a tadpole to an adult frog or caterpillar to a
butterfly that can then reproduce. Plants undergo a simi-
lar metamorphosis that leads to the production of a
flower. Unlike the frog that loses its tail, plants just keep
adding on structures to existing structures with their
meristems. At germination, most plants are incapable of
producing a flower, even if all the environmental cues are
optimal. Internal developmental changes allow plants to
obtain competence to respond to external and/or internal
signals that trigger flower formation. This transition is re-
ferred to as phase change. Phase change can be morpho-
logically obvious or very subtle. Take a look at an oak tree
in the winter. The lower leaves will still be clinging to the
branches, while the upper ones will be gone (figure
41.24a). Those lower branches were initiated by a juvenile
meristem. The fact that they did not respond to environ-
mental cues and drop their leaves indicates that they are
young branches and have not made a phase change. Ivy
also has distinctive juvenile and adult phases of growth
(figure 41.24b). Stem tissue produced by a juvenile meris-
tem initiates adventitious roots that can cling to walls. If
824 Part XI Plant Growth and Reproduction
41.3 The environment influences flowering.
Phase change
Juvenile Adult
Floral promoters,
floral inhibitors
Flowering
Temperature
Light
FIGURE 41.23
Factors involved in initiating flowering. This is a model of environmentally cued and
internally processed events that result in a shoot meristem initiating flowers.
FIGURE 41.24
Phase change.(a) The lower
branches of this oak tree represent
the juvenile phase of development
and cling to their leaves in the
winter. The lower leaves are not
able to form an abscission layer and
break off the tree in the fall. Such
visible changes are marks of phase
change, but the real test is whether
or not the plant is able to flower. (b)
Juvenile ivy (left) makes adventitious
roots and has an alternating leaf
arrangement. Adult ivy (right)
cannot make adventitious roots and
has leaves with a different
morphology that are arranged on an
upright stem in a spiral.
(a)
(b)
you look at very old brick buildings that are covered with
ivy, you will notice the uppermost branches are falling off
because they have transitioned to the adult phase of
growth and have lost the ability to produce adventitious
roots. It is important to remember that even though a
plant has reached the adult stage of development, it may
or may not produce reproductive structures. Other factors
may be necessary to trigger flowering.
Generally it is easier to get a plant to revert from an
adult to vegetative state than to induce phase change exper-
imentally. Applications of gibberellins and severe pruning
can cause reversion. There is evidence in peas and Ara-
bidopsis for genetically controlled repression of flowering.
The embryonic flower mutant of Arabidopsis flowers almost
immediately (figure 41.25), which is consistent with the hy-
pothesis that the wild-type allele suppresses flowering. It is
possible that flowering is the default state and that mecha-
nisms have evolved to delay flowering. This delay allows
the plant to store more energy to be allocated for repro-
duction.
The best example of inducing the juvenile to adult
transition comes from the construction of transgenic
plants that overexpress a gene necessary for flowering,
that is found in many species. This gene, LEAFY, was
cloned in Arabidopsis and its promoter was replaced with a
viral promoter that results in constant, high levels of
LEAFY transcription. This gene construct was then in-
troduced into cultured aspen cells which were used to re-
generate plants. When LEAFY is
overexpressed in aspen, flowering oc-
curs in weeks instead of years (figure
41.26). Phase change requires both
sufficient signal and the ability to per-
ceive the signal. Some plants acquire
competence in the shoot to perceive a
signal of a certain intensity. Others
acquire competence to produce suffi-
cient promotive signal(s) and/or de-
crease inhibitory signal(s).
Plants become reproductively
competent through changes in
signaling and perception. The
transition to the adult stage of
development where reproduction
is possible is called phase change.
Plants in the adult phase of
development may or may not
produce reproductive structures
(flowers), depending on
environmental cues.
Chapter 41 How Plants Grow in Response to Their Environment 825
FIGURE 41.25
In Arabidopsis, the embryonic flower gene may repress
flowering. The embryonic flowermutant flowers upon
germination.
(a) (b)
FIGURE 41.26
Overexpression of a flowering gene can accelerate phase change. (a) An aspen tree
normally grows for several years before producing flowers. (b) Overexpression of the
Arabidopsis flowering gene, LEAFY, causes rapid flowering in a transgenic aspen.
Pathways Leading to Flower
Production
The environment can promote or repress flowering. In
some cases, it can be relatively neutral. Light can be a sig-
nal that long, summer days have arrived in a temperate cli-
mate and conditions are favorable for reproduction. In
other cases, plants depend on light to accumulate sufficient
amounts of sucrose to fuel reproduction, but flower inde-
pendently of the length of day. Temperature can also be
used as a clue. Gibberellins are important and have been
linked to the vernalization pathway. Clearly, reproductive
success would be unlikely in the middle of a blizzard. As-
suming regulation of reproduction first arose in more con-
stant tropical environments, many of the day length and
temperature controls would have evolved as plants colo-
nized more temperate climates. Plants can rely primarily on
one pathway, but all three pathways can be present. The
complexity of the flowering pathways has been dissected
physiologically. Now analysis of flowering mutants is pro-
viding insight into the molecular mechanisms of the floral
pathways. The redundancy of pathways to flowering en-
sures that there will be another generation.
Light-Dependent Pathway
Flowering requires much energy accumulated via photosyn-
thesis. Thus, all plants require light for flowering, but this is
distinct from the photoperiodic, or light-dependent, flow-
ering pathway. Aspects of growth and development in
most plants are keyed to changes in the proportion of
light to dark in the daily 24-hour cycle (day length). This
provides a mechanism for organisms to respond to sea-
sonal changes in the relative length of day and night. Day
length changes with the seasons; the farther from the
equator, the greater the variation. Flowering responses of
plants to day length fall into several basic categories.
When the daylight becomes shorter than a critical
length, flowering is initiated in short-day plants (figure
41.27). When the daylight becomes longer than a critical
length, flowering is initiated in long-day plants. Other
plants, such as snapdragons, roses, and many native to the
tropics (for example, tomatoes), will flower when mature
regardless of day length, as long as they have received
enough light for normal growth. These are referred to as
day-neutral plants. Several grasses (for example, Indian
grass, Sorghastrum nutans), as well as ivy, have two critical
826 Part XI Plant Growth and Reproduction
Long-day plants Short-day plants
Short length of dark
required for bloom
Iris
Midnight
Noon
6 P.M. 6 A.M.
6 P.M. 6 A.M.
6 P.M. 6 A.M.
Early
summer
Late
fall Midnight
Noon
Flash of light
Noon
(a)
(b)
Goldenrod
Long length of dark
required for bloom
FIGURE 41.27
How flowering responds to day length.
(a) This iris is a long-day plant that is
stimulated by short nights to flower in the
spring. The goldenrod is a short-day plant
that, throughout its natural distribution in
the northern hemisphere, is stimulated by
long nights to flower in the fall. (b) If the
long night of winter is artificially
interrupted by a flash of light, the
goldenrod will not flower, and the iris will.
In each case, it is the duration of
uninterrupted darkness that determines
when flowering will occur.
photoperiods; they will not flower if the days are too
long, and they also will not flower if the days are too
short. In some species, there is a sharp distinction be-
tween long and short days. In others, flowering occurs
more rapidly or slowly depending on the length of day.
These plants rely on other flowering pathways as well
and are called facultative long- or short-day plants. The
garden pea is an example of a facultative long-day plant.
In all of these plants, it is actually the length of darkness
(night), not the length of day, that is physiologically sig-
nificant. Using light as a cue permits plants to flower
when abiotic environmental conditions are optimal, polli-
nators are available, and competition for resources with
other plants may be less. For example, the spring
ephemerals flower in the woods before the canopy leafs
out, blocking sunlight necessary for photosynthesis.
At middle latitudes, most long-day plants flower in the
spring and early summer; examples of such plants include
clover, irises, lettuce, spinach, and hollyhocks. Short-day
plants usually flower in late summer and fall, and include
chrysanthemums, goldenrods, poinsettias, soybeans, and
many weeds. Commercial plant growers use these re-
sponses to day length to bring plants into flower at spe-
cific times. For example, photoperiod is manipulated in
greenhouses so poinsettias flower just in time for the
winter holidays (figure 41.28). The geographic distribu-
tion of certain plants may be determined by flowering re-
sponses to day length.
Photoperiod is perceived by several different forms of
phytochrome and also a blue-light-sensitive molecule
(cryptochrome). The conformational change in a light re-
ceptor molecule triggers a cascade of events that leads to
the production of a flower. There is a link between light
and the circadian rhythm regulated by an internal clock
that facilitates or inhibits flowering. At a molecular level
the gaps between light signaling and production of flowers
are rapidly filling in and the control mechanisms are quite
complex. Here is one example of how day length affects a
specific flowering gene in Arabidopsis, a facultative long-day
plant that flowers in response to both far-red and blue
light. Red light inhibits flowering. The gene CONSTANS
(CO) is expressed under long days but not short days. The
loss of CO product does not alter when a plant flowers
under short days, but delays flowering under long days.
What happens is that the gene is positively regulated by
cryptochrome that perceives blue light under long days.
Cryptochrome appears to inhibit the inhibition of flower-
ing by phytochrome B exposed to red light. Simply put,
flowering is promoted by repressing a gene that represses
flowering! CO is a transcription factor that turns on other
genes which results in the expression of LEAFY. As dis-
cussed in the section on phase change, LEAFY is one of the
key genes that “tells” a meristem to switch over to flower-
ing. We will see that other pathways also converge on this
important gene.
The Flowering Hormone: Does It Exist? The Holy
Grail in plant biology has been a flowering hormone,
quested unsuccessfully for more than 50 years. A consider-
able amount of evidence demonstrates the existence of sub-
stances that promote flowering and substances that inhibit
it. Grafting experiments have shown that these substances
can move from leaves to shoots. The complexity of their
interactions, as well as the fact that multiple chemical mes-
sengers are evidently involved, has made this scientifically
and commercially interesting search very difficult, and to
this day, the existence of a flowering hormone remains
strictly hypothetical. We do know that LEAFY can be ex-
pressed in the vegetative as well as the reproductive por-
tions of plants. Clearly, information about day length gath-
ered by leaves is transmitted to shoot apices. Given that
there are multiple pathways to flowering, several signals
may be facilitating communication between leaves and
shoots. We also know that roots can be a source of floral
inhibitors affecting shoot development.
Chapter 41 How Plants Grow in Response to Their Environment 827
FIGURE 41.28
Manipulation of photoperiod in greenhouses ensures that
short-day poinsettias flower in time for the winter holidays.
Note that the colorful “petals” are actually sepals. Even after
flowering is induced, there are many developmental events leading
to the production of species-specific flowers.
Temperature-Dependent Pathway
Lysenko solved the problem of winter wheat seed rotting in
the fields in Russia by chilling the seeds and planting them
in the spring. Winter wheat would not flower without a pe-
riod of chilling, called vernalization. Unfortunately a great
many problems, including mistreatment of Russian geneti-
cists, resulted from this scientifically significant discovery.
Lysenko erroneously concluded that he had converted one
species, winter wheat, to another, spring wheat, by simply
altering the environment. There was a shift from science to
politics. Genetics and Darwinian evolution were suspect for
half a century. Social history aside, the valuable lesson here
is that cold temperatures can accelerate or permit flowering
in many species. As with light, this ensures that plants
flower at more optimal times.
Vernalization may be necessary for seeds or plants in
later stages of development. Analysis of mutants in
Arabidopsis and pea indicate that vernalization is a sepa-
rate flowering pathway that may be linked to the hor-
mone gibberellin. In this pathway, repression may also
lead to flowering. High levels of one of the genes in the
pathway may block the promotion of flowering by gib-
berellins. When plants are chilled, there is less of this
gene product and gibberellin activity may increase. It is
known that gibberellins enhance the expression of
LEAFY. One proposal is that both the vernalization and
autonomous pathways share a common intersection af-
fecting gibberellin promotion of flowering. Weigel has
shown that gibberellin actually binds the promoter of the
LEAFY gene, so its effect on flowering is direct. The con-
nection between gibberellin levels and temperature also
needs to be understood.
Autonomous Pathway
The autonomous pathway to flowering is independent of
external cues except for basic nutrition. Presumably this
was the first pathway to evolve. Day-neutral plants often
depend primarily on the autonomous pathway which allows
plants to “count” and “remember.” A field of day-neutral
tobacco will produce a uniform number of nodes before
flowering. If the shoots of these plants are removed at dif-
ferent positions, axillary buds will grow out and produce
the same number of nodes as the removed portion of the
shoot (figure 41.29). At a certain point in development
shoots become committed or determined to flower (figure
41.30). The upper axillary buds of flowering tobacco will
remember their position when rooted or grafted. The ter-
minal shoot tip becomes florally determined about four
nodes before it initiates a flower. In some other species,
this commitment is less stable and/or occurs later.
How do shoots know where they are and at some point
“remember” that information? It is clear that there are in-
hibitory signals from the roots. If bottomless pots are con-
tinuously placed over a growing tobacco plant and filled
with soil, flowering is delayed by the formation of adventi-
tious roots (figure 41.31). Control experiments with leaf re-
moval show that it is the addition of roots and not the loss
of leaves that delays flowering. A balance between floral
promoting and inhibiting signals may regulate when flow-
ering occurs in the autonomous pathway and the other
pathways as well.
828 Part XI Plant Growth and Reproduction
Intact
plant
Shoot
removed
Replacement
shoot
Intact
plant
Shoot
removed
Replacement
shoot
(a) Upper axillary bud removed
(b) Lower axillary bud removed
Shoot
removed
here
Shoot
removed
here
FIGURE 41.29
Plants can count.When axillary buds of flowering tobacco
plants are released from apical dominance by removing the main
shoot, they replace the number of nodes that were initiated by the
main shoot. (After McDaniel 1996.)
Determination for flowering is tested at the organ or
whole plant level by changing the environment and ascer-
taining whether or not the fate has changed. How does flo-
ral determination correlate with molecular level changes?
In Arabidopsis, floral determination correlates with the in-
crease of LEAFY gene expression and has occurred by the
time a second flowering gene, APETALA1, is expressed.
Because all three flowering pathways appear to converge
with increased levels of LEAFY, this determination event
should occur in species with a variety of balances among
the pathways.
Plants use light receptor molecules to measure the
length of night. This information is then used to signal
pathways that promote or inhibit flowering. Light
receptors in the leaves trigger events that result in
changes in the shoot meristem. Vernalization is the
requirement for a period of chilling before a plant can
flower. The autonomous pathway leads to flowering
independent of environmental cues. Plants integrate
information about position in regulating flowering and
both promoters and inhibitors of flowering are
important.
Chapter 41 How Plants Grow in Response to Their Environment 829
(a) Shoot florally determined (b) Shoot not florally determined
Shoot
removed
here
Intact
plant
Intact
plant
Shoot
removed
Shoot
removed
Rooted
shoot
Flowering
rooted
shoot
Rooted
shoot
Flowering
rooted
shoot
Shoot
removed
here
FIGURE 41.30
Plants can remember.At a certain point in the flowering process, shoots become committed to making a flower. This is called floral
determination. (a) Florally determined axillary buds “remember” their position when rooted in a pot. That is, they produce the same
number of nodes that they would have if they had grown out on the plant, and then they flower. (b) Those that are not yet florally
determined cannot remember how many nodes they have left, so they start counting again. That is, they develop like a seedling and then
flower. (After McDaniel 1996.)
Control plants:
no treatment
Experimental plants:
pot-on-pot treatment
Control plants:
Lower leaves were
continually removed
FIGURE 41.31
Roots can inhibit flowering.Adventitious roots formed as
bottomless pots were continuously placed over growing tobacco
plants, delaying flowering. The delay in flowering is caused by the
roots, not the loss of the leaves. This was shown by removing leaves
on control plants at the same time and in the same position as leaves
on experimental plants that became buried as pots were added.
Identity Genes and the Formation of
Floral Meristem and Floral Organs
Arabidopsis and snapdragon are valuable model systems
for identifying flowering genes and understanding their
interactions. The three pathways, discussed in the previ-
ous section, lead to an adult meristem becoming a floral
meristem by either activating or repressing the inhibition
of floral meristem identity genes (figure 41.32). Two of
the key floral meristem identity genes are LEAFY and
APETALA1. These genes establish the meristem as a
flower meristem. They then turn on floral organ identity
genes. The floral organ identity genes define four con-
centric whorls moving inward in the floral meristem as
sepal, petal, stamen, and carpel. Meyerowitz and Coen
proposed a model, called the ABC model, to explain how
three classes of floral organ identity genes could specify
four distinct organ types (figure 41.33). The ABC model
proposes that three classes of organ identity genes (A, B,
and C) specify the floral organs in the four floral whorls.
By studying mutants the researchers have determined the
following:
1. Class Agenes alone specify the sepals.
2. Class Aand class B genes together specify the petals.
3. Class B and class C genes together specify the
stamens.
4. Class Cgenes alone specify the carpels.
The beauty of their ABC model is that it is entirely testable
by making different combinations of floral organ identity
mutants. Each class of genes is expressed in two whorls,
yielding four different combinations of the gene products.
When any one class is missing, there are aberrant floral or-
gans in predictable positions.
It is important to recognize that this is actually only the
beginning of the making of a flower. These organ identity
genes are transcription factors that turn on many more
genes that will actually give rise to the three-dimensional
flower. There are also genes that “paint” the petals. Com-
plex biochemical pathways lead to the accumulation of an-
thocyanin pigments in vacuoles. These pigments can be or-
ange, red, or purple and the actual color is influenced by
pH and by the shape of the petal.
The Formation of Gametes
The ovule within the carpel has origins more ancient than
the angiosperms. Floral parts are modified leaves, and
within the ovule is the female gametophyte. This next
generation develops from placental tissue in the ovary. A
megaspore mother cell develops and meiotically gives rise
to the embryo sac. Usually two layers of integument tissue
form around this embryo sac and will become the seed
coat. Genes responsible for the initiation of integuments
and also those responsible for the formation of the integu-
ment have been identified. Some also affect leaf structure.
This chapter has focused on the complex and elegant
process that gives rise to the reproductive structure called
the flower. It is indeed a metamorphosis, but the subtle
shift from mitosis to meiosis in the megaspore mother cell
leading to the development of a haploid,
gamete-producing gametophyte is per-
haps even more critical. The same can be
said for pollen formation in the anther of
the stamen. As we will see in the next
chapter, the flower houses the haploid
generations that will produce gametes.
The flower also functions to increase the
probability that male and female gametes
from different (or sometimes the same
plant) will unite.
Floral structures form as a result of
floral meristem identity genes
turning on floral organ identity genes
which specify where sepals, petals,
stamens, and carpels will form. This
is followed by organ development
which involves many complex
pathways that account for floral
diversity among species.
830 Part XI Plant Growth and Reproduction
Repression of floral inhibitors
Activation of floral meristem identity genes
Phytochrome and
cryptochrome
Gibberellin
production
Light
Cold
temperature
Light
dependent
pathway
Autonomous
pathway
Temperature
dependent
pathway
Adult meristem Floral meristem
FIGURE 41.32
Model for flowering.The light-dependent, temperature-dependent, and autonomous
flowering pathways promote the formation of floral meristems from adult meristems by
repressing floral inhibitors and activating floral meristem identity genes.
Chapter 41 How Plants Grow in Response to Their Environment 831
Whorl 1
Sepal
Whorl 2
Petal
Whorl 3
Stamen
Whorl 4
Carpel
AA
A
and
B
A
and
B
B
and
C
B
and
C
C
CC
B
and
C
B
and
C
B
and
C
B
and
C
C
Wild type
floral meristem
Sepals
Petals
Stamens
Carpels
Development
Development
Development
Development
Cross-section of wild type flower
–A mutant
floral meristem:
missing gene
class A
–B mutant
floral meristem:
missing gene
class B
–C mutant
floral meristem:
missing gene
class C
Carpels
Carpels
Stamens
Stamens
Sepals
Sepals
Carpels
Carpels
AA AA
A
A
A
C
CC
Cross-section of –A mutant flower
Cross-section of –B mutant flower
Cross-section of –C mutant flower
Sepals
Petals
Petals
Sepals
A
and
B
A
and
B
A
and
B
A
and
B
FIGURE 41.33
ABC model for floral organ
specification.
Letters labeling whorls indicate
which gene classes are active.
When Afunction is lost (-A), C
expands to the first and second
whorls. When Bfunction is lost
(-B), both outer two whorls have
just Afunction, and both inner
two whorls have just Cfunction;
none of the whorls have dual
gene function. When Cfunction
is lost (-C), Aexpands into the
inner two whorls. These new
combinations of gene expression
patterns alter which floral
structures form in each whorl.
(Model proposed by Coen and
Meyerowitz, 1991.)
Larger predators, microbes, water, and wind often present
a plant with rapid immediate stress. Response, to be effec-
tive, must also be immediate. There is little time for
growth, and plants instead invoke a variety of other kinds
of responses. Many environmental cues trigger rapid and
reversible localized plant movements, for example. The
rapid folding of leaves can startle a potential predator. Leaf
folding can also prevent water loss by reducing the surface
area available for transpiration. Some localized plant move-
ments are triggered by unpredictable environmental sig-
nals. Other movements are tied into daily internal rhythms
established by cyclic environmental signals like light and
temperature. Plants lack a nervous system in the conven-
tional sense. Some of the rapid signaling, however, is the
result of electric charge moving through an organ as a wave
of membrane ion exchange, not unlike that seen in animals.
This is translated into movement by changing the turgor
pressure of cells.
Turgor Movement
Turgor is pressure within a living cell resulting from diffu-
sion of water into it. If water leaves turgid cells (ones with
turgor pressure), the cells may collapse, causing plant
movement; conversely, water entering a limp cell may also
cause movement as the cell once more becomes turgid.
Many other plants, including those of the legume family
(Fabaceae), exhibit leaf movements in response to touch or
other stimuli. After exposure to a stimulus, the changes in
leaf orientation are mostly associated with rapid turgor
pressure changes in pulvini (singular: pulvinus), which are
multicellular swellings located at the base of each leaf or
leaflet. When leaves with pulvini, such as those of the sen-
sitive plant (Mimosa pudica), are stimulated by wind, heat,
touch, or, in some instances, intense light, an electrical sig-
nal is generated. The electrical signal is translated into a
chemical signal, with potassium ions, followed by water,
migrating from the cells in one half of a pulvinus to the in-
tercellular spaces in the other half. The loss of turgor in
half of the pulvinus causes the leaf to “fold.” The move-
ments of the leaves and leaflets of the sensitive plant are es-
pecially rapid; the folding occurs within a second or two
after the leaves are touched (figure 41.34). Over a span of
about 15 to 30 minutes after the leaves and leaflets have
folded, water usually diffuses back into the same cells from
which it left, and the leaf returns to its original position.
832 Part XI Plant Growth and Reproduction
41.4 Many short-term responses to the environment do not require growth.
Pulvinus
Vascular tissue
Cells retaining
turgor
Cells losing
turgor
(a) (b)
FIGURE 41.34
Sensitive plant (Mimosa pudica).(a) The
blades of Mimosaleaves are divided into
numerous leaflets; at the base of each leaflet
is a swollen structure called a pulvinus.
(b) Changes in turgor cause leaflets to fold
in response to a stimulus. (c) When leaves
are touched (center two leaves above), they
fold due to loss of turgor.
The leaves of some plants with similar mechanisms may
track the sun, with their blades oriented at right angles to
it; how their orientation is directed is, however, poorly un-
derstood. Such leaves can move quite rapidly (as much as
15°an hour). This movement maximizes photosynthesis
and is analogous to solar panels that are designed to track
the sun.
Some of the most familiar of these reversible changes
are seen in leaves and flowers that “open” during the day
and “close” at night. For example, the flowers of four
o’clocks open at 4 P.M.and evening primrose petals open at
night. The blades of plant leaves that exhibit such a daily
shift in position may not actually fold; instead, their orien-
tation may be changed as a result of turgor movements.
Bean leaves are horizontal during the day when their pul-
vini are turgid, but become more or less vertical at night as
the pulvini lose turgor (figure 41.35). These sleep move-
ments reduce water loss from transpiration during the
night, but maximize photosynthetic surface area during the
day. In these cases, the movement is closely tied to an in-
ternal rhythm.
Circadian Clocks
How do leaves know when to “sleep”? They have endoge-
nous circadian clocks that set a rhythm with a period of
about 24 hours (actually it is closer to 22 or 23 hours).
While there are shorter and much longer, naturally occur-
ring rhythms, circadian rhythms are particularly common
and widespread because of the day-night cycle on earth.
Jean de Mairan, a French astronomer, first identified circa-
dian rhythms in 1729. He studied the sensitive plant which,
in addition to having a touch response, closes its leaflets
and leaves at night like the bean plant described above.
When de Mairan put the plants in total darkness, they con-
tinued “sleeping” and “waking” just as they did when ex-
posed to night and day. This is one of four characteristics
of a circadian rhythm: it must continue to run in the ab-
sence of external inputs. It must be about 24 hours in dura-
tion and can be reset or entrained. (Perhaps you’ve experi-
enced entrainment when traveling to a different time zone
in the form of jet-lag recovery.) The fourth characteristic is
that the clock can compensate for differences in tempera-
ture. This is quite unique when you consider what you
know about biochemical reactions; most rates of reactions
vary significantly based on temperature. Circadian clocks
exist in many organisms and appear to have evolved inde-
pendently multiple times. The mechanism behind the clock
is not fully understood, but is being actively investigated at
the molecular level.
Turgor movements of plants are reversible and involve
changes in the turgor pressure of specific cells.
Circadian clocks are endogenous timekeepers that keep
plant movements and other responses synchronized
with the environment.
Chapter 41 How Plants Grow in Response to Their Environment 833
FIGURE 41.35
Sleep movements in bean leaves.In
the bean plant, leaf blades are oriented
horizontally during the day and
vertically at night.
12:00 NOON 3:00 P.M.
10:00 P.M. 12:00 MIDNIGHT
Plant Defense Responses
Interactions between plants and other organisms can be
symbiotic (for example, nitrogen-fixing bacteria and
mycorrhizae) or pathogenic. In evolutionary terms, these
two types of interactions may simply be opposite sides of
the same coin. The interactions have many common as-
pects and are the result of coevolution between two species
that signal and respond to each other. In the case of
pathogens, the microbe or pest is “winning,” at least for
that second in evolutionary time. In chapter 38, we dis-
cussed surface barriers the plant constructs to block inva-
sion. In this section, we will focus on cellular level re-
sponses to attacks by microbes and animals.
Recognizing the Invader
Half a century ago, Flor proposed that there is a plant re-
sistance gene (R) whose product interacts with that of a
pathogen avirulence gene (avr). This is called the gene-for-
gene model and several pairs of avr and R genes have been
cloned in different species pathogenized by microbes,
fungi, and insects, in one case. This has been motivated
partially by the agronomic benefit of identifying genes that
can be added to protect other plants from invaders. Much
is now known about the signal transduction pathways that
follow the recognition of the pathogen by the R gene.
These pathways lead to the triggering of the hypersensitive
response (HR) which leads to rapid cell death around the
source of the invasion and also a longer-term resistance
(figure 41.36). There is not always a gene-for-gene re-
sponse, but plants still have defense responses to pathogens
and also mechanical wounding. Some of the response path-
ways may be similar. Also, oligosaccharins in the cell walls
may serve as recognition and signaling molecules.
While our focus is on invaders outside the plant king-
dom, more is being learned about how parasitic plants in-
vade other plants. There are specific molecules released
from the root hairs of the host that the parasitic plant rec-
ognizes and responds to with invasive action. Less is known
about the host response and so far the different defense
genes that are activated appear to be ineffective.
Responding to the Invader
When a plant is attacked and there is gene-for-gene recog-
nition, the HR response leads to very rapid cell death
around the site of attack. This seals off the wounded tissue
to prevent the pathogen or pest from moving into the rest
of the plant. Hydrogen peroxide and nitric oxide are pro-
duced and may signal a cascade of biochemical events re-
sulting in the localized death of host cells. They may also
have negative effects on the pathogen, although antioxidant
abilities have coevolved in pathogens. Other antimicrobial
agents produced include the phytoalexins which are chemi-
cal defense agents. A variety of pathogenesis-related genes
(PR genes) are expressed and their proteins can either func-
tion as antimicrobial agents or signals for other events that
protect the plant.
In the case of virulent invaders (no R gene recognition),
there are changes in local cell walls that at least partially
block the movement of the pathogen or pest farther into
the plant. In this case there is not an HR response and the
local plant cells are not suicidal.
When an insect takes a mouthful of a leaf, defense
responses are also triggered. Mechanical damage causes re-
sponses that have some similar components, but the reac-
tion may be slower. Biochemically, it is distinct from some
of the events triggered by signals in the insect’s mouth.
Such responses are collectively called wound responses.
Wound responses are a challenge in designing other types
of experiments with plants that involve cutting or otherwise
mechanically damaging the tissue. It is important to run
control experiments to be sure you are answering your
question and not observing a wound response.
Preparing for Future Attacks
In addition to the HR or other local responses, plants are
capable of a systemic response to a pathogen or pest at-
tack. This is called a systemic acquired response (SAR).
Several pathways lead to broad-ranging resistance that
lasts for a period of days. The signals that induce SAR in-
clude salicylic acid and jasmonic acid. Salicylic acid is the
active ingredient in aspirin too! SAR allows the plant to
respond more quickly if it is attacked again. However, this
is not the same as the human immune response where an-
tibodies (proteins) that recognize specific antigens (for-
eign proteins) persist in the body. SAR is neither as spe-
cific or long lasting.
Plants defend themselves from invasion in ways
reminiscent of the animal immune system. When an
invader is recognized, localized cell death seals off the
infected area.
834 Part XI Plant Growth and Reproduction
Chapter 41 How Plants Grow in Response to Their Environment 835
Plant
cell
Plant
cells
HR
R protein
Microbial
protein
Hypersensitive response (HR):
local cell death seals off pathogen
Systemic acquired response (SAR):
temporary broad-ranging resistance to pathogen
Pathogenic
microbial
attack
Signal
molecule
Signal
molecule
SAR
FIGURE 41.36
Plant defense responses. In the gene-for-gene response, a cascade of events is triggered leading to local cell death (HR response) and
longer-term resistance in the rest of the plant (SAR).
836 Part XI Plant Growth and Reproduction
Chapter 41
Summary Questions Media Resources
41.1 Plant growth is often guided by environmental cues.
? Tropisms in plants are growth responses to external
stimuli, such as light, gravity, or contact.
? Dormancy is a plant adaptation that carries a plant
through unfavorable seasons or periods of drought.
1.In general, which part of a
plant is positively phototropic?
What is the adaptive significance
of this reaction?
? Auxin migrates away from light and promotes the
elongation of plant cells on the dark side, causing
stems to bend in the direction of light.
? Cytokinins are necessary for mitosis and cell division
in plants. They promote growth of lateral buds and
inhibit formation of lateral roots.
? Gibberellins, along with auxin, play a major role in
stem elongation in most plants. They also tend to
hasten the germination of seeds and to break
dormancy in buds.
2.How does auxin affect the
plasticity of the plant cell walls?
3.Where are most cytokinins
produced? From what
biomolecule do cytokinins
appear to be derived?
4.What plant hormones could
be lacking in genetically dwarfed
plants?
41.2 The hormones that guide growth are keyed to the environment.
? The transition of a shoot meristem from vegetative to
adult development is called phase change. During
phase change, plants gain competence to produce a
floral signal(s) and or perceive a signal.
? The light-dependent pathway uses information from
light receptor molecules integrated with a biological
clock to determine if the length of night is sufficient
for flowering.
? The autonomous path functions independently of
environmental cues. Internal floral inhibitor(s) from
roots and leaves and floral promoter(s) from leaves
move through the plant.
5.A plant has undergone phase
change. Although it is an adult,
it does not flower. How might
you get this plant to flower?
6.You have recently moved
from Canada to Mexico and
brought some seeds from your
favorite plants. They germinate
and produce beautiful leaves, but
never flower. What went wrong?
41.3 The environment influences flowering.
? Changes in turgor pressure reflect responses to
environmental signals that can protect plants from
predation.
? Other reversible movements in plants are caused by
changes in turgor pressure that are regulated by
internal circadian rhythms.
? Plants have the ability to recognize and respond to
invaders through cellular level recognition and
response.
7.How are motor cells involved
in the function of the pulvinus?
What happens in the motor cells
of the sensitive plant (Mimosa
pudica) when its leaves are
touched?
8.In what ways can a plant
protect itself from pathogenic
microbes? From animals?
41.4 Many short-term responses to the environment do not require growth.
www.mhhe.com/raven6e www.biocourse.com
? Photoperiod
? Hormones
? Student Research:
Plant Growth
? Student Research:
Selection in Flowering
Plants
837
42
Plant Reproduction
Concept Outline
42.1 Angiosperms have been incredibly successful, in
part, because of their reproductive strategies.
Rise of the Flowering Plants. Animal and wind
dispersal of pollen increases genetic variability in a species.
Seed and fruit dispersal mechanisms allow offspring to
colonize distant regions. Other features such as shortened
life cycles may also have been responsible for the rapid
diversification of the flowering plants.
Evolution of the Flower. A complete flower has four
whorls, containing protective sepals, attractive petals, male
stamens, and female ovules.
42.2 Flowering plants use animals or wind to transfer
pollen between flowers.
Formation of Angiosperm Gametes. The male
gametophytes are the pollen grains, and the female
gametophyte is the embryo sac.
Pollination. Evolutionary modifications of flowers have
enhanced effective pollination.
Self-Pollination. Self-pollination is favored in stable
environments, but outcrossing enhances genetic variability.
Fertilization. Angiosperms use two sperm cells, one to
fertilize the egg, the other to produce a nutrient tissue
called endosperm.
42.3 Many plants can clone themselves by asexual
reproduction.
Asexual Reproduction. Some plants do without sexual
reproduction, instead cloning new individuals from parts of
themselves.
42.4 How long do plants and plant organs live?
The Life Span of Plants. Clonal plants can live
indefinitely through their propagules. Parts of plants
senesce and die. Some plants reproduce sexually only once
and die.
T
he remarkable evolutionary success of flowering plants
can be linked to their reproductive strategies (figure
42.1). The evolution and development of flowers has been
discussed in chapters 37 and 41. Here we explore reproduc-
tive strategies in the angiosperms and how their unique fea-
tures, flowers and fruits, have contributed to their success.
This is, in part, a story of coevolution between plants and
animals that ensures greater genetic diversity by dispersing
plant gametes widely. However, in a stable environment,
there are advantages to maintaining the status quo geneti-
cally. Asexual reproduction is a strategy to clonally propa-
gate individuals. An unusual twist to sexual reproduction in
some flowering plants is that senescence and death of the
parent plant follows.
FIGURE 42.1
Reproductive success in flowering plants. Unique reproductive
systems and strategies have coevolved between plants and animals,
accounting for almost 250,000 flowering plants inhabiting all but
the harshest environments on earth.
form varies from cacti, grasses, and daisies to aquatic
pondweeds. Most shrubs and trees (other than conifers and
Ginkgo) are also in this phylum. This chapter focuses on re-
production in angiosperms (figure 42.2) because of their
tremendous success and many uses by humans. Virtually all
838 Part XI Plant Growth and Reproduction
Rise of the Flowering Plants
Most of the plants we see daily are angiosperms. The
250,000 species of flowering plants range in size from al-
most microscopic herbs to giant Eucalyptus trees, and their
42.1 Angiosperms have been incredibly successful, in part, because of their
reproductive strategies.
Anther
Microspore
mother cell (2n)
Megaspore
mother cell (2n)
MEIOSIS
Pollen grains
(microgametophyte) (n)
Pollen grain
Stigma
Tube cell
nucleus
Sperm
cells
Formation of
pollen tube (n)
Pollen
tube
Egg
DOUBLE
FERTILIZATION
Endosperm (3n)
Seed (2n)
Seed coat
Embryo
Adult
sporophyte (2n)
with flowers
Carpel
Ovary
Ovule
MEIOSIS
Eight-nucleate embryo sac
(megagametophyte) (n)
Pollen sac
Young
embryo (2n)
Pollen
tube
cell
FIGURE 42.2
Angiosperm life cycle. Eggs form within the embryo sac inside the ovules, which, in turn, are enclosed in the carpels. The pollen grains,
meanwhile, are formed within the sporangia of the anthers and are shed. Fertilization is a double process. A sperm and an egg come
together, producing a zygote; at the same time, another sperm fuses with the polar nuclei to produce the endosperm. The endosperm is
the tissue, unique to angiosperms, that nourishes the embryo and young plant.
of our food is derived, directly or indirectly, from flowering
plants; in fact, more than 90% of the calories we consume
come from just over 100 species. Angiosperms are also
sources of medicine, clothing, and building materials.
While the other plant phyla also provide resources, they
are outnumbered seven to one by the angiosperms. For ex-
ample, there are only about 750 extant gymnosperm
species!
Why Are the Angiosperms Successful?
When flowering plants originated, Africa and South Amer-
ica were still connected to each other, as well as to Antarc-
tica and India, and, via Antarctica, to Australia and New
Zealand (figure 42.3). These landmasses formed the great
continent known as Gondwanaland. In the north, Eurasia
and North America were united, forming another super-
continent called Laurasia. The huge landmass formed by
the union of South America and Africa spanned the equator
and probably had a climate characterized by extreme tem-
peratures and aridity in its interior. Similar climates occur
in the interiors of major continents at present. Much of the
early evolution of angiosperms may have taken place in
patches of drier and less favorable habitat found in the inte-
rior of Gondwanaland. Many features of flowering plants
seem to correlate with successful growth under arid and
semiarid conditions.
The transfer of pollen between flowers of separate
plants, sometimes over long distances, ensures outcrossing
(cross-pollination between individuals of the same
species) and may have been important in the early success
of angiosperms. The various means of effective fruit dis-
persal that evolved in the group were also significant in
the success of angiosperms (see chapter 40). The rapid
life cycle of some of the angiosperms (Arabidopsis can go
from seed to adult flowering plant in 24 days) was an-
other factor. Asexual reproduction has given many inva-
sive species a competitive edge. Xylem vessels and other
anatomical and morphological features of the an-
giosperms correlate with their biological success. As early
angiosperms evolved, all of these advantageous features
became further elaborated and developed, and the pace of
their diversification accelerated.
The Rise to Dominance
Angiosperms began to dominate temperate and tropical
terrestrial communities about 80 to 90 million years ago,
during the second half of the Cretaceous Period. We can
document the relative abundance of different groups of
plants by studying fossils that occur at the same time and
place. In rocks more than 80 million years old, the fossil
remains of plant phyla other than angiosperms, includ-
ing lycopods, horsetails, ferns, and gymnosperms, are
most common. Angiosperms arose in temperate and
tropical terrestrial communities in a relatively short
time.
At about the time that angiosperms became abundant in
the fossil record, pollen, leaves, flowers, and fruits of some
families that still survive began to appear. For example,
representatives of the magnolia, beech, and legume fami-
lies, which were in existence before the end of the Creta-
ceous Period (65 million years ago), are alive and flourish-
ing today.
A number of insect orders that are particularly associ-
ated with flowers, such as Lepidoptera (butterflies and
moths) and Diptera (flies), appeared or became more
abundant during the rise of angiosperms. Plants and in-
sects have clearly played a major role in each other’s pat-
terns of evolution, and their interactions continue to be
of fundamental importance. Additional animals, includ-
ing birds and mammals, now assist in pollination and
seed dispersal.
By 80 to 90 million years ago, angiosperms were
dominant in terrestrial habitats throughout the world.
Chapter 42 Plant Reproduction 839
Equator
Gondwanaland
Laurasia
FIGURE 42.3
The alignment of the continents when the angiosperms first
appeared in the fossil record about 130 million years ago.
Africa, Madagascar, South America, India, Australia, and
Antarctica were all connected and part of the huge continent of
Gondwanaland, which eventually separated into the discrete
landmasses we have today.
Evolution of the Flower
Pollination in angiosperms does not involve direct contact
between the pollen grain and the ovule. Pollen matures
within the anthers and is transported, often by insects,
birds, or other animals, to the stigma of another flower.
When pollen reaches the stigma, it germinates, and a
pollen tube grows down, carrying the sperm nuclei to the
embryo sac. After double fertilization takes place, develop-
ment of the embryo and endosperm begins. The seed ma-
tures within the ripening fruit; the germination of the seed
initiates another life cycle.
Successful pollination in many angiosperms depends on
the regular attraction of pollinators such as insects, birds,
and other animals, so that pollen is transferred between
plants of the same species. When animals disperse pollen,
they perform the same functions for flowering plants that
they do for themselves when they actively search out mates.
The relationship between plant and pollinator can be quite
intricate. Mutations in either partner can block reproduc-
tion. If a plant flowers at the “wrong” time, the pollinator
may not be available. If the morphology of the flower or
pollinator is altered, there may be physical barriers to polli-
nation. Clearly floral morphology has coevolved with polli-
nators and the result is much more complex and diverse
than the initiation of four distinct whorls of organs de-
scribed in chapter 40.
Characteristics of Floral Evolution
The evolution of the angiosperms is a focus of chapter 37.
Here we need to keep in mind that the diversity of an-
giosperms is partly due to the evolution of a great variety of
floral phenotypes that may enhance the effectiveness of
pollination. All floral organs are thought to have evolved
from leaves. In early angiosperms, these organs maintain
the spiral phyllotaxy often found in leaves. The trend has
been toward four distinct whorls. A complete flower has four
whorls of parts (calyx, corolla, androecium, and gynoe-
cium), while an incomplete flower lacks one or more of the
whorls (figure 42.4).
In both complete and incomplete flowers, the calyx usu-
ally constitutes the outermost whorl; it consists of flattened
appendages, called sepals, which protect the flower in the
bud. The petals collectively make up the corolla and may
be fused. Petals function to attract pollinators. While these
two outer whorls of floral organs are sterile, they can en-
hance reproductive success.
Androecium (from the Greek andros, “man”, + oikos,
“house”) is a collective term for all the stamens (male
structures) of a flower. Stamens are specialized structures
that bear the angiosperm microsporangia. There are simi-
lar structures bearing the microsporangia in the pollen
cones of gymnosperms. Most living angiosperms have sta-
mens whose filaments (“stalks”) are slender and often
threadlike, and whose four microsporangia are evident at
the apex in a swollen portion, the anther. Some of the
more primitive angiosperms have stamens that are flattened
and leaflike, with the sporangia producing from the upper
or lower surface.
The gynoecium (from the Greek gyne, “woman,” +
oikos, “house”) is a collective term for all the female parts of
a flower. In most flowers, the gynoecium, which is unique
to angiosperms, consists of a single carpel or two or more
fused carpels. Single or fused carpels are often referred to
as the simple or compound pistils, respectively. Most flow-
ers with which we are familiar—for example, those of
tomatoes and oranges—have a single compound pistil. In
other mostly primitive flowers—for example, buttercups
and stonecups—there may be several to many separate pis-
tils, each formed from a single carpel. Ovules (which de-
velop into seeds) are produced in the pistil’s swollen lower
portion, the ovary, which usually narrows at the top into a
slender, necklike style with a pollen-receptive stigma at its
apex. Sometimes the stigma is divided, with the number of
stigma branches indicating how many carpels are in the
particular pistil. Carpels are essentially inrolled floral leaves
with ovules along the margins. It is possible that the first
carpels were leaf blades that folded longitudinally; the mar-
gins, which had hairs, did not actually fuse until the fruit
developed, but the hairs interlocked and were receptive to
pollen. In the course of evolution, there is evidence the
hairs became localized into a stigma, a style was formed,
and the fusing of the carpel margins ultimately resulted in a
pistil. In many modern flowering plants, the carpels have
become highly modified and are not visually distinguish-
able from one another unless the pistil is cut open.
Trends of Floral Specialization
Two major evolutionary trends led to the wide diversity
of modern flowering plants: (1) separate floral parts have
840 Part XI Plant Growth and Reproduction
Anther
Microspore
mother cell (2n)
Megaspore
mother cell (2n)
MEIO
Ovary
Ovule
MEIOSIS
FIGURE 42.4
Structure of an angiosperm flower.
grouped together, or fused, and (2) floral parts have been
lost or reduced (figure 42.5). In the more advanced an-
giosperms, the number of parts in each whorl has often
been reduced from many to few. The spiral patterns of at-
tachment of all floral parts in primitive angiosperms have,
in the course of evolution, given way to a single whorl at
each level. The central axis of many flowers has short-
ened, and the whorls are close to one another. In some
evolutionary lines, the members of one or more whorls
have fused with one another, sometimes joining into a
tube. In other kinds of flowering plants, different whorls
may be fused together. Whole whorls may even be lost
from the flower, which may lack sepals, petals, stamens,
carpels, or various combinations of these structures. Mod-
ifications often relate to pollination mechanisms and, in
some cases like the grasses, wind has replaced animals for
pollen dispersal.
While much floral diversity is the result of natural se-
lection related to pollination, it is important to recognize
the impact breeding (artificial selection) has had on
flower morphology. Humans have selected for practical
or aesthetic traits that may have little adaptive value to
species in the wild. Maize (corn), for example, has been
selected to satisfy the human palate. Human intervention
ensures the reproductive success of each generation;
while in a natural setting modern corn would not have
the same protection from herbivores as its ancestors, and
the fruit dispersal mechanism would be quite different
(see figure 21.13). Floral shops sell heavily bred species
with modified petals, often due to polyploidy, that en-
hance their economic value, but not their ability to at-
tract pollinators. In making inferences about symbioses
between flowers and pollinators, be sure to look at native
plants that have not been genetically altered by human
intervention.
Trends in Floral Symmetry
Other trends in floral evolution have affected the symmetry
of the flower (figure 42.6). Primitive flowers such as those
of buttercups are radically symmetrical; that is, one could
draw a line anywhere through the center and have two
roughly equal halves. Flowers of many advanced groups are
bilaterally symmetrical; that is, they are divisible into two
equal parts along only a single plane. Examples of such
flowers are snapdragons, mints, and orchids. Such bilater-
ally symmetrical flowers are also common among violets
and peas. In these groups, they are often associated with
advanced and highly precise pollination systems. Bilateral
symmetry has arisen independently many times. In snap-
dragons, the cyclodia gene regulates floral symmetry, and in
its absence flowers are more radial (figure 42.7). Here the
evolutionary introduction of a single gene is sufficient to
cause a dramatic change in morphology. Whether the same
gene or functionally similar genes arose in parallel in other
species is an open question.
The first angiosperms likely had numerous free, spirally
arranged flower parts. Modification of floral parts
appears to be closely tied to pollination mechanisms.
More recently, horticulturists have bred plants for
aesthetic reasons resulting in an even greater diversity
of flowers.
Chapter 42 Plant Reproduction 841
FIGURE 42.5
Trends in floral
specialization.
Wild geranium,
Geranium
maculatum.The
petals are reduced
to five each, the
stamens to ten.
FIGURE 42.6
Bilateral
symmetry in an
orchid. While
primitive flowers
are usually radially
symmetrical,
flowers of many
advanced groups,
such as the orchid
family
(Orchidaceae), are
bilaterally
symmetrical.
(a) (b)
FIGURE 42.7
Genetic regulation of asymmetry in flowers. (left) Snapdragon
flowers normally have bilateral symmetry. (right) The cyclodiagene
regulates floral symmetry, and cyclodiamutant snapdragons have
radially symmetrical flowers.
Formation of Angiosperm Gametes
Reproductive success depends on uniting the gametes (egg
and sperm) found in the embryo sacs and pollen grains of
flowers. As mentioned previously, plant sexual life cycles
are characterized by an alternation of generations, in which
a diploid sporophyte generation gives rise to a haploid ga-
metophyte generation. In angiosperms, the gametophyte
generation is very small and is completely enclosed within
the tissues of the parent sporophyte. The male gameto-
phytes, or microgametophytes, are pollen grains. The fe-
male gametophyte, or megagametophyte, is the embryo
sac. Pollen grains and the embryo sac both are produced in
separate, specialized structures of the angiosperm flower.
Like animals, angiosperms have separate structures for
producing male and female gametes (figure 42.8), but the
reproductive organs of angiosperms are different from
those of animals in two ways. First, in angiosperms, both
male and female structures usually occur together in the
same individual flower (with exceptions noted in chapter
38). Second, angiosperm reproductive structures are not
permanent parts of the adult individual. Angiosperm flow-
ers and reproductive organs develop seasonally, at times of
the year most favorable for pollination. In some cases, re-
productive structures are produced only once and the par-
ent plant dies. It is significant that the germ line for an-
giosperms is not set aside early in development, but forms
quite late, as detailed in chapter 40.
842 Part XI Plant Growth and Reproduction
42.2 Flowering plants use animals or wind to transfer pollen between flowers.
Anther
Microspore
mother cell (2n)
Meiosis
Microspores (n)
Megaspores (n)
Mitosis
Mitosis
Pollen grains (n)
(microgametophyte)
Tube cell
nucleus
Generative
cell
Ovule
Megaspore
mother cell (2n)
Surviving
megaspore
Antipodals
Polar
nuclei
Degenerated
megaspores
8-nucleate embryo sac
(megagametophyte) (n)
Synergids
Egg
cell
Meiosis
FIGURE 42.8
Formation of pollen grains and the embryo sac. Diploid (2n) microspore mother cells are housed in the anther and divide by meiosis to
form four haploid (n) microspores. Each microspore develops by mitosis into a pollen grain. The generative cell within the pollen grain
will later divide to form two sperm cells. Within the ovule, one diploid megaspore mother cell divides by meiosis to produce four haploid
megaspores. Usually only one of the megaspores will survive, and the other three will degenerate. The surviving megaspore divides by
mitosis to produce an embryo sac with eight nuclei.
Pollen Formation
Pollen grains form in the two pollen sacs located in the
anther. Each pollen sac contains specialized chambers in
which the microspore mother cells are enclosed and pro-
tected. The microspore mother cells undergo meiosis to
form four haploid microspores. Subsequently, mitotic di-
visions form four pollen grains. Inside each pollen grain
is a generative cell; this cell will later divide to produce
two sperm cells.
Pollen grain shapes are specialized for specific flower
species. As discussed in more detail later in the chapter,
fertilization requires that the pollen grain grow a tube that
penetrates the style until it encounters the ovary. Most
pollen grains have a furrow from which this pollen tube
emerges; some grains have three furrows (figure 42.9).
Embryo Sac Formation
Eggs develop in the ovules of the angiosperm flower.
Within each ovule is a megaspore mother cell. Each
megaspore mother cell undergoes meiosis to produce
four haploid megaspores. In most plants, only one of
these megaspores, however, survives; the rest are ab-
sorbed by the ovule. The lone remaining megaspore un-
dergoes repeated mitotic divisions to produce eight hap-
loid nuclei that are enclosed within a seven-celled
embryo sac. Within the embryo sac, the eight nuclei are
arranged in precise positions. One nucleus is located near
the opening of the embryo sac in the egg cell. Two are
located in a single cell in the middle of the embryo sac
and are called polar nuclei; two nuclei are contained in
cells called synergids that flank the egg cell; and the
other three nuclei reside in cells called the antipodals, lo-
cated at the end of the sac, opposite the egg cell (figure
42.10). The first step in uniting the sperm in the pollen
grain with the egg and polar nuclei is to get pollen ger-
minating on the stigma of the carpel and growing toward
the embryo sac.
In angiosperms, both male and female structures often
occur together in the same individual flower. These
reproductive structures are not a permanent part of the
adult individual and the germ line is not set aside early
in development.
Chapter 42 Plant Reproduction 843
FIGURE 42.9
Pollen grains. (a) In the Easter lily,
Lilium candidum,the pollen tube
emerges from the pollen grain through
the groove or furrow that occurs on one
side of the grain. (b) In a plant of the
sunflower family, Hyoseris longiloba,three
pores are hidden among the
ornamentation of the pollen grain. The
pollen tube may grow out through any
one of them.
(a) (b)
2 Antipodals
(3rd antipodal
not visible)
2 Polar nuclei
Egg
Synergids
FIGURE 42.10
A mature embryo sac of a lily. The eight haploid nuclei
produced by mitotic divisions of the haploid megaspore are
labeled.
Pollination
Pollination is the process by which pollen is
placed on the stigma. Pollen may be carried
to the flower by wind or by animals, or it
may originate within the individual flower
itself. When pollen from a flower’s anther
pollinates the same flower’s stigma, the
process is called self-pollination.
Pollination in Early Seed Plants
Early seed plants were pollinated passively,
by the action of the wind. As in present-day
conifers, great quantities of pollen were shed
and blown about, occasionally reaching the
vicinity of the ovules of the same species. In-
dividual plants of any given species must
grow relatively close to one another for such
a system to operate efficiently. Otherwise, the chance that
any pollen will arrive at the appropriate destination is very
small. The vast majority of windblown pollen travels less
than 100 meters. This short distance is significant com-
pared with the long distances pollen is routinely carried by
certain insects, birds, and other animals.
Pollination by Animals
The spreading of pollen from plant to plant by pollinators
visiting flowers of specific angiosperm species has played an
important role in the evolutionary success of the group. It
now seems clear that the earliest angiosperms, and perhaps
their ancestors also, were insect-pollinated, and the coevo-
lution of insects and plants has been important for both
groups for over 100 million years. Such interactions have
also been important in bringing about increased floral spe-
cialization. As flowers become increasingly specialized, so
do their relationships with particular groups of insects and
other animals.
Bees. Among insect-pollinated angiosperms, the most
numerous groups are those pollinated by bees (figure
42.11). Like most insects, bees initially locate sources of
food by odor, then orient themselves on the flower or
group of flowers by its shape, color, and texture. Flowers
that bees characteristically visit are often blue or yellow.
Many have stripes or lines of dots that indicate the location
of the nectaries, which often occur within the throats of
specialized flowers. Some bees collect nectar, which is used
as a source of food for adult bees and occasionally for lar-
vae. Most of the approximately 20,000 species of bees visit
flowers to obtain pollen. Pollen is used to provide food in
cells where bee larvae complete their development.
Only a few hundred species of bees are social or semi-
social in their nesting habits. These bees live in colonies, as
do the familiar honeybee, Apis mellifera, and the bumble-
bee, Bombus. Such bees produce several generations a year
and must shift their attention to different kinds of flowers
as the season progresses. To maintain large colonies, they
also must use more than one kind of flower as a food source
at any given time.
Except for these social and semi-social bees and about
1000 species that are parasitic in the nests of other bees, the
great majority of bees—at least 18,000 species—are soli-
tary. Solitary bees in temperate regions characteristically
have only a single generation in the course of a year. Often
they are active as adults for as little as a few weeks a year.
Solitary bees often use the flowers of a given group of
plants almost exclusively as sources of their larval food.
The highly constant relationships of such bees with those
flowers may lead to modifications, over time, in both the
flowers and the bees. For example, the time of day when
the flowers open may correlate with the time when the bees
appear; the mouthparts of the bees may become elongated
in relation to tubular flowers; or the bees’ pollen-collecting
apparatuses may be adapted to the pollen of the plants that
they normally visit. When such relationships are estab-
lished, they provide both an efficient mechanism of pollina-
tion for the flowers and a constant source of food for the
bees that “specialize” on them.
Insects Other Than Bees. Among flower-visiting in-
sects other than bees, a few groups are especially promi-
nent. Flowers such as phlox, which are visited regularly by
butterflies, often have flat “landing platforms” on which
butterflies perch. They also tend to have long, slender flo-
ral tubes filled with nectar that is accessible to the long,
coiled proboscis characteristic of Lepidoptera, the order of
insects that includes butterflies and moths. Flowers like
jimsonweed, evening primrose, and others visited regularly
by moths are often white, yellow, or some other pale color;
they also tend to be heavily scented, thus serving to make
the flowers easy to locate at night.
844 Part XI Plant Growth and Reproduction
FIGURE 42.11
Pollination by a
bumblebee. As this
bumblebee, Bombus,
squeezes into the
bilaterally symmetrical,
advanced flower of a
member of the mint
family, the stigma
contacts its back and
picks up any pollen that
the bee may have
acquired during a visit to
a previous flower.
Birds. Several interesting groups of plants are regularly
visited and pollinated by birds, especially the humming-
birds of North and South America and the sunbirds of
Africa (figure 42.12). Such plants must produce large
amounts of nectar because if the birds do not find enough
food to maintain themselves, they will not continue to visit
flowers of that plant. Flowers producing large amounts of
nectar have no advantage in being visited by insects because
an insect could obtain its energy requirements at a single
flower and would not cross-pollinate the flower. How are
these different selective forces balanced in flowers that are
“specialized” for hummingbirds and sunbirds?
Ultraviolet light is highly visible to insects. Carotenoids,
yellow or orange pigments frequently found in plants, are
responsible for the colors of many flowers, such as sunflow-
ers and mustard. Carotenoids reflect both in the yellow
range and in the ultraviolet range, the mixture resulting in
a distinctive color called “bee’s purple.” Such yellow flow-
ers may also be marked in distinctive ways normally invisi-
ble to us, but highly visible to bees and other insects (figure
42.13). These markings can be in the form of a bull’s-eye
or a landing strip.
Red does not stand out as a distinct color to most in-
sects, but it is a very conspicuous color to birds. To most
insects, the red upper leaves of poinsettias look just like the
other leaves of the plant. Consequently, even though the
flowers produce abundant supplies of nectar and attract
hummingbirds, insects tend to bypass them. Thus, the red
color both signals to birds the presence of abundant nectar
and makes that nectar as inconspicuous as possible to in-
sects. Red is also seen again in fruits that are dispersed by
birds.
Other Animals. Other animals including bats and small
rodents may aid in pollination. The signals here also are
species specific. These animals also assist in dispersing the
seeds and fruits that result from pollination. Monkeys are
attracted to orange and yellow and will be effective in dis-
persing those fruits.
Wind-Pollinated Angiosperms
Many angiosperms, representing a number of different
groups, are wind-pollinated—a characteristic of early seed
plants. Among them are such familiar plants as oaks,
birches, cottonwoods, grasses, sedges, and nettles. The
flowers of these plants are small, greenish, and odorless;
their corollas are reduced or absent (see figures 42.14 and
42.15). Such flowers often are grouped together in fairly
large numbers and may hang down in tassels that wave
about in the wind and shed pollen freely. Many wind-
pollinated plants have stamen- and carpel-containing flow-
ers separated among individuals or on a single individual. If
the pollen-producing and ovule-bearing flowers are sepa-
rated, it is certain that pollen released to the wind will
reach a flower other than the one that sheds it, a strategy
that greatly promotes outcrossing. Some wind-pollinated
plants, especially trees and shrubs, flower in the spring, be-
fore the development of their leaves can interfere with the
wind-borne pollen. Wind-pollinated species do not depend
on the presence of a pollinator for species survival.
Bees are the most frequent and characteristic
pollinators of flowers. Insects often are attracted by the
odors of flowers. Bird-pollinated flowers are
characteristically odorless and red, with the nectar not
readily accessed by insects.
Chapter 42 Plant Reproduction 845
FIGURE 42.12
Hummingbirds and flowers. A long-tailed hermit hummingbird
extracts nectar from the flowers of Heliconia imbricatain the
forests of Costa Rica. Note the pollen on the bird’s beak.
Hummingbirds of this group obtain nectar primarily from long,
curved flowers that more or less match the length and shape of
their beaks.
FIGURE 42.13
How a bee sees a flower. (a) The yellow flower of
Ludwigia peruviana(Onagraceae) photographed in normal
light and (b) with a filter that selectively transmits
ultraviolet light. The outer sections of the petals reflect
both yellow and ultraviolet, a mixture of colors called
“bee’s purple”; the inner portions of the petals reflect
yellow only and therefore appear dark in the photograph
that emphasizes ultraviolet reflection. To a bee, this
flower appears as if it has a conspicuous central bull’s-eye.
(a) (b)
Self-Pollination
All of the modes of pollination that we have considered
thus far tend to lead to outcrossing, which is as highly ad-
vantageous for plants as it is for eukaryotic organisms gen-
erally. Nevertheless, self-pollination also occurs among an-
giosperms, particularly in temperate regions. Most of the
self-pollinating plants have small, relatively inconspicuous
flowers that shed pollen directly onto the stigma, some-
times even before the bud opens. You might logically ask
why there are many self-pollinated plant species if out-
crossing is just as important genetically for plants as it is for
animals. There are two basic reasons for the frequent oc-
currence of self-pollinated angiosperms:
1. Self-pollination obviously is ecologically advanta-
geous under certain circumstances because self-
pollinators do not need to be visited by animals to
produce seed. As a result, self-pollinated plants expend
less energy in the production of pollinator attractants
and can grow in areas where the kinds of insects or
other animals that might visit them are absent or very
scarce—as in the Arctic or at high elevations.
2. In genetic terms, self-pollination produces progenies
that are more uniform than those that result from out-
crossing. Remember that because meiosis is involved,
there is still recombination and the offspring will not
be identical to the parent. However, such progenies
may contain high proportions of individuals well-
adapted to particular habitats. Self-pollination in nor-
mally outcrossing species tends to produce large num-
bers of ill-adapted individuals because it brings
together deleterious recessive genes; but some of these
combinations may be highly advantageous in particu-
lar habitats. In such habitats, it may be advantageous
for the plant to continue self-pollinating indefinitely.
This is the main reason many self-pollinating plant
species are weeds—not only have humans made weed
habitats uniform, but they have also spread the weeds
all over the world.
Factors That Promote Outcrossing
Outcrossing, as we have stressed, is of critical importance
for the adaptation and evolution of all eukaryotic organ-
isms. Often flowers contain both stamens and pistils, which
increase the likelihood of self-pollination. One strategy to
promote outcrossing is to separate stamens and pistils.
In various species of flowering plants—for example, wil-
lows and some mulberries—staminate and pistillate flowers
may occur on separate plants. Such plants, which produce
only ovules or only pollen, are called dioecious, from the
Greek words for “two houses.” Obviously, they cannot self-
pollinate and must rely exclusively on outcrossing. In other
kinds of plants, such as oaks, birches, corn (maize), and
pumpkins, separate male and female flowers may both be
produced on the same plant. Such plants are called monoe-
cious, meaning “one house” (figure 42.14). In monoecious
plants, the separation of pistillate and staminate flowers,
which may mature at different times, greatly enhances the
probability of outcrossing.
Even if, as usually is the case, functional stamens and
pistils are both present in each flower of a particular plant
species, these organs may reach maturity at different times.
Plants in which this occurs are called dichogamous. If the
stamens mature first, shedding their pollen before the stig-
mas are receptive, the flower is effectively staminate at that
time. Once the stamens have finished shedding pollen, the
stigma or stigmas may then become receptive, and the
flower may become essentially pistillate (figures 42.15 and
42.16). This has the same effect as if the flower completely
lacked either functional stamens or functional pistils; its
outcrossing rate is thereby significantly increased.
846 Part XI Plant Growth and Reproduction
FIGURE 42.14
Staminate and
pistillate flowers
of a birch, Betula.
Birches are
monoecious; their
staminate flowers
hang down in long,
yellowish tassels,
while their pistillate
flowers mature into
clusters of small,
brownish, conelike
structures.
FIGURE 42.15
Wind-pollinated
flowers. The large
yellow anthers,
dangling on very
slender filaments, are
hanging out, about to
shed their pollen to the
wind; later, these
flowers will become
pistillate, with long,
feathery stigmas—well
suited for trapping
windblown pollen—
sticking far out of them.
Many grasses, like this
one, are therefore
dichogamous.
Many flowers are constructed such
that the stamens and stigmas do not
come in contact with each other.
With such an arrangement, there is a
natural tendency for the pollen to be
transferred to the stigma of another
flower rather than to the stigma of its
own flower, thereby promoting
outcrossing.
Even when a flower’s stamens and
stigma mature at the same time, genetic
self-incompatibility, which is wide-
spread in flowering plants, increases
outcrossing. Self-incompatibility results
when the pollen and stigma recognize
each other as being genetically related
and fertilization is blocked (figure
42.17). Self-incompatibility is con-
trolled by the S (self-incompatibility)
locus. There are many alleles at the S
locus that regulate recognition re-
sponses between the pollen and stigma.
There are two types of self-incompati-
bility. Gametophytic self-incompatibil-
ity depends on the haploid S locus of
the pollen and the diploid S locus of the stigma. If either of
the S alleles in the stigma match the pollen S allele, pollen
tube growth stops before it reaches the embryo sac. Petunias
have gametophytic self-incompatibility. In the case of sporo-
phytic self-incompatibility, such as in broccoli, both S alleles
of the pollen parent are important; if the alleles in the stigma
match with either of the pollen parent S alleles, the haploid
pollen will not germinate.
Much is being learned about the cellular basis of this
recognition and the signal transduction pathways that
block the successful growth of the pollen tube. These
pollen recognition mechanisms may have had their ori-
gins in a common ancestor of the gymnosperms. Fossils
with pollen tubes from the Carboniferous are consistent
with the hypothesis that they had highly evolved pollen-
recognition systems. These may have been systems that
recognized foreign pollen that predated self-recognition
systems.
Self-pollinated angiosperms are frequent where there is
a strong selective pressure to produce large numbers of
genetically uniform individuals adapted to specific,
relatively uniform habitats. Outcrossing in plants may
be promoted through dioecism, monoecism, self-
incompatibility, or the physical separation or different
maturation times of the stamens and pistils.
Outcrossing promotes genetic diversity.
Chapter 42 Plant Reproduction 847
(a) (b)
FIGURE 42.16
Dichogamy, as illustrated by the flowers of fireweed, Epilobium angustifolium. More
than 200 years ago (in the 1790s) fireweed, which is outcrossing, was one of the first plant
species to have its process of pollination described. First, the anthers shed pollen, and then
the style elongates above the stamens while the four lobes of the stigma curl back and
become receptive. Consequently, the flowers are functionally staminate at first, becoming
pistillate about two days later. The flowers open progressively up the stem, so that the
lowest are visited first. Working up the stem, the bees encounter pollen-shedding,
staminate-phase flowers and become covered with pollen, which they then carry to the
lower, functionally pistillate flowers of another plant. Shown here are flowers in (a) the
staminate phase and (b) the pistillate phase.
S
1
S
2
pollen parent
S
1
S
1
S
1
S
1
S
2
S
2
S
2
S
2
S
2
S
3
carpel of
pollen recipient
S
1
S
2
pollen parent
S
2
S
3
carpel of
pollen recipient
(a) Gametophytic self-incompatibility (b) Sporophytic self-incompatibility
X
XX
FIGURE 42.17
Self-pollination can be genetically
controlled so self-pollen is not recognized.
(a) Gametophytic self-incompatibility is
determined by the haploid pollen genotype. (b)
Sporophytic self-incompatibility recognizes the
genotype of the diploid pollen parent, not just
the haploid pollen genotype. In both cases, the
recognition is based on the Slocus, which has
many different alleles. The subscript numbers
indicate the Sallele genotype. In gametophytic
self-incompatibility, the block comes after
pollen tube germination. In sporophytic self-
incompatibility, the pollen tube fails to
germinate.
Fertilization
Fertilization in angiosperms is a complex, somewhat un-
usual process in which two sperm cells are utilized in a
unique process called double fertilization. Double fertil-
ization results in two key developments: (1) the fertiliza-
tion of the egg, and (2) the formation of a nutrient sub-
stance called endosperm that nourishes the embryo. Once
a pollen grain has been spread by wind, by animals, or
through self-pollination, it adheres to the sticky, sugary
substance that covers the stigma and begins to grow a
pollen tube that pierces the style (figure 42.18). The
pollen tube, nourished by the sugary substance, grows
until it reaches the ovule in the ovary. Meanwhile, the
generative cell within the pollen grain tube cell divides to
form two sperm cells.
The pollen tube eventually reaches the embryo sac in
the ovule. At the entry to the embryo sac, the tip of the
pollen tube bursts and releases the two sperm cells. Simul-
taneously, the two nuclei that flank the egg cell disinte-
grate, and one of the sperm cells fertilizes the egg cell,
forming a zygote. The other sperm cell fuses with the two
polar nuclei located at the center of the embryo sac, form-
ing the triploid (3n) primary endosperm nucleus. The pri-
mary endosperm nucleus eventually develops into the en-
dosperm.
Once fertilization is complete, the embryo develops by
dividing numerous times. Meanwhile, protective tissues en-
close the embryo, resulting in the formation of the seed.
The seed, in turn, is enclosed in another structure called
the fruit. These typical angiosperm structures evolved in
response to the need for seeds to be dispersed over long
distances to ensure genetic variability.
In double fertilization, angiosperms utilize two sperm
cells. One fertilizes the egg, while the other helps form
a substance called endosperm that nourishes the
embryo.
848 Part XI Plant Growth and Reproduction
Generative
cell
Tube
cell
Stigma
Style
Ovary
Ovule
Carpel
Pollination
Embryo
sac
Tube cell
Sperm cells
Tube cell nucleus
Growth of
pollen tube
Pollen tube
Double fertilizationRelease of sperm cells
Pollen grain
Zygote
(2n)
Antipodals
Polar nuclei
Egg cell
Synergids
Endosperm
(3n)
FIGURE 42.18
The formation of the pollen tube and double fertilization. When pollen lands on the stigma of a flower, the pollen tube cell grows
toward the embryo sac, forming a pollen tube. While the pollen tube is growing, the generative cell divides to form two sperm cells. When
the pollen tube reaches the embryo sac, it bursts through one of the synergids and releases the sperm cells. In a process called double
fertilization, one sperm cell nucleus fuses with the egg cell to form the diploid (2n) zygote, and the other sperm cell nucleus fuses with the
two polar nuclei to form the triploid (3n) endosperm nucleus.
Asexual Reproduction
While self-pollination reduces genetic variability, asexual
reproduction results in genetically identical individuals be-
cause only mitotic cell divisions occur. In the absence of
meiosis, individuals that are highly adapted to a relatively
unchanging environment persist for the same reasons that
self-pollination is favored. Should conditions change dra-
matically, there will be less variation in the population for
natural selection to act upon and the species may be less
likely to survive. Asexual reproduction is also used in agri-
culture and horticulture to propagate a particularly desir-
able plant whose traits would be altered by sexual repro-
duction, even self-pollination. Most roses and potatoes for
example, are vegetatively propagated.
Vegetative Reproduction
In a very common form of asexual reproduction called veg-
etative reproduction, new plant individuals are simply
cloned from parts of adults (figure 42.19). The forms of
vegetative reproduction in plants are many and varied.
Stolons. Some plants reproduce by means of runners, or
stolons—long, slender stems that grow along the surface of
the soil. In the cultivated strawberry, for example, leaves,
flowers, and roots are produced at every other node on the
runner. Just beyond each second node, the tip of the run-
ner turns up and becomes thickened. This thickened por-
tion first produces adventitious roots and then a new shoot
that continues the runner.
Rhizomes. Underground stems, or rhizomes, are also
important reproductive structures, particularly in grasses
and sedges. Rhizomes invade areas near the parent plant,
and each node can give rise to a new flowering shoot. The
noxious character of many weeds results from this type of
growth pattern, and many garden plants, such as irises, are
propagated almost entirely from rhizomes. Corms, bulbs,
and tubers are rhizomes specialized for storage and repro-
duction. White potatoes are propagated artificially from
tuber segments, each with one or more “eyes.” The eyes, or
“seed pieces,” of potato give rise to the new plant.
Suckers. The roots of some plants—for example, cherry,
apple, raspberry, and blackberry—produce “suckers,” or
sprouts, which give rise to new plants. Commercial vari-
eties of banana do not produce seeds and are propagated by
suckers that develop from buds on underground stems.
When the root of a dandelion is broken, as it may be if one
attempts to pull it from the ground, each root fragment
may give rise to a new plant.
Adventitious Leaves. In a few species, even the leaves
are reproductive. One example is the house plant Kalancho?
daigremontiana, familiar to many people as the “maternity
plant,” or “mother of thousands.” The common names of
this plant are based on the fact that numerous plantlets
arise from meristematic tissue located in notches along the
leaves. The maternity plant is ordinarily propagated by
means of these small plants, which, when they mature, drop
to the soil and take root.
Apomixis
In certain plants, including some citruses, certain grasses
(such as Kentucky bluegrass), and dandelions, the em-
bryos in the seeds may be produced asexually from the
parent plant. This kind of asexual reproduction is known
as apomixis. The seeds produced in this way give rise to
individuals that are genetically identical to their parents.
Thus, although these plants reproduce asexually by
cloning diploid cells in the ovule, they also gain the ad-
vantage of seed dispersal, an adaptation usually associated
with sexual reproduction. As you will see in chapter 43,
embryos can also form via mitosis when plant tissues are
cultured. In general, vegetative reproduction, apomixis,
and other forms of asexual reproduction promote the
exact reproduction of individuals that are particularly
well suited to a certain environment or habitat. Asexual
reproduction among plants is far more common in harsh
or marginal environments, where there is little margin
for variation. There is a greater proportion of asexual
plants in the arctic, for example, than in temperate
regions.
Plants that reproduce asexually clone new individuals
from portions of the root, stem, leaves, or ovules of
adult individuals. The asexually produced progeny are
genetically identical to the parent individual.
Chapter 42 Plant Reproduction 849
42.3 Many plants can clone themselves by asexual reproduction.
FIGURE 42.19.
Vegetative reproduction. Small plants arise from notches along
the leaves of the house plant Kalancho? daigremontiana.
The Life Span of Plants
Plant Life Spans Vary Greatly
Once established, plants live for highly
variable periods of time, depending on
the species. Life span may or may not
correlate with reproductive strategy.
Woody plants, which have extensive
secondary growth, nearly always live
longer than herbaceous plants, which
have limited or no secondary growth.
Bristlecone pine, for example, can live
upward of 4000 years. Some herba-
ceous plants send new stems above the
ground every year, producing them
from woody underground structures.
Others germinate and grow, flowering
just once before they die. Shorter-lived
plants rarely become very woody be-
cause there is not enough time for the
accumulation of secondary tissues. De-
pending on the length of their life cy-
cles, herbaceous plants may be annual, biennial, or
perennial, while woody plants are generally perennial
(figure 42.20). Determining life span is even more com-
plicated for clonally reproducing organisms. Aspen trees
form huge clones from asexual reproduction of their
roots. Collectively, an aspen clone may form the largest
“organism” on earth. Other asexually reproducing plants
may cover less territory but live for thousands of years.
Creosote bushes in the Mojave Desert have been identi-
fied that are up to 12,000 years old!
Annual Plants
Annual plants grow, flower, and form fruits and seeds
within one growing season; they then die when the
process is complete. Many crop plants are annuals, includ-
ing corn, wheat, and soybeans. Annuals generally grow
rapidly under favorable conditions and in proportion to
the availability of water or nutrients. The lateral mer-
istems of some annuals, like sunflowers or giant ragweed,
do produce poorly developed secondary tissues, but most
are entirely herbaceous. Annuals typically die after flower-
ing once, the developing flowers or embryos using hor-
monal signaling to reallocate nutrients so the parent plant
literally starves to death. This can be demonstrated by
comparing a population of bean plants where the beans
are continually picked with a population where the beans
are left on the plant. The frequently picked population
will continue to grow and yield beans much longer than
the untouched population. The process that leads to the
death of a plant is called senescence.
Biennial Plants
Biennial plants, which are much less common than an-
nuals, have life cycles that take two years to complete.
During the first year, biennials store photosynthate in
underground storage organs. During the second year of
growth, flowering stems are produced using energy
stored in the underground parts of the plant. Certain
crop plants, including carrots, cabbage, and beets, are bi-
ennials, but these plants generally are harvested for food
during their first season, before they flower. They are
grown for their leaves or roots, not for their fruits or
seeds. Wild biennials include evening primroses, Queen
Anne’s lace, and mullein. Many plants that are considered
biennials actually do not flower until they are three or
more years of age, but all biennial plants flower only
once before they die.
Perennial Plants
Perennial plants continue to grow year after year and
may be herbaceous, as are many woodland, wetland, and
prairie wildflowers, or woody, as are trees and shrubs.
The majority of vascular plant species are perennials.
Herbaceous perennials rarely experience any secondary
growth in their stems; the stems die each year after a pe-
riod of relatively rapid growth and food accumulation.
Food is often stored in the plants’ roots or underground
stems, which can become quite large in comparison to
their less substantial aboveground counterparts.
850 Part XI Plant Growth and Reproduction
42.4 How long do plants and plant organs live?
FIGURE 42.20
Annual and perennial plants. Plants live
for very different lengths of time. (a)
Desert annuals complete their entire life
span in a few weeks. (b) Some trees, such
as the giant redwood (Sequoiadendron
giganteum), which occurs in scattered
groves along the western slopes of the
Sierra Nevada in California, live 2000
years or more.
Trees and shrubs generally flower repeatedly, but
there are exceptions. Bamboo lives for many seasons as a
vegetative plant, but senesces and dies after flowering.
The same is true for at least one tropical tree which
achieves great heights before flowering and senescing.
Considering the tremendous amount of energy that goes
into the growth of a tree, this particular reproductive
strategy is quite curious.
Trees and shrubs are either deciduous, with all the
leaves falling at one particular time of year and the plants
remaining bare for a period, or evergreen, with the leaves
dropping throughout the year and the plants never appear-
ing completely bare. In northern temperate regions,
conifers are the most familiar evergreens; but in tropical
and subtropical regions, most angiosperms are evergreen,
except where there is a severe seasonal drought. In these
areas, many angiosperms are deciduous, losing their leaves
during the drought and thus conserving water.
Organ Abscission
Senescence is an important developmental process that
leads to the death of an organ, shoot, or the whole plant.
Annual and biennial plants undergo whole plant senes-
cence, but individual organs on any plant can also senesce
and be shed. The process by which leaves or petals are shed
is called abscission.
One advantage to organ senescence is that nutrient sinks
can be dispensed with. For example, shaded leaves that are
no longer photosynthetically productive can be shed.
Petals, which are modified leaves, may senesce once polli-
nation occurs. Orchid flowers remain fresh for long periods
of time, even in a florist shop. However, once pollination
occurs, a hormonal change is triggered that leads to petal
senescence. This makes sense in terms of allocation of en-
ergy resources, as the petals are no longer necessary to at-
tract a pollinator. On a larger scale, deciduous plants in
temperate areas produce new leaves in the spring and then
lose them in the fall. In the tropics, however, the produc-
tion and subsequent loss of leaves in some species is corre-
lated with wet and dry seasons. Evergreen plants, such as
most conifers, usually have a complete change of leaves
every two to seven years, periodically losing some but not
all of their leaves.
Abscission involves changes that take place in an abscis-
sion zone at the base of the petiole (figure 42.21). Young
leaves produce hormones (especially cytokinins) that in-
hibit the development of specialized layers of cells in the
abscission zone. Hormonal changes take place as the leaf
ages, however, and two layers of cells become differenti-
ated. (Despite the name, abscisic acid is not involved in
this process.) A protective layer, which may be several cells
wide, develops on the stem side of the petiole base.
These cells become impregnated with suberin, which, as
you will recall, is a fatty substance that is impervious to
moisture. A separation layer develops on the side of the
leaf blade; the cells of the separation layer sometimes di-
vide, swell, and become gelatinous. When temperatures
drop, the duration and intensity of light diminishes as the
days grow shorter, or other environmental changes
occur, enzymes break down the pectins in the middle
lamellae of the separation cells. Wind and rain can then
easily separate the leaf from the stem. Left behind is a
sealed leaf scar that is protected from bacteria and other
disease organisms.
As the abscission zone develops, the green chlorophyll
pigments present in the leaf break down, revealing the yel-
lows and oranges of other pigments, such as carotenoids,
that previously had been masked by the intense green col-
ors. At the same time, water-soluble red or blue pigments
called anthocyanins and betacyanins may also accumulate in
the vacuoles of the leaf cells—all contributing to an array of
fall colors in leaves (see figure 41.7a).
Annual plants complete their whole growth cycle within
a single year. Biennial plants flower only once, normally
after two seasons of growth. Perennials flower
repeatedly and live for many years. Abscission occurs
when a plant sheds its organs.
Chapter 42 Plant Reproduction 851
Petiole
Separation layer
Suberized cells
of protective
layer
Axillary bud
FIGURE 42.21
Leaf abscission. The abscission zone of a leaf. Hormonal changes
in this zone cause abscission. Two layers of cells in the abscission
zone differentiate into a protective layer and a separation layer. As
pectins in the separation layer break down, wind and rain can
easily separate the leaf from the stem.
852 Part XI Plant Growth and Reproduction
Chapter 42
Summary Questions Media Resources
42.1 Angiosperms have been incredibly successful, in part, because of their reproductive strategies.
? Angiosperms have been successful because they can
be relatively drought-resistant, and smaller
herbaceous angiosperms have relatively short life
cycles. Most important, however, are their flowers
and fruits. Flowers make possible the precise transfer
of pollen and, therefore, outcrossing, even when the
stationary individual plants are widely separated.
Fruits, with their complex adaptations, facilitate the
wide dispersal of angiosperms.
? Modification of floral parts, especially petals, has
been key in facilitating pollination. Bilateral
symmetry has evolved independently, multiple times.
1. What characteristics of early
angiosperms are thought to
contribute to their success.
2. What flower whorl is
collectively made up of petals?
With which other flower parts
are the petals of most flowers
homologous?
3. What is an androecium? Of
which flower parts is it
composed?
www.mhhe.com/raven6e www.biocourse.com
? Bees are the most frequent and constant pollinators
of flowers. Insects often are attracted by the odors of
flowers rather than color. Birds are attracted to red
flowers, but not odors.
? Self-pollination reduces genetic variability among
offspring. Outcrossing increases genetic diversity.
? Outcrossing in different angiosperms is promoted by
the separation of stamens and carpels into different
flowers, or even into different individuals.
4. What does it mean if a plant is
dichogamous? Of what
advantage is it to the plant?
5. Is it more likely that a flower
visited by a social or a solitary
bee will become highly
specialized toward that bee?
Why?
42.2 Flowering plants use animals or wind to transfer pollen between flowers.
? In asexual reproduction, plants clone new individuals
from portions of adult roots, stems, leaves, or ovules.
? The progeny produced by asexual reproduction are
all genetically identical to the parent individual, even
when they are produced in the ovules (apomixis).
6. Why would a plant capable of
sexual reproduction reproduce
asexually?
7. You have just cloned a gene
responsible for apomixis. Several
corn breeders are very interested
in your gene. Why?
42.3 Many plants can clone themselves by asexual reproduction.
? Plants can live for a single season or thousands of
years.
? For annual and biennial plants, reproduction triggers
senescence and death.
? Asexually reproducing plants can form clones that
cover huge areas and/or live for many thousands of
years.
? Plant organs and shoots can senesce and die while the
whole plant thrives. Organ senescence is an efficient
way to maximize the use of energy resources.
8. Some plants flower once and
die; others flower multiple times,
reaching great heights and
diameters. What are the relative
advantages of the two strategies?
9. How and why does leaf
abscission occur?
42.4 How long do plants and plant organs live?
? Asexual Reproduction
? Gamete formation
? Fertilization
853
43
Plant Genomics
Concept Outline
43.1 Genomic organization is much more varied in
plants than in animals.
Overview of Plant Genomics. As agrarian societies
formed, people began to select for desirable traits. Until
relatively recently, plant biologists focused their research
efforts on variation in chromosomes, but work is now
shifting increasingly to the molecular level.
Organization of Plant Genomes. Plant genomes are
more complex than those of other eukaryotic organisms
due to the presence of multiple chromosome copies and
extensive amounts of DNA with repetitive sequences.
Comparative Genome Mapping and Model Systems.
RFLP and AFLP techniques are useful for mapping traits in
plant genomes. Despite the technical success in sequencing
the Arabidopsis genome and other genomes, we still don’t
know what most of these genes do and how the proteins
they encode function in physiology and development.
43.2 Advances in plant tissue culture are
revolutionizing agriculture.
Overview of Plant Tissue Culture. Because plants are
totipotent, bits of tissue can be used to regenerate whole
plants.
Types of Plant Tissue Cultures. Plant cells, tissues, and
organs can be grown in an artificial culture medium, and
some cells can be directed to generate whole plants.
Applications of Plant Tissue Culture. Plant tissue
cultures can be used for the production of plant products,
propagation of horticultural plants, and crop improvement.
43.3 Plant biotechnology now affects every aspect of
agriculture.
World Population in Relation to Advances Made in
Crop Production. It is uncertain whether advances made
in crop production by improved farming practices and crop
breeding can provide for an increasing world population.
Plant Biotechnology for Agricultural Improvement.
Plants can be genetically engineered to have altered levels
of oils and amino acids and to provide vaccines.
Methods of Plant Transformation. The genetic
engineering of plants is based upon introduction of foreign
DNA into plant cells.
B
y selective breeding favoring desired traits, people have
been genetically modifying plants since agrarian soci-
eties began. All of our key modern crops are the result of
this long effort. Today, we have even more powerful tools,
recombinant DNA technologies that are the subject of this
chapter. This chapter looks ahead to the impact of these
new technologies on the future of plants and our study of
plant biology (figure 43.1). Both the Arabidopsis and rice
genomes are essentially sequenced. Not only can we expect
to learn much about the molecular basis of plant physiology
and development from these rich databases; we will surely
gain a far deeper understanding of plant evolution.
FIGURE 43.1
Golden rice. Rice is the dietary staple of almost half the world’s
population, but it lacks vitamin A. Vitamin A deficiency leads to
vision and immunity problems. Genetically engineered rice that
produces vitamin A has now been developed. The rice is golden
because a biosynthetic pathway has been genetically modified to
produce gold-colored beta-carotene, a precursor to vitamin A.
Here, while rice is mixed in with golden rice. The intensity of
golden color indicates the amount of pro-vitamin A present.
regions of sequence repeats, sequence inversions, or trans-
posable element insertions, which further modify their ge-
netic content. Traditionally, variation in chromosome in-
versions and ploidy has been used to build up a picture of
how plant species have evolved (figure 43.3). Increasingly,
researchers are turning to studying the organization of
plant DNA sequences to obtain important information
about the evolutionary history of a plant species.
People have been genetically engineering plants for
centuries by selecting for desired traits. Traditionally,
biologists have examined variation among plants at the
chromosome level; today, researchers are focusing
more of their efforts at the DNA sequence level.
854 Part XI Plant Growth and Reproduction
Overview of Plant Genomics
Early Approaches
While the term genetic engineering is commonly used to
describe plants and animals modified using recombinant
DNA technology, people have actually been genetic engi-
neers for thousands of years. As agrarian societies formed,
changes in the gene pool within crop species began. For ex-
ample, seed dispersal was selected against in maize and
wheat. Without the ability to disperse seed, these domesti-
cated plants are completely dependent on humans for seed
dispersal. Rice was converted from a perennial plant to an
annual plant without the seed dormancy mechanisms present
in wild rice. Parts of the plant that were of most dietary
value to humans and domesticated animals have been se-
lected for increased size. These include seeds, fruits, and
storage organs like roots in the case of carrots. All of these
changes were accomplished without knowledge of particu-
lar genes, by selecting and propagating individuals with the
desired traits.
Breeding Strategies to Enhance Yield
At the beginning of the twentieth century, a growing un-
derstanding of genetics increased the rate of crop improve-
ment. Among the most dramatic agricultural developments
was the introduction of hybrid corn. As corn breeding pro-
gressed, highly inbred lines began to have decreased yield
as deleterious recessive genes became homozygous. George
Harrison Shull found that crossing two different inbred
lines gave rise to offspring with “hybrid vigor.” The yield
increased fourfold! Hybrid corn now grows in almost all
fields in the United States. Hybrid rice developed by the
International Rice Research Institute in the Philippines has
increased yield 20%.
Breeders have now turned to specific genes to optimize
food quality (see figure 43.1). Only a small percentage of
the genes and their function have been identified, but we
start this century with technologically powerful new ways
to understand genomes.
Studying Plant Genomes
Plant genomes are more complex than other eukaryotic
genomes, and analysis reveals many evolutionary flips and
turns of the DNA sequences over time. Plants show widely
different chromosome numbers and varied ploidy levels
(figure 43.2). Overall, the size of plant genomes (both num-
ber of chromosomes and total nucleotide base-pairs) ex-
hibits the greatest variation of any kingdom in the biologi-
cal world. For example, tulips contain over 170 times as
much DNA as the small weed Arabidopsis thaliana (table
43.1). The DNA of plants, like animals, can also contain
Table 43.1 Genome Size of Plants
Genome Size
Common (Millions of
Scientific Name Name Base-Pairs)
Arabidopsis thaliana Arabidopsis 145
Prunus persica Peach 262
Ricinus communis Castor bean 323
Citrus sinensis Orange 367
Oryza sativa spp. javanica Rice 424
Petunia parodii Petunia 1,221
Pisum sativum Garden pea 3,947
Avena sativa Oats 11,315
Tulipa spp. Garden tulip 24,704
Source: From Plant Biochemistry and Molecular Biology, by P. J. Lea and R.
C. Leegods, eds. Copyright ? 1993 John Wiley & Sons, Limited.
Reproduced with permission.
43.1 Genomic organization is much more varied in plants than in animals.
Haploid Diploid Polyploid
FIGURE 43.2
Chromosome numbers possible in
plant genomes. Haploid: a set of
chromosomes without their pairs; for
example, the chromosome number present
in a gamete. Diploid: a single set of
chromosome pairs. Polyploid: multiple sets of chromosome pairs;
for example, bananas have a triple set of chromosomes and are
therefore polyploid.
Chapter 43 Plant Genomics 855
Diploid: 2H115477
Tetraploid: 4H115477
Hexaploid: 6H115477
Diploid: 2H115477
Triticum monococcum (2n =14)
AA
Sterile hybrid (1n =14)
AB
Sterile hybrid(1n =21)
ABC
Triticum turgidum (2n =28)
AABB
Triticum aestivum (2n =42)
AABBCC
Triticum tauschii (2n =14)
CC
Diploid: 2H115477
Triticum searsii (2n =14)
BB
Chromosome doubling
Chromosome doubling
FIGURE 43.3
Evolutionary history of wheat. Domestic wheat arose in southwestern Asia in the hilly country of what is now Iraq. In this region, there
is a rich assembly of grasses of the genus Triticum. Domestic wheat (T. aestivum) is a polyploid species of Triticum that arose through two
so-called “allopolyploid” events. (1) Two different diploid species, AA and BB, hybridized to form an AB polyploid; the species were so
different that A and B chromosomes could not pair in meiosis, so the AB polyploid was sterile. However, in some plants the chromosome
number spontaneously doubled due to a failure of chromosomes to separate in meiosis, producing a fertile tetraploid species AABB. This
wheat is used in the production of pasta. (2) In a similar fashion, the tetraploid species AABB hybridized with another diploid species CC to
produce the hexaploid T. aestivum, AABBCC. This bread wheat is commonly used throughout the world.
Organization of Plant
Genomes
Low-, Medium-, and High-
Copy-Number DNA
Most seed plants contain quantities of
DNA that greatly exceed their needs
for coding and regulatory function.
Hence, for plants, a very small percent-
age of the genome may actually encode
genes involved in the production of
protein. This portion of the genome
which encodes most of the transcribed
genes is often referred to as “low-copy-
number DNA,” because the DNA se-
quences comprising these genes are
present in single or small numbers of
copies. How do plants function with so
much extra DNA inserted into the
genome? It appears that most of these
sequence alterations occur in noncoding
regions.
“Medium-copy-number DNA” is
composed largely of DNA sequences
that encode ribosomal RNA (rRNA), a
key element of the cellular machinery
that translates transcribed messenger
RNA (mRNA) into protein. In plant
genomes, rRNA genes may be repeated
several hundred to several thousand
times. This is in contrast to animal cells, where only 100 to
200 rRNA genes are normally present. The extent of vari-
ability in plant genomes with respect to the number of
rRNA genes and mutations in them has provided a useful
tool for analyzing the evolutionary patterns of plant species.
Plant cells may also contain excess DNA in their
genomes in the form of highly repetitive sequences, or
“high-copy-number DNA.” At present, the function of
this high-copy-number DNA in plant genomes is un-
known. Roughly half the maize genome is composed of
such retroviral-like DNA. RNA retroviruses like HIV use
their host genomes to make DNA copies that then insert
into the host genome. Clearly, the effects of some retro-
viruses can be lethal. How maize came to tolerate such a
large amount of this foreign DNA is an evolutionary
mystery.
Sequence Replication and Inversion
High-copy-number DNA sequences in the plant genome
may be short, such as the nucleotide sequence “GAA,” or
much longer, involving up to several hundred nucleotides.
Moreover, the number of copies of an individual high-
copy repetitive DNA sequence can total from 10,000 to
100,000. There are several possibilities for how high-copy
repetitive DNA sequences may be organized within a
plant genome (figure 43.4a). Several copies of a single
repetitive DNA sequence may be present together in the
same orientation, in a pattern called “simple tandem
array.” Alternatively, repetitive DNA sequences can be
dispersed among single-copy DNA in the same orienta-
tion (“repeat/single-copy interspersion”) or the opposite
orientation (“inverted repeats”). In addition, groups of
repetitive DNA sequences can also occur together in plant
genomes in a variety of possible arrangements, such as a
“compound tandem array” or a “repeat/repeat intersper-
sion.” The presence of repetitive DNA can vastly increase
the size of a plant genome, making it difficult to find and
characterize individual single-copy genes. Characterizing
single-copy genes can thus become a sort of “needle-in-
the-haystack” hunt.
A variety of mechanisms can account for the presence
of highly repetitive DNA sequences in plant genomes.
Repetitive sequences can be generated by DNA sequence
amplification in which multiple rounds of DNA replica-
tion occur for specific chromosomal regions. Unequal
crossing over of the chromosomes during meiosis or mi-
tosis (translocation) or the action of transposable ele-
ments (see next section) can also generate repetitive
sequences.
856 Part XI Plant Growth and Reproduction
(a) Different arrangements of repeated and
inverted DNA sequences
(b) Transposable element excision and
reinsertion
Simple tandem array
Repeat/single-copy interspersion
Inverted repeats
Compound tandem array
Repeat/repeat interspersion
Single-copy gene
Transposable
element
FIGURE 43.4
Organization of repeated DNA sequences and the mechanism of transposable
elements in altering gene function. (a) Repeated DNA sequences can occur in plant
genomes in several different arrangements. The arrows represent repeated DNA
sequences. Arrows of the same size and color represent DNA sequences which are
identical to each other. The direction of the arrowhead indicates the orientation of the
DNA sequence. (b) Transposable elements can be a source of repetitive DNA that alters
gene function. Following excision from its original location, a transposable element may
reinsert in the single-copy DNA sequence comprising a gene and alter the gene’s function.
Transposable Elements
Transposable elements, described in chapter 18, are special
sequences of DNA with the ability to move from place to
place in the genome. They can excise from one site at un-
predictable times and reinsert in another site. For this rea-
son, transposable elements have been called “jumping
genes.” Transposable elements often insert into coding re-
gions or regulatory regions of a gene and so affect expres-
sion of that gene, resulting in a mutation that may or may
not be detectable (figure 43.4b). Barbara McClintock won
the Nobel Prize in 1983 for her work describing transpos-
able elements in corn (see figure 18.23).
Due to their capacity to replicate independently and to
move through the genome, transposable elements can also
be involved in generating repetitive DNA sequences. This
is believed to be the case with the extensive retroviral-like
insertions in maize. Retention of the repetitive DNA se-
quence at a particular site in the genome would involve in
each instance a mutation in the transposable element itself
which removes its capacity to transpose.
Chloroplast Genome and Its Evolution
The chloroplast is a plant organelle that functions in pho-
tosynthesis, and it can independently replicate in the plant
cell. Plant chloroplasts have their own specific DNA, which
is separate from that present in the nucleus. This DNA is
maternally inherited and encodes unique chloroplast pro-
teins. Many of the proteins encoded by chloroplast DNA
are involved in photosynthesis. Chloroplasts are thought to
have originated from a photosynthetic prokaryote that be-
came part of a plant cell by endosymbiosis. In support of
this concept, research has shown that chloroplast DNA has
many prokaryote-like features. Chloroplast DNA is present
as circular loops of double-stranded DNA similar to
prokaryotic chromosomal DNA. Moreover, chloroplast
DNA contains genes for ribosomes that are very similar to
those present in prokaryotes.
The DNA in chloroplasts of all land plants has about the
same number of genes (~100), and they are present in about
the same order (figure 43.5). In contrast to the evolution of
the DNA in the plant cell nucleus, chloroplast DNA has
evolved at a more conservative pace, and therefore shows a
more interpretable evolutionary pattern when scientists
study DNA sequence similarities. Chloroplast DNA is also
not subject to modification caused by transposable ele-
ments and mutations due to recombination. Over time,
there appears to have been some genetic exchange between
the nuclear and chloroplast genomes. For example, the key
enzyme in the Calvin cycle of photosynthesis (RUBISCO)
consists of a large and small subunit. The small subunit is
encoded in the nuclear genome. The protein it encodes has
a targeting sequence that allows it to enter the chloroplast
and combine with large subunits. The evolutionary history
of the localization of these genes is a puzzle.
A characteristic feature of the chloroplast genome is the
presence of two identical inverted repeats in the DNA se-
quence. Other DNA sequence inversions or deletions
occur rarely, but when they do occur, they provide a char-
acter or a tool to analyze evolutionary relationships be-
tween plants. For instance, a large inversion in chloroplast
DNA is found in the Asteraceae, or sunflower family, and
not in other plant families. While previous work on the
evolutionary relationships between plants has emphasized
the comparative analysis of plant anatomy or morphology,
there is increasing use of plant molecular data such as
chloroplast DNA sequences. When considered together,
morphological and molecular information can provide a
clearer understanding of the evolutionary processes that
govern biological diversity.
Plant nuclear genomes may contain large amounts of
DNA in comparison to other eukaryotic organisms, but
only a small amount of this DNA represents functional
genes. Excess DNA in plant genomes can result from
increased chromosome copy number (polyploidy), and
DNA sequence repeats. Chloroplast genomes evolve
more slowly than nuclear genomes and can provide
important evolutionary information.
Chapter 43 Plant Genomics 857
Typical plant chloroplast genome
Small single-copy region
(~18 kb)
Large single-copy region
(~87 kb)
Inverted repeat
(~25 kb)
FIGURE 43.5
Chloroplast genome. A schematic drawing of a typical plant
chloroplast genome indicates two regions containing single-copy
genes, one containing about 87,000 nucleotides (87 kb) and
another about 18 kb, and two symmetrical inverted repeats, each
containing about 25 kb. Chloroplast DNA does not show
recombination events that are common in the nuclear genome. It
is thus a good subject for DNA phylogenetic analysis.
Comparative Genome Mapping
and Model Systems
Knowledge of plant genomes has been growing with the
advent of new techniques to study DNA sequences, such as
gene mapping and chromosome synteny. An increased un-
derstanding of plant genomes can lead to better manipula-
tion of genetic traits such as crop yield, disease resistance,
growth abilities, nutritive qualities, or drought tolerance.
Multiple genes could encode each of these traits. By
genome mapping model plants, plant biologists can lay a
foundation for future plant breeding and for an under-
standing of plant evolution at the genetic level. One such
model system, rice, has been chosen because it has a high
level of synteny with other grains. In a genomic sense, “rice
is wheat.” This provides a strong argument for rice as a
model system. The other model system that has been se-
lected in plants is Arabidopsis. This small weed that is a
member of the mustard family has an unusually small
genome with only 20% repetitive DNA (see table 43.1)
which has made it possible to sequence the entire genome.
Getting down to the level of individual base pairs is a step-
wise process, as described below.
RFLP and AFLP as Tools to Map Genomes
and Detect Polymorphisms
The classical approach to locating genes in linear order on
chromosomes involves making crosses between plants with
known genes identified by mutations. The frequency of re-
combination is used to calculate distance (see chapter 13).
The result is a genetic or linkage map. This approach is
limited to genes with alleles that can be phenotypically
identified. Much more of the genome can be mapped using
RFLPs (restriction fragment length polymorphisms) which
need not have a macroscopic phenotype. This approach,
described in detail in chapter 19 (see figures 19.2, 19.4,
19.9, and 19.10), involves analysis of the RFLP map, or the
pattern of DNA fragments, produced when DNA is treated
with restriction enzymes that cleave at specific sites. RFLP
mapping can identify important regions of the genome at a
glance, while sequence data require sophisticated com-
puter-based searching and matching systems. A comparison
of the RFLP maps of parents and progeny can give an indi-
cation of the heritability of gene traits and of heritable loci
that are characteristic of traits. If the trait and the RFLP
co-segregate, you have a direct link between the trait and
the DNA sequence. Moreover, after full genomes are se-
quenced at the nucleotide level, the genetic identification
of RFLP markers in regions of interest will be facilitated.
Remember, RFLPs are chunks of DNA that may contain a
part of one or more genes. Currently, the most dense
RFLP map is in rice where 2000 DNA sequences have
been mapped onto 12 chromosomes.
Another tool that utilizes sequence variability is
AFLPs, or amplified fragment length polymorphisms.
Hybridizing DNA primers with genomic DNA fragments
that have been cut with restriction enzymes, usually EcoRI
and MseI, and then subsequently amplified using the poly-
merase chain reaction (PCR) generates AFLP maps. The
resulting PCR products, which represent each piece of
DNA cut by a restriction enzyme, are separated by size
858 Part XI Plant Growth and Reproduction
(a)
(b) (c)
FIGURE 43.6
AFLP fingerprint pattern from normal and
“hypernodulating” soybeans. It is still not known what
determines the nodule number in (a) a normal soybean root versus
(b) a “hypernodulating” mutant. The slight genetic differences
between these plants can be evaluated by AFLP (c). The banding
pattern changes indicate what genetic markers are linked to the
“hypernodulation” mutation. Lane 1: normal soybean DNA; Lane
2: “hypernodulating” soybean DNA.
via gel electrophoresis. The band sizes on an AFLP gel
tend to show more polymorphisms than those found with
RFLP mapping because the entire genome is visible on
the gel (figure 43.6). Both RFLPs and AFLPs (among
many other tools for genome analysis) can provide mark-
ers of traits which are inherited from parents to progeny
through crosses.
DNA Microarrays
How can DNA sequences be made available to researchers,
other than as databases of electronic information? DNA mi-
croarrays are a way to link sequences with the study of gene
function and make DNA sequences available to many. Also
called biochips or “genes on chips,” these convenient assays
for the presence of a particular version of a gene were dis-
cussed in chapter 19. To prepare a particular DNA microar-
ray, fragments of DNA are deposited on a microscope slide
by a robot at indexed locations. Up to 10,000 spots can be
displayed over an area of only 3.24 cm
2
(figure 43.7). The
primary applications of microarrays are to determine which
genes are expressed developmentally in certain tissues or in
response to environmental factors. RNA from these tissues
can be isolated and used as a probe for these microarrays.
Only those sequences that are expressed in the tissues will
be present to hybridize to the spot on the microarray.
Chapter 43 Plant Genomics 859
DNA
Arabidopsis
genome
DNA
microarray
DNA
Flower-specific mRNA
(sample 1)
Reverse transcriptase
Fluorescent nucleotide
Reverse transcriptase
Different fluorescent nucleotide
cDNA probe
cDNA probe
Leaf-specific mRNA
(sample 2)
Probe 1
Probe 2
Strong
signal from
probe 2
Weak
signal from
probe 2 Strong
signal
from
probe 1
Weak
signal
from
probe 1
Similar
signals from
both probes
Mix
Hybridize
3. Samples of mRNA are obtained, for instance from
two different tissues. Probes for each sample are
prepared using a different fluorescent nucleotide for
each sample.
4. The two probes are mixed and
hybridized with the microarray.
Fluorescent signals on the
microarray are analyzed.
2. DNA is printed onto
a microscope slide.
1. Target DNA is amplified by PCR.
Robotic
quill
FIGURE 43.7
Microarrays. Microarrays are created by robotically placing DNA onto a microscope slide. The microarray can then be probed with RNA
from tissues of interest to identify expressed DNA. The microarray with hybridized probes is analyzed and often displayed as a false-color
image. If a gene is frequently expressed in one of the samples, the fluorescent signal will be strong (red or green) where the gene is located
on the microarray. If a gene is rarely expressed in one of the samples, the signal will be weak (pink or light green). A yellow color indicates
genes that are expressed at similar levels in each sample.
Plant Genome Projects
The potential of having complete ge-
nomic sequences of plants is tremen-
dous and about to be realized now that
the Arabidopsis Genome Project is es-
sentially complete. This project repre-
sents a new paradigm in the way biol-
ogy is done. The international effort
brought together research teams with
the expertise and tenacity to apply new
sequencing technology to an entire
genome, rather than single genes.
Powerful databases are being con-
structed to make this information ac-
cessible to all. The completely se-
quenced Arabidopsis genome will have
far-reaching uses in agricultural
breeding and evolutionary analysis.
This information can be expected to
help plant breeders in the future be-
cause the localization of genes in one
plant species can help indicate where
that gene might also be located in an-
other species (figure 43.8). In plant
genomes, local gene order seems to be more conserved
than the nucleotide sequences of homologous genes.
Thus, the complete genomic sequence of Arabidopsis
thaliana will facilitate gene cloning from many plant
species, using information on relative genomic location as
well as similarity of sequences.
Sequencing the rice genome provides a model for a
small monocot genome. Rice was selected, in part, be-
cause its genome is 6, 10, and 40 times smaller than
maize, barley, and wheat. These grains represent a major
food source for humans. By understanding the rice
genome at the level of its DNA sequence, it should be
much easier to identify and isolate genes from grains with
larger genomes. Even though these plants diverged more
than 50 million years ago, the chromosomes of rice, corn,
barley, wheat, and other grass crops show extensive con-
served arrangements of segments (synteny) (figure 43.9).
DNA sequence analysis of cereal grains will be important
for identifying genes associated with disease resistance,
crop yield, nutritional quality, and growth capacity. It will
also be possible to construct an approximate map of the
ancestral cereal genome.
Functional Genomics and Proteomics
Sequencing the Arabidopsis and rice genome represent
major technological accomplishments. A new field of
bioinformatics takes advantage of high-end computer
technology to analyze the growing gene databases, look
for relationships among genomes, and hypothesize func-
tions of genes based on sequence. Genomics (the study of
genomes) is now shifting gears and moving back to
hypothesis-driven science. Again, an international com-
munity of researchers has come together with a plan to
assign function to all of the 20,000 to 25,000 Arabidopsis
genes by 2010 (Project 2010). In many ways, the goal is
to ultimately answer the questions we have raised in
chapters 37 through 42. One of the first steps is to deter-
mine when and where these genes are expressed. Each
step beyond that will require additional enabling technol-
ogy. Research will move from genomics to proteomics
(the study of all proteins in an organism). Proteins are
much more difficult to study because of posttranslational
modification and formation of complexes of proteins.
This information will be essential in understanding cell
biology, physiology, development, and evolution. For ex-
ample, how are similar genes used in different plants to
create biochemically and morphologically distinct organ-
isms? So, in many ways, we continue to ask the same
questions that even Mendel asked, but at a much differ-
ent level of organization.
Restriction fragment length polymorphisms (RFLPs)
and amplified fragment length polymorphisms (AFLPs)
represent important tools for mapping genetic traits in
plant genomes. Due to its short life cycle, small size,
and small genome, the mustard relative Arabidopsis
thaliana is being used as a model plant for genetic
studies. The genome of rice is also essentially
sequenced and will be a valuable model for other
monocot cereal grains such as wheat, barley, oats, and
corn. Assigning function to these genes is the next
challenge.
860 Part XI Plant Growth and Reproduction
FIGURE 43.8
Future directions in the genetic engineering of vegetable oils?
Chapter 43 Plant Genomics 861
123 456
78 9101 12
ABCD
FGH I
12345678
1234567
910
Rice genome
Sugar cane
chromosome segments
Wheat
chromosome segments
Corn chromosome segments
Genomic alignment
(segment rearrangement)
Rice
Corn
Wheat
S
ug
ar ca
n
e
FIGURE 43.9
Grain genomes are rearrangements of similar chromosome segments. Shades of the same color represent pieces of DNA that are
conserved among the different species but have been rearranged. By splitting the individual chromosomes of major grass species into
segments, and rearranging the segments, researchers have found that the genome components of rice, sugar cane, corn, and wheat are
highly conserved. This implies that the order of the segments in the ancestral grass genome has been rearranged by recombination as the
grasses have evolved. Data: G. Moore, K. M. Devos, Z. Wang, and M. D. Gale: “Grasses, line up and form a circle,” Current Biology 1995,
vol. 5, pp. 737-739.
862 Part XI Plant Growth and Reproduction
Overview of Plant Tissue Culture
One of the major hopes for the plant genome projects is
using newly identified genes for biotechnology, and ad-
vances in tissue culture are facilitating this. Having an
agriculturally valuable gene in hand is just the beginning.
With methods discussed in chapter 19 and below, desir-
able genes can be introduced into plants, yielding trans-
genic cells and tissues. Whole plants can than be regener-
ated using tissue culture. While animals can now be
cloned, the process is much simpler in plants. Many so-
matic (not germ-line) plant cells are totipotent, which
means they can express portions of their previously unex-
pressed genes and develop into whole plants under the
right conditions.
The successful culture of plant cells, tissues, or organs
requires utilizing the proper plant starting material, ap-
propriate nutrient medium, and timing of hormonal
treatments to maximize growth potential and drive differ-
entiation (figure 43.10). Most plant tissue cultures are
initiated from explants, or small sections of tissue re-
moved from an intact plant under sterile conditions.
After being placed on a sterile growth medium contain-
ing nutrients, vitamins, and combinations of plant growth
regulators, cells present in the explant will begin to di-
vide and proliferate. Under appropriate culture condi-
tions, plant cells can multiply and form organs (roots,
shoots, embryos, leaf primordia, and so on) and can even
regenerate a whole plant. The regeneration of a whole
plant from tissue-cultured plant cells represents an im-
portant step in the production of genetically engineered
plants. Using plant tissue cultures, genetic manipulation
can be conducted at the level of single cells in culture,
and whole plants can then be produced bearing the intro-
duced genetic trait.
Plant cells growing in culture can also be used for the
mass production of genetically identical plants (clones)
with valuable inheritable traits. For example, this ap-
proach of clonal propagation using plant tissue culture is
commonly used in the commercial production of many
ornamental plants such as chrysanthemums and ferns. As
we will describe next, different types of cultures can be
generated based on the initial type of plant tissue used for
the explant and on the composition of the growth
medium.
It is often possible to regenerate an entire plant from
one or a few cells. Depending on the plant tissue used
and the growth medium selected, different types of
cultures may be produced.
43.2 Advances in plant tissue culture are revolutionizing agriculture.
(a)
(b)
(c)
FIGURE 43.10
Culture of orchid plants. While the natural maturation of a
single orchid plant may take up to seven years, commercial
growers can produce thousands of cultured orchids in a relatively
short time. (a) The apical meristem is removed from an orchid
plant. (b) The meristematic tissue is grown in flasks containing
hormone-enhanced media, and roots and shoots begin to form. (c)
The plantlets are then separated and grown to maturity.
Types of Plant Tissue Cultures
Depending on the type of plant tissue used as the explant
and the composition of the growth medium, a variety of
different types of plant tissue cultures can be generated.
These different types of plant tissue cultures have applica-
tions both in basic plant research and commercial plant
production.
Callus Culture
Callus culture refers to the growth of unorganized masses of
plant cells in culture. To generate a callus culture, an ex-
plant, usually containing a region of meristematic cells, is
incubated on a growth medium containing certain plant
growth regulators such as auxin and cytokinin (figure
43.11). The cells grow from the explant and divide to form
an undifferentiated mass of cells called a callus. This unor-
ganized mass of growing cells is analogous to a plant
tumor. Cells can proliferate indefinitely if they are periodi-
cally transferred to fresh growth media. However, if the
callus cells are transferred to a growth medium containing
a different combination of plant growth regulators, the
cells can be directed to differentiate into roots and/or
shoots. This process of converting unorganized growth
into the production of shoots and roots is called organo-
genesis, and it represents one means by which a whole
plant can be regenerated from tissue culture cells. When a
plantlet produced by organogenesis is large enough, it can
be transferred to a large container with nutrients or soil
and grown to maturity.
Chapter 43 Plant Genomics 863
(a)
(b) (c) (d)
FIGURE 43.11
Callus culture. (a) An explant is
incubated on growth media. (b)
The cells grow and divide and form
a callus. (c) The callus cells, grown
on media containing new plant
growth regulators, differentiate
into plant parts. (d) After the
plantlet is large enough, it is grown
to maturity in soil.
Cell Suspension Culture
Plant cell suspension culture involves the growth of single
or small groups of plant cells in a liquid growth medium.
Cell suspension cultures are usually initiated by the trans-
fer of plant callus cells into a liquid medium containing a
combination of plant growth regulators and chemicals that
promote the disaggregation of the cells into single cells or
small clumps of cells (figure 43.12). Continued cell growth
requires that the liquid cultures be shaken at low speed to
promote aeration and chemical exchange with the
medium. Suspension cell cultures are often used in re-
search applications where access to single cells is impor-
tant. The suspension bath can provide an efficient means
for selecting out cells with desirable traits such as herbi-
cide tolerance or salt tolerance because the bath is in uni-
form contact with all the cells at once. This differs from
callus culture, where only those cells in contact with the
solid medium can be selected by chemical additions to the
medium. Suspension cultures can also provide a conve-
nient means for producing and collecting the plant chemi-
cals cells secrete. These can include important plant
metabolites, such as food products, oils, and medicinal
chemicals. In addition, plant suspension cell cultures can
often be used to produce whole plants via a process known
as somatic cell embryogenesis (figure 43.13). For some
plants, this provides a more convenient means of regener-
ating a whole plant after genetic engineering takes place at
the single-cell level. In somatic cell embryogenesis, plant
suspension culture cells are transferred to a medium con-
taining a combination of growth regulators that drive dif-
ferentiation and organization of the cells to form individ-
ual embryos. Under a dissection microscope, these
embryos can be isolated and transferred to a new growth
medium, where they grow into individual plants.
864 Part XI Plant Growth and Reproduction
FIGURE 43.12
Cell suspension culture. Plant cells can be grown as individual
cells or small groups of cells in a liquid culture medium. Liquid
suspension culture of plant cells ensures that most cells are in
contact with the growth medium.
(a)
(e)(d)
(b) (c)
FIGURE 43.13
Somatic cell embryogenesis. A large
number of plants can be cloned from a
single soybean seed via somatic cell
embryogenesis. (a) Immature soybean seeds
placed on culture medium. (b) Embryos
appear on the seeds after two weeks in
culture. (c) Four embryos at different stages
of development (globular, heart, torpedo,
and plantlet). (d) Seedlings with shoots and
roots. (e) Mature soybean plants.
Protoplast Isolation and Culture
Protoplasts are plant cells that have had their thick cell
walls removed by an enzymatic process, leaving behind a
plant cell enclosed only by the plasma membrane. Plant
protoplasts have been extremely useful in research on the
plant plasma membrane, a structure normally inaccessible
due to its close association with the cell wall. Within hours
of their isolation, plant protoplasts usually begin to resyn-
thesize cell walls, so this process has also been useful in
studies on cell wall production in plants. Plant protoplasts
are also more easily transformed with foreign DNA using
approaches such as electroporation (see the subsequent
section). In addition, protoplasts isolated from different
plants can be forced to fuse together to form a hybrid. If
they are regenerated into whole plants, these hybrids
formed from protoplast fusion can represent genetic com-
binations that would never occur in nature. Hence, proto-
plast fusion can provide an additional means of genetic en-
gineering, allowing beneficial traits from one plant to be
incorporated into another plant despite broad differences
between the species. When either single or fused proto-
plasts are transferred to a culture growth medium, cell wall
regeneration takes place, followed by cell division to form
a callus (figure 43.14). Once a callus is formed, whole
plants can be produced either by organogenesis or by so-
matic cell embryogenesis in culture.
Chapter 43 Plant Genomics 865
(a) (b) (c)
(d) (e) (f)
FIGURE 43.14
Protoplast regeneration. Different stages in the recovery of intact plants from single plant protoplasts of evening primrose. (a) Individual
plant protoplasts. (b) Regeneration of the cell wall and the beginning of cell division. (c, d) Aggregates of plant cells resulting from cell
division which can form a callus. (e) Production of somatic cell embryos from the callus. ( f ) Recovery of a plantlet from the somatic cell
embryo through the process described in figure 43.13.
Anther/Pollen Culture
In flowers, the anthers are the
anatomical structures that contain the
pollen. In normal flower develop-
ment, the anthers mature and open to
allow pollen dispersal. In anther cul-
ture, anthers are excised from the
flowers of a plant and then trans-
ferred to an appropriate growth
medium. After a short period of time,
pollen cells can be manipulated to
form individual plantlets, which can
be grown in culture and used to pro-
duce mature plants. The development
of these plantlets usually proceeds
through the formation of embryos
(figure 43.15). Plants produced by an-
ther/pollen culture can be haploid be-
cause they were originally derived
from pollen cells that have undergone
meiosis. However, these plants may
be sterile and thus not useful for
breeding or genetic manipulation. On
the other hand, plants derived from
anther/pollen culture can be treated
at an early stage with chemical agents
such as colchicine, which allows chro-
mosome duplication. Chromosome
duplication results in the conversion
of sterile haploid plants into fertile
diploid organisms. Under these con-
ditions, plants can be produced that
are homozygous for every single trait,
even those which tend to be recessive
traits. On a cautionary note, not every cell exposed to
colchicine becomes diploid. Some have unusual ploidy
levels, and they can be screened for chromosome num-
ber. The homozygous plants are useful tools, allowing
breeders to introduce a normally recessive trait.
Plant Organ Culture
Plant organs can also be grown under culture conditions,
and this has provided a useful tool in the study of plant
organ development. For example, pollinated flowers of a
plant such as a tomato can be excised and transferred to a
culture flask containing an appropriate medium. Over
time, the ovular portion of the plant will develop into a
tomato fruit that will eventually turn red and ripen. Sec-
tions of plant roots can also be excised and transferred to
a liquid growth medium. In this medium, the roots can
proliferate extensively, forming both primary and sec-
ondary root branches (figure 43.16).
Many plant cells are totipotent; a whole plant may be
regenerated from a single plant cell. Depending upon
the explant type, culture medium, and combinations of
plant growth regulators, it is possible to grow plant
cells, tissues, or organs in sterile cultures.
866 Part XI Plant Growth and Reproduction
(a) (b)
(c)
FIGURE 43.15
Anther culture. Callus formation from
maize pollen. Anthers containing pollen
can be regenerated on tissue culture
medium. The pollen in the anthers contain
a haploid set of chromosomes, which can
be doubled to form a homozygous diploid
cell. Regenerated homozygous diploid
plants are important for plant breeding
purposes. (a) Maize anthers in culture
medium. (b) Callus formation from pollen.
(c) Callus and shoot formation.
FIGURE 43.16
Plant organ
culture. Plant
roots growing in a
liquid culture
medium. From
small excised
sections of plant
roots, the roots
will grow and
proliferate with
extensive lateral
root formation
(branching).
Applications of Plant
Tissue Culture
In addition to the applications already
described, plant tissue cultures have a
variety of uses both in agriculture and
in industry.
Suspension Cultures as
Biological Factories
An important industrial application of
plant tissue culture involves the use of
plant cells as biological factories.
Large-scale suspension cultures can be
grown to produce antimicrobial com-
pounds, antitumor alkaloids, vitamins,
insecticides, and food flavors. Plant
roots can also be grown in liquid cul-
ture, creating a mesh of roots that can
produce a number of useful plant com-
pounds.
Horticultural Uses
Plants with valuable traits can be mass
propagated through tissue-culture
cloning. In this application of plant tis-
sue culture, hundreds or even thou-
sands of genetically identical plants can
be produced by vegetative asexual
propagation from one plant source.
This has been extensively used in the
flower industry where genetically iden-
tical plants can be produced from a su-
perior parent plant. Propagation of
plant tissue in the sterile environment
of the growth medium can also help in
the production disease-free plants, such as those cultured
from the meristematic (apical dome) tissue untouched by
viruses or other diseases because it is new growth. This ap-
proach has been particularly useful in the culture of dis-
ease-free orchids and raspberries.
Somaclonal Variation
Plant tissue culture also has a problematic side effect that
can be used as an asset under certain conditions. During
periods of extended growth of plant cells in callus or sus-
pension cell culture, various parts of the plant genome may
become more or less “active” due to a release of control
over gene expression. Transposable elements may also be-
come more active, and chromosomal rearrangements may
occur. Sometimes you end up with unusual numbers of
chromosomes. This altered control provides a new source
of genetic diversity that can result in novel traits which
were not even present in the original plant material used as
the explant to start the cultures (figure 43.17). This in-
creased genetic diversity following extended time in tissue
culture is called somaclonal variation. It can be problem-
atic if the desired goal is the propagation or production of
identical plant clones. However, somaclonal variation, in-
duced by intentionally growing plant cells in tissue over a
longer time period, can be very useful to generate novel
plants with traits not currently present in a given gene
pool. These traits can be identified either at the tissue cul-
ture stage (for example, disease resistance or heat tolerance)
or following the regeneration of whole plants by either
organogenesis or embryogenesis (plant size, photosynthetic
rates, and so forth).
Plant cell, tissue, and organ cultures have important
applications in agriculture and industry.
Chapter 43 Plant Genomics 867
(a) (b)
(c)
FIGURE 43.17
Somaclonal variation.
Regeneration of plants
from tissue culture can
produce plants that are
not similar to their
parents due to
chromosomal alterations.
This variability can be
used to select plants with
altered traits. These
maize plants show
evidence of somaclonal
variation. (a) Yellow leaf
stripe. (b) Dwarf maize.
(c) Yellow leaf tip.
Plant biotechnology provides an efficient means to pro-
duce an array of novel products and tools for use by our
global society. Agricultural biotechnology has the poten-
tial to increase farming revenue, lower the cost of raw ma-
terials, and improve environmental quality. Plant genetic
engineering is becoming a key tool for improving crop
production.
World Population in Relation to
Advances Made in Crop Production
Due in a large part to scientific advances in crop breeding
and farming techniques, world food production has dou-
bled since 1960. Moreover, productivity of agricultural
land and water usage has tripled over this time period.
While major genetic improvements have been made in
crops through crop breeding, this can be a slow process.
Furthermore, most crops grown in the United States pro-
duce less than 50% of their genetic potential. These short-
falls in yield are due in large part to the inability of crops to
tolerate or adapt to environmental stresses (salt, water, and
temperature), pests, and disease (figure 43.18).
The world now farms an area the size of South America,
but without the scientific advances of the past 30 years,
farmland equaling the entire western hemisphere would be
required to feed the world. Nevertheless, the world popula-
tion is expected to double to 12 billion by the first half of
this century, and it is not clear whether current levels of
food production can keep pace with this rate of population
growth. Many believe the exploitation of conventional crop
breeding programs may have reached their limit. The ques-
tion is how best to feed billions of additional people with-
out destroying much of the planet in the process. In this re-
spect, the disappearance of tropical rain forests, wetlands,
and other vital habitats will accelerate unless agriculture
becomes more productive and less taxing to the environ-
ment. Advances in our understanding of plant reproduction
from the molecular to the ecosystems levels are providing
tools to further protect natural environments by preventing
the spread of modified genes to wild populations.
Although improved farming practices and crop breeding
have increased crop yields, it is uncertain whether these
approaches can keep pace with the food demands of an
ever-increasing world population.
868 Part XI Plant Growth and Reproduction
43.3 Plant biotechnology now affects every aspect of agriculture.
FIGURE 43.18
Corn crop
productivity well
below its genetic
potential due to
drought stress. Corn
production can be
limited by water
deficiencies due to
drought during the
growing season in dry
climates.
Plant Biotechnology
for Agricultural Improvement
It seems certain that plant genetic engineering will play a
major role in resolving the problem of feeding an increas-
ing world population. The nutritional quality of crop plants
is being improved by increasing the levels of nutrients they
contain, such as beta-carotene and vitamins A, C, and E,
which may protect people from health problems such as
cancer and heart disease. Biotechnology is now being em-
ployed to improve the quality of seed grains, increase pro-
tein levels in forage crops, and transform plants to improve
their resistance to disease, insects, herbicides, and viruses.
Other stresses on plants, such as heat or salt, can be im-
proved by engineering higher tolerance levels.
Compared with approaches that rely on plant breed-
ing, genetic engineering can compress the time frame re-
quired for the development of improved crop varieties.
Moreover, in genetic engineering, genetic barriers, such
as pollen compatibility with the pistil, no longer limit the
introduction of advantageous traits from diverse species
into crop plants. Once a useful trait has been identified at
the level of individual genes and their DNA sequences,
the incorporation of this trait into a crop plant requires
only the introduction of the DNA bearing these genes
into the crop plant genome. The process of incorporating
foreign DNA into an existing plant genome is called
plant transformation. At present, there are several ap-
proaches for plant transformation; the use of Agrobac-
terium tumefaciens in this process was described in chapter
19. This approach works best if the plant being trans-
formed is a dicot. However, many food crops, such as the
cereal grains (rice, wheat, corn, barley, oats, and so on)
are monocots. Two additional plant transformation
methods that can be used with both dicots and monocots
are discussed in the next section.
Useful Traits That Can Be Introduced into Plants
Although plant transformation represents a relatively new
technology, extensive efforts are underway to utilize this
approach to develop plants and food products with benefi-
cial characteristics. We discussed a variety of biotechnolog-
ical applications for crop improvement in chapter 19. Fur-
ther applications of this approach involve modifications of
nutritional quality of foods, phytoremediation, production
of plastics, and using plants as “edible vaccines.”
Improved Nutritional Quality of Food Crops. Approx-
imately 75% of the world’s production of oils and fats
come from plant sources. For medical and dietary reasons,
there is a trend away from the use of animal fats and toward
the use of high-quality vegetable oils. Genetic engineering
has allowed researchers to modify seed oil biochemistry to
produce “designer oils” for edible and nonedible products.
One technique modifies canola oil to replace cocoa butter
as a source of saturated fatty acids; others modify the en-
zyme ACP desaturase for the creation of monounsaturated
fatty acids in transgenic plants. High-lauric acid canola has
been planted in several countries and used in both foods
and soaps.
Attempts are also underway to modify the amino acid
contents of various plant seeds to present a more complete
nutritional diet to the consumer. A high-lysine corn seed is
being developed; this would cut down on the need for ly-
sine supplements that are currently added to livestock feed.
Biotechnology has the potential to make plant foods
healthier and more nutritious for human consumption.
Fruits and vegetables, such as tomatoes, may be engineered
to contain increased levels of vitamins A and C and beta-
carotene, which, when included in the human diet, may
help protect against chronic diseases.
Phytoremediation. Cleaning up environmental toxins to
reclaim polluted land is an ongoing challenge. Genetically
modified plants offer an enticing solution. Work is pro-
gressing on plants that accumulate heavy metals at high
concentrations. These plants can then be harvested. Be-
cause most of their biomass is water, the dried plants allow
for the collection of the metals in a small area. Organic
compounds that pose hazards to human health have the po-
tential to be taken up by plants and broken down into
harmless components. Modified biochemical pathways are
being used to break down toxic substances. Modified
poplars, for example, have been engineered to break down
TNT.
Plants Bearing Vaccines for Human Diseases. An-
other very interesting application of plant genetic engi-
neering includes the introduction of “vaccine genes” into
edible plants. Here, genes encoding the antigen (for ex-
ample, a viral coat protein) for a particular human
pathogen is introduced into the genome of an edible plant
such as a banana, tomato, or apple via plant transforma-
tion. This antigen protein would then be present in the
cells of the edible plant, and a human individual that con-
sumed the plant would develop antibodies against the
pathogenic organism. Currently, researchers are trying to
develop such edible vaccines for a coat protein of hepatitis
B, an enterotoxin B of E. coli, and a viral capsid protein of
the Norwalk virus. The measles gene has been introduced
into tobacco as a model system and is now being intro-
duced into lettuce and rice. This is a terrific advance for
tropical areas where it is difficult to keep the traditional
vaccine cold (remember that proteins degrade rapidly as
the temperature increases).
Genetic engineering of crop plants has allowed
researchers to alter the oil content, amino acid
composition, and vitamin content of food crops.
Genetic engineering may also allow the production of
food crops bearing “edible vaccines.”
Chapter 43 Plant Genomics 869
Methods of Plant Transformation
Plant Transformation Using the Particle Gun
Using a “gun” to blast plant cells does not seem like a suit-
able method for introducing foreign DNA into a plant
genome. However, it works, and many whole plants have
been regenerated after foreign DNA is shot through the
cell wall and then integrated into the plant genome. The
particle gun utilizes microscopic gold particles coated with
the foreign DNA, shooting these particles into plant cells
at high velocity. Acceleration of the particles to a sufficient
velocity to pass through the plant cell wall can be achieved
by a burst of high-pressure helium gas or an electrical dis-
charge (figure 43.19a). Only a few cells actually receive the
foreign DNA and survive this treatment. These cells are
identified with the help of a selectable marker also present
on the foreign DNA. The selectable marker allows only
those cells receiving the foreign DNA to survive on a par-
ticular growth medium (figure 43.20). The selectable
markers include genes for resistance to a herbicide or an-
tibiotic. Plant cells which survive growth in the selection
medium are then tested for the presence of the foreign
gene(s) of interest.
Plant Transformation Using Electroporation
Foreign DNA can also be “shocked” into cells that lack a
cell wall, such as the plant protoplasts described earlier. A
pulse of high-voltage electricity in a solution containing
plant protoplasts and DNA briefly opens up small pores in
the protoplasts’ plasma membranes, allowing the foreign
DNA to enter the cell (figure 43.19b). Ideally, the DNA in-
corporates into one of the plant’s chromosomes. Following
electroporation, the protoplasts are transferred to a growth
medium for cell wall regeneration, cell division, and, even-
tually, the regeneration of whole plants. As with the use of
the particle gun, a selectable marker is typically present in
the foreign DNA, and protoplasts containing foreign DNA
are selected based upon their ability to survive and prolifer-
ate in a growth medium containing the selection treatment
(antibiotic or herbicide). Once regenerated from electropo-
rated protoplasts, whole plants can then be evaluated for
the presence of the beneficial trait.
Plant biotechnology may play an important role in the
further improvement of crop plants. The particle gun
and electroporation are useful methods for introducing
foreign DNA into plants.
870 Part XI Plant Growth and Reproduction
Discharge chamber
HV(+) HV(–)
DNA coated gold particles on film
Retaining screen
Target cells/tissue
(a)
– +
DNA
Cells
Voltage
applied
Transformed
cell
(b)
FIGURE 43.19
Methods for plant transformation. (a) The particle gun is one
method for introducing foreign DNA into plant cells. Here an
electrical discharge propels DNA-coated gold particles into plant
cells or tissue. A retaining screen reduces cellular damage
associated with bombardment by only allowing the DNA-coated
particles to pass and retaining fragments of the mounting film. (b)
Foreign DNA can also be introduced into plant protoplasts by
electroporation. A brief pulse of electricity generates pores in the
plasma membrane, allowing DNA to enter the cells.
FIGURE 43.20
Regeneration after transformation with the use of a selectable marker. Stages in the
recovery of a plant containing foreign DNA introduced by the “particle gun” method for
plant cell transformation. A selectable marker, in this case a gene for resistance to
herbicide, aids in the identification and recovery of plants containing the DNA insert. (a)
Embryonic callus just prior to particle gun bombardment. (b) Following bombardment,
callus cells containing the foreign DNA are indicated by color from the gus gene used as a
tag or label on the foreign DNA. (c) Shoot formation in the transformed plants growing on
a selective medium. Here, the gene for herbicide resistance in the transformed plants
allows growth on the selective medium containing the herbicide. Nontransformed plants
do not contain the herbicide resistance gene and do not grow well. (d) Production of
plantlets from transformed plants growing on the selective medium. (e) Comparison of
growth on the selection medium for transformed plants bearing the herbicide resistance
gene (left) and a nontransformed plant (right). ( f ) Mature transgenic plants resulting from
this process.
Chapter 43 Plant Genomics 871
(a) (b)
(c) (d) (e)
(f)
872 Part XI Plant Growth and Reproduction
Chapter 43
Summary Questions Media Resources
43.1 Genomic organization is much more varied in plants than in animals.
? Plant genomes are very large in comparison to other
eukaryotes, mainly due to a high amount of repetitive
DNA.
? Plant genomes can be compared with one another by
mapping the locations of certain genes or gene traits
in various plants. RFLPs and AFLPs can be used to
map plant DNA.
? Arabidopsis thaliana has a small genome, for a plant.
This complete genome is essentially sequenced, so all
genes and their positions are known.
? The molecular maps of the genomes of rice and other
grains demonstrate remarkable similarity.
? Functional genomics and proteomics will allow us to
understand and utilize the information in fully se-
quenced plant genomes.
1. Describe mechanisms for the
generation of highly repetitive
DNA in plants.
2. What characteristics of
Arabidopsis thaliana make it
useful as a model system in
genetic studies and for the
sequencing of its entire genome?
Why is rice useful as a model
system for the analysis of the
genome of a monocot plant?
3. Why will microarrays be
useful in functional genomics?
4. What type of questions can be
asked now that the Arabidopsis
and rice genomes are essentially
sequenced?
? With the addition of appropriate combinations of
plant growth regulators (auxin, cytokinin), plant cells
in culture can be directed to form organs, embryos,
or whole plants.
? Anther cultures can produce haploid plants or plants
that are homozygous for all traits.
? Plant tissue culture has a number of practical
applications, including the industrial production of
plant chemicals, clonal propagation of horticultural
plants, and the generation of disease-free plants.
? Growth of plant cells in tissue culture over extended
time results in an increase in genetic variation called
somaclonal variation. This variation can extend
beyond the traits present in the gene pool and can
generate novel genetic variations in breeding studies.
5. Describe how whole plants
can be regenerated from tissue-
cultured plant cells using either
organogenesis or somatic cell
embryogenesis. Which approach
requires the use of suspension
cell cultures?
6. How are plant protoplasts
generated, and what is protoplast
fusion? How can plant
protoplasts be used to generate
hybrid plants that would not
occur in nature?
43.2 Advances in plant tissue culture are revolutionizing agriculture.
? Genetic engineering and biotechnology can be
utilized to improve the quality of food crops, increase
disease resistance, and improve the tolerance of crops
to environmental stress.
? A key aspect of plant genetic engineering is the
introduction of foreign DNA into plant cells. This
can be achieved using a particle gun or
electroporation.
7. Describe how the particle gun
and electroporation can be used
to introduce foreign DNA into
plant cells. Which approach
requires the use of plant
protoplasts? Why?
8. How can a plant be
“engineered” to produce an
edible vaccine ?
43.3 Plant biotechnology now affects every aspect of agriculture.
www.mhhe.com/raven6e www.biocourse.com
? Scientists on Science:
Plant Biotechnology
? Student Research:
Plant Crop Protection
873
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Part
XII
Animal Diversity
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874 Part XII Animal Diversity
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875
44
The Noncoelomate
Animals
Concept Outline
44.1 Animals are multicellular heterotrophs without
cell walls.
Some General Features of Animals. Animals lack cell
walls and move more rapidly and in more complex ways
than other organisms. The animal kingdom is divided into
animals without symmetry and tissues, and animals with
symmetry and tissues.
Five Key Transitions in Body Plan. Over the course of
animal evolution, the animal body plan has undergone
many changes, five of them significant.
44.2 The simplest animals are not bilaterally
symmetrical.
Parazoa. Sponges are the most primitive animals, without
either tissues or, for the most part, symmetry.
Eumetazoa: The Radiata. Cnidarians and ctenophorans
have distinct tissues and radial symmetry.
44.3 Acoelomates are solid worms that lack a body
cavity.
Eumetazoa: The Bilaterian Acoelomates. Flatworms
are the simplest bilaterally symmetrical animals; they lack a
body cavity, but possess true organs.
44.4 Pseudocoelomates have a simple body cavity.
The Pseudocoelomates. Nematodes and rotifers possess
a simple body cavity.
44.5 The coming revolution in animal taxonomy will
likely alter traditional phylogenies.
Reevaluating How the Animal Body Plan Evolved. The
use of molecular data will most likely generate changes to
traditional animal phylogenies.
W
e will now explore the great diversity of animals, the
result of a long evolutionary history. Animals, con-
stituting millions of species, are among the most abundant
living things. Found in every conceivable habitat, they be-
wilder us with their diversity. We will start with the sim-
plest members of the animal kingdom—sponges, jellyfish,
and simple worms. These animals lack a body cavity called
a coelom, and are thus called noncoelomates (figure 44.1).
The major organization of the animal body first evolved in
these animals, a basic body plan upon which all the rest of
animal evolution has depended. In chapters 45 through 48,
we will consider the more complex animals. Despite their
great diversity, you will see that all animals have much in
common.
FIGURE 44.1
A noncoelomate: a marine flatworm. Some of the earliest
invertebrates to evolve, marine flatworms possess internal organs
but lack a true cavity called a coelom.
the zygote. Consequently, with a few exceptions, there is
no counterpart among animals to the alternation of haploid
(gametophyte) and diploid (sporophyte) generations char-
acteristic of plants (see chapter 32).
Embryonic Development. Most animals have a similar
pattern of embryonic development. The zygote first under-
goes a series of mitotic divisions, called cleavage, and be-
comes a solid ball of cells, the morula, then a hollow ball of
cells, the blastula. In most animals, the blastula folds in-
ward at one point to form a hollow sac with an opening at
one end called the blastopore. An embryo at this stage is
called a gastrula. The subsequent growth and movement
of the cells of the gastrula produce the digestive system,
also called the gut or intestine. The details of embryonic
development differ widely from one phylum of animals to
another and often provide important clues to the evolu-
tionary relationships among them.
The Classification of Animals
Two subkingdoms are generally recognized within the
kingdom Animalia: Parazoa—animals that for the most part
lack a definite symmetry and possess neither tissues nor or-
gans, mostly comprised of the sponges, phylum Porifera;
and Eumetazoa—animals that have a definite shape and
symmetry and, in most cases, tissues organized into organs
and organ systems. Although very different in structure,
both types evolved from a common ancestral form (figure
44.2) and possess the most fundamental animal traits.
All eumetazoans form distinct embryonic layers during
development that differentiate into the tissues of the adult
animal. Eumetazoans of the subgroup Radiata (having ra-
dial symmetry) have two layers, an outer ectoderm and an
inner endoderm, and thus are called diploblastic. All
other eumetazoans, the Bilateria (having bilateral symme-
try), are triploblastic and produce a third layer, the meso-
derm, between the ectoderm and endoderm. No such lay-
ers are present in sponges.
The major phyla of animals are listed in table 44.1. The
simplest invertebrates make up about 14 phyla. In this
chapter, we will discuss 8 of these 14 phyla and focus in de-
tail on 4 major phyla: phylum Porifera (sponges), which
lacks any tissue organization; phylum Cnidaria (radially
symmetrical jellyfish, hydroids, sea anemones, and corals);
phylum Platyhelminthes (bilaterally symmetrical flat-
worms); and phylum Nematoda (nematodes), a phylum that
includes both free-living and parasitic roundworms.
Animals are complex multicellular organisms typically
characterized by high mobility and heterotrophy. Most
animals also possess internal tissues and organs and
reproduce sexually.
876 Part XII Animal Diversity
Some General Features of Animals
Animals are the eaters or consumers of the earth. They are
heterotrophs and depend directly or indirectly on plants,
photosynthetic protists (algae), or autotrophic bacteria for
nourishment. Animals are able to move from place to place
in search of food. In most, ingestion of food is followed by
digestion in an internal cavity.
Multicellular Heterotrophs. All animals are multicellu-
lar heterotrophs. The unicellular heterotrophic organisms
called Protozoa, which were at one time regarded as simple
animals, are now considered to be members of the kingdom
Protista, the large and diverse group we discussed in chap-
ter 35.
Diverse in Form. Almost all animals (99%) are inverte-
brates, lacking a backbone. Of the estimated 10 million liv-
ing animal species, only 42,500 have a backbone and are re-
ferred to as vertebrates. Animals are very diverse in form,
ranging in size from ones too small to see with the naked
eye to enormous whales and giant squids. The animal king-
dom includes about 35 phyla, most of which occur in the
sea. Far fewer phyla occur in fresh water and fewer still
occur on land. Members of three phyla, Arthropoda (spi-
ders and insects), Mollusca (snails), and Chordata (verte-
brates), dominate animal life on land.
No Cell Walls. Animal cells are distinct among multicel-
lular organisms because they lack rigid cell walls and are
usually quite flexible. The cells of all animals but sponges
are organized into structural and functional units called tis-
sues, collections of cells that have joined together and are
specialized to perform a specific function; muscles and
nerves are tissues types, for example.
Active Movement. The ability of animals to move more
rapidly and in more complex ways than members of other
kingdoms is perhaps their most striking characteristic and
one that is directly related to the flexibility of their cells
and the evolution of nerve and muscle tissues. A remark-
able form of movement unique to animals is flying, an abil-
ity that is well developed among both insects and verte-
brates. Among vertebrates, birds, bats, and pterosaurs
(now-extinct flying reptiles) were or are all strong fliers.
The only terrestrial vertebrate group never to have had fly-
ing representatives is amphibians.
Sexual Reproduction. Most animals reproduce sexually.
Animal eggs, which are nonmotile, are much larger than
the small, usually flagellated sperm. In animals, cells
formed in meiosis function directly as gametes. The hap-
loid cells do not divide by mitosis first, as they do in plants
and fungi, but rather fuse directly with each other to form
44.1 Animals are multicellular heterotrophs without cell walls.
Chapter 44 The Noncoelomate Animals 877
?
Radiata (Cnidaria and Ctenophora)
Porifera (sponges)
Platyhelminthes (flatworms)
Nematoda (roundworms)
Rotifera (rotifers)
Mollusca Annelida
Arthropoda
Lophophorates Echinodermata
Chordata
Ancestral protist
Multicellularity
Tissues
Bilateral symmetry
Body cavity
Coelom
Deuterostome development,
Endoskeleton
Notochord,
Segmentation,
Jointed
appendages
Jointed appendages,
Exoskeleton
Segmentation
Protostome development
Pseudocoel
No body cavity
Radial symmetry
No true tissues
Parazoa
Radiata
Acoelomates Pseudocoelomates
Eumetazoa
Bilateria
Protostomes
Segmented
Coelomates
Deuterostomes
Segmented
1
1
2
2
3
3
4
4
5
5
FIGURE 44.2
A possible phylogeny of the major groups of the kingdom Animalia. Transitions in the animal body plan are identified along the
branches; the five key advances are the evolution of tissues, bilateral symmetry, a body cavity, protostome and deuterostome development,
and segmentation.
878 Part XII Animal Diversity
Table 44.1 The Major Animal Phyla
Approximate
Number of
Phylum Typical Examples Key Characteristics Named Species
Arthropoda
(arthropods)
Mollusca
(mollusks)
Chordata
(chordates)
Platyhelminthes
(flatworms)
Nematoda
(roundworms)
Annelida
(segmented
worms)
Beetles, other
insects, crabs,
spiders
Snails, oysters,
octopuses,
nudibranchs
Mammals, fish,
reptiles, birds,
amphibians
Planaria,
tapeworms,
liver flukes
Ascaris, pinworms,
hookworms, Filaria
Earthworms,
polychaetes,
beach tube
worms,
leeches
Most successful of all animal phyla;
chitinous exoskeleton covering segmented
bodies with paired, jointed appendages;
many insect groups have wings
Soft-bodied coelomates whose bodies are
divided into three parts: head-foot, visceral
mass, and mantle; many have shells; almost
all possess a unique rasping tongue, called a
radula; 35,000 species are terrestrial
Segmented coelomates with a notochord;
possess a dorsal nerve cord, pharyngeal
slits, and a tail at some stage of life; in
vertebrates, the notochord is replaced
during development by the spinal column;
20,000 species are terrestrial
Solid, unsegmented, bilaterally symmetrical
worms; no body cavity; digestive cavity, if
present, has only one opening
Pseudocoelomate, unsegmented, bilaterally
symmetrical worms; tubular digestive tract
passing from mouth to anus; tiny; without
cilia; live in great numbers in soil and
aquatic sediments; some are important
animal parasites
Coelomate, serially segmented, bilaterally
symmetrical worms; complete digestive
tract; most have bristles called setae on
each segment that anchor them during
crawling
1,000,000
110,000
42,500
20,000
12,000
+
12,000
Chapter 44 The Noncoelomate Animals 879
Table 44.1 The Major Animal Phyla (continued)
Approximate
Number of
Phylum Typical Examples Key Characteristics Named Species
Cnidaria
(cnidarians)
Echinodermata
(echinoderms)
Porifera
(sponges)
Bryozoa
(moss animals)
Rotifera
(wheel animals)
Jellyfish, hydra,
corals, sea
anemones
Sea stars, sea
urchins, sand
dollars, sea
cucumbers
Barrel sponges,
boring sponges,
basket sponges,
vase sponges
Bowerbankia,
Plumatella, sea
mats, sea moss
Rotifers
Soft, gelatinous, radially symmetrical
bodies whose digestive cavity has a single
opening; possess tentacles armed with
stinging cells called cnidocytes that shoot
sharp harpoons called nematocysts; almost
entirely marine
Deuterostomes with radially symmetrical
adult bodies; endoskeleton of calcium
plates; five-part body plan and unique
water vascular system with tube feet; able
to regenerate lost body parts; marine
Asymmetrical bodies without distinct
tissues or organs; saclike body consists of
two layers breached by many pores;
internal cavity lined with food-filtering
cells called choanocytes; most marine (150
species live in fresh water)
Microscopic, aquatic deuterostomes that
form branching colonies, possess circular
or U-shaped row of ciliated tentacles for
feeding called a lophophore that usually
protrudes through pores in a hard
exoskeleton; also called Ectoprocta
because the anus or proct is external to the
lophophore; marine or freshwater
Small, aquatic pseudocoelomates with a
crown of cilia around the mouth
resembling a wheel; almost all live in
fresh water
10,000
6,000
5,150
4,000
2,000
880 Part XII Animal Diversity
Table 44.1 The Major Animal Phyla (continued)
Approximate
Number of
Phylum Typical Examples Key Characteristics Named Species
Five phyla of
minor worms
Brachiopoda
(lamp shells)
Velvet worms, acorn
worms, arrow worms,
giant tube worms
Comb jellies, sea
walnuts
Phoronis
Nanaloricus
mysticus
Chaetognatha (arrow worms): coelomate
deuterostomes; bilaterally symmetrical;
large eyes (some) and powerful jaws
Hemichordata (acorn worms): marine
worms with dorsal and ventral nerve cords
Onychophora (velvet worms):
protostomes with a chitinous exoskeleton;
evolutionary relics
Pogonophora (tube worms): sessile deep-
sea worms with long tentacles; live within
chitinous tubes attached to the ocean floor
Nemertea (ribbon worms): acoelomate,
bilaterally symmetrical marine worms with
long extendable proboscis
Like bryozoans, possess a lophophore, but
within two clamlike shells; more than
30,000 species known as fossils
Lingula
Ctenophora
(sea walnuts)
Phoronida
(phoronids)
Loricifera
(loriciferans)
Gelatinous, almost transparent, often
bioluminescent marine animals; eight
bands of cilia; largest animals that use
cilia for locomotion; complete digestive
tract with anal pore
Lophophorate tube worms; often live in
dense populations; unique U-shaped gut,
instead of the straight digestive tube of
other tube worms
Tiny, bilaterally symmetrical, marine
pseudocoelomates that live in spaces
between grains of sand; mouthparts include
a unique flexible tube; a recently discovered
animal phylum (1983)
980
250
100
12
6
Five Key Transitions in Body Plan
1. Evolution of Tissues
The simplest animals, the Parazoa, lack both defined tis-
sues and organs. Characterized by the sponges, these ani-
mals exist as aggregates of cells with minimal intercellular
coordination. All other animals, the Eumetazoa, have dis-
tinct tissues with highly specialized cells. The evolution of
tissues is the first key transition in the animal body plan.
2. Evolution of Bilateral Symmetry
Sponges also lack any definite symmetry, growing asym-
metrically as irregular masses. Virtually all other animals
have a definite shape and symmetry that can be defined
along an imaginary axis drawn through the animal’s body.
Animals with symmetry belong to either the Radiata, ani-
mals with radial symmetry, or the Bilateria, animals with
bilateral symmetry.
Radial Symmetry. Symmetrical bodies first evolved in
marine animals belonging to two phyla: Cnidaria (jellyfish,
sea anemones, and corals) and Ctenophora (comb jellies).
The bodies of members of these two phyla, the Radiata, ex-
hibit radial symmetry, a body design in which the parts of
the body are arranged around a central axis in such a way
that any plane passing through the central axis divides the
organism into halves that are approximate mirror images
(figure 44.3a).
Bilateral Symmetry. The bodies of all other animals, the
Bilateria, are marked by a fundamental bilateral symme-
try, a body design in which the body has a right and a left
half that are mirror images of each other (figure 44.3b). A
bilaterally symmetrical body plan has a top and a bottom,
better known respectively as the dorsal and ventral portions
of the body. It also has a front, or anterior end, and a back,
or posterior end. In some higher animals like echinoderms
(starfish), the adults are radially symmetrical, but even in
them the larvae are bilaterally symmetrical.
Bilateral symmetry constitutes the second major evolu-
tionary advance in the animal body plan. This unique form
of organization allows parts of the body to evolve in differ-
ent ways, permitting different organs to be located in dif-
ferent parts of the body. Also, bilaterally symmetrical ani-
mals move from place to place more efficiently than
radially symmetrical ones, which, in general, lead a sessile
or passively floating existence. Due to their increased mo-
bility, bilaterally symmetrical animals are efficient in seek-
ing food, locating mates, and avoiding predators.
During the early evolution of bilaterally symmetrical an-
imals, structures that were important to the organism in
monitoring its environment, and thereby capturing prey or
avoiding enemies, came to be grouped at the anterior end.
Other functions tended to be located farther back in the
body. The number and complexity of sense organs are
much greater in bilaterally symmetrical animals than they
are in radially symmetrical ones.
Much of the nervous system in bilaterally symmetrical
animals is in the form of major longitudinal nerve cords. In
a very early evolutionary advance, nerve cells became
grouped around the anterior end of the body. These nerve
cells probably first functioned mainly to transmit impulses
from the anterior sense organs to the rest of the nervous
system. This trend ultimately led to the evolution of a defi-
nite head and brain area, a process called cephalization, as
well as to the increasing dominance and specialization of
these organs in the more advanced animal phyla.
Chapter 44 The Noncoelomate Animals 881
(a)
(b)
Ventral
Dorsal
Anterior
Posterior
F
rontal
plane
Sagittal plane
T
r
ansv
erse plane
FIGURE 44.3
A comparison of radial and bilateral symmetry. (a) Radially
symmetrical animals, such as this sea anemone, can be bisected
into equal halves in any two-dimensional plane. (b) Bilaterally
symmetrical animals, such as this squirrel, can only be bisected
into equal halves in one plane (the sagittal plane).
3. Evolution of a Body Cavity
A third key transition in the evolution
of the animal body plan was the evolu-
tion of the body cavity. The evolution
of efficient organ systems within the
animal body was not possible until a
body cavity evolved for supporting or-
gans, distributing materials, and foster-
ing complex developmental interac-
tions.
The presence of a body cavity allows
the digestive tract to be larger and
longer. This longer passage allows for
storage of undigested food, longer ex-
posure to enzymes for more complete
digestion, and even storage and final
processing of food remnants. Such an
arrangement allows an animal to eat a
great deal when it is safe to do so and
then to hide during the digestive process, thus limiting the
animal’s exposure to predators. The tube within the body
cavity architecture is also more flexible, thus allowing the
animal greater freedom to move.
An internal body cavity also provides space within which
the gonads (ovaries and testes) can expand, allowing the ac-
cumulation of large numbers of eggs and sperm. Such stor-
age capacity allows the diverse modifications of breeding
strategy that characterize the more advanced phyla of ani-
mals. Furthermore, large numbers of gametes can be stored
and released when the conditions are as favorable as possi-
ble for the survival of the young animals.
Kinds of Body Cavities. Three basic kinds of body plans
evolved in the Bilateria. Acoelomates have no body cavity.
Pseudocoelomates have a body cavity called the pseudo-
coel located between the mesoderm and endoderm. A third
way of organizing the body is one in which the fluid-filled
body cavity develops not between endoderm and meso-
derm, but rather entirely within the mesoderm. Such a
body cavity is called a coelom, and animals that possess
such a cavity are called coelomates. In coelomates, the gut
is suspended, along with other organ systems of the animal,
within the coelom; the coelom, in turn, is surrounded by a
layer of epithelial cells entirely derived from the mesoderm.
The portion of the epithelium that lines the outer wall of
the coelom is called the parietal peritoneum, and the por-
tion that covers the internal organs suspended within the
cavity is called the visceral peritoneum (figure 44.4).
The development of the coelom poses a problem—cir-
culation—solved in pseudocoelomates by churning the
fluid within the body cavity. In coelomates, the gut is
again surrounded by tissue that presents a barrier to dif-
fusion, just as it was in solid worms. This problem is
solved among coelomates by the development of a circu-
latory system, a network of vessels that carries fluids to
parts of the body. The circulating fluid, or blood, carries
nutrients and oxygen to the tissues and removes wastes
and carbon dioxide. Blood is usually pushed through the
circulatory system by contraction of one or more muscu-
lar hearts. In an open circulatory system, the blood
passes from vessels into sinuses, mixes with body fluid,
and then reenters the vessels later in another location. In
a closed circulatory system, the blood is physically sep-
arated from other body fluids and can be separately con-
trolled. Also, blood moves through a closed circulatory
system faster and more efficiently than it does through an
open system.
The evolutionary relationship among coelomates,
pseudocoelomates, and acoelomates is not clear. Acoelo-
mates, for example, could have given rise to coelomates,
but scientists also cannot rule out the possibility that
acoelomates were derived from coelomates. The different
phyla of pseudocoelomates form two groups that do not
appear to be closely related.
Advantages of a Coelom. What is the functional dif-
ference between a pseudocoel and a coelom? The answer
has to do with the nature of animal embryonic develop-
ment. In animals, development of specialized tissues in-
volves a process called primary induction in which one
of the three primary tissues (endoderm, mesoderm, and
ectoderm) interacts with another. The interaction re-
quires physical contact. A major advantage of the coelo-
mate body plan is that it allows contact between meso-
derm and endoderm, so that primary induction can occur
during development. For example, contact between
mesoderm and endoderm permits localized portions of
the digestive tract to develop into complex, highly spe-
cialized regions like the stomach. In pseudocoelomates,
mesoderm and endoderm are separated by the body cav-
ity, limiting developmental interactions between these
tissues that ultimately limits tissue specialization and
development.
882 Part XII Animal Diversity
Ectoderm
Ectoderm
Mesoderm
Endoderm
Pseudocoel
Ectoderm
Mesoderm
Visceral
peritoneum
Coelomic
cavity
Endoderm
Parietal
peritoneum
Mesoderm
Endoderm
Acoelomate Pseudocoelomate Coelomate
FIGURE 44.4
Three body plans for bilaterally symmetrical animals.
Chapter 44 The Noncoelomate Animals 883
4. The Evolution of Protostome and
Deuterostome Development
Two outwardly dissimilar large phyla, Echinodermata
(starfish) and Chordata (vertebrates), together with two
smaller phyla, have a series of key embryological features
different from those shared by the other animal phyla. Be-
cause it is extremely unlikely that these features evolved
more than once, it is believed that these four phyla share a
common ancestry. They are the members of a group called
the deuterostomes. Members of the other coelomate ani-
mal phyla are called protostomes. Deuterostomes evolved
from protostomes more than 630 million years ago.
Deuterostomes, like protostomes, are coelomates. They
differ fundamentally from protostomes, however, in the
way in which the embryo grows. Early in embryonic
growth, when the embryo is a hollow ball of cells, a por-
tion invaginates inward to form an opening called the
blastopore. The blastopore of a protostome becomes the
animal’s mouth, and the anus develops at the other end. In
a deuterostome, by contrast, the blastopore becomes the
animal’s anus, and the mouth develops at the other end
(figure 44.5).
Deuterostomes differ in many other aspects of embryo
growth, including the plane in which the cells divide. Per-
haps most importantly, the cells that make up an embry-
onic protostome each contain a different portion of the
regulatory signals present in the egg, so no one cell of the
embryo (or adult) can develop into a complete organism. In
marked contrast, any of the cells of a deuterostome can de-
velop into a complete organism.
5. The Evolution of Segmentation
The fifth key transition in the animal body plan involved
the subdivision of the body into segments. Just as it is effi-
cient for workers to construct a tunnel from a series of
identical prefabricated parts, so segmented animals are “as-
sembled” from a succession of identical segments. During
the animal’s early development, these segments become
most obvious in the mesoderm but later are reflected in the
ectoderm and endoderm as well. Two advantages result
from early embryonic segmentation:
1. In annelids and other highly segmented animals, each
segment may go on to develop a more or less com-
plete set of adult organ systems. Damage to any one
segment need not be fatal to the individual because
the other segments duplicate that segment’s func-
tions.
2. Locomotion is far more effective when individual
segments can move independently because the animal
as a whole has more flexibility of movement. Because
the separations isolate each segment into an individ-
ual skeletal unit, each is able to contract or expand
autonomously in response to changes in hydrostatic
pressure. Therefore, a long body can move in ways
that are often quite complex.
Segmentation, also referred to as metamerism, underlies
the organization of all advanced animal body plans. In
some adult arthropods, the segments are fused, but seg-
mentation is usually apparent in their embryological devel-
opment. In vertebrates, the backbone and muscular areas
are segmented, although segmentation is often disguised in
the adult form. True segmentation is found in only three
phyla: the annelids, the arthropods, and the chordates, al-
though this trend is evident in many phyla.
Five key transitions in body design are responsible for
most of the differences we see among the major animal
phyla: the evolution of (1) tissues, (2) bilateral
symmetry, (3) a body cavity, (4) protostome and
deuterostome development, and (5) segmentation.
Invagination in
early embryo
Invagination in
early embryo
Blastopore
Anus
Anus
Mouth
Mouth
(a) Protostomes (b) Deuterostomes
Blastopore
Ectoderm
Mesoderm
cells
Endoderm
Ectoderm
Endoderm
Mesoderm
FIGURE 44.5
The fate of the blastopore. (a) In protostomes, the blastopore becomes the animal’s mouth. (b) In deuterostomes, the blastopore
becomes the animal’s anus.
Parazoa
The sponges are Parazoans, animals
that lack tissues and organs and a defi-
nite symmetry. However, sponges,
like all animals, have true, complex
multicellularity, unlike their protistan
ancestors. The body of a sponge con-
tains several distinctly different types
of cells whose activities are loosely co-
ordinated with one another. As we
will see, the coordination between cell
types in the eumetazoans increases
and becomes quite complex.
The Sponges
There are perhaps 5000 species of
marine sponges, phylum Porifera,
and about 150 species that live in
fresh water. In the sea, sponges are
abundant at all depths. Although
some sponges are tiny, no more than
a few millimeters across, some, like
the loggerhead sponges, may reach 2
meters or more in diameter. A few
small ones are radially symmetrical,
but most members of this phylum
completely lack symmetry. Many
sponges are colonial. Some have a
low and encrusting form, while oth-
ers may be erect and lobed, some-
times in complex patterns. Although
larval sponges are free-swimming,
adults are sessile, or anchored in
place to submerged objects.
Sponges, like all animals, are
composed of multiple cell types (see
figure 44.7), but there is relatively
little coordination among sponge
cells. A sponge seems to be little
more than a mass of cells embedded
in a gelatinous matrix, but these
cells recognize one another with a
high degree of fidelity and are specialized for different
functions of the body.
The basic structure of a sponge can best be understood
by examining the form of a young individual. A small,
anatomically simple sponge first attaches to a substrate
and then grows into a vaselike shape. The walls of the
“vase” have three functional layers. First, facing into the
internal cavity are specialized flagellated cells called
choanocytes, or collar cells. These cells line either the
entire body cavity or, in many large and more complex
sponges, specialized chambers. Sec-
ond, the bodies of sponges are
bounded by an outer epithelial layer
consisting of flattened cells some-
what like those that make up the ep-
ithelia, or outer layers, of other ani-
mal phyla. Some portions of this
layer contract when touched or ex-
posed to appropriate chemical stim-
uli, and this contraction may cause
some of the pores to close. Third,
between these two layers, sponges
consist mainly of a gelatinous, pro-
tein-rich matrix called the mesohyl,
within which various types of amoe-
boid cells occur. In addition, many
kinds of sponges have minute nee-
dles of calcium carbonate or silica
known as spicules, or fibers of a
tough protein called spongin, or
both, within this matrix. Spicules
and spongin strengthen the bodies of
the sponges in which they occur. A
spongin skeleton is the model for the
bathtub sponge, once the skeleton of
a real animal, but now largely known
from its cellulose and plastic mimics.
Sponges feed in a unique way.
The beating of flagella that line the
inside of the sponge draws water in
through numerous small pores; the
name of the phylum, Porifera, refers
to this system of pores. Plankton and
other small organisms are filtered
from the water, which flows through
passageways and eventually is forced
out through an osculum, a special-
ized, larger pore (figure 44.6).
Choanocytes. Each choanocyte
closely resembles a protist with a sin-
gle flagellum (figure 44.7), a similarity
that reflects its evolutionary deriva-
tion. The beating of the flagella of the
many choanocytes that line the body cavity draws water in
through the pores and through the sponge, thus bringing
in food and oxygen and expelling wastes. Each choanocyte
flagellum beats independently, and the pressure they create
collectively in the cavity forces water out of the osculum. In
some sponges, the inner wall of the body cavity is highly
convoluted, increasing the surface area and, therefore, the
number of flagella that can drive the water. In such a
sponge, 1 cubic centimeter of sponge can propel more than
20 liters of water a day.
884 Part XII Animal Diversity
44.2 The simplest animals are not bilaterally symmetrical.
Sponges
Cnidar
ians
Flatw
or
ms
Nematodes
Mollusks Annelids
Ar
thropods
Echinoder
ms
Chordates
FIGURE 44.6
Aplysina longissima. This beautiful, bright
blue and yellow elongated sponge is found
on deep regions of coral reefs. The oscula
are ringed with yellow.
Reproduction in Sponges. Some sponges will re-form
themselves once they have passed through a silk mesh.
Thus, as you might suspect, sponges frequently repro-
duce by simply breaking into fragments. If a sponge
breaks up, the resulting fragments usually are able to re-
constitute whole new individuals. Sexual reproduction is
also exhibited by sponges, with some mature individuals
producing eggs and sperm. Larval sponges may undergo
their initial stages of development within the parent.
They have numerous external, flagellated cells and are
free-swimming. After a short planktonic stage, they settle
down on a suitable substrate, where they begin their
transformation into adults.
Sponges probably represent the most primitive animals,
possessing multicellularity but neither tissue-level
development nor body symmetry. Their cellular
organization hints at the evolutionary ties between the
unicellular protists and the multicellular animals.
Sponges are unique in the animal kingdom in
possessing choanocytes, special flagellated cells whose
beating drives water through the body cavity.
Chapter 44 The Noncoelomate Animals 885
The body of a sponge
is lined with cells called
choanocytes and is
perforated by many tiny
pores through which
water enters.
Sponges are multicellular,
containing many different cell
types, such as amoebocytes
and choanocytes.
The beating flagella of the many choanocytes
draw water in through the pores, through the
sponge, and eventually out through the
osculum.
When a choanocyte beats its flagellum,
water is drawn down through openings in
its collar, where food particles become
trapped. The particles are then devoured
by endocytosis.
Each choanocyte is exactly
like a type of unicellular
protist called a
choanoflagellate. It
seems certain that these
protists are the ancestors
of the sponges, and
probably of all animals.
Between the outer wall and the body
cavity of the sponge body are amoeboid
cells called amoebocytes that secrete hard
mineral needles called spicules and tough
protein fibers called spongin. These structures
strengthen and protect the sponge.
Osculum
Pore
Water
Pore
Spicule
Spongin
Choanocyte
Choanocyte
Collar
Flagellum
Nucleus
Amoebocyte
Epithelial
wall
PHYLUM PORIFERA: Multicellularity
FIGURE 44.7
The body of a sponge is multicellular. The first evolutionary advance seen in animals is complex multicellularity, in which individuals
are composed of many highly specialized kinds of cells.
Eumetazoa: The
Radiata
The subkingdom Eumetazoa contains
animals that evolved the first key
transition in the animal body plan:
distinct tissues. Two distinct cell layers
form in the embryos of these animals:
an outer ectoderm and an inner endo-
derm. These embryonic tissues give
rise to the basic body plan, differenti-
ating into the many tissues of the
adult body. Typically, the outer cov-
ering of the body (called the epider-
mis) and the nervous system develop
from the ectoderm, and the layer of
digestive tissue (called the gastroder-
mis) develops from the endoderm. A
layer of gelatinous material, called the
mesoglea, lies between the epidermis
and gastrodermis and contains the
muscles in most eumetazoans.
Eumetazoans also evolved true
body symmetry and are divided into
two major groups. The Radiata in-
cludes two phyla of radially symmet-
rical organisms, Cnidaria (pro-
nounced ni-DAH-ree-ah), the
cnidarians—hydroids, jellyfish, sea
anemones, and corals—and
Ctenophora (pronounced tea-NO-fo-
rah), the comb jellies, or ctenophores.
All other eumetazoans are in the Bila-
teria and exhibit a fundamental bilat-
eral symmetry.
The Cnidarians
Cnidarians are nearly all marine, al-
though a few live in fresh water.
These fascinating and simply con-
structed animals are basically gelati-
nous in composition. They differ
markedly from the sponges in organi-
zation; their bodies are made up of
distinct tissues, although they have not evolved true organs.
These animals are carnivores. For the most part, they do not
actively move from place to place, but rather capture their
prey (which includes fishes, crustaceans, and many other
kinds of animals) with the tentacles that ring their mouths.
Cnidarians may have two basic body forms, polyps and
medusae (figure 44.8). Polyps are cylindrical and are usu-
ally found attached to a firm substrate. They may be soli-
tary or colonial. In a polyp, the mouth faces away from the
substrate on which the animal is growing, and, therefore,
often faces upward. Many polyps build up a chitinous or
calcareous (made up of calcium car-
bonate) external or internal skeleton,
or both. Only a few polyps are free-
floating. In contrast, most medusae
are free-floating and are often um-
brella-shaped. Their mouths usually
point downward, and the tentacles
hang down around them. Medusae,
particularly those of the class
Scyphozoa, are commonly known as
jellyfish because their mesoglea is
thick and jellylike.
Many cnidarians occur only as
polyps, while others exist only as
medusae; still others alternate be-
tween these two phases during their
life cycles. Both phases consist of
diploid individuals. Polyps may re-
produce asexually by budding; if they
do, they may produce either new
polyps or medusae. Medusae repro-
duce sexually. In most cnidarians, fer-
tilized eggs give rise to free-swim-
ming, multicellular, ciliated larvae
known as planulae. Planulae are com-
mon in the plankton at times and may
be dispersed widely in the currents.
A major evolutionary innovation in
cnidarians, compared with sponges, is
the internal extracellular digestion of
food (figure 44.9). Digestion takes
place within a gut cavity, rather than
only within individual cells. Digestive
enzymes are released from cells lining
the walls of the cavity and partially
break down food. Cells lining the gut
subsequently engulf food fragments
by phagocytosis.
The extracellular fragmentation
that precedes phagocytosis and intra-
cellular digestion allows cnidarians to
digest animals larger than individual
cells, an important improvement
over the strictly intracellular diges-
tion of sponges.
Nets of nerve cells coordinate contraction of cnidarian
muscles, apparently with little central control. Cnidarians
have no blood vessels, respiratory system, or excretory
organs.
On their tentacles and sometimes on their body surface,
cnidarians bear specialized cells called cnidocytes. The name
of the phylum Cnidaria refers to these cells, which are highly
distinctive and occur in no other group of organisms. Within
each cnidocyte is a nematocyst, a small but powerful “har-
poon.” Each nematocyst features a coiled, threadlike tube.
Lining the inner wall of the tube is a series of barbed spines.
886 Part XII Animal Diversity
Sponges
Cnidar
ians
Flatw
or
ms
Nematodes
Mollusks Annelids
Ar
thropods
Echinoder
ms
Chordates
Gastrovascular
cavity
Gastrovascular cavity
Mouth
Epidermis
Mesoglea
Gastrodermis
Tentacles
Polyp
Medusa
FIGURE 44.8
Two body forms of cnidarians, the
medusa and the polyp. These two phases
alternate in the life cycles of many
cnidarians, but a number—including the
corals and sea anemones, for example—
exist only as polyps. Both forms have two
fundamental layers of cells, separated by a
jellylike layer called the mesoglea.
Cnidarians use the threadlike tube to spear their prey and
then draw the harpooned prey back with the tentacle contain-
ing the cnidocyte. Nematocysts may also serve a defensive
purpose. To propel the harpoon, the cnidocyte uses water
pressure. Before firing, the cnidocyte builds up a very high in-
ternal osmotic pressure. This is done by using active transport
to build a high concentration of ions inside, while keeping its
wall impermeable to water. Within the undischarged nemato-
cyst, osmotic pressure reaches about 140 atmospheres.
When a flagellum-like trigger on the cnidocyte is stim-
ulated to discharge, its walls become permeable to water,
which rushes inside and violently pushes out the barbed
filament. Nematocyst discharge is one of the fastest cellu-
lar processes in nature. The nematocyst is pushed out-
ward so explosively that the barb can penetrate even the
hard shell of a crab. A toxic protein often produced a
stinging sensation, causing some cnidarians to be called
“stinging nettles.”
Chapter 44 The Noncoelomate Animals 887
PHYLUM CNIDARIA: Tissues and radial symmetry
Hydra
Cross-section
Stinging cell (cnidocyte)
with nematocyst
Trigger
Discharged
nematocyst
Undischarged
nematocyst
Filament
Tentacles
Hydra and other jellyfish are radially
symmetrical, with parts arranged around a
central axis like petals of a daisy.
The cells of cnidarians are organized into tissues.
A major innovation of hydra and jellyfish is extracellular
digestion of food—that is, digestion within a gut cavity.
Tentacles and body have stinging
cells (cnidocytes) that contain small
but very powerful harpoons called
nematocysts.
The harpoon is propelled by osmotic
pressure and is one of the fastest and
most powerful processes in nature.
Hydra use nematocysts to
spear prey and then draw the
wounded prey back to the hydra.
The barb explodes out of the
stinging cell at a high velocity and
can even penetrate the hard shell
of a crustacean.
Hydra and jellyfish are
carnivores that capture their
prey with tentacles that ring
their mouth.
Mouth
Sensory cell
Gastrodermis
Mesoglea
Epidermis
Cnidocyte
FIGURE 44.9
Eumetazoans all have tissues and symmetry. The cells of a cnidarian like this Hydra are organized into specialized tissues. The interior
gut cavity is specialized for extracellular digestion—that is, digestion within a gut cavity rather than within individual cells. Cnidarians are
also radially symmetrical.
Classes of Cnidarians
There are four classes of cnidarians: Hydrozoa (hydroids),
Scyphozoa (jellyfish), Cubozoa (box jellyfish), and Antho-
zoa (anemones and corals).
Class Hydrozoa: The Hydroids. Most of the approxi-
mately 2700 species of hydroids (class Hydrozoa) have both
polyp and medusa stages in their life cycle (figure 44.10).
Most of these animals are marine and colonial, such as
Obelia and the already mentioned, very unusual Por-
tuguese man-of-war. Some of the marine hydroids are bio-
luminescent.
A well-known hydroid is the abundant freshwater genus
Hydra, which is exceptional in that it has no medusa stage
and exists as a solitary polyp. Each polyp sits on a basal
disk, which it can use to glide around, aided by mucous se-
cretions. It can also move by somersaulting, bending over
and attaching itself to the substrate by its tentacles, and
then looping over to a new location. If the polyp detaches
itself from the substrate, it can float to the surface.
Class Scyphozoa: The Jellyfish. The approximately 200
species of jellyfish (class Scyphozoa) are transparent or
translucent marine organisms, some of a striking orange,
blue, or pink color (figure 44.11). These animals spend
most of their time floating near the surface of the sea. In all
of them, the medusa stage is dominant—much larger and
more complex than the polyps. The medusae are bell-
shaped, with hanging tentacles around their margins. The
polyp stage is small, inconspicuous, and simple in structure.
The outer layer, or epithelium, of a jellyfish contains a
number of specialized epitheliomuscular cells, each of
which can contract individually. Together, the cells form a
muscular ring around the margin of the bell that pulses
rhythmically and propels the animal through the water. Jel-
lyfish have separate male and female individuals. After fer-
tilization, planulae form, which then attach and develop
into polyps. The polyps can reproduce asexually as well as
budding off medusae. In some jellyfish that live in the open
ocean, the polyp stage is suppressed, and planulae develop
directly into medusae.
888 Part XII Animal Diversity
Feeding
polyp
Medusa bud
Reproductiv
polyp
e
Ovary
Medusae
Eggs
Sperm
Sexual
reproduction
Zygote
Testis
Blastula
Free-swimming
planula larva
Settles down to
start new colony
Young colony and
asexual budding
Mature
colony
FIGURE 44.10
The life cycle of Obelia, a marine
colonial hydroid. Polyps reproduce by
asexually budding, forming colonies. They
may also give rise to medusae, which
reproduce sexually via gametes. These
gametes fuse, producing zygotes that
develop into planulae, which, in turn, settle
down to produce polyps.
FIGURE 44.11
Class Scyphozoa. Jellyfish, Aurelia aurita.
Class Cubozoa: The Box Jellyfish.
Until recently the cubozoa were con-
sidered an order of Scyphozoa. As
their name implies, they are box-
shaped medusa (the polyp stage is in-
conspicuous and in many cases not
known). Most are only a few cm in
height, although some are 25 cm tall.
A tentacle or group of tentacles is
found at each corner of the box (figure
44.12). Box jellies are strong swim-
mers and voracious predators of fish.
Stings of some species can be fatal to
humans.
Class Anthozoa: The Sea Anemones
and Corals. By far the largest class of
cnidarians is Anthozoa, the “flower
animals” (from the Greek anthos,
meaning “flower”). The approxi-
mately 6200 species of this group are
solitary or colonial marine animals.
They include stonelike corals, soft-
bodied sea anemones, and other
groups known by such fanciful names
as sea pens, sea pansies, sea fans, and
sea whips (figure 44.13). All of these
names reflect a plantlike body topped
by a tuft or crown of hollow tentacles.
Like other cnidarians, anthozoans use
these tentacles in feeding. Nearly all
members of this class that live in shal-
low waters harbor symbiotic algae,
which supplement the nutrition of
their hosts through photosynthesis.
Fertilized eggs of anthozoans usually
develop into planulae that settle and
develop into polyps; no medusae are
formed.
Sea anemones are a large group of
soft-bodied anthozoans that live in
coastal waters all over the world and
are especially abundant in the tropics.
When touched, most sea anemones re-
tract their tentacles into their bodies
and fold up. Sea anemones are highly
muscular and relatively complex organ-
isms, with greatly divided internal cavi-
ties. These animals range from a few
millimeters to about 10 centimeters in
diameter and are perhaps twice that
high.
Corals are another major group of
anthozoans. Many of them secrete
tough outer skeletons, or exoskele-
tons, of calcium carbonate and are
thus stony in texture. Others, includ-
ing the gorgonians, or soft corals, do
not secrete exoskeletons. Some of the
hard corals help form coral reefs,
which are shallow-water limestone
ridges that occur in warm seas. Al-
though the waters where coral reefs
develop are often nutrient-poor, the
coral animals are able to grow actively
because of the abundant algae found
within them.
The Ctenophorans (Comb
Jellies)
The members of this small phylum
range from spherical to ribbonlike and
are known as comb jellies or sea wal-
nuts. Traditionally, the roughly 90 ma-
rine species of ctenophores (phylum
Ctenophora) were considered closely
related to the cnidarians. However,
ctenophores are structurally more com-
plex than cnidarians. They have anal
pores, so that water and other sub-
stances pass completely through the an-
imal. Comb jellies, abundant in the
open ocean, are transparent and usually
only a few centimeters long. The mem-
bers of one group have two long, re-
tractable tentacles that they use to cap-
ture their prey.
Ctenophores propel themselves
through water with eight comblike
plates of fused cilia that beat in a coor-
dinated fashion (figure 44.14). They are
the largest animals that use cilia for lo-
comotion. Many ctenophores are biolu-
minescent, giving off bright flashes of
light particularly evident in the open
ocean at night.
Cnidarians and ctenophores have
tissues and radial symmetry.
Cnidarians have a specialized kind
of cell called a cnidocyte.
Ctenophores propel themselves
through the water by means of
eight comblike plates of fused cilia.
Chapter 44 The Noncoelomate Animals 889
FIGURE 44.12
Class Cubozoa. Box jelly, Chironex fleckeri.
FIGURE 44.13
Class Anthozoa. The sessile soft-bodied
sea anemone.
FIGURE 44.14
A comb jelly (phylum Ctenophora).
Note the comblike plates along the ridges
of the base.
Eumetazoa: The
Bilaterian Acoelomates
The Bilateria are characterized by
the second key transition in the ani-
mal body plan, bilateral symmetry,
which allowed animals to achieve
high levels of specialization within
parts of their bodies. The simplest
bilaterians are the acoelomates; they
lack any internal cavity other than
the digestive tract. As discussed ear-
lier, all bilaterians have three em-
bryonic layers during development:
ectoderm, endoderm, and meso-
derm. We will focus our discussion
of the acoelomates on the largest
phylum of the group, the flatworms.
Phylum Platyhelminthes: The
Flatworms
Phylum Platyhelminthes consists of some 20,000 species.
These ribbon-shaped, soft-bodied animals are flattened
dorsoventrally, from top to bottom. Flatworms are among
the simplest of bilaterally symmetrical animals, but they do
have a definite head at the anterior end and they do possess
organs. Their bodies are solid: the only internal space con-
sists of the digestive cavity (figure 44.15).
Flatworms range in size from a millimeter or less to
many meters long, as in some tapeworms. Most species of
flatworms are parasitic, occurring within the bodies of
many other kinds of animals (figure
44.16). Other flatworms are free-liv-
ing, occurring in a wide variety of
marine and freshwater habitats, as
well as moist places on land. Free-
living flatworms are carnivores and
scavengers; they eat various small an-
imals and bits of organic debris.
They move from place to place by
means of ciliated epithelial cells,
which are particularly concentrated
on their ventral surfaces.
Those flatworms that have a diges-
tive cavity have an incomplete gut,
one with only one opening. As a re-
sult, they cannot feed, digest, and
eliminate undigested particles of food
simultaneously, and thus, flatworms
cannot feed continuously, as more ad-
vanced animals can. Muscular con-
tractions in the upper end of the gut
cause a strong sucking force allowing flatworms to ingest
their food and tear it into small bits. The gut is branched
and extends throughout the body, functioning in both di-
gestion and transport of food. Cells that line the gut engulf
most of the food particles by phagocytosis and digest
them; but, as in the cnidarians, some of these particles are
partly digested extracellularly. Tapeworms, which are par-
asitic flatworms, lack digestive systems. They absorb their
food directly through their body walls.
890 Part XII Animal Diversity
44.3 Acoelomates are solid worms that lack a body cavity.
Sponges
Cnidar
ians
Flatw
or
ms
Nematodes
Mollusks Annelids
Ar
thropods
Echinoder
ms
Chordates
Eyespot
Opening
to pharynx
Protruding
pharynx
Intestinal diverticulum
Intestine
Testis
Epidermis
Parenchymal muscle
Longitudinal muscles
Intestine
Circular muscles
Oviduct
Nerve cord Sperm duct
FIGURE 44.15
Architecture of a solid worm. This organism is Dugesia, the familiar
freshwater “planaria” of many biology laboratories.
Unlike cnidarians, flatworms have an excretory system,
which consists of a network of fine tubules (little tubes) that
runs throughout the body. Cilia line the hollow centers of
bulblike flame cells located on the side branches of the
tubules (see figure 58.9). Cilia in the flame cells move water
and excretory substances into the tubules and then to exit
pores located between the epidermal cells. Flame cells were
named because of the flickering movements of the tuft of
cilia within them. They primarily regulate the water bal-
ance of the organism. The excretory function of flame cells
appears to be a secondary one. A large proportion of the
metabolic wastes excreted by flatworms diffuses directly
into the gut and is eliminated through the mouth.
Like sponges, cnidarians, and ctenophorans, flatworms
lack circulatory systems for the transport of oxygen and
food molecules. Consequently, all flatworm cells must be
within diffusion distance of oxygen and food. Flatworms
have thin bodies and highly branched digestive cavities that
make such a relationship possible.
The nervous system of flatworms is very simple. Like
cnidarians, some primitive flatworms have only a nerve
net. However, most members of this phylum have longi-
tudinal nerve cords that constitute a simple central ner-
vous system.
Free-living members of this phylum have eyespots on
their heads. These are inverted, pigmented cups containing
light-sensitive cells connected to the nervous system. These
eyespots enable the worms to distinguish light from dark;
worms move away from strong light.
The reproductive systems of flatworms are complex.
Most flatworms are hermaphroditic, with each individual
containing both male and female sexual structures. In many
of them, fertilization is internal. When they mate, each
partner deposits sperm in the copulatory sac of the other.
The sperm travel along special tubes to reach the eggs. In
most free-living flatworms, fertilized eggs are laid in co-
coons strung in ribbons and hatch into miniature adults. In
some parasitic flatworms, there is a complex succession of
distinct larval forms. Flatworms are also capable of asexual
regeneration. In some genera, when a single individual is
divided into two or more parts, each part can regenerate an
entirely new flatworm.
Chapter 44 The Noncoelomate Animals 891
Scolex
Scolex attached
to intestinal wall
Repeated proglottid
segments
Uterus
Hooks
Sucker
Solid worms are
bilaterally symmetrical
acoelomates. Their
bodies are composed
of solid layers of tissues
surrounding a central
gut. The body of many
flatworms is soft and
flattened, like a piece of
tape or ribbon.
Each proglottid segment
contains reproductive organs.
When segments of a worm
pass out of humans in feces,
embryos may be ingested by
cattle or another human,
transmitting the parasite
to a new host. Embryos of
the tapeworms are released
through a single genital pore
on each proglottid segment.
Genital
pore
Tapeworms are parasites that
attach by their heads to the
intestinal wall of a host
organism. The body of a
mature tapeworm may reach
10 meters in length—longer
than a truck.
PHYLUM PLATYHELMINTHES: Bilateral symmetry
Most solid worms
have a highly branched
gut that brings food
near all tissues for
absorption directly
across the body wall.
Tapeworms are a special
case in that they have
solid bodies that lack a
digestive cavity.
Proglottid
FIGURE 44.16
The evolution of bilateral symmetry. Acoelomate solid worms like this beef tapeworm, Taenia saginata, are bilaterally symmetrical. In
addition, all bilaterians have three embryonic layers and exhibit cephalization.
Class Turbellaria: Turbellarians.
Only one of the three classes of flat-
worms, the turbellarians (class
Turbellaria) are free-living. One of
the most familiar is the freshwater
genus Dugesia, the common planaria
used in biology laboratory exercises.
Other members of this class are
widespread and often abundant in
lakes, ponds, and the sea. Some also
occur in moist places on land.
Class Trematoda: The Flukes.
Two classes of parasitic flatworms
live within the bodies of other ani-
mals: flukes (class Trematoda) and
tapeworms (class Cestoda). Both
groups of worms have epithelial lay-
ers resistant to the digestive enzymes
and immune defenses produced by
their hosts—an important feature in
their parasitic way of life. However,
they lack certain features of the free-
living flatworms, such as cilia in the
adult stage, eyespots, and other sen-
sory organs that lack adaptive signif-
icance for an organism that lives
within the body of another animal.
Flukes take in food through their mouth, just like their
free-living relatives. There are more than 10,000 named
species, ranging in length from less than 1 millimeter to
more than 8 centimeters. Flukes attach themselves within
the bodies of their hosts by means of suckers, anchors, or
hooks. Some have a life cycle that involves only one host,
usually a fish. Most have life cycles involving two or more
hosts. Their larvae almost always occur in snails, and there
may be other intermediate hosts. The final host of these
flukes is almost always a vertebrate.
To human beings, one of the most important flat-
worms is the human liver fluke, Clonorchis sinensis. It lives
in the bile passages of the liver of humans, cats, dogs, and
pigs. It is especially common in Asia. The worms are 1 to
2 centimeters long and have a complex life cycle. Al-
though they are hermaphroditic, cross-fertilization usu-
ally occurs between different individuals. Eggs, each con-
taining a complete, ciliated first-stage larva, or
miracidium, are passed in the feces (figure 44.17). If they
reach water, they may be ingested by a snail. Within the
snail an egg transforms into a sporocyst—a baglike struc-
ture with embryonic germ cells. Within the sporocysts are
produced rediae, which are elongated, nonciliated larvae.
These larvae continue growing within the snail, giving
rise to several individuals of the tadpole-like next larval
stage, cercariae.
Cercariae escape into the water, where they swim about
freely. If they encounter a fish of the family Cyprinidae—
the family that includes carp and goldfish—they bore into
the muscles or under the scales, lose their tails, and trans-
form into metacercariae within cysts in the muscle tissue.
If a human being or other mammal eats raw infected fish,
the cysts dissolve in the intestine, and the young flukes mi-
grate to the bile duct, where they mature. An individual
fluke may live for 15 to 30 years in the liver. In humans, a
heavy infestation of liver flukes may cause cirrhosis of the
liver and death.
Other very important flukes are the blood flukes of
genus Schistosoma. They afflict about 1 in 20 of the world’s
population, more than 200 million people throughout trop-
ical Asia, Africa, Latin America, and the Middle East.
Three species of Schistosoma cause the disease called schis-
tosomiasis, or bilharzia. Some 800,000 people die each year
from this disease.
Recently, there has been a great deal of effort to control
schistosomiasis. The worms protect themselves in part
from the body’s immune system by coating themselves with
a variety of the host’s own antigens that effectively render
the worm immunologically invisible (see chapter 57). De-
spite this difficulty, the search is on for a vaccine that
would cause the host to develop antibodies to one of the
antigens of the young worms before they protect them-
selves with host antigens. This vaccine would prevent hu-
mans from infection. The disease can be cured with drugs
after infection.
892 Part XII Animal Diversity
Adult fluke
Egg containing
miracidium
Miracidium hatches
after being eaten
by snail
Sporocyst
Redia
Cercaria
Metacercarial
cysts in fish
muscle
Bile duct
Liver
Raw, infected fish
is consumed by
humans or other
mammals
FIGURE 44.17
Life cycle of the human liver fluke, Clonorchis sinensis.
Class Cestoda: The Tapeworms. Class Cestoda is the
third class of flatworms; like flukes, they live as parasites
within the bodies of other animals. In contrast to flukes,
tapeworms simply hang on to the inner walls of their hosts
by means of specialized terminal attachment organs and ab-
sorb food through their skins. Tapeworms lack digestive
cavities as well as digestive enzymes. They are extremely
specialized in relation to their parasitic way of life. Most
species of tapeworms occur in the intestines of vertebrates,
about a dozen of them regularly in humans.
The long, flat bodies of tapeworms are divided into
three zones: the scolex, or attachment organ; the unseg-
mented neck; and a series of repetitive segments, the
proglottids (see figure 44.16). The scolex usually bears
several suckers and may also have hooks. Each proglottid is
a complete hermaphroditic unit, containing both male and
female reproductive organs. Proglottids are formed contin-
uously in an actively growing zone at the base of the neck,
with maturing ones moving farther back as new ones are
formed in front of them. Ultimately the proglottids near
the end of the body form mature eggs. As these eggs are
fertilized, the zygotes in the very last segments begin to dif-
ferentiate, and these segments fill with embryos, break off,
and leave their host with the host’s feces. Embryos, each
surrounded by a shell, emerge from the proglottid through
a pore or the ruptured body wall. They are deposited on
leaves, in water, or in other places where they may be
picked up by another animal.
The beef tapeworm Taenia saginata occurs as a juvenile
in the intermuscular tissue of cattle but as an adult in the
intestines of human beings. A mature adult beef tapeworm
may reach a length of 10 meters or more. These worms at-
tach themselves to the intestinal wall of their host by a
scolex with four suckers. The segments that are shed from
the end of the worm pass from the human in the feces and
may crawl onto vegetation. The segments ultimately rup-
ture and scatter the embryos. Embryos may remain viable
for up to five months. If they are ingested by cattle, they
burrow through the wall of the intestine and ultimately
reach muscle tissues through the blood or lymph vessels.
About 1% of the cattle in the United States are infected,
and some 20% of the beef consumed is not federally in-
spected. When infected beef is eaten rare, infection of hu-
mans by these tapeworms is likely. As a result, the beef
tapeworm is a frequent parasite of humans.
Phylum Nemertea: The Ribbon Worms
The phylogenetic relationship of phylum Nemertea (figure
44.18) to other free-living flatworms is unclear. Ne-
merteans are often called ribbon worms or proboscis
worms. These aquatic worms have the body plan of a flat-
worm, but also possess a fluid-filled sac that may be a prim-
itive coelom. This sac serves as a hydraulic power source
for their proboscis, a long muscular tube that can be thrust
out quickly from a sheath to capture prey. Shaped like a
thread or a ribbon, ribbon worms are mostly marine and
consist of about 900 species. Ribbon worms are large, often
10 to 20 centimeters and sometimes many meters in length.
They are the simplest animals that possess a complete di-
gestive system, one that has two separate openings, a
mouth and an anus. Ribbon worms also exhibit a circula-
tory system in which blood flows in vessels. Many impor-
tant evolutionary trends that become fully developed in
more advanced animals make their first appearance in the
Nemertea.
The acoelomates, typified by flatworms, are the most
primitive bilaterally symmetrical animals and the
simplest animals in which organs occur.
Chapter 44 The Noncoelomate Animals 893
FIGURE 44.18
A ribbon worm, Lineus (phylum
Nemertea). This is the simplest animal
with a complete digestive system.
The Pseudocoelomates
All bilaterians except solid worms
possess an internal body cavity, the
third key transition in the animal
body plan. Seven phyla are character-
ized by their possession of a pseudo-
coel (see figure 44.4). Their evolu-
tionary relationships remain unclear,
with the possibility that the pseudo-
coelomate condition arose indepen-
dently many times. The pseudocoel
serves as a hydrostatic skeleton—one
that gains its rigidity from being filled
with fluid under pressure. The ani-
mals’ muscles can work against this
“skeleton,” thus making the move-
ment of pseudocoelomates far more
efficient than that of the acoelomates.
Pseudocoelomates lack a defined
circulatory system; this role is per-
formed by the fluids that move within
the pseudocoel. Most pseudocoelo-
mates have a complete, one-way di-
gestive tract that acts like an assembly
line. Food is first broken down, then
absorbed, and then treated and
stored.
Phylum Nematoda:
The Roundworms
Nematodes, eelworms, and other
roundworms constitute a large phy-
lum, Nematoda, with some 12,000
recognized species. Scientists estimate
that the actual number might ap-
proach 100 times that many. Members
of this phylum are found everywhere.
Nematodes are abundant and diverse
in marine and freshwater habitats, and
many members of this phylum are parasites of animals
(figure 44.19) and plants. Many nematodes are micro-
scopic and live in soil. It has been estimated that a spade-
ful of fertile soil may contain, on the average, a million
nematodes.
Nematodes are bilaterally symmetrical, unsegmented
worms. They are covered by a flexible, thick cuticle,
which is molted as they grow. Their muscles constitute a
layer beneath the epidermis and extend along the length
of the worm, rather than encircling its body. These lon-
gitudinal muscles pull both against the cuticle and the
pseudocoel, which forms a hydrostatic skeleton. When
nematodes move, their bodies whip
about from side to side.
Near the mouth of a nematode, at
its anterior end, are usually 16 raised,
hairlike, sensory organs. The mouth is
often equipped with piercing organs
called stylets. Food passes through
the mouth as a result of the sucking
action of a muscular chamber called
the pharynx. After passing through a
short corridor into the pharynx, food
continues through the other portions
of the digestive tract, where it is bro-
ken down and then digested. Some of
the water with which the food has
been mixed is reabsorbed near the end
of the digestive tract, and material that
has not been digested is eliminated
through the anus (figure 44.20).
Nematodes completely lack flagella
or cilia, even on sperm cells. Repro-
duction in nematodes is sexual, with
sexes usually separate. Their develop-
ment is simple, and the adults consist of
very few cells. For this reason, nema-
todes have become extremely important
subjects for genetic and developmental
studies (see chapter 17). The 1-mil-
limeter-long Caenorhabditis elegans ma-
tures in only three days, its body is
transparent, and it has only 959 cells. It
is the only animal whose complete de-
velopmental cellular anatomy is known.
About 50 species of nematodes, in-
cluding several that are rather com-
mon in the United States, regularly
parasitize human beings. The most se-
rious common nematode-caused dis-
ease in temperate regions is trichi-
nosis, caused by worms of the genus
Trichinella. These worms live in the
small intestine of pigs, where fertilized female worms
burrow into the intestinal wall. Once it has penetrated
these tissues, each female produces about 1500 live
young. The young enter the lymph channels and travel to
muscle tissue throughout the body, where they mature
and form highly resistant, calcified cysts. Infection in
human beings or other animals arises from eating under-
cooked or raw pork in which the cysts of Trichinella are
present. If the worms are abundant, a fatal infection can
result, but such infections are rare; only about 20 deaths
in the United States have been attributed to trichinosis
during the past decade.
894 Part XII Animal Diversity
44.4 Pseudocoelomates have a simple body cavity.
Sponges
Cnidar
ians
Flatw
or
ms
Nematodes
Mollusks Annelids
Ar
thropods
Echinoder
ms
Chordates
FIGURE 44.19
Trichinella nematode encysted in pork.
The serious disease trichinosis can result
from eating undercooked pork or bear meat
containing such cysts.
Phylum Rotifera: Rotifers
Phylum Rotifera includes common, small, bilaterally sym-
metrical, basically aquatic animals that have a crown of
cilia at their heads. Rotifers are pseudocoelomates but are
very unlike nematodes. They have several features that
suggest their ancestors may have resembled flatworms.
There are about 2000 species of this phylum. While a few
rotifers live in soil or in the capillary water in cushions of
mosses, most occur in fresh water, and they are common
everywhere. Very few rotifers are marine. Most rotifers
are between 50 and 500 micrometers in length, smaller
than many protists.
Rotifers have a well-developed food-processing appara-
tus. A conspicuous organ on the tip of the head called the
corona gathers food. It is composed of a circle of cilia
which sweeps their food into their mouths, as well as
being used for locomotion. Rotifers are often called
“wheel animals” because the cilia, when they are beating
together, resemble the movement of spokes radiating
from a wheel.
A Relatively New Phylum: Cycliophora
In December 1995, two Danish biologists reported the dis-
covery of a strange new kind of creature, smaller than a pe-
riod on a printed page. The tiny organism had a striking
circular mouth surrounded by a ring of fine, hairlike cilia
and has so unusual a life cycle that they assigned it to an
entirely new phylum, Cycliophora (Greek for “carrying a
small wheel”). There are only about 35 known animal
phyla, so finding a new one is extremely rare! When the
lobster to which it is attached starts to molt, the tiny sym-
biont begins a bizarre form of sexual reproduction. Dwarf
males emerge, with nothing but brains and reproductive
organs. Each dwarf male seeks out another female sym-
biont on the molting lobster and fertilizes its eggs, generat-
ing free-swimming individuals that can seek out another
lobster and renew the life cycle.
The pseudocoelomates, including nematodes and
rotifers, all have fluid-filled pseudocoels.
Chapter 44 The Noncoelomate Animals 895
The pseudocoel of a nematode
separates the endoderm-lined gut from
the rest of the body. The digestive tract
is one-way: food enters the mouth at
one end of the worm and leaves
through the anus at the other end.
The nematode's body is covered
with a flexible, thick cuticle that is shed
as the worm grows. Muscles extend
along the length of the body rather
than encircling it, which allows the
worm to flex its body to move through
the soil.
Roundworms are bilaterally
symmetrical, cylindrical, unsegmented
worms. Most nematodes are very
small, less than a millimeter long—
hundreds of thousands may live in a
handful of fertile soil.
An adult nematode consists of very
few cells. Caenorhabditis elegans has
exactly 959 cells and is the only animal
whose complete cellular anatomy is
known.
Pseudocoel
PHYLUM NEMATODA: Body cavity
Nematodes have excretory ducts that
permit them to conserve water and live
on land. Other roundworms possess
excretory cells called flame cells.Intestine
Intestine
Oviduct
Cuticle
Nerve cord
Ovary
Ovary
Muscle
Uterus
Uterus
Excretory duct
Anus
Genital pore
Pharynx
Mouth
Excretory pore
FIGURE 44.20
The evolution of a simple body cavity. The major innovation in body design in roundworms (phylum Nematoda) is a body cavity
between the gut and the body wall. This cavity is the pseudocoel. It allows chemicals to circulate throughout the body and prevents organs
from being deformed by muscle movements.
Reevaluating How the Animal Body
Plan Evolved
The great diversity see in the body plan of animals is diffi-
cult to fit into any one taxonomic scheme. Biologists have
traditionally inferred the general relationships among the
35 animal phyla by examining what seemed to be funda-
mental characters—segmentation, possession of a coelom,
and so on. The general idea has been that such characters
are most likely to be conserved during a group’s evolution.
Animal phyla that share a fundamental character are more
likely to be closely related to each other than to other phyla
that do not exhibit the character. The phylogeny presented
in figure 44.2 is a good example of the sort of taxonomy
this approach has generated.
However, not every animal can be easily accommodated
by this approach. Take, for example, the myzostomids (fig-
ure 44.21), an enigmatic and anatomically bizarre group of
marine animals that are parasites or symbionts of echino-
derms. Myzostomid fossils are found associated with echin-
oderms since the Ordovician, so the myzostomid-echino-
derm relationship is a very ancient one. Their long history
of obligate association has led to the loss or simplification
of many myzostomid body elements, leaving them, for ex-
ample, with no body cavity (they are acoelomates) and only
incomplete segmentation.
This character loss has led to considerable disagreement
among taxonomists. However, while taxonomists have dis-
agreed about the details, all have generally allied myzosto-
mids is some fashion with the annelids, sometimes within
the polychaetes, sometimes as a separate phylum closely al-
lied to the annelids.
Recently, this view has been challenged. New taxonomi-
cal comparisons using molecular data have come to very
different conclusions. Researchers examined two compo-
nents of the protein synthesis machinery, the small riboso-
mal subunit rRNA gene, and an elongation factor gene
(called 1 alpha). The phylogeny they obtain does not place
the myzostomids in with the annelids. Indeed, they find
that the myzostomids have no close links to the annelids at
all. Instead, surprisingly, they are more closely allied with
the flatworms!
This result hints strongly that the key morphological
characters that biologists have traditionally used to con-
struct animal phylogenies—segmentation, coeloms,
jointed appendages, and the like—are not the conservative
characters we had supposed. Among the myzostomids
these features appear to have been gained and lost again
during the course of their evolution. If this unconservative
evolutionary pattern should prove general, our view of the
evolution of the animal body plan, and how the various an-
imal phyla relate to one another, will soon be in need of
major revision.
Molecular Phylogenies
The last decade has seen a wealth of new molecular se-
quence data on the various animal groups. The animal phy-
logenies that these data suggest are often significantly at
odds with the traditional phylogeny used in this text and
presented in figure 44.2. One such phylogeny, developed
from ribosomal RNA studies, is presented in figure 44.22.
It is only a rough outline; in the future more data should
allow us to resolve relationships within groupings. Still, it is
clear that major groups are related in very different ways in
the molecular phylogeny than in the more traditional one.
At present, molecular phylogenetic analysis of the ani-
mal kingdom is in its infancy. Molecular phylogenies devel-
oped from different molecules often tend to suggest differ-
ent evolutionary relationships. However, the childhood of
this approach is likely to be short. Over the next few years,
a mountain of additional molecular data can be anticipated.
As more data are brought to bear, we can hope that the
confusion will lessen, and that a consensus phylogeny will
emerge. When and if this happens, it is likely to be very
different from the traditional view.
The use of molecular data to construct phylogenies is
likely to significantly alter our understanding of
relationships among the animal phyla.
896 Part XII Animal Diversity
44.5 The coming revolution in animal taxonomy will likely alter traditional
phylogenies.
FIGURE 44.21
A taxonomic puzzle. Myzostoma martenseni has no body cavity
and incomplete segmentation. Animals such as this present a
classification challenge, causing taxonomists to reconsider
traditional animal phylogenies based on fundamental characters.
Chapter 44 The Noncoelomate Animals 897
Vertebrates
Cephalochordates
Urochordates
Hemichordates
Echinoderms
Brachiopods
Bryozoans
Phoronids
Sipunculans
Mollusks
Echiurians
Pogonophorans
Annelids
Onychophorans
Tardigrades
Arthropods
Gnathostomulids
Rotifers
Gastrotrichs
Nematodes
Priapulids
Kinorhynchs
Platyhelminthes
Nemerteans
Entoprocts
Ctenophorans
Cnidarians
Poriferans
Poriferans
Cnidarians
Ctenophorans
Gastrotrichs
Nematodes
Priapulids
Kinorhynchs
Onychophorans
Tardigrades
Arthropods
Bryozoans
Entoprocts
Platyhelminthes
Pogonophorans
Brachiopods
Phoronids
Nemerteans
Annelids
Echiurans
Mollusks
Sipunculans
Gnathostomulids
Rotifers
Vertebrates
Cephalochordates
Urochordates
Hemichordates
Echinoderms
Bilateria
Bilateria
Deuterostomes Deuterostomes
Protostomes
Protostomes
Lophophorates
Ecdysozoans
Radiata
Acoelomates
Pseudocoelomates
Coelomates
Lophotrochozoans
(a) Traditional phylogeny (b) Molecular phylogeny
FIGURE 44.22
Traditional versus molecular animal phylogenies. (a) Traditional phylogenies are based on fundamental morphological characters.
(After L. H. Hyman, The Invertebrates, 1940.) (b) More recent phylogenies are often based on molecular analyses, this one on comparisons
of rRNA sequence differences among the animal phyla. (After Adoutte, et al., Proc. Nat. Acad. Sci 97: p. 4454, 2000.)
898 Part XII Animal Diversity
Chapter 44
Summary Questions Media Resources
44.1 Animals are multicellular heterotrophs without cell walls.
? Animals are heterotrophic, multicellular, and usually
have the ability to move. Almost all animals
reproduce sexually. Animal cells lack rigid cell walls
and digest their food internally.
? The kingdom Animalia is divided into two
subkingdoms: Parazoa, which includes only the
asymmetrical phylum Porifera, and Eumetazoa,
characterized by body symmetry.
1. What are the characteristics
that distinguish animals from
other living organisms?
2. What are the two
subkingdoms of animals? How
do they differ in terms of
symmetry and body
organization?
? The sponges (phylum Porifera) are characterized by
specialized, flagellated cells called choanocytes. They
do not possess tissues or organs, and most species lack
symmetry in their body organization.
? Cnidarians (phylum Cnidaria) are predominantly
marine animals with unique stinging cells called
cnidocytes, each of which contains a specialized
harpoonlike apparatus, or nematocyst.
3. From what kind of ancestor
did sponges probably evolve?
4. What are the specialized
cells used by a sponge to capture
food?
5. What are the two ways
sponges reproduce? What do
larval sponges look like?
6. What is a planula?
44.2 The simplest animals are not bilaterally symmetrical.
? Acoelomates lack an internal cavity, except for the
digestive system, and are the simplest animals that
have organs.
? The most prominent phylum of acoelomates,
Platyhelminthes, includes the free-living flatworms
and the parasitic flukes and tapeworms.
? Ribbon worms (phylum Nemertea) are similar to
free-living flatworms, but have a complete digestive
system and a circulatory system in which the blood
flows in vessels.
7. What body plan do members
of the phylum Platyhelminthes
possess? Are these animals
parasitic or free-living? How do
they move from place to place?
8. How are tapeworms
different from flukes? How do
tapeworms reproduce?
44.3 Acoelomates are solid worms that lack a body cavity.
? Pseudocoelomates, exemplified by the nematodes
(phylum Nematoda), have a body cavity that develops
between the mesoderm and the endoderm.
? Rotifers (phylum Rotifera), or wheel animals, are very
small freshwater pseudocoelomates.
9. Why are nematodes
structurally unique in the animal
world?
10. How do rotifers capture
food?
44.4 Pseudocoelomates have a simple body cavity.
? Molecular data are suggesting animal phylogenies
that are in considerable disagreement with traditional
phylogenies.
11. With what group are
myzostomids most closely allied?
44.5 The coming revolution in animal taxonomy will likely alter
traditional phylogenies.
www.mhhe.com/raven6e www.biocourse.com
? Activity: Invertebrates
? Characteristics of
Invertebrates
? Body Organization
? Symmetry in Nature
? Posterior to Anterior
? Sagittal Plane
? Frontal to Coronal
Plane
? Transvere/Cross-
sectional Planes
? Sponges
? Radical Phyla
? Bilateral Acoelomates
? Student Research:
Parasitic Flatworms
? Pseudocoelomates
? Student Research:
Molecular Phylogeny
of Gastropods
899
45
Mollusks and
Annelids
Concept Outline
45.1 Mollusks were among the first coelomates.
Coelomates. Next to the arthropods, mollusks comprise
the second most diverse phylum and include snails, clams,
and octopuses. There are more terrestrial mollusk species
than terrestrial vertebrates!
Body Plan of the Mollusks. The mollusk body plan is
characterized by three distinct sections, a unique rasping
tongue, and distinctive free-swimming larvae also found in
annelid worms.
The Classes of Mollusks. The three major classes of
mollusks are the gastropods (snails and slugs), the bivalves
(oysters and clams), and the cephalopods (octopuses and
squids). While they seem very different at first glance, on
closer inspection, they all have the same basic mollusk body
plan.
45.2 Annelids were the first segmented animals.
Segmented Animals. Annelids are segmented coelomate
worms, most of which live in the sea. The annelid body is
composed of numerous similar segments.
Classes of Annelids. The three major classes of annelids
are the polychaetes (marine worms), the oligochaetes
(earthworms and related freshwater worms), and the
hirudines (leeches).
45.3 Lophophorates appear to be a transitional group.
Lophophorates. The three phyla of lophophorates share
a unique ciliated feeding structure, but differ in many other
ways.
A
lthough acoelomates and pseudocoelomates have proven
very successful, a third way of organizing the animal
body has also evolved, one that occurs in the bulk of the ani-
mal kingdom. We will begin our discussion of the coelomate
animals with mollusks, which include such animals as clams,
snails, slugs, and octopuses. Annelids (figure 45.1), such as
earthworms, leeches, and seaworms, are also coelomates, but
in addition, were the earliest group of animals to evolve seg-
mented bodies. The lophophorates, a group of marine ani-
mals united by a distinctive feeding structure called the
lophophore, have features intermediate between those of pro-
tostomes and deuterostomes and will also be discussed in this
chapter. The remaining groups of coelomate animals will be
discussed in chapters 46, 47, and 48.
FIGURE 45.1
An annelid, the Christmas-tree worm, Spirobranchus
giganteus. Mollusks and annelids inhabit both terrestrial and
aquatic habitats. They are large and successful groups, with some
of their most spectacular members represented in marine
environments.
As a group, mollusks are an impor-
tant source of food for humans. Oys-
ters, clams, scallops, mussels, octopuses,
and squids are among the culinary deli-
cacies that belong to this large phylum.
Mollusks are also of economic signifi-
cance to us in many other ways. For ex-
ample, pearls are produced in oysters,
and the material called mother-of-
pearl, often used in jewelry and other
decorative objects, is produced in the
shells of a number of different mol-
lusks, but most notably in the snail
called abalone. Mollusks are not wholly
beneficial to humans, however. Bivalve
mollusks called shipworms burrow
through wood submerged in the sea,
damaging boats, docks, and pilings.
The zebra mussel has recently invaded
North American ecosystems from Eu-
rope via the ballast water of cargo ships
from Europe, wreaking havoc in many
aquatic ecosystems. Slugs and terres-
trial snails often cause extensive dam-
age to garden flowers, vegetables, and
crops. Other mollusks serve as hosts to
the intermediate stages of many serious
parasites, including several nematodes
and flatworms, which we discussed in
chapter 44.
Mollusks range in size from almost
microscopic to huge, although most
measure a few centimeters in their
largest dimension. Some, however, are
minute, while others reach formidable
sizes. The giant squid, which is occa-
sionally cast ashore but has rarely been
observed in its natural environment,
may grow up to 21 meters long!
Weighing up to 250 kilograms, the
giant squid is the largest invertebrate
and, along with the giant clam (figure 45.4), the heaviest.
Millions of giant squid probably inhabit the deep regions of
the ocean, even though they are seldom caught. Another
large mollusk is the bivalve Tridacna maxima, the giant
clam, which may be as long as 1.5 meters and may weigh as
much as 270 kilograms.
Mollusks are the second-largest phylum of animals in
terms of named species; mollusks exhibit a variety of
body forms and live in many different environments.
900 Part XII Animal Diversity
Coelomates
The evolution of the coelom was a sig-
nificant advance in the structure of the
animal body. Coelomates have a new
body design that repositions the fluid
and allows the development of com-
plex tissues and organs. This new body
plan also made it possible for animals
to evolve a wide variety of different
body architectures and to grow to
much larger sizes than acoelomate ani-
mals. Among the earliest groups of
coelomates were the mollusks and the
annelids.
Mollusks
Mollusks (phylum Mollusca) are an ex-
tremely diverse animal phylum, second
only to the arthropods, with over
110,000 described species. Mollusks
include snails, slugs, clams, scallops,
oysters, cuttlefish, octopuses, and
many other familiar animals (figure
45.2). The durable shells of some mol-
lusks are often beautiful and elegant;
they have long been favorite objects
for professional scientists and amateurs
alike to collect, preserve, and study.
Chitons and nudibranchs are less fa-
miliar marine mollusks. Mollusks are
characterized by a coelom, and while
there is extraordinary diversity in this
phylum, many of the basic compo-
nents of the mollusk body plan can be
seen in figure 45.3.
Mollusks evolved in the oceans, and
most groups have remained there. Ma-
rine mollusks are widespread and
often abundant. Some groups of mol-
lusks have invaded freshwater and terrestrial habitats, in-
cluding the snails and slugs that live in your garden. Ter-
restrial mollusks are often abundant in places that are at
least seasonally moist. Some of these places, such as the
crevices of desert rocks, may appear very dry, but even
these habitats have at least a temporary supply of water at
certain times. There are so many terrestrial mollusks that
only the arthropods have more species adapted to a ter-
restrial way of life. The 35,000 species of terrestrial mol-
lusks far outnumber the roughly 20,000 species of terres-
trial vertebrates.
45.1 Mollusks were among the first coelomates.
Sponges
Cnidarians
Flatworms
Nematodes
Mollusks Annelids
Arthropods
Echinoderms
Chordates
FIGURE 45.2
A mollusk. The blue-ringed octopus is one
of the few mollusks dangerous to humans.
Strikingly beautiful, it is equipped with a
sharp beak and poison glands—divers give
it a wide berth!
Chapter 45 Mollusks and Annelids 901
PHYLUM MOLLUSCA: Coelom
Radula
Mantle
Digestive
gland
Stomach
Gonad
Intestine
Heart
Coelom
Nephridium
Shell
Gill
Mantle cavity
Anus
Retractor musclesFootNerve collar
Mouth
The mantle is a heavy fold of tissue
wrapped around the mollusk body like a
cape. The cavity between the mantle and
the body contains gills, which capture
oxygen from the water passing through
the mantle cavity. In some mollusks, like
snails, the mantle secretes a hard
outer shell.
Many mollusks are carnivores. They locate
prey using chemosensory structures. Within
the mouth of a snail are horny jaws and a
unique rasping tongue called a radula.
Snails creep along the ground on a
muscular foot. Squid can shoot through
the water by squeezing water out of the
mantle cavity, in a kind of jet propulsion.
Mollusks were among the first
animals to develop an efficient
excretory system. Tubular structures
called nephridia gather wastes
from the coelom and discharge
them into the mantle cavity.
Snails have a three-chambered heart
and an open circulation system. The
coelom is confined to a small cavity
around the heart.
FIGURE 45.3
Evolution of the coelom. A generalized mollusk body plan is shown above. The body cavity of a mollusk is a coelom, which is completely
enclosed within the mesoderm. This allows physical contact between the mesoderm and the endoderm, permitting interactions that lead to
development of highly specialized organs such as a stomach.
FIGURE 45.4
Giant clam. Second only to the
arthropods in number of described species,
members of the phylum Mollusca occupy
almost every habitat on earth. This giant
clam, Tridacna maxima, has a green color
that is caused by the presence of symbiotic
dinoflagellates (zooxanthellae). Through
photosynthesis, the dinoflagellates
probably contribute most of the food
supply of the clam, although it remains a
filter feeder like most bivalves. Some
individual giant clams may be nearly 1.5
meters long and weigh up to 270
kilograms.
Body Plan of the Mollusks
In their basic body plan (figure 45.5), mollusks have dis-
tinct bilateral symmetry. Their digestive, excretory, and
reproductive organs are concentrated in a visceral mass,
and a muscular foot is their primary mechanism of loco-
motion. They may also have a differentiated head at the
anterior end of the body. Folds (often two) arise from the
dorsal body wall and enclose a cavity between themselves
and the visceral mass; these folds constitute the mantle. In
some mollusks the mantle cavity acts as a lung; in others it
contains gills. Gills are specialized portions of the mantle
that usually consist of a system of filamentous projections
rich in blood vessels. These projections greatly increase
the surface area available for gas exchange and, therefore,
the animal’s overall respiratory potential. Mollusk gills are
very efficient, and many gilled mollusks extract 50% or
more of the dissolved oxygen from the water that passes
through the mantle cavity. Finally, in most members of
this phylum, the outer surface of the mantle also secretes a
protective shell.
A mollusk shell consists of a horny outer layer, rich in
protein, which protects the two underlying calcium-rich
layers from erosion. The middle layer consists of densely
packed crystals of calcium carbonate. The inner layer is
pearly and increases in thickness throughout the animal’s
life. When it reaches a sufficient thickness, this layer is
used as mother-of-pearl. Pearls themselves are formed
when a foreign object, like a grain of sand, becomes lodged
between the mantle and the inner shell layer of bivalve
mollusks (two-shelled), including clams and oysters. The
mantle coats the foreign object with layer upon layer of
shell material to reduce irritation caused by the object.
The shell of mollusks serves primarily for protection.
Many species can withdraw for protection into their shell
if they have one.
In aquatic mollusks, a continuous stream of water passes
into and out of the mantle cavity, drawn by the cilia on the
gills. This water brings in oxygen and, in the case of the bi-
valves, also brings in food; it also carries out waste materi-
als. When the gametes are being produced, they are fre-
quently carried out in the same stream.
The foot of a mollusk is muscular and may be adapted
for locomotion, attachment, food capture (in squids and oc-
topuses), or various combinations of these functions. Some
mollusks secrete mucus, forming a path that they glide
along on their foot. In cephalopods—squids and octo-
puses—the foot is divided into arms, also called tentacles.
In some pelagic forms, mollusks that are perpetually free-
swimming, the foot is modified into wing-like projections
or thin fins.
One of the most characteristic features of all the mol-
lusks except the bivalves is the radula, a rasping, tongue-
like organ used for feeding. The radula consists primarily
of dozens to thousands of microscopic, chitinous teeth
arranged in rows (figure 45.6). Gastropods (snails and
their relatives) use their radula to scrape algae and other
food materials off their substrates and then to convey this
food to the digestive tract. Other gastropods are active
predators, some using a modified radula to drill through
the shells of prey and extract the food. The small holes
often seen in oyster shells are produced by gastropods
that have bored holes to kill the oyster and extract its
body for food.
The circulatory system of all mollusks except
cephalopods consists of a heart and an open system in
which blood circulates freely. The mollusk heart usually
has three chambers, two that collect aerated blood from the
gills, while the third pumps it to the other body tissues. In
mollusks, the coelom takes the form of a small cavity
around the heart.
902 Part XII Animal Diversity
Radula
Gut
Gill
Gut
Gill
Radula
Foot
Mantle
Mantle
cavity
Shell
GutGillTentacles
Foot
Mantle
Mantle
cavity
Shell
Radula
Gut
GillRadula Foot
Mantle
Mantle cavity
Shell
Gut
Gill
Foot
Mantle
Mantle cavity
Shell
Cephalopods
Chitons
Bivalves
Hypothetical Ancestor
Gastropods
FIGURE 45.5
Body plans among the mollusks.
Nitrogenous wastes are removed
from the mollusk by one or two tubu-
lar structures called nephridia. A typi-
cal nephridium has an open funnel, the
nephrostome, which is lined with
cilia. A coiled tubule runs from the
nephrostome into a bladder, which in
turn connects to an excretory pore.
Wastes are gathered by the nephridia
from the coelom and discharged into
the mantle cavity. The wastes are then
expelled from the mantle cavity by the
continuous pumping of the gills. Sug-
ars, salts, water, and other materials are
reabsorbed by the walls of the
nephridia and returned to the animal’s
body as needed to achieve an appropri-
ate osmotic balance.
In animals with a closed circulatory
system, such as annelids, cephalopod
mollusks, and vertebrates, the coiled
tubule of a nephridium is surrounded
by a network of capillaries. Wastes are
extracted from the circulatory system
through these capillaries and are trans-
ferred into the nephridium, then sub-
sequently discharged. Salts, water, and
other associated materials may also be
reabsorbed from the tubule of the
nephridium back into the capillaries.
For this reason, the excretory systems
of these coelomates are much more ef-
ficient than the flame cells of the
acoelomates, which pick up substances
only from the body fluids. Mollusks
were one of the earliest evolutionary
lines to develop an efficient excretory
system. Other than chordates, coelo-
mates with closed circulation have sim-
ilar excretory systems.
Reproduction in Mollusks
Most mollusks have distinct male and female individuals, al-
though a few bivalves and many gastropods are hermaphro-
ditic. Even in hermaphroditic mollusks, cross-fertilization is
most common. Remarkably, some sea slugs and oysters are
able to change from one sex to the other several times dur-
ing a single season.
Most aquatic mollusks engage in external fertilization.
The males and females release their gametes into the water,
where they mix and fertilization occurs. Gastropods more
often have internal fertilization, however, with the male in-
serting sperm directly into the female’s body. Internal fer-
tilization is one of the key adaptations that allowed gas-
tropods to colonize the land.
Many marine mollusks have free-swimming larvae called
trochophores (figure 45.7a), which closely resemble the lar-
val stage of many marine annelids. Trochophores swim by
means of a row of cilia that encircles the middle of their
body. In most marine snails and in bivalves, a second free-
swimming stage, the veliger, follows the trochophore stage.
This veliger stage, has the beginnings of a foot, shell, and
mantle (figure 45.7b). Trochophores and veligers drift widely
in the ocean currents, dispersing mollusks to new areas.
Mollusks were among the earliest animals to evolve an
efficient excretory system. The mantle of mollusks not
only secretes their protective shell, but also forms a
cavity that is essential to respiration.
Chapter 45 Mollusks and Annelids 903
(a) (b)
FIGURE 45.6
Structure of the radula in a snail. (a) The radula consists of chitin and is covered with
rows of teeth. (b) Enlargement of the rasping teeth on a radula.
(a) (b)
FIGURE 45.7
Stages in the molluscan life cycle. (a) The trochophore larva of a mollusk. Similar larvae,
as you will see, are characteristic of some annelid worms as well as a few other phyla.
(b) Veliger stage of a mollusk.
The Classes of Mollusks
There are seven classes of mollusks. We will examine four
classes of mollusks as representatives of the phylum: (1)
Polyplacophora—chitons; (2) Gastropoda—snails, slugs,
limpets, and their relatives; (3) Bivalvia—clams, oysters,
scallops, and their relatives; and (4) Cephalopoda—squids,
octopuses, cuttlefishes, and nautilus. By studying living
mollusks and the fossil record, some scientists have de-
duced that the ancestral mollusk was probably a dorsoven-
trally flattened, unsegmented, wormlike animal that glided
on its ventral surface. This animal may also have had a
chitinous cuticle and overlapping calcareous scales. Other
scientists believe that mollusks arose from segmented an-
cestors and became unsegmented secondarily.
Class Polyplacophora: The Chitons
Chitons are marine mollusks that have oval bodies with
eight overlapping calcareous plates. Underneath the plates,
the body is not segmented. Chitons creep along using a
broad, flat foot surrounded by a groove or mantle cavity in
which the gills are arranged. Most chitons are grazing her-
bivores that live in shallow marine habitats, but some live at
depths of more than 7000 meters.
Class Gastropoda: The Snails and Slugs
The class Gastropoda contains about 40,000 described
species of snails, slugs, and similar animals. This class is
primarily a marine group, but it also contains many fresh-
water and terrestrial mollusks (figure 45.8). Most gas-
tropods have a shell, but some, like slugs and nudibranchs,
have lost their shells through the course of evolution. Gas-
tropods generally creep along on a foot, which may be
modified for swimming.
The heads of most gastropods have a pair of tentacles
with eyes at the ends. These tentacles have been lost in
some of the more advanced forms of the class. Within the
mouth cavity of many members of this class are horny jaws
and a radula.
During embryological development, gastropods undergo
torsion. Torsion is the process by which the mantle cavity
and anus are moved from a posterior location to the front
of the body, where the mouth is located. Torsion is
brought about by a disproportionate growth of the lateral
muscles; that is, one side of the larva grows much more
rapidly than the other. A 120-degree rotation of the vis-
ceral mass brings the mantle cavity above the head and
twists many internal structures. In some groups of gas-
tropods, varying degrees of detorsion have taken place. The
coiling, or spiral winding, of the shell is a separate process.
This process has led to the loss of the right gill and right
nephridium in most gastropods. Thus, the visceral mass of
gastropods has become bilaterally asymmetrical during the
course of evolution.
Gastropods display extremely varied feeding habits.
Some are predatory, others scrape algae off rocks (or
aquarium glass), and others are scavengers. Many are herbi-
vores, and some terrestrial ones are serious garden and
agricultural pests. The radula of oyster drills is used to bore
holes in the shells of other mollusks, through which the
contents of the prey can be removed. In cone shells, the
radula has been modified into a kind of poisonous harpoon,
which is shot with great speed into the prey.
Sea slugs, or nudibranchs, are active predators; a few
species of nudibranchs have the extraordinary ability to ex-
tract the nematocysts from the cnidarian polyps they eat
and transfer them through their digestive tract to the sur-
face of their gills intact and use them for their own protec-
tion. Nudibranchs are interesting in that they get their
name from their gills, which instead of being enclosed
within the mantle cavity are exposed along the dorsal sur-
face (nudi, “naked”; branch; “gill”).
In terrestrial gastropods, the empty mantle cavity, which
was occupied by gills in their aquatic ancestors, is extremely
rich in blood vessels and serves as a lung, in effect. This
structure evolved in animals living in environments with
plentiful oxygen; it absorbs oxygen from the air much more
effectively than a gill could, but is not as effective under
water.
Class Bivalvia: The Bivalves
Members of the class Bivalvia include the clams, scallops,
mussels, and oysters. Bivalves have two lateral (left and
right) shells (valves) hinged together dorsally (figure 45.9).
A ligament hinges the shells together and causes them to
gape open. Pulling against this ligament are one or two
large adductor muscles that can draw the shells together.
904 Part XII Animal Diversity
FIGURE 45.8
A gastropod mollusk. The terrestrial snail, Allogona townsendiana.
The mantle secretes the shells and ligament and envelops
the internal organs within the pair of shells. The mantle is
frequently drawn out to form two siphons, one for an in-
coming and one for an outgoing stream of water. The
siphons often function as snorkels to allow bivalves to filter
water through their body while remaining almost com-
pletely buried in sediments. A complex folded gill lies on
each side of the visceral mass. These gills consist of pairs of
filaments that contain many blood vessels. Rhythmic beat-
ing of cilia on the gills creates a pattern of water circula-
tion. Most bivalves are sessile filter-feeders. They extract
small organisms from the water that passes through their
mantle cavity.
Bivalves do not have distinct heads or radulas, differing
from gastropods in this respect (see figure 45.5). However,
most have a wedge-shaped foot that may be adapted, in dif-
ferent species, for creeping, burrowing, cleansing the ani-
mal, or anchoring it in its burrow. Some species of clams
can dig into sand or mud very rapidly by means of muscular
contractions of their foot.
Bivalves disperse from place to place largely as larvae.
While most adults are adapted to a burrowing way of life,
some genera of scallops can move swiftly through the water
by using their large adductor muscles to clap their shells to-
gether. These muscles are what we usually eat as “scallops.”
The edge of a scallop’s body is lined with tentacle-like pro-
jections tipped with complex eyes.
There are about 10,000 species of bivalves. Most species
are marine, although many also live in fresh water. Over
500 species of pearly freshwater mussels, or naiads, occur in
the rivers and lakes of North America.
Class Cephalopoda: The Octopuses, Squids, and
Nautilus
The more than 600 species of the class Cephalopoda—oc-
topuses, squids, and nautilus—are the most intelligent of
the invertebrates. They are active marine predators that
swim, often swiftly, and compete successfully with fish.
The foot has evolved into a series of tentacles equipped
with suction cups, adhesive structures, or hooks that seize
prey efficiently. Squids have 10 tentacles (figure 45.10); oc-
topuses, as indicated by their name, have eight; and the
nautilus, about 80 to 90. Once the tentacles have snared the
prey, it is bitten with strong, beaklike paired jaws and
pulled into the mouth by the tonguelike action of the
radula.
Cephalopods have highly developed nervous systems,
and their brains are unique among mollusks. Their eyes are
very elaborate, and have a structure much like that of verte-
brate eyes, although they evolved separately (see chapter
55). Many cephalopods exhibit complex patterns of behav-
ior and a high level of intelligence; octopuses can be easily
trained to distinguish among classes of objects. Most mem-
bers of this class have closed circulatory systems and are the
only mollusks that do.
Although they evolved from shelled ancestors, living
cephalopods, except for the few species of nautilus, lack an
external shell. Like other mollusks, cephalopods take water
into the mantle cavity and expel it through a siphon.
Cephalopods have modified this system into a means of jet
propulsion. When threatened, they eject water violently
and shoot themselves through the water.
Most octopus and squid are capable of changing color to
suit their background or display messages to one another.
They accomplish this feat through the use of their chro-
matophores, pouches of pigments embedded in the epithe-
lium.
Gastropods typically live in a hard shell. Bivalves have
hinged shells but do not have a distinct head area.
Cephalopods possess well-developed brains and are the
most intelligent invertebrates.
Chapter 45 Mollusks and Annelids 905
FIGURE 45.9
A bivalve. The file shell, Lima scabra, opened, showing tentacles.
FIGURE 45.10
A cephalopod. Squids are active predators, competing effectively
with fish for prey.
Segmented Animals
A key transition in the animal body
plan was segmentation, the building of a
body from a series of similar segments.
The first segmented animals to evolve
were most likely annelid worms, phy-
lum Annelida (figure 45.11). One ad-
vantage of having a body built from re-
peated units (segments) is that the
development and function of these
units can be more precisely controlled,
at the level of individual segments or
groups of segments. For example, dif-
ferent segments may possess different
combinations of organs or perform dif-
ferent functions relating to reproduc-
tion, feeding, locomotion, respiration,
or excretion.
Annelids
Two-thirds of all annelids live in the sea (about 8000
species), and most of the rest, some 3100 species, are
earthworms. Annelids are characterized by three principal
features:
1. Repeated segments. The body of an annelid worm
is composed of a series of ring-like segments running
the length of the body, looking like a stack of donuts
or roll of coins (figure 45.12). Internally, the seg-
ments are divided from one another by partitions
called septa, just as bulkheads separate the segments
of a submarine. In each of the cylindrical segments,
the excretory and locomotor organs are repeated.
The fluid within the coelom of each segment creates
a hydrostatic (liquid-supported) skeleton that gives
the segment rigidity, like an inflated balloon. Muscles
within each segment push against the fluid in the
coelom. Because each segment is separate, each can
expand or contract independently. This lets the worm
move in complex ways.
2. Specialized segments. The anterior (front) seg-
ments of annelids have become modified to contain
specialized sensory organs. Some are sensitive to
light, and elaborate eyes with lenses and retinas
have evolved in some annelids. A well-developed
cerebral ganglion, or brain, is contained in one an-
terior segment.
3. Connections. Although partitions separate the seg-
ments, materials and information do pass between
segments. Annelids have a closed circulatory system
that carries blood from one segment to another. A
ventral nerve cord connects the nerve centers or gan-
glia in each segment with one another and the brain.
These neural connections are critical features that
allow the worm to function and behave as a unified
and coordinated organism.
Body Plan of the Annelids
The basic annelid body plan is a tube within a tube, with
the internal digestive tract—a tube running from mouth to
anus—suspended within the coelom. The tube that makes
up the digestive tract has several portions—the pharynx,
esophagus, crop, gizzard, and intestine—that are special-
ized for different functions.
Annelids make use of their hydrostatic skeleton for lo-
comotion. To move, annelids contract circular muscles
running around each segment. Doing so squeezes the
segment, causing the coelomic fluid to squirt outwards,
like a tube of toothpaste. Because the fluid is trapped in
the segment by the septa, instead of escaping like tooth-
paste, the fluid causes the segment to elongate and get
much thinner. By then contracting longitudinal muscles
that run along the length of the worm, the segment is re-
turned to its original shape. In most annelid groups, each
segment typically possesses setae, bristles of chitin that
help anchor the worms during locomotion. By extending
the setae in some segments so that they anchor in the
substrate and retracting them in other segments, the
worm can squirt its body, section by section, in either
direction.
906 Part XII Animal Diversity
45.2 Annelids were the first segmented animals.
Sponges
Cnidarians
Flatworms
Nematodes
Mollusks Annelids
Arthropods
Echinoderms
Chordates
FIGURE 45.11
A polychaete annelid. Nereis virens is a
wide-ranging, predatory, marine polychaete
worm equipped with feathery parapodia for
movement and respiration, as well as jaws
for hunting. You may have purchased Nereis
as fishing bait!
Unlike the arthropods and most mollusks, most annelids
have a closed circulatory system. Annelids exchange oxygen
and carbon dioxide with the environment through their body
surfaces; most lack gills or lungs. However, much of their
oxygen supply reaches the different parts of their bodies
through their blood vessels. Some of these vessels at the ante-
rior end of the worm body are enlarged and heavily muscular,
serving as hearts that pump the blood. Earthworms have five
pulsating blood vessels on each side that serve as hearts, help-
ing to pump blood from the main dorsal vessel, which is their
major pumping structure, to the main ventral vessel.
The excretory system of annelids consists of ciliated,
funnel-shaped nephridia generally similar to those of mol-
lusks. These nephridia—each segment has a pair—collect
waste products and transport them out of the body through
the coelom by way of specialized excretory tubes.
Annelids are a diverse group of coelomate animals
characterized by serial segmentation. Each segment in
the annelid body has its own circulatory, excretory,
neural elements, and setae.
Chapter 45 Mollusks and Annelids 907
PHYLUM ANNELIDA: Segmentation
Segments
Setae
Clitellum
Mouth
Brain
Pharynx
Hearts
Esophagus
Dorsal
blood
vessel
Intestine
Longitudinal
muscleSepta
Male
gonads
Female
gonads
Nerve
cord
Ventral
blood
vessel
NephridiumCircular
muscle
Each segment contains
a set of excretory
organs and a nerve center.
Segments are connected by the circulatory
and nervous systems. A series of hearts
at the anterior (front) end pump the blood.
A well-developed brain located in an anterior
segment coordinates the activities of all
segments.
Each segment has a coelom. Muscles squeeze
the fluid of the coelom, making each segment
rigid, like an inflated balloon. Because each
segment can contract independently, a worm
can crawl by lengthening some segments
while shortening others.
Earthworms crawl by
anchoring bristles called
setae to the ground and
pulling against them.
Polychaete annelids have
a flattened body and swim
or crawl by flexing it.
FIGURE 45.12
The evolution of segmentation. Marine polychaetes and earthworms (phylum Annelida) were most likely the first organisms to evolve a
body plan based on partly repeated body segments. Segments are separated internally from each other by septa.
Classes of Annelids
The roughly 12,000 described species of annelids occur in
many different habitats. They range in length from as lit-
tle as 0.5 millimeter to the more than 3-meter length of
some polychaetes and giant Australian earthworms. There
are three classes of annelids: (1) Polychaeta, which are
free-living, almost entirely marine bristleworms, comprising
some 8000 species; (2) Oligochaeta, terrestrial earthworms
and related marine and freshwater worms, with some 3100
species; and (3) Hirudinea, leeches, mainly freshwater
predators or bloodsuckers, with about 500 species. The an-
nelids are believed to have evolved in the sea, with poly-
chaetes being the most primitive class. Oligochaetes seem to
have evolved from polychaetes, perhaps by way of brackish
water to estuaries and then to streams. Leeches share with
oligochaetes an organ called a clitellum, which secretes a
cocoon specialized to receive the eggs. It is generally agreed
that leeches evolved from oligochaetes, specializing in their
bloodsucking lifestyle as external parasites.
Class Polychaeta: The Polychaetes
Polychaetes (class Polychaeta) include clamworms, plume
worms, scaleworms, lugworms, twin-fan worms, sea mice,
peacock worms, and many others. These worms are often
surprisingly beautiful, with unusual forms and sometimes
iridescent colors (figure 45.13; see also figure 45.1). Poly-
chaetes are often a crucial part of marine food chains, as
they are extremely abundant in certain habitats.
Some polychaetes live in tubes or permanent burrows of
hardened mud, sand, mucuslike secretions, or calcium car-
bonate. These sedentary polychaetes are primarily filter
feeders, projecting a set of feathery tentacles from the tubes
in which they live that sweep the water for food. Other
polychaetes are active swimmers, crawlers, or burrowers.
Many are active predators.
Polychaetes have a well-developed head with specialized
sense organs; they differ from other annelids in this re-
spect. Their bodies are often highly organized into distinct
regions formed by groups of segments related in function
and structure. Their sense organs include eyes, which range
from simple eyespots to quite large and conspicuous stalked
eyes.
Another distinctive characteristic of polychaetes is the
paired, fleshy, paddlelike flaps, called parapodia, on most
of their segments. These parapodia, which bear bristlelike
setae, are used in swimming, burrowing, or crawling. They
also play an important role in gas exchange because they
greatly increase the surface area of the body. Some poly-
chaetes that live in burrows or tubes may have parapodia
featuring hooks to help anchor the worm. Slow crawling is
carried out by means of the parapodia. Rapid crawling and
swimming is by undulating the body. In addition, the poly-
chaete epidermis often includes ciliated cells which aid in
respiration and food procurement.
The sexes of polychaetes are usually separate, and fertil-
ization is often external, occurring in the water and away
from both parents. Unlike other annelids, polychaetes usu-
ally lack permanent gonads, the sex organs that produce
gametes. They produce their gametes directly from germ
cells in the lining of the coelom or in their septa. Fertiliza-
tion results in the production of ciliated, mobile trochophore
larvae similar to the larvae of mollusks. The trochophores
develop for long periods in the plankton before beginning
to add segments and thus changing to a juvenile form that
more closely resembles the adult form.
Class Oligochaeta:
The Earthworms
The body of an earthworm (class Oligochaeta) consists of
100 to 175 similar segments, with a mouth on the first and
an anus on the last. Earthworms seem to eat their way
through the soil because they suck in organic and other
material by expanding their strong pharynx. Everything
that they ingest passes through their long, straight digestive
tracts. One region of this tract, the gizzard, grinds up the
organic material with the help of soil particles.
The material that passes through an earthworm is de-
posited outside of its burrow in the form of castings that
908 Part XII Animal Diversity
FIGURE 45.13
A polychaete. The shiny bristleworm, Oenone fulgida.
consist of irregular mounds at the opening of a burrow. In
this way, earthworms aerate and enrich the soil. A worm
can eat its own weight in soil every day.
In view of the underground lifestyle that earthworms
have evolved, it is not surprising that they have no eyes.
However, earthworms do have numerous light-, chemo-,
and touch-sensitive cells, mostly concentrated in segments
near each end of the body—those regions most likely to en-
counter light or other stimuli. Earthworms have fewer
setae than polychaetes and no parapodia or head region.
Earthworms are hermaphroditic, another way in which
they differ from most polychaetes. When they mate (figure
45.14), their anterior ends point in opposite directions, and
their ventral surfaces touch. The clitellum is a thickened
band on an earthworm’s body; the mucus it secretes holds
the worms together during copulation. Sperm cells are re-
leased from pores in specialized segments of one partner
into the sperm receptacles of the other, the process going
in both directions simultaneously.
Two or three days after the worms separate, the clitel-
lum of each worm secretes a mucous cocoon, surrounded
by a protective layer of chitin. As this sheath passes over
the female pores of the body—a process that takes place as
the worm moves—it receives eggs. As it subsequently
passes along the body, it incorporates the sperm that were
deposited during copulation. Fertilization of the eggs takes
place within the cocoon. When the cocoon finally passes
over the end of the worm, its ends pinch together. Within
the cocoon, the fertilized eggs develop directly into young
worms similar to adults.
Class Hirudinea: The Leeches
Leeches (class Hirudinea) occur mostly in fresh water, al-
though a few are marine and some tropical leeches occupy
terrestrial habitats. Most leeches are 2 to 6 centimeters
long, but one tropical species reaches up to 30 centimeters.
Leeches are usually flattened dorsoventrally, like flat-
worms. They are hermaphroditic, and develop a clitellum
during the breeding season; cross-fertilization is obligatory
as they are unable to self-fertilize.
A leech’s coelom is reduced and continuous throughout
the body, not divided into individual segments as in the
polychaetes and oligochaetes. Leeches have evolved suckers
at one or both ends of the body. Those that have suckers at
both ends move by attaching first one and then the other
end to the substrate, looping along. Many species are also
capable of swimming. Except for one species, leeches have
no setae.
Some leeches have evolved the ability to suck blood
from animals. Many freshwater leeches live as external par-
asites. They remain on their hosts for long periods and
suck their blood from time to time.
The best-known leech is the medicinal leech, Hirudo
medicinalis (figure 45.15). Individuals of Hirudo are 10 to 12
centimeters long and have bladelike, chitinous jaws that
rasp through the skin of the victim. The leech secretes an
anticoagulant into the wound to prevent the blood from
clotting as it flows out, and its powerful sucking muscles
pump the blood out quickly once the hole has been opened.
Leeches were used in medicine for hundreds of years to
suck blood out of patients whose diseases were mistakenly
believed to be caused by an excess of blood. Today, Euro-
pean pharmaceutical companies still raise and sell leeches,
but they are used to remove excess blood after certain surg-
eries. Following the surgery, blood may accumulate be-
cause veins may function improperly and fail to circulate
the blood. The accumulating blood “turns off” the arterial
supply of fresh blood, and the tissue often dies. When
leeches remove the excess blood, new capillaries form in
about a week, and the tissues remain healthy.
Segmented annelids evolved in the sea. Earthworms are
their descendents, as are parasitic leeches.
Chapter 45 Mollusks and Annelids 909
FIGURE 45.14
Earthworms mating. The anterior ends are pointing in opposite
directions.
FIGURE 45.15
Hirudo medicinalis, the medicinal leech, is seen here feeding on a
human arm. Leeches uses chitinous, bladelike jaws to make an
incision to access blood and secrete an anticoagulant to keep the
blood from clotting. Both the anticoagulant and the leech itself
have made important contributions to modern medicine.
Lophophorates
Three phyla of marine animals—Phoronida, Ectoprocta,
and Brachiopoda—are characterized by a lophophore, a
circular or U-shaped ridge around the mouth bearing one
or two rows of ciliated, hollow tentacles. Because of this
unusual feature, they are thought to be related to one an-
other. The lophophore presumably arose in a common an-
cestor. The coelomic cavity of lophophorates extends into
the lophophore and its tentacles. The lophophore functions
as a surface for gas exchange and as a food-collection
organ. Lophophorates use the cilia of their lophophore to
capture the organic detritus and plankton on which they
feed. Lophophorates are attached to their substrate or
move slowly.
Lophophorates share some features with mollusks, an-
nelids and arthropods (all protostomes) and share others
with deuterostomes. Cleavage in lophophorates is mostly
radial, as in deuterostomes. The formation of the coelom
varies; some lophophorates resemble protostomes in this
respect, others deuterostomes. In the Phoronida, the
mouth forms from the blastopore, while in the other two
phyla, it forms from the end of the embryo opposite the
blastopore. Molecular evidence shows that the ribosomes
of all lophophorates are decidedly protostome-like, lending
strength to placing them within the protostome phyla. De-
spite the differences among the three phyla, the unique
structure of the lophophore seems to indicate that the
members share a common ancestor. Their relationships
continue to present a fascinating puzzle.
Phylum Phoronida: The Phoronids
Phoronids (phylum Phoronida) superficially resemble
common polychaete tube worms seen on dock pilings but
have many important differences. Each phoronid secretes
a chitinous tube and lives out its life within it (figure
45.16). They also extend tentacles to feed and quickly
withdraw them when disturbed, but the resemblance to
the tube worm ends there. Instead of a straight tube-
within-a-tube body plan, phoronids have a U-shaped gut.
Only about 10 phoronid species are known, ranging in
length from a few millimeters to 30 centimeters. Some
species lie buried in sand, others are attached to rocks ei-
ther singly or in groups. Phoronids develop as proto-
stomes, with radial cleavage and the anus developing sec-
ondarily.
Phylum Ectoprocta: The Bryozoans
Ectoprocts (phylum Ectoprocta) look like tiny, short ver-
sions of phoronids (figure 45.17). They are small—usually
less than 0.5 millimeter long—and live in colonies that look
like patches of moss on the surfaces of rocks, seaweed, or
other submerged objects (in fact, their common name bry-
ozoans translates from Greek as “moss-animals”). The
name Ectoprocta refers to the location of the anus (proct),
which is external to the lophophore. The 4000 species in-
clude both marine and freshwater forms—the only nonma-
rine lophophorates. Individual ectoprocts secrete a tiny
chitinous chamber called a zoecium that attaches to rocks
and other members of the colony. Individuals communicate
chemically through pores between chambers. Ectoprocts
develop as deuterostomes, with the mouth developing sec-
ondarily; cleavage is radial.
910 Part XII Animal Diversity
45.3 Lophophorates appear to be a transitional group.
LophophoreTentacles of
lophophore
Anus
Mouth
Nephridium
Body wall
Intestine
Testis
Ovary
FIGURE 45.16
Phoronids (phylum Phoronida). A phoronid, such as Phoronis,
lives in a chitinous tube that the animal secretes to form the outer
wall of its body. The lophophore consists of two parallel,
horseshoe-shaped ridges of tentacles and can be withdrawn into
the tube when the animal is disturbed.
Phylum Brachiopoda: The
Brachiopods
Brachiopods, or lamp shells, superfi-
cially resemble clams, with two calci-
fied shells (figure 45.18). Many
species attach to rocks or sand by a
stalk that protrudes through an open-
ing in one shell. The lophophore lies
within the shell and functions when
the brachiopod’s shells are opened
slightly. Although a little more than
300 species of brachiopods (phylum
Brachiopoda) exist today, more than
30,000 species of this phylum are
known as fossils. Because brachiopods
were common in the earth’s oceans
for millions of years and because their
shells fossilize readily, they are often
used as index fossils to define a partic-
ular time period or sediment type.
Brachiopods develop as deutero-
stomes and show radial cleavage.
The three phyla of lophophorates
probably share a common
ancestor, and they show a mixture
of protostome and deuterostome
characteristics.
Chapter 45 Mollusks and Annelids 911
Anus
Intestine
Mouth
Stomach
Lophophore
Retracted
lophophore
Retractor
muscle
Zoecium
(a) (b)
FIGURE 45.17
Ectoprocts (phylum Ectoprocta). (a) A small portion of a colony of the freshwater ectoproct, Plumatella (phylum Ectoprocta),
which grows on the underside of rocks. The individual at the left has a fully extended lophophore, the structure characteristic of
the three lophophorate phyla. The tiny individuals of Plumatella disappear into their shells when disturbed. (b) Plumatella repens, a
freshwater bryozoan.
Pedicel
Digestive
gland
Stomach
Nephridium
Intestine
Muscle
Coelom
Mouth Spiral portion of lophophore
Lateral arm of lophophore
Mantle
Ventral
(pedicel)
valve
Dorsal
(brachial)
valve
Gonad
FIGURE 45.18
Brachiopods (phylum
Brachiopoda). (a) The
lophophore lies within two
calcified shells, or valves.
(b) The brachiopod,
Terebratolina septentrionalsi, is
shown here slightly opened so
that the lophophore is visible.
(a)
(b)
912 Part XII Animal Diversity
Chapter 45
Summary Questions Media Resources
45.1 Mollusks were among the first coelomates.
? Mollusks contain a true body cavity, or coelom,
within the embryonic mesoderm and were among the
first coelomate animals.
? The mollusks constitute the second largest phylum of
animals in terms of named species. Their body plan
consists of distinct parts: a head, a visceral mass, and a
foot.
? Of the seven classes of mollusks, the gastropods
(snails and slugs), bivalves (clams and scallops), and
cephalopods (octopuses, squids, and nautilus), are
best known.
? Gastropods typically live in a hard shell. During
development, one side of the embryo grows more
rapidly than the other, producing a characteristic
twisting of the visceral mass.
? Members of the class Bivalvia have two shells hinged
together dorsally and a wedge-shaped foot. They lack
distinct heads and radulas. Most bivalves are filter-
feeders.
? Octopuses and other cephalopods are efficient and
often large predators. They possess well-developed
brains and are the most intelligent invertebrates.
1. What is the basic body plan of
a mollusk? Where is the mantle
located? Why is it important in
the mollusks? What occurs in
the mantle cavity of aquatic
mollusks?
2. What is a radula? Do all
classes of mollusks possess this
structure? How is it used in
different types of mollusks?
3. How does the mollusk
excretory structure work? Why
is it better than the flame cells of
acoelomates?
4. What is a trochophore? What
is a veliger?
5. Do bivalves generally disperse
as larvae or adults? Explain.
? Segmentation is a characteristic seen only in
coelomate animals at the annelid evolutionary level
and above. Segmentation, or the repetition of body
regions, greatly facilitates the development of
specialized regions of the body.
? Annelids are worms with bodies composed of
numerous similar segments, each with its own
circulatory, excretory, and neural elements, and array
of setae. There are three classes of annelids, the
largely marine Polychaeta, the largely terrestrial
Oligochaeta, and the largely freshwater Hirudinea.
6. What evolutionary advantages
does segmentation confer upon
an organism?
7. What are annelid setae? What
function do they serve? What
are parapodia? What class of
annelids possess them?
8. How do earthworms obtain
their nutrients? What sensory
structures do earthworms
possess? How do these animals
reproduce?
45.2 Annelids were the first segmented animals.
? The lophophorates consist of three phyla of marine
animals—Phoronida, Ectoprocta, and Brachiopoda—
characterized by a circular or U-shaped ridge, the
lophophore, around the mouth.
? Some lophophorates have characteristics like
protostomes, others like deuterostomes. All are
characterized by a lophophore and are thought to
share a common ancestor.
9. What prominent feature
characterizes the lophophorate
animals? What are the functions
of this feature?
45.3 Lophophorates appear to be a transitional group.
www.mhhe.com/raven6e www.biocourse.com
? Mollusks
? Annelids
? Student Research:
Growth in
Earthworms
913
46
Arthropods
Concept Outline
46.1 The evolution of jointed appendages has made
arthropods very successful.
Jointed Appendages and an Exoskeleton. Arthropods
probably evolved from annelids, and with their jointed
appendages and an exoskeleton, have successfully invaded
practically every habitat on earth.
Classification of Arthropods. Arthropods have been
traditionally divided into three groups based on
morphological characters. However, recent research
suggests a restructuring of arthropod classification is
needed.
General Characteristics of Arthropods. Arthropods
have segmented bodies, a chitinous exoskeleton, and often
have compound eyes. They have open circulatory systems.
In some groups, a series of tubes carry oxygen to the
organs, and unique tubules eliminate waste.
46.2 The chelicerates all have fangs or pincers.
Class Arachnida: The Arachnids. Spiders and scorpions
are predators, while most mites are herbivores.
Class Merostomata: Horseshoe Crabs. Among the
most ancient of living animals, horseshoe crabs are thought
to have evolved from trilobites.
Class Pycnogonida: The Sea Spiders. The spiders that
are common in marine habitats differ greatly from
terrestrial spiders.
46.3 Crustaceans have branched appendages.
Crustaceans. Crustaceans are unique among living
arthropods because virtually all of their appendages are
branched.
46.4 Insects are the most diverse of all animal groups.
Classes Chilopoda and Diplopoda: The Centipedes and
Millipedes. Centipedes and millipedes are highly
segmented, with legs on each segment.
Class Insecta: The Insects. Insects are the largest group
of organisms on earth. They are the only invertebrate
animals that have wings and can fly.
Insect Life Histories. Insects undergo simple or
complete metamorphosis.
T
he evolution of segmentation among annelids marked
the first major innovation in body structure among
coelomates. An even more profound innovation was the de-
velopment of jointed appendages in arthropods, a phylum
that almost certainly evolved from an annelid ancestor.
Arthropod bodies are segmented like those of annelids, but
the individual segments often exist only during early devel-
opment and fuse into functional groups as adults. In
arthropods like the wasp above (figure 46.1), jointed ap-
pendages include legs, antennae, and a complex array of
mouthparts. The functional flexibility provided by such a
broad array of appendages has made arthropods the most
successful of animal groups.
FIGURE 46.1
An arthropod. One of the major arthropod groups is represented
here by Polistes, the common paper wasp (class Insecta).
component of plants, and shares
similar properties of toughness and
flexibility. Together, the chitin and
protein provide an external covering
that is both very strong and capable
of flexing in response to the contrac-
tion of muscles attached to it. In
most crustaceans, the exoskeleton is
made even tougher, although less
flexible, with deposits of calcium
salts. However, there is a limitation.
The exoskeleton must be much
thicker to bear the pull of the mus-
cles in large insects than in small
ones. That is why you don’t see bee-
tles as big as birds, or crabs the size
of a cow—the exoskeleton would be
so thick the animal couldn’t move its
great weight. Because this size limi-
tation is inherent in the body design
of arthropods, there are no large
arthropods—few are larger than
your thumb.
The Arthropods
Arthropods, especially the largest
class—insects—are by far the most
successful of all animals. Well over
1,000,000 species—about two-thirds
of all the named species on earth—
are members of this phylum (figure
46.2). One scientist recently esti-
mated, based on the number and di-
versity of insects in tropical forests,
that there might be as many as 30
million species in this one class
alone. About 200 million insects are
alive at any one time for each
human! Insects and other arthro-
pods (figure 46.3) abound in every
habitat on the planet, but they espe-
cially dominate the land, along with
flowering plants and vertebrates.
The majority of arthropod species
consist of small animals, mostly about
a millimeter in length. Members of
the phylum range in adult size from about 80 micrometers
long (some parasitic mites) to 3.6 meters across (a gigantic
crab found in the sea off Japan).
Arthropods, especially insects, are of enormous eco-
nomic importance and affect all aspects of human life.
They compete with humans for food of every kind, play a
key role in the pollination of certain crops, and cause bil-
914 Part XII Animal Diversity
Jointed Appendages
and an Exoskeleton
With the evolution of the first an-
nelids, many of the major innova-
tions of animal structure had already
appeared: the division of tissues into
three primary types (endoderm,
mesoderm, and ectoderm), bilateral
symmetry, a coelom, and segmenta-
tion. With arthropods, two more in-
novations arose—the development of
jointed appendages and an exoskeleton.
Jointed appendages and an exoskele-
ton have allowed arthropods (phy-
lum Arthropoda) to become the most
diverse phylum.
Jointed Appendages
The name “arthropod” comes from
two Greek words, arthros, “jointed,”
and podes, “feet.” All arthropods have
jointed appendages. The numbers of
these appendages are reduced in the
more advanced members of the phy-
lum. Individual appendages may be
modified into antennae, mouthparts
of various kinds, or legs. Some ap-
pendages, such as the wings of certain
insects, are not homologous to the
other appendages; insect wings
evolved separately.
To gain some idea of the impor-
tance of jointed appendages, imagine
yourself without them—no hips,
knees, ankles, shoulders, elbows,
wrists, or knuckles. Without jointed
appendages, you could not walk or
grasp any object. Arthropods use
jointed appendages such as legs for
walking, antennae to sense their envi-
ronment, and mouthparts for feeding.
Exoskeleton
The arthropod body plan has a sec-
ond major innovation: a rigid external skeleton, or ex-
oskeleton, made of chitin and protein. In any animal, the
skeleton functions to provide places for muscle attach-
ment. In arthropods, the muscles attach to the interior
surface of their hard exoskeleton, which also protects the
animal from predators and impedes water loss. Chitin is
chemically similar to cellulose, the dominant structural
46.1 The evolution of jointed appendages has made arthropods very successful.
Sponges
Cnidarians
Flatworms
Nematodes
Mollusks Annelids
Arthropods
Echinoderms
Chordates
Beetles
Bacteria
Protists
Plants
Mollusks
Chordates
Other animals
Flies
Butterflies,
moths
Bees, wasps
Other insects
Other arthropods
Spiders
Crustaceans
Fungi
FIGURE 46.2
Arthropods are a successful group.
About two-thirds of all named species are
arthropods. About 80% of all arthropods
are insects, and about half of the named
species of insects are beetles.
lions of dollars of damage to crops, before and after har-
vest. They are by far the most important herbivores in all
terrestrial ecosystems and are a valuable food source as
well. Virtually every kind of plant is eaten by one or
more species of insect. Diseases spread by insects cause
enormous financial damage each year and strike every
kind of domesticated animal and plant, as well as human
beings.
Arthropods are segmented protostomes with jointed
appendages. Arthropods are the most successful of all
animal groups.
Chapter 46 Arthropods 915
The jointed appendages of insects
are all connected to the central body
region, the thorax. There are three pairs
of legs attached there and, most often,
two pairs of wings (some insects like
flies have retained only one wing pair).
The wings are sheets of chitin and protein.
Insects eliminate wastes by collecting
circulatory fluid osmotically in Malpighian
tubules that extend from the gut into the
blood and then reabsorbing the fluid, but
not the wastes.
Insects breathe through small tubes
called tracheae that pass throughout the
body and are connected to the outside
by special openings called spiracles.
Insects have
complex sensory
organs located on
the head, including
a single pair of
antennae and
compound eyes
composed of many
independent visual
units.
Arthropods have been
the most successful
of all animals. Two-thirds
of all named species
on earth are arthropods.
Antenna
Eye
Head
Thorax Air sac
Malpighian
tubules
Abdomen
Rectum
Poison
sac
Sting
Midgut
Spiracles
Mouthparts
PHYLUM ARTHROPODA: Jointed appendages and exoskeleton
FIGURE 46.3
The evolution of jointed appendages and an exoskeleton. Insects and other arthropods (phylum Arthropoda) have a coelom,
segmented bodies, and jointed appendages. The three body regions of an insect (head, thorax, and abdomen) are each actually composed of
a number of segments that fuse during development. All arthropods have a strong exoskeleton made of chitin. One class, the insects, has
evolved wings that permit them to fly rapidly through the air.
Classification of the Arthropods
Arthropods are among the oldest of animals, first appearing
in the Precambrian over 600 million years ago. Ranging in
size from enormous to microscopic, all arthropods share a
common heritage of segmented bodies and jointed ap-
pendages, a powerful combination for generating novel
evolutionary forms. Arthropods are the most diverse of all
the animal phyla, with more species than all other animal
phyla combined, most of them insects.
Origin of the Arthropods
Taxonomists have long held that there is a close relation-
ship between the annelids and the arthropods, the two
great segmented phyla. Velvet worms (phylum Ony-
chophora), known from the Burgess Shale (where upside-
down fossils were called Hallucigenia) and many other early
Cambrian deposits, have many features in common with
both annelids and arthropods. Some recent molecular stud-
ies have supported the close relationship between annelids
and arthropods, others have not.
Traditional Classification
Members of the phylum Arthropoda have been tradition-
ally divided into three subphyla, based largely on morpho-
logical characters.
1. Trilobites (the extinct trilobites). Trilobites, com-
mon in the seas 250 million years ago, were the first
animals whose eyes were capable of a high degree of
resolution.
2. Chelicerates (spiders, horseshoe crabs, sea spiders).
These arthropods lack jaws. The foremost ap-
pendages of their bodies are mouthparts called che-
licerae (figure 46.4a) that function in feeding, usually
pincers or fangs .
3. Mandibulates (crustaceans, insects, centipedes, milli-
pedes). These arthropods have biting jaws, called
mandibles (figure 46.4b). In mandibulates, the most
anterior appendages are one or more pairs of sensory
antennae, and the next appendages are the mandibles.
Among the mandibulates, insects have traditionally
been set apart from the crustaceans, grouped instead
with the myriapods (centipedes and millipedes) in a
taxon called Tracheata. This phylogeny, still widely
employed, dates back to benchmark work by the
great comparative biologist Robert Snodgrass in the
1930s. He pointed out that insects, centipedes, and
millipedes are united by several seemingly powerful
attributes:
A tracheal respiratory system. Trachea are small,
branched air ducts that transmit oxygen from
openings in the exoskeleton to every cell of the
body.
Use of Malpighian tubules for excretion. Malpighian
tubules are slender projections from the digestive
tract which collect and filter body fluids, emptying
wastes into the hindgut.
Uniramous (single-branched) legs. All crustacean ap-
pendages are basically biramous, or “two-
branched” (figure 46.5), although some of these
appendages have become single-branched by re-
duction in the course of their evolution. Insects, by
contrast, have uniramous, or single-branched,
mandibles and other appendages.
Doubts about the Traditional Approach
Recent research is casting doubt on the wisdom of this
taxonomic decision. The problem is that the key morpho-
logical traits used to define the Tracheata are not as pow-
erful taxonomically as had been assumed. Taxonomists
have traditionally assumed a character like branching ap-
916 Part XII Animal Diversity
Eyes
Chelicera
Pedipalp
Antenna
Mandible
(a) Chelicerate
(b) Mandibulate
FIGURE 46.4
Chelicerates and mandibulates. In the chelicerates, such as a
spider (a), the chelicerae are the foremost appendages of the body.
In contrast, the foremost appendages in the mandibulates, such as
an ant (b), are the antennae, followed by the mandibles.
Exopodite
Endopodite
Crayfish maxilliped
(biramous)
Insect appendage
(uniramous)
FIGURE 46.5
Mandibulate appendages. A biramous leg in a crustacean
(crayfish) and a uniramous leg in an insect.
pendages to be a fundamental one, conserved over the
course of evolution, and thus suitable for making taxo-
nomic distinctions.
However, modern molecular biology now tells us that
this is not a valid assumption. The branching of arthro-
pod legs, for example, turns out to be controlled by a sin-
gle gene. The pattern of appendages among arthropods is
orchestrated by a family of genes called homeotic (Hox)
genes, described in detail in chapter 17. A single one of
these Hox genes, called Distal-less, has recently been
shown to initiate development of unbranched limbs in in-
sects and branched limbs in crustaceans. The same
Distal-less gene is found in many animal phyla, including
vertebrates.
A Revolutionary New Phylogeny
In recent years a mass of accumulating morphological and
molecular data has led many taxonomists to suggest new
arthropod taxonomies. The most revolutionary of these,
championed by Richard Brusca of Columbia University,
considers crustaceans to be the basic arthropod group, and
insects a close sister group (figure 46.6).
Morphological Evidence. The most recent morphologi-
cal study of arthropod phylogeny, reported in 1998, was
based on 100 conserved anatomical features of the central
nervous system. It concluded insects were more closely re-
lated to crustaceans than to any other arthropod group.
They share a unique pattern of segmental neurons, and
many other features.
Molecular Evidence. Molecular phylogenies based on
18S rRNA sequences, the 18S rDNA gene, the elongation
factor EF-1a, and the RNA polymerase II gene, all place
insects as a sister group to crustaceans, not myriapods, and
arising from within the crustaceans. In conflict with 150
years of morphology-based thinking, these conclusions are
certain to engender lively discussion.
Arthropods have traditionally been classified into
arachnids and other chelicerates that lack jaws and have
fang mouthparts, and mandibulates (crustaceans and
tracheates) with biting jaws. A revised arthropod
taxonomy considers Tracheata to be the products
convergent evolution, with insects and crustaceans
sister groups.
Chapter 46 Arthropods 917
Ancestral
arthropod
Ancestral
arthropod
Trilobites (extinct) Trilobites (extinct)
Eurypterids (extinct)
Horseshoe crabs
Horseshoe crabs
Arachnids
Arachnids
Sea spiders
Sea spiders
Chelicerates
Chelicerates
Crustaceans
Crustaceans
Mandibulates
Traditional Phylogeny Revised Phylogeny
Insects
Insects
Tracheata
Centipedes
Centipedes
Millipedes
Millipedes
Myriapoda
Modern
crustaceans
Eurypterids (extinct)
A crustacean?
FIGURE 46.6
A proposed revision of arthropod phylogeny. Accumulating evidence supports the hypothesis that insects and modern crustaceans are
sister groups, having evolved from the same ancient crustacean ancestor in the Precambrian. This implies that insects may be viewed as
“flying crustaceans,” and that the traditional Tracheata taxon, which places centipedes, millipedes, and insects together, is in fact a
polyphyletic group.
General Characteristics of
Arthropods
Arthropod bodies are segmented like annelids, a phylum to
which at least some arthropods are clearly related. Mem-
bers of some classes of arthropods have many body seg-
ments. In others, the segments have become fused together
into functional groups, or tagmata (singular, tagma), such
as the head and thorax of an insect (figure 46.7). This fus-
ing process, known as tagmatization, is of central impor-
tance in the evolution of arthropods. In most arthropods,
the original segments can be distinguished during larval de-
velopment. All arthropods have a distinct head, sometimes
fused with the thorax to form a tagma called the
cephalothorax.
Exoskeleton
The bodies of all arthropods are covered by an exoskeleton,
or cuticle, that contains chitin. This tough outer covering,
against which the muscles work, is secreted by the epider-
mis and fused with it. The exoskeleton remains fairly flexi-
ble at specific points, allowing the exoskeleton to bend and
appendages to move. The exoskeleton protects arthropods
from water loss and helps to protect them from predators,
parasites, and injury.
Molting. Arthropods periodically undergo ecdysis, or
molting, the shedding of the outer cuticular layer. When
they outgrow their exoskeleton, they form a new one un-
derneath. This process is controlled by hormones. When
the new exoskeleton is complete, it becomes separated from
the old one by fluid. This fluid dissolves the chitin and pro-
tein and, if it is present, calcium carbonate, from the old
exoskeleton. The fluid increases in volume until, finally,
the original exoskeleton cracks open, usually along the
back, and is shed. The arthropod emerges, clothed in a
new, pale, and still somewhat soft exoskeleton. The arthro-
pod then “puffs itself up,” ultimately expanding to full size.
The blood circulation to all parts of the body aids them in
this expansion, and many insects and spiders take in air to
assist them. The expanded exoskeleton subsequently hard-
ens. While the exoskeleton is soft, the animal is especially
vulnerable. At this stage, arthropods often hide under
stones, leaves, or branches.
Compound Eye
Another important structure in many arthropods is the
compound eye (figure 46.8a). Compound eyes are com-
posed of many independent visual units, often thousands
of them, called ommatidia. Each ommatidium is covered
with a lens and linked to a complex of eight retinular cells
918 Part XII Animal Diversity
Cephalothorax
(fused head and
thorax)
Abdomen
Abdomen
(a) Scorpion (b) Honeybee
Head
Thorax
FIGURE 46.7
Arthropod evolution from many to few body segments. The
(a) scorpion and the (b) honeybee are arthropods with different
numbers of body segments.
Ommatidium
Optic nerve
Nerve
fiber
Corneal lens
Crystalline core
Rhabdom
Retinular cells
Pigment
cells
Cross section
of
ommatidium
FIGURE 46.8
The compound eye. (a) The compound eyes found in insects are complex structures. (b) Three ocelli are visible between the compound
eyes of the robberfly (order Diptera).
(a) (b)
and a light-sensitive central core, or rhabdom. Com-
pound eyes among insects are of two main types: apposi-
tion eyes and superposition eyes. Apposition eyes are
found in bees and butterflies and other insects that are ac-
tive during the day. Each ommatidium acts in isolation,
surrounded by a curtain of pigment cells that blocks the
passage of light from one to another. Superposition eyes,
such as those found in moths and other insects that are
active at night, are designed to maximize the amount of
light that enters each ommatidium. At night, the pigment
in the pigment cells is concentrated at the top of the cells
so that the low levels of light can be received by many dif-
ferent ommatidia. During daylight, the pigment in the
pigment cells is evenly dispersed throughout the cells, al-
lowing the eye to function much like an apposition eye.
The pigment in the pigment cells gives the arthropod eye
its color, but it is not the critical pigment needed for vi-
sion. The visual pigment is located in an area called the
rhabdom found in the center of the ommatidium. The in-
dividual images from each ommatidium are combined in
the arthropod’s brain to form its image of the external
world.
Simple eyes, or ocelli, with single lenses are found in
the other arthropod groups and sometimes occur together
with compound eyes, as is often the case in insects (figure
46.8b). Ocelli function in distinguishing light from dark-
ness. The ocelli of some flying insects, namely locusts and
dragonflies, function as horizon detectors and help the in-
sect visually stabilize its course in flight.
Circulatory System
In the course of arthropod evolution, the coelom has be-
come greatly reduced, consisting only of cavities that
house the reproductive organs and some glands. Arthro-
pods completely lack cilia, both on the external surfaces of
the body and on the internal organs. Like annelids,
arthropods have a tubular gut that extends from the
mouth to the anus. In the next paragraphs we will discuss
the circulatory, respiratory, excretory, and nervous sys-
tems of the arthropods (figure 46.9).
The circulatory system of arthropods is open; their
blood flows through cavities between the internal organs
and not through closed vessels. The principal component
of an insect’s circulatory system is a longitudinal vessel
called the heart. This vessel runs near the dorsal surface of
the thorax and abdomen. When it contracts, blood flows
into the head region of the insect.
When an insect’s heart relaxes, blood returns to it
through a series of valves. These valves are located in the
posterior region of the heart and allow the blood to flow
inward only. Thus, blood from the head and other anterior
portions of the insect gradually flows through the spaces
between the tissues toward the posterior end and then back
through the one-way valves into the heart. Blood flows
most rapidly when the insect is running, flying, or other-
wise active. At such times, the blood efficiently delivers nu-
trients to the tissues and removes wastes from them.
Nervous System
The central feature of the arthropod nervous system is a
double chain of segmented ganglia running along the ani-
mal’s ventral surface. At the anterior end of the animal are
three fused pairs of dorsal ganglia, which constitute the
brain. However, much of the control of an arthropod’s ac-
tivities is relegated to ventral ganglia. Therefore, the ani-
mal can carry out many functions, including eating, move-
ment, and copulation, even if the brain has been removed.
The brain of arthropods seems to be a control point, or in-
hibitor, for various actions, rather than a stimulator, as it is
in vertebrates.
Chapter 46 Arthropods 919
Rectum
Malpighian
tubules
Heart
Ovary
Tympanal
organ
Compound eye
Ocelli
Head
Thorax
Abdomen
Antennae
Spiracles
Mouth
Nerve ganglia
Brain
Aorta
Crop
Stomach
Gastric
ceca
(a) (b)
FIGURE 46.9
A grasshopper (order Orthoptera). This grasshopper illustrates the major structural features of the insects, the most numerous group of
arthropods. (a) External anatomy. (b) Internal anatomy.
Respiratory System
Insects and other members of subphylum Uniramia, which
are fundamentally terrestrial, depend on their respiratory
rather than their circulatory system to carry oxygen to their
tissues. In vertebrates, blood moves within a closed circula-
tory system to all parts of the body, carrying the oxygen
with it. This is a much more efficient arrangement than ex-
ists in arthropods, in which all parts of the body need to be
near a respiratory passage to obtain oxygen. As a result, the
size of the arthropod body is much more limited than that
of the vertebrates. Along with the brittleness of their chitin
exoskeletons, this feature of arthropod design places severe
limitations on size.
Unlike most animals, the arthropods have no single
major respiratory organ. The respiratory system of most
terrestrial arthropods consists of small, branched, cuticle-
lined air ducts called tracheae (figure 46.10). These tra-
cheae, which ultimately branch into very small tracheoles,
are a series of tubes that transmit oxygen throughout the
body. Tracheoles are in direct contact with individual cells,
and oxygen diffuses directly across the cell membranes. Air
passes into the tracheae by way of specialized openings in
the exoskeleton called spiracles, which, in most insects,
can be opened and closed by valves. The ability to prevent
water loss by closing the spiracles was a key adaptation that
facilitated the invasion of the land by arthropods. In many
insects, especially larger ones, muscle contraction helps to
increase the flow of gases in and out of the tracheae. In
other terrestrial arthropods, the flow of gases is essentially
a passive process.
Many spiders and some other chelicerates have a unique
respiratory system that involves book lungs, a series of
leaflike plates within a chamber. Air is drawn in and ex-
pelled out of this chamber by muscular contraction. Book
lungs may exist alongside tracheae, or they may function
instead of tracheae. One small class of marine chelicerates,
the horseshoe crabs, have book gills, which are analogous
to book lungs but function in water. Tracheae, book lungs,
and book gills are all structures found only in arthropods
and in the phylum Onychophora, which have tracheae.
Crustaceans lack such structures and have gills.
Excretory System
Though there are various kinds of excretory systems in dif-
ferent groups of arthropods, we will focus here on the
unique excretory system consisting of Malpighian tubules
that evolved in terrestrial uniramians. Malpighian tubules are
slender projections from the digestive tract that are attached
at the junction of the midgut and hindgut (see figure 46.3).
Fluid passes through the walls of the Malpighian tubules to
and from the blood in which the tubules are bathed. As this
fluid passes through the tubules toward the hindgut, nitroge-
nous wastes are precipitated as concentrated uric acid or
guanine. These substances are then emptied into the hindgut
and eliminated. Most of the water and salts in the fluid are
reabsorbed by the hindgut and rectum and returned to the
arthropod’s body. Malpighian tubules are an efficient mech-
anism for water conservation and were another key adapta-
tion facilitating invasion of the land by arthropods.
All arthropods have a rigid chitin and protein
exoskeleton that provides places for muscle attachment,
protects the animal from predators and injury, and,
most important, impedes water loss. Many arthropods
have compound eyes. Arthropods have an open
circulatory system. Many arthropods eliminate
metabolic wastes by a unique system of Malpighian
tubules. Most terrestrial insects have a network of tubes
called tracheae that transmit oxygen from the outside to
the organs.
920 Part XII Animal Diversity
Spiracles
Spiracle
Tracheoles
Trachea
FIGURE 46.10
Tracheae and tracheoles. Tracheae and tracheoles are connected to the exterior by specialized openings called spiracles and carry oxygen
to all parts of a terrestrial insect’s body. (a) The tracheal system of a grasshopper. (b) A portion of the tracheal system of a cockroach.
(a) (b)
Class Arachnida:
The Arachnids
Chelicerates (subphylum Chelicerata) are a distinct evolu-
tionary line of arthropods in which the most anterior ap-
pendages have been modified into chelicerae, which often
function as fangs or pincers. By far the largest of the three
classes of chelicerates is the largely terrestrial Arachnida,
with some 57,000 named species; it includes spiders, ticks,
mites, scorpions, and daddy longlegs. Arachnids have a pair
of chelicerae, a pair of pedipalps, and four pairs of walking
legs. The chelicerae are the foremost appendages; they
consist of a stout basal portion and a movable fang often
connected to a poison gland.
The next pair of appendages, pedipalps, resemble legs
but have one less segment and are not used for locomotion.
In male spiders, they are specialized copulatory organs. In
scorpions, the pedipalps are large pincers.
Most arachnids are carnivorous. The main exception is
mites, which are largely herbivorous. Most arachnids can
ingest only preliquified food, which they often digest ex-
ternally by secreting enzymes into their prey. They can
then suck up the digested material with their muscular,
pumping pharynx. Arachnids are primarily, but not exclu-
sively, terrestrial. Some 4000 known species of mites and
one species of spider live in fresh water, and a few mites
live in the sea. Arachnids breathe by means of tracheae,
book lungs, or both.
Order Opiliones: The Daddy Longlegs
A familiar group of arachnids consists of the daddy long-
legs, or harvestmen (order Opiliones). Members of this
order are easily recognized by their oval, compact bodies
and extremely long, slender legs (figure 46.11). They
respire by means of a primary pair of tracheae and are un-
usual among the arachnids in that they engage in direct
copulation. The males have a penis, and the females an
ovipositor, or egg-laying organ which deposits their eggs
in cracks and crevices. Most daddy longlegs are predators
of insects and other arachnids, but some live on plant juices
and many scavenge dead animal matter. The order includes
about 5000 species.
Order Scorpiones: The Scorpions
Scorpions (order Scorpiones) are arachnids whose pedi-
palps are modified into pincers. Scorpions use these pincers
to handle and tear apart their food (figure 46.12). The ven-
omous stings of scorpions are used mainly to stun their
prey and less commonly in self-defense. The stinging appa-
ratus is located in the terminal segment of the abdomen. A
scorpion holds its abdomen folded forward over its body
when it is moving about. The elongated, jointed abdomens
of scorpions are distinctive; in most chelicerates, the ab-
dominal segments are more or less fused together and ap-
pear as a single unit.
Scorpions are probably the most ancient group of terres-
trial arthropods; they are known from the Silurian Period,
some 425 million years ago. Adults of this order of arach-
nids range in size from 1 to 18 centimeters. There are some
1200 species of scorpions, all terrestrial, which occur
throughout the world. They are most common in tropical,
subtropical, and desert regions. The young are born alive,
with 1 to 95 in a given brood.
Chapter 46 Arthropods 921
46.2 The chelicerates all have fangs or pincers.
FIGURE 46.11
A harvestman, or daddy longlegs.
FIGURE 46.12
The scorpion, Uroctonus mordax. This photograph shows the
characteristic pincers and segmented abdomen, ending in a
stinging apparatus, raised over the animal’s back. The white mass
is comprised of the scorpion’s young.
Order Araneae: The Spiders
There are about 35,000 named
species of spiders (order Araneae).
These animals play a major role in
virtually all terrestrial ecosystems.
They are particularly important as
predators of insects and other small
animals. Spiders hunt their prey or
catch it in silk webs of remarkable di-
versity. The silk is formed from a
fluid protein that is forced out of
spinnerets on the posterior portion of
the spider’s abdomen. The webs and
habits of spiders are often distinctive.
Some spiders can spin gossamer floats
that allow them to drift away in the
breeze to a new site.
Many kinds of spiders, like the familiar wolf spiders and
tarantulas, do not spin webs but instead hunt their prey ac-
tively. Others, called trap-door spiders, construct silk-lined
burrows with lids, seizing their prey as it passes by. One
species of spider, Argyroneta aquatica, lives in fresh water,
spending most of its time below the surface. Its body is sur-
rounded by a bubble of air, while its legs, which are used
both for underwater walking and for swimming, are not.
Several other kinds of spiders walk about freely on the sur-
face of water.
Spiders have poison glands leading through their che-
licerae, which are pointed and used to bite and paralyze
prey. Some members of this order, such as the black widow
and brown recluse (figure 46.13), have bites that are poiso-
nous to humans and other large mammals.
Order Acari: Mites and Ticks
The order Acari, the mites and ticks, is the largest in terms
of number of species and the most diverse of the arachnids.
Although only about 30,000 species of mites and ticks have
been named, scientists that study the group estimate that
there may be a million or more members of this order in
existence.
Most mites are small, less than 1 millimeter long, but
adults of different species range from 100 nanometers to 2
centimeters. In most mites, the cephalothorax and ab-
domen are fused into an unsegmented ovoid body. Respira-
tion occurs either by means of tracheae or directly through
the exoskeleton. Many mites pass through several distinct
stages during their life cycle. In most, an inactive eight-
legged prelarva gives rise to an active six-legged larva,
which in turn produces a succession of three eight-legged
stages and, finally, the adult males and females.
Mites and ticks are diverse in structure and habitat.
They are found in virtually every terrestrial, freshwater,
and marine habitat known and feed on fungi, plants, and
animals. They act as predators and as internal and external
parasites of both invertebrates and vertebrates.
Many mites produce irritating bites and diseases in hu-
mans. Mites live in the hair follicles and wax glands of your
forehead and nose, but usually cause no symptoms.
Ticks are blood-feeding ectoparasites, parasites that
occur on the surface of their host (figure 46.14). They are
larger than most other mites and cause discomfort by suck-
ing the blood of humans and other animals. Ticks can carry
many diseases, including some caused by viruses, bacteria,
and protozoa. The spotted fevers (Rocky Mountain spotted
fever is a familiar example) are caused by bacteria carried
by ticks. Lyme disease is apparently caused by spirochaetes
transmitted by ticks. Red-water fever, or Texas fever, is an
important tick-borne protozoan disease of cattle, horses,
sheep, and dogs.
Scorpions, spiders, and mites are all arachnids, the
largest class of chelicerates.
922 Part XII Animal Diversity
FIGURE 46.13
Two common poisonous spiders. (a) The black widow spider, Latrodectus mactans.
(b) The brown recluse spider, Loxosceles reclusa. Both species are common throughout
temperate and subtropical North America, but bites are rare in humans.
FIGURE 46.14
Ticks (order Acari). Ticks (the large one is engorged) on the
hide of a tapir in Peru. Many ticks spread diseases in humans and
other vertebrates.
(a) (b)
Class Merostomata: Horseshoe
Crabs
A second class of chelicerates is the horseshoe crabs (class
Merostomata). There are three genera of horseshoe crabs.
One, Limulus (figure 46.15), is common along the East
Coast of North America. The other two genera live in the
Asian tropics. Horseshoe crabs are an ancient group, with
fossils virtually identical to Limulus dating back 220 million
years to the Triassic Period. Other members of the class,
the now-extinct eurypterans, are known from 400 million
years ago. Horseshoe crabs may have been derived from
trilobites, a relationship suggested by
the appearance of their larvae. Individ-
uals of Limulus grow up to 60 centime-
ters long. They mature in 9 to 12 years
and have a life span of 14 to 19 years.
Limulus individuals live in deep water,
but they migrate to shallow coastal wa-
ters every spring, emerging from the
sea to mate on moonlit nights when
the tide is high.
Horseshoe crabs feed at night, pri-
marily on mollusks and annelids. They
swim on their backs by moving their
abdominal plates. They can also walk
on their four pairs of legs, protected
along with chelicerae and pedipalps by
their shell (figure 46.16).
Horseshoe crabs are a very ancient
group.
Class Pycnogonida: The Sea Spiders
The third class of chelicerates is the sea spiders (class Pyc-
nogonida). Sea spiders are common in coastal waters, with
more than 1000 species in the class. These animals are not
often observed because many are small, only about 1 to 3
centimeters long, and rather inconspicuous. They are
found in oceans throughout the world but are most abun-
dant in the far north and far south. Adult sea spiders are
mostly external parasites or predators of other animals like
sea anemones (figure 46.17).
Sea spiders have a sucking proboscis in a mouth located at
its end. Their abdomen is much reduced,
and their body appears to consist almost
entirely of the cephalothorax, with no
well-defined head. Sea spiders usually
have four, or less commonly five or six,
pairs of legs. Male sea spiders carry the
eggs on their legs until they hatch, thus
providing a measure of parental care. Sea
spiders completely lack excretory and
respiratory systems. They appear to carry
out these functions by direct diffusion,
with waste products flowing outward
through the cells and oxygen flowing in-
ward through them. Sea spiders are not
closely related to either of the other two
classes of chelicerates.
Sea spiders are very common in
marine habitats. They are not
closely related to terrestrial spiders.
Chapter 46 Arthropods 923
FIGURE 46.15
Limulus. Horseshoe crabs, emerging from
the sea to mate at the edge of Delaware
Bay, New Jersey, in early May.
Operculum Walking legs
Book gills
Pedipalp
Chelicera
Mouth
Carapace
Cephalothorax
Abdomen
Telson
FIGURE 46.16
Diagram of a horseshoe crab, Limulus, from below. This diagram illustrates the
principal features of this archaic animal.
FIGURE 46.17
A marine pycnogonid. The sea spider,
Pycnogonum littorale (yellow animal),
crawling over a sea anemone.
Crustaceans
The crustaceans (subphylum Crustacea) are a large group
of primarily aquatic organisms, consisting of some 35,000
species of crabs, shrimps, lobsters, crayfish, barnacles,
water fleas, pillbugs, and related groups (table 46.1). Most
crustaceans have two pairs of antennae, three types of
chewing appendages, and various numbers of pairs of legs.
All crustacean appendages, with the possible exception of
the first pair of antennae, are basically biramous. In some
crustaceans, appendages appear to have only a single
branch; in those cases, one of the branches has been lost
during the course of evolutionary specialization. The
nauplius larva stage through which all crustaceans pass
(figure 46.18) provides evidence that all members of this
diverse group are descended from a common ancestor.
The nauplius hatches with three pairs of appendages and
metamorphoses through several stages before reaching
maturity. In many groups, this nauplius stage is passed in
the egg, and development of the hatchling to the adult
form is direct.
Crustaceans differ from insects but resemble cen-
tipedes and millipedes in that they have appendages on
their abdomen as well as on their thorax. They are the
only arthropods with two pairs of antennae. Their
mandibles likely originated from a pair of limbs that took
on a chewing function during the course of evolution, a
process that apparently occurred independently in the
common ancestor of the terrestrial mandibulates. Many
crustaceans have compound eyes. In addition, they have
delicate tactile hairs that project from the cuticle all over
the body. Larger crustaceans have feathery gills near the
bases of their legs. In smaller members of this class, gas
exchange takes place directly through the thinner areas of
the cuticle or the entire body. Most crustaceans have sep-
arate sexes. Many different kinds of specialized copulation
occur among the crustaceans, and the members of some
orders carry their eggs with them, either singly or in egg
pouches, until they hatch.
Decapod Crustaceans
Large, primarily marine crustaceans such as shrimps, lob-
sters, and crabs, along with their freshwater relatives, the
crayfish, are collectively called decapod crustaceans (figure
46.19). The term decapod means “ten-footed.” In these ani-
mals, the exoskeleton is usually reinforced with calcium
carbonate. Most of their body segments are fused into a
cephalothorax covered by a dorsal shield, or carapace,
which arises from the head. The crushing pincers common
in many decapod crustaceans are used in obtaining food,
for example, by crushing mollusk shells.
In lobsters and crayfish, appendages called swimmerets
occur in lines along the ventral surface of the abdomen and
are used in reproduction and swimming. In addition, flat-
tened appendages known as uropods form a kind of com-
pound “paddle” at the end of the abdomen. These animals
may also have a telson, or tail spine. By snapping its ab-
domen, the animal propels itself through the water rapidly
and forcefully. Crabs differ from lobsters and crayfish in
proportion; their carapace is much larger and broader and
the abdomen is tucked under it.
924 Part XII Animal Diversity
Table 46.1 Tradional Classification of the Phylum
Arthropoda
Subphylum Characteristics Members
Chelicerata Mouthparts are chelicerae The chelicerates:
spiders,
horseshoe crabs,
mites
Crustacea Mouthparts are mandibles; The crustaceans:
biramous appendages lobsters, crabs,
shrimp, isopods,
barnacles
Uniramia Mouthparts are mandibles; Chilopods
uniramous appendages (centipedes),
diplopods
(millipedes), and
insects
46.3 Crustaceans have branched appendages.
FIGURE 46.18
Although crustaceans are diverse, they have fundamentally
similar larvae. The nauplius larva of a crustacean is an important
unifying feature found in all members of this group.
Terrestrial and Freshwater Crustaceans
Although most crustaceans are marine, many occur in fresh
water and a few have become terrestrial. These include pill-
bugs and sowbugs, the terrestrial members of a large order
of crustaceans known as the isopods (order Isopoda). About
half of the estimated 4500 species of this order are terres-
trial and live primarily in places that are moist, at least sea-
sonally. Sand fleas or beach fleas (order Amphipoda) are
other familiar crustaceans, many of which are semiterres-
trial (intertidal) species.
Along with the larvae of larger species, minute crus-
taceans are abundant in the plankton. Especially significant
are the tiny copepods (order Copepoda; figure 46.20),
which are among the most abundant multicellular organ-
isms on earth.
Sessile Crustaceans
Barnacles (order Cirripedia; figure 46.21) are a group of
crustaceans that are sessile as adults. Barnacles have free-
swimming larvae, which ultimately attach their heads to a
piling, rock, or other submerged object and then stir food
into their mouth with their feathery legs. Calcareous plates
protect the barnacle’s body, and these plates are usually at-
tached directly and solidly to the substrate. Although most
crustaceans have separate sexes, barnacles are hermaphro-
ditic, but they generally cross-fertilize.
Crustaceans include marine, freshwater, and terrestrial
forms. All possess a nauplius larval stage and branched
appendages.
Chapter 46 Arthropods 925
Cheliped
Eye
Cephalothorax Abdomen
Swimmerets
Telson
Uropod
Antenna
Antennule
Walking legs
FIGURE 46.19
Decapod crustacean. A lobster, Homarus americanus. The principal features are labeled.
FIGURE 46.20
Freshwater crustacean. A copepod with attached eggs, a member
of an abundant group of marine and freshwater crustaceans (order
Copepoda), most of which are a few millimeters long. Copepods
are important components of the plankton.
FIGURE 46.21
Gooseneck barnacles, Lepas anatifera, feeding.
These are stalked barnacles; many others lack a stalk.
Millipedes, centipedes, and insects, three dis-
tinct classes, are uniramian mandibulates. They
respire by means of tracheae and excrete their
waste products through Malpighian tubules.
These groups were certainly derived from
annelids, probably ones similar to the
oligochaetes, which they resemble in their
embryology.
Classes Chilopoda
and Diplopoda: The
Centipedes and Millipedes
The centipedes (class Chilopoda) and millipedes
(class Diplopoda) both have bodies that consist
of a head region followed by numerous seg-
ments, all more or less similar and nearly all
bearing paired appendages. Although the name
centipede would imply an animal with a hundred
legs and the name millipede one with a thousand, adult cen-
tipedes usually have 30 or more legs, adult millipedes 60 or
more. Centipedes have one pair of legs on each body seg-
ment (figure 46.22), millipedes two (figure 46.23). Each
segment of a millipede is a tagma that originated during
the group’s evolution when two ancestral segments fused.
This explains why millipedes have twice as many legs per
segment as centipedes.
In both centipedes and millipedes, fertilization is in-
ternal and takes place by direct transfer of sperm. The
sexes are separate, and all species lay eggs. Young milli-
pedes usually hatch with three pairs of legs; they experi-
ence a number of growth stages, adding segments and
legs as they mature, but do not change in general
appearance.
Centipedes, of which some 2500 species are known,
are all carnivorous and feed mainly on insects. The ap-
pendages of the first trunk segment are modi-
fied into a pair of poison fangs. The poison is
often quite toxic to human beings, and many
centipede bites are extremely painful, some-
times even dangerous.
In contrast, most millipedes are herbivores,
feeding mainly on decaying vegetation. A few
millipedes are carnivorous. Many millipedes can
roll their bodies into a flat coil or sphere be-
cause the dorsal area of each of their body seg-
ments is much longer than the ventral one.
More than 10,000 species of millipedes have
been named, but this is estimated to be no more
than one-sixth of the actual number of species
that exists. In each segment of their body, most
millipedes have a pair of complex glands that
produces a bad-smelling fluid. This fluid is ex-
uded for defensive purposes through openings along the
sides of the body. The chemistry of the secretions of dif-
ferent millipedes has become a subject of considerable in-
terest because of the diversity of the compounds involved
and their effectiveness in protecting millipedes from at-
tack. Some produce cyanide gas from segments near their
head end. Millipedes live primarily in damp, protected
places, such as under leaf litter, in rotting logs, under bark
or stones, or in the soil.
Centipedes are segmented hunters with one pair of legs
on each segment. Millipedes are segmented herbivores
with two pairs of legs on each segment.
926 Part XII Animal Diversity
46.4 Insects are the most diverse of all animal groups.
FIGURE 46.22
A centipede. Centipedes, like this member of the genus Scolopendra, are active
predators.
FIGURE 46.23
A millipede. Millipedes, such as this Sigmoria individual, are herbivores.
Class Insecta: The Insects
The insects, class Insecta, are by far the largest group of or-
ganisms on earth, whether measured in terms of numbers
of species or numbers of individuals. Insects live in every
conceivable habitat on land and in fresh water, and a few
have even invaded the sea. More than half of all the named
animal species are insects, and the actual proportion is
doubtless much higher because millions of additional forms
await detection, classification, and naming. Approximately
90,000 described species occur in the United States and
Canada, and the actual number of species in this area prob-
ably approaches 125,000. A hectare of lowland tropical for-
est is estimated to be inhabited by as many as 41,000
species of insects, and many suburban gardens may have
1500 or more species. It has been estimated that approxi-
mately a billion billion (10
18
) individual insects are alive at
any one time. A glimpse at the enormous diversity of in-
sects is presented in figure 46.24 and later in table 46.2.
Chapter 46 Arthropods 927
(a)
(b)
(d) (e) (f)
(c)
FIGURE 46.24
Insect diversity. (a) Luna moth, Actias luna. Luna moths and their relatives are among the most spectacular insects (order Lepidoptera).
(b) Soldier fly, Ptecticus trivittatus (order Diptera). (c) Boll weevil, Anthonomus grandis. Weevils are one of the largest groups of beetles
(order Coleoptera). (d) A thorn-shaped leafhopper, Umbonica crassicornis (order Hemiptera). (e) Copulating grasshoppers (order
Orthoptera). (f) Termite, Macrotermes bellicosus (order Isoptera). The large, sausage-shaped individual is a queen, specialized for laying
eggs; most of the smaller individuals around the queen are nonreproductive workers, but the larger individual at the lower left is a
reproductive male.
External Features
Insects are primarily a terrestrial group, and most, if not
all, of the aquatic insects probably had terrestrial ances-
tors. Most insects are relatively small, ranging in size from
0.1 millimeter to about 30 centimeters in length or
wingspan. Insects have three body sections, the head, tho-
rax, and abdomen; three pairs of legs, all attached to the
thorax; and one pair of antennae. In addition, they may
have one or two pairs of wings. Insect mouthparts all have
the same basic structure but are modified in different
groups in relation to their feeding habits (figure 46.25).
Most insects have compound eyes, and many have ocelli
as well.
The insect thorax consists of three segments, each with
a pair of legs. Legs are completely absent in the larvae of
certain groups, for example, in most members of the
order Diptera (flies) (figure 46.26). If two pairs of wings
are present, they attach to the middle and posterior seg-
ments of the thorax. If only one pair of wings is present, it
usually attaches to the middle segment. The thorax is al-
most entirely filled with muscles that operate the legs and
wings.
The wings of insects arise as saclike outgrowths of the
body wall. In adult insects, the wings are solid except for
the veins. Insect wings are not homologous to the other ap-
pendages. Basically, insects have two pairs of wings, but in
some groups, like flies, the second set has been reduced to a
pair of balancing knobs called halteres during the course of
evolution. Most insects can fold their wings over their ab-
domen when they are at rest; but a few, such as the dragon-
flies and damselflies (order Odonata), keep their wings
erect or outstretched at all times.
Insect forewings may be tough and hard, as in beetles.
If they are, they form a cover for the hindwings and usu-
ally open during flight. The tough forewings also serve a
protective function in the order Orthoptera, which in-
cludes grasshoppers and crickets. The wings of insects are
made of sheets of chitin and protein; their strengthening
veins are tubules of chitin and protein. Moths and butter-
flies have wings that are covered with detachable scales
that provide most of their bright colors (figure 46.27). In
some wingless insects, such as the springtails or silverfish,
wings never evolved. Other wingless groups, such as fleas
and lice, are derived from ancestral groups of insects that
had wings.
Internal Organization
The internal features of insects resemble those of the
other arthropods in many ways. The digestive tract is a
tube, usually somewhat coiled. It is often about the same
length as the body. However, in the order Hemiptera,
which consists of the leafhoppers, cicadas, and related
groups, and in many flies (order Diptera), the digestive
tube may be greatly coiled and several times longer than
the body. Such long digestive tracts are generally found in
928 Part XII Animal Diversity
(a) (b) (c)
FIGURE 46.25
Modified mouthparts in three kinds of insects. Mouthparts are
modified for (a) piercing in the mosquito, Culex, (b) sucking nectar
from flowers in the alfalfa butterfly, Colias, and (c) sopping up
liquids in the housefly, Musca domestica.
FIGURE 46.26
Larvae of a mosquito, Culex pipiens. The aquatic larvae of
mosquitoes are quite active. They breathe through tubes from the
surface of the water, as shown here. Covering the water with a
thin film of oil causes them to drown.
insects that have sucking mouthparts
and feed on juices rather than on
protein-rich solid foods because they
offer a greater opportunity to absorb
fluids and their dissolved nutrients.
The digestive enzymes of the insect
also are more dilute and thus less effec-
tive in a highly liquid medium than in a
more solid one. Longer digestive tracts
give these enzymes more time to work
while food is passing through.
The anterior and posterior regions
of an insect’s digestive tract are lined
with cuticle. Digestion takes place pri-
marily in the stomach, or midgut; and
excretion takes place through
Malpighian tubules. Digestive enzymes
are mainly secreted from the cells that
line the midgut, although some are
contributed by the salivary glands near
the mouth.
The tracheae of insects extend
throughout the body and permeate its
different tissues. In many winged in-
sects, the tracheae are dilated in vari-
ous parts of the body, forming air sacs.
These air sacs are surrounded by mus-
cles and form a kind of bellows system
to force air deep into the tracheal sys-
tem. The spiracles, a maximum of 10
on each side of the insect, are paired
and located on or between the seg-
ments along the sides of the thorax
and abdomen. In most insects, the
spiracles can be opened by muscular action. Closing the
spiracles at times may be important in retarding water
loss. In some parasitic and aquatic groups of insects, the
spiracles are permanently closed. In these groups, the tra-
cheae run just below the surface of the insect, and gas ex-
change takes place by diffusion.
The fat body is a group of cells located in the insect
body cavity. This structure may be quite large in relation
to the size of the insect, and it serves as a food-storage
reservoir, also having some of the functions of a verte-
brate liver. It is often more prominent in immature in-
sects than in adults, and it may be completely depleted
when metamorphosis is finished. Insects that do not feed
as adults retain their fat bodies and live on the food stored
in them throughout their adult lives (which may be very
short).
Sense Receptors
In addition to their eyes, insects have several characteristic
kinds of sense receptors. These include sensory hairs,
which are usually widely distributed over their bodies. The
sensory hairs are linked to nerve cells
and are sensitive to mechanical and
chemical stimulation. They are partic-
ularly abundant on the antennae and
legs—the parts of the insect most
likely to come into contact with other
objects.
Sound, which is of vital importance
to insects, is detected by tympanal or-
gans in groups such as grasshoppers
and crickets, cicadas, and some
moths. These organs are paired struc-
tures composed of a thin membrane,
the tympanum, associated with the
tracheal air sacs. In many other
groups of insects, sound waves are de-
tected by sensory hairs. Male mosqui-
toes use thousands of sensory hairs on
their antennae to detect the sounds
made by the vibrating wings of female
mosquitoes.
Sound detection in insects is impor-
tant not only for protection but also
for communication. Many insects
communicate by making sounds, most
of which are quite soft, very high-
pitched, or both, and thus inaudible to
humans. Only a few groups of insects,
especially grasshoppers, crickets, and
cicadas, make sounds that people can
hear. Male crickets and longhorned
grasshoppers produce sounds by rub-
bing their two front wings together.
Shorthorned grasshoppers do so by
rubbing their hind legs over specialized areas on their
wings. Male cicadas vibrate the membranes of air sacs lo-
cated on the lower side of the most anterior abdominal
segment.
In addition to using sound, nearly all insects communi-
cate by means of chemicals or mixtures of chemicals
known as pheromones. These compounds, extremely di-
verse in their chemical structure, are sent forth into the
environment, where they are active in very small amounts
and convey a variety of messages to other individuals.
These messages not only convey the attraction and recog-
nition of members of the same species for mating, but
they also mark trails for members of the same species, as
in the ants.
All insects possess three body segments (tagmata): the
head, the thorax, and the abdomen. The three pairs of
legs are attached to the thorax. Most insects have
compound eyes, and many have one or two pairs of
wings. Insects possess sophisticated means of sensing
their environment, including sensory hairs, tympanal
organs, and chemoreceptors.
Chapter 46 Arthropods 929
FIGURE 46.27
Scales on the wing of Parnassius
imperator, a butterfly from China. Scales
of this sort account for most of the colored
patterns on the wings of butterflies and
moths.
Insect Life Histories
Most young insects hatch from fertilized eggs laid outside
their mother’s body. The zygote develops within the egg
into a young insect, which escapes by chewing through or
bursting the shell. Some immature insects have specialized
projections on the head that assist in this process. In a few
insects, eggs hatch within the mother’s body.
During the course of their development, young insects
undergo ecdysis a number of times before they become
adults. Most insects molt four to eight times during the
course of their development, but some may molt as many
as 30 times. The stages between the molts are called in-
stars. When an insect first emerges following ecdysis, it is
pale, soft, and especially susceptible to predators. Its ex-
oskeleton generally hardens in an hour or two. It must
grow to its new size, usually by taking in air or water, dur-
ing this brief period.
There are two principal kinds of metamorphosis in in-
sects: simple metamorphosis and complete metamor-
phosis (figure 46.28). In insects with simple metamorpho-
sis, immature stages are often called nymphs. Nymphs are
usually quite similar to adults, differing mainly in their
smaller size, less well-developed wings, and sometimes
color. In insect orders with simple metamorphosis, such as
mayflies and dragonflies, nymphs are aquatic and extract
oxygen from the water through gills. The adult stages are
terrestrial and look very different from the nymphs. In
other groups, such as grasshoppers and their relatives,
nymphs and adults live in the same habitat. Such insects
usually change gradually during their life cycles with re-
spect to wing development, body proportions, the appear-
ance of ocelli, and other features.
In complete metamorphosis, the wings develop inter-
nally during the juvenile stages and appear externally
only during the resting stage that immediately precedes
the final molt (figure 46.28b). During this stage, the in-
sect is called a pupa or chrysalis, depending on the
group to which it belongs. A pupa or chrysalis does not
normally move around much, although mosquito pupae
do move around freely. A large amount of internal body
reorganization takes place while the insect is a pupa or
chrysalis.
More than 90% of the insects, including the members of
all of the largest and most successful orders, display com-
plete metamorphosis. The juvenile stages and adults often
live in distinct habitats, have different habits, and are usu-
ally extremely different in form. In these insects, develop-
ment is indirect. The immature stages, called larvae, are
often wormlike, differing greatly in appearance from the
adults. Larvae do not have compound eyes. They may be
legless or have legs or leg-like appendages on the abdomen.
They usually have chewing mouthparts, even in those or-
ders in which the adults have sucking mouthparts; chewing
mouthparts are the primitive condition in these groups.
When larvae and adults play different ecological roles, they
do not compete directly for the same resources, an advan-
tage to the species.
Pupae do not feed and are usually relatively inactive. As
pupae, insects are extremely vulnerable to predators and
parasites, but they are often covered by a cocoon or some
other protective structure. Groups of insects with complete
metamorphosis include moths and butterflies; beetles; bees,
wasps, and ants; flies; and fleas.
Some species of insects exhibit no dramatic change in
form from immature stages to adult. This type of develop-
ment is called ametabolus (meaning without change) and
is seen in the most primitive orders of insects such as the
silverfish and springtails.
Hormones control both ecdysis and metamorphosis.
Molting hormone, or ecdysone, is released from a gland
in the thorax when that gland has been stimulated by brain
hormone, which in turn is produced by neurosecretory
cells and released into the blood. The effects of the molting
induced by ecdysone are determined by juvenile hormone,
which is present during the immature stages but declines in
quantity as the insect passes through successive molts.
When the level of juvenile hormone is relatively high, the
molt produces another larva; when it is lower, it produces
the pupa and then the final development of the adult.
Insects undergo either simple or complete
metamorphosis.
930 Part XII Animal Diversity
Egg
(a)
(b)
Egg Early
larva
Nymphs
Full-sized
larva
Pupa
Adult chinch bug
Adult housefly
FIGURE 46.28
Metamorphosis. (a) Simple metamorphosis in a chinch bug
(order Hemiptera), and (b) complete metamorphosis in a housefly,
Musca domestica (order Diptera).
Chapter 46 Arthropods 931
Table 46.2 Major Orders of Insects
Approximate
Typical Number of
Order Examples Key Characteristics Named Species
Coleoptera
Diptera
Lepidoptera
Hymenoptera
Hemiptera
Orthoptera
Odonata
Isoptera
Siphonaptera
Beetles
Flies
Butterflies,
moths
Bees, wasps,
ants
True bugs,
bedbugs,
leafhoppers
Grasshoppers,
crickets, roaches
Dragonflies
Termites
Fleas
The most diverse animal order; two pairs of wings;
front pair of wings is a hard cover that partially
protects the transparent rear pair of flying wings;
heavily armored exoskeleton; biting and chewing
mouthparts; complete metamorphosis
Some that bite people and other mammals are considered
pests; front flying wings are transparent;
hind wings are reduced to knobby balancing organs;
sucking, piercing, and lapping mouthparts; complete
metamorphosis
Often collected for their beauty; two pairs of broad, scaly,
flying wings, often brightly colored; hairy body; tubelike,
sucking mouthparts; complete metamorphosis
Often social, known to many by their sting; two pairs
of transparent flying wings; mobile head and well-
developed eyes; often possess stingers; chewing and
sucking mouthparts; complete metamorphosis
Often live on blood; two pairs of wings, or wingless;
piercing, sucking mouthparts; simple metamorphosis
Known for their jumping; two pairs of wings or
wingless; among the largest insects; biting and
chewing mouthparts in adults; simple metamorphosis
Among the most primitive of the insect order; two pairs
of transparent flying wings; large, long, and slender body;
chewing mouthparts; simple metamorphosis
One of the few types of animals able to eat wood; two
pairs of wings, but some stages are wingless; social insects;
there are several body types with division of labor;
chewing mouthparts; simple metamorphosis
Small, known for their irritating bites; wingless; small
flattened body with jumping legs; piercing and sucking
mouthparts; complete metamorphosis
350,000
120,000
120,000
100,000
60,000
20,000
5,000
2,000
1,200
932 Part XII Animal Diversity
Chapter 46
Summary Questions Media Resources
46.1 The evolution of jointed appendages has made arthropods very successful.
? Jointed appendages and an exoskeleton greatly
expanded locomotive and manipulative capabilities
for the arthropod phyla, the most successful of all
animals in terms of numbers of individuals, species,
and ecological diversification.
? Traditionally, arthropods have been grouped into
three subphyla based on morphological characters but
new research is calling this classification of the
arthropods into question.
? Like annelids, arthropods have segmented bodies, but
some of their segments have become fused into
tagmata during the course of evolution. All possess a
rigid external skeleton, or exoskeleton.
1. What are the advantages of an
exoskeleton? What occurs
during ecdysis? What controls
this process?
2. What type of circulatory
system do arthropods have?
Describe the direction of blood
flow. What helps to maintain
this one-way flow?
3. What are Malpighian tubules?
How do they work? What other
system are they connected to?
How does this system process
wastes? How does it regulate
water loss?
? Chelicerates consist of three classes: Arachnida
(spiders, ticks, mites, and scorpions); Merostomata
(horseshoe crabs); and Pycnogonida (sea spiders).
? Spiders, the best known arachnids, have a pair of
chelicerae, a pair of pedipalps, and four pairs of
walking legs. Spiders secrete digestive enzymes into
their prey, then suck the contents out.
4. Into what two groups are
arthropods traditionally divided?
Describe each group in terms of
its mouthparts and appendages,
and give several examples of
each.
46.2 The chelicerates all have fangs or pincers.
? Crustaceans comprise some 35,000 species of crabs,
shrimps, lobsters, barnacles, sowbugs, beach fleas,
and many other groups. Their appendages are
basically biramous, and their embryology is
distinctive.
5. On which parts of the body
do crustaceans possess legs?
6. How do biramous and
uniramous appendages differ?
46.3 Crustaceans have branched appendages.
? Centipedes and millipedes are segmented uniramia.
Centipedes are hunters with one pair of legs per
segment, and millipedes are herbivores with two pairs
of legs per segment.
? Insects have three body segments, three pairs of legs,
and often one or two pairs of wings. Many have
complex eyes and other specialized sensory
structures.
? Insects exhibit either simple metamorphosis, moving
through a succession of forms relatively similar to the
adult, or complete metamorphosis, in which an often
wormlike larva becomes a usually sedentary pupa, and
then an adult.
7. How are millipedes and
centipedes similar to each other?
How do they differ?
8. What type of digestive system
do most insects possess? What
digestive adaptations occur in
insects that feed on juices low in
protein? Why?
9. What is an instar as it relates
to insect metamorphosis? What
are the two different kinds of
metamorphosis in insects? How
do they differ?
46.4 Insects are the most diverse of all animal groups.
www.mhhe.com/raven6e www.biocourse.com
? Arthropods
? Enhancement
Chapter: Arthropod
Taxonomy,
Sections 1 and 2
? Enhancement
Chapter: Arthropod
Taxonomy, Section 3
? Enhancement
Chapter: Arthropod
Taxonomy, Section 4
? Student Research:
Insect Behavior
933
47
Echinoderms
Concept Outline
47.1 The embryos of deuterostomes develop quite
differently from those of protostomes.
Protostomes and Deuterostomes. Deuterostomes—the
echinoderms, chordates, and a few other groups—share a
mode of development that is quite different from other
animals.
47.2 Echinoderms are deuterostomes with an
endoskeleton.
Deuterostomes. Echinoderms are bilaterally symmetrical
as larvae but metamorphose to radially symmetrical adults.
Echinoderm Body Plan. Echinoderms have an
endoskeleton and a unique water-vascular system seen in no
other phylum.
47.3 The six classes of echinoderms are all radially
symmetrical as adults.
Class Crinoidea: The Sea Lilies and Feather Stars.
Crinoids are the only echinoderms that are attached to the
sea bottom for much of their lives.
Class Asteroidea: The Sea Stars. Sea stars, also called
starfish, are five-armed mobile predators.
Class Ophiuroidea: The Brittle Stars. Brittle stars are
quite different from the sea stars for whom they are
sometimes mistaken.
Class Echinoidea: The Sea Urchins and Sand Dollars.
Sea urchins and sand dollars have five-part radial symmetry
but lack arms.
Classes Holothuroidea and Concentricycloidea: Sea
Cucumbers and Sea Daisies. Sea cucumbers are soft-
bodied echinoderms without arms. The most recently
discovered class of echinoderms, sea daisies are tiny,
primitive echinoderms that live at great depths.
E
chinoderms, which include the familiar starfish, have
been described as a “noble group especially designed
to puzzle the zoologist.” They are bilaterally symmetrical
as larvae, but undergo a bizarre metamorphosis to a radially
symmetrical adult (figure 47.1). A compartment of the
coelom is transformed into a unique water-vascular system
that uses hydraulic power to operate a multitude of tiny
tube feet that are used in locomotion and food capture.
Some echinoderms have an endoskeleton of dermal plates
beneath the skin, fused together like body armor. Many
have miniature jawlike pincers scattered over their body
surface, often on stalks and sometimes bearing poison
glands. This collection of characteristics is unique in the
animal kingdom.
FIGURE 47.1
An echinoderm. Brittle star, Ophiothrix, a member of the largest
group of echinoderms.
to all of the phyla that exhibit it. In deuterostome (Greek,
deuteros, “second,” and stoma, “mouth”) development, the
blastopore gives rise to the organism’s anus, and the mouth
develops from a second pore that arises in the blastula later
in development.
Deuterostomes represent a revolution in embryonic de-
velopment. In addition to the pattern of blastopore forma-
tion, deuterostomes differ from protostomes in a number of
other fundamental embryological features:
1. The progressive division of cells during embryonic
growth is called cleavage. The cleavage pattern relative
to the embryo’s polar axis determines how the cells
will array. In nearly all protostomes, each new cell
buds off at an angle oblique to the polar axis. As a re-
sult, a new cell nestles into the space between the
older ones in a closely packed array. This pattern is
called spiral cleavage because a line drawn through a
sequence of dividing cells spirals outward from the
polar axis (figure 47.2).
In deuterostomes, the cells divide parallel to and at
right angles to the polar axis. As a result, the pairs of
cells from each division are positioned directly above
934 Part XII Animal Diversity
Protostomes and Deuterostomes
The coelomates we have met so far—the mollusks, annelids,
and arthropods—exhibit essentially the same kind of embry-
ological development, starting as a hollow ball of cells, a
blastula, which indents to form a two-layer-thick ball with a
blastopore opening to the outside. Also in this group, the
mouth (stoma) develops from or near the blastopore (figure
47.2). This same pattern of development, in a general sense,
is seen in all noncoelomate animals. An animal whose
mouth develops in this way is called a protostome (from
the Greek words protos, “first,” and stoma, “mouth”). If such
an animal has a distinct anus or anal pore, it develops later
in another region of the embryo. The fact that this kind of
developmental pattern is so widespread in diverse phyla sug-
gests that it is the original pattern for animals as a whole and
that it was characteristic of the common ancestor of all eu-
metazoan animals.
A second distinct pattern of embryological development
occurs in the echinoderms, the chordates, and a few other
smaller related phyla. The consistency of this pattern of de-
velopment, and its distinctiveness from that of the proto-
stomes suggests that it evolved once, in a common ancestor
47.1 The embryos of deuterostomes develop quite differently from those of
protostomes.
1 cell
1 cell
2 cells 4 cells 8 cells 16 cells 32 cells
2 cells 4 cells 8 cells 16 cells 32 cells
Protostomes
Deuterostomes
FIGURE 47.2
Embryonic development in protostomes and deuterostomes. Cleavage of the egg produces a hollow ball of cells called the blastula.
Invagination of the blastula produces the blastopore and archenteron. In protostomes, embryonic cells cleave in a spiral pattern and
become tightly packed. The blastopore becomes the animal’s mouth, and the coelom originates from a mesodermal split.
and below one another; this process gives rise to a
loosely packed array of cells. This pattern is called ra-
dial cleavage because a line drawn through a se-
quence of dividing cells describes a radius outward
from the polar axis.
2. Protostomes exhibit determinate development. In
this type of development, each embryonic cell has a
predetermined fate in terms of what kind of tissue it
will form in the adult. Before cleavage begins, the
chemicals that act as developmental signals are local-
ized in different parts of the egg. Consequently, the
cell divisions that occur after fertilization separate dif-
ferent signals into different daughter cells. This
process specifies the fate of even the very earliest em-
bryonic cells. Deuterostomes, on the other hand, dis-
play indeterminate development. The first few cell
divisions of the egg produce identical daughter cells.
Any one of these cells, if separated from the others,
can develop into a complete organism. This is possible
because the chemicals that signal the embryonic cells
to develop differently are not localized until later in
the animal’s development.
3. In all coelomates, the coelom originates from meso-
derm. In protostomes, this occurs simply and directly:
the cells simply move away from one another as the
coelomic cavity expands within the mesoderm. How-
ever, in deuterostomes, whole groups of cells usually
move around to form new tissue associations. The
coelom is normally produced by an evagination of the
archenteron—the central tube within the gastrula,
also called the primitive gut. This tube, lined with en-
doderm, opens to the outside via the blastopore and
eventually becomes the gut cavity.
The first abundant and well-preserved animal fossils are
nearly 600 million years old; they occur in the Ediacara se-
ries of Australia and similar formations elsewhere. Among
these fossils, many represent groups of animals that no
longer exist. In addition, these ancient rocks bear evidence
of the coelomates, the most advanced evolutionary line of
animals, and it is remarkable that their two major subdivi-
sions were differentiated so early. In the coelomates, it
seems likely that all deuterostomes share a common proto-
stome ancestor—a theory that is supported by evidence
from comparison of rRNA and other molecular studies. The
event, however, occurred very long ago and presumably did
not involve groups of organisms that closely resemble any
that are living now.
In deuterostomes, the egg cleaves radially, and the
blastopore becomes the anus. In protostomes, the egg
cleaves spirally, and the blastopore becomes the mouth.
Chapter 47 Echinoderms 935
Mesoderm splits
Archenteron outpockets
to form coelom
Mouth forms
from blastopore
Anus forms
from blastopore
Blastula
Blastula
Blastopore
Mesoderm
Blastopore
Archenteron Coelom
Archenteron Coelom
Coelom
Coelom
Anus
Mouth
Anus
Mouth
FIGURE 47.2 (continued)
In deuterostomes, embryonic cells cleave radially and form a loosely packed array. The blastopore becomes the animal’s anus, and the mouth
develops at the other end. The coelom originates from an evagination, or outpouching, of the archenteron in deuterostomes.
Deuterostomes
Mollusks, annelids, and arthropods are
protostomes. However, the echinoderms
are characterized by deuterostome develop-
ment, a key evolutionary advance. The
endoskeleton makes its first appearance in
the echinoderms also.
The Echinoderms
Deuterostomate marine animals called
echinoderms appeared nearly 600 mil-
lion years ago (figure 47.3). Echino-
derms (phylum Echinodermata) are an
ancient group of marine animals con-
sisting of about 6000 living species and
are also well represented in the fossil
record. The term echinoderm means
“spiny skin” and refers to an en-
doskeleton composed of hard calcium-
rich plates just beneath the delicate skin (figure 47.4).
When they first form, the plates are enclosed in living tis-
sue and so are truly an endoskeleton, although in adults
they frequently fuse, forming a hard shell. Another inno-
vation in echinoderms is the development of a hydraulic
system to aid in movement or feeding. Called a water-
vascular system, this fluid-filled system is composed of a
central ring canal from which five radial canals extend out
into the body and arms.
Many of the most familiar animals seen along the
seashore, sea stars (starfish), brittle stars, sea urchins, sand
dollars, and sea cucumbers, are echinoderms. All are radi-
ally symmetrical as adults. While some other kinds of ani-
mals are radially symmetrical, none have the complex
organ systems of adult echinoderms. Echinoderms are well
represented not only in the shallow
waters of the sea but also in its abyssal
depths. In the oceanic trenches, which
are the deepest regions of the oceans,
sea cucumbers account for more than
90% of the biomass! All of them are
bottom-dwellers except for a few
swimming sea cucumbers. The adults
range from a few millimeters to more
than a meter in diameter (for one
species of sea star) or in length (for a
species of sea cucumber).
There is an excellent fossil record
of the echinoderms, extending back
into the Cambrian. However, despite
this wealth of information, the origin
of echinoderms remains unclear.
They are thought to have evolved
from bilaterally symmetrical ances-
tors because echinoderm larvae are
bilateral. The radial symmetry that is
the hallmark of echinoderms develops later, in the adult
body. Many biologists believe that early echinoderms
were sessile and evolved radiality as an adaptation to the
sessile existence. Bilaterality is of adaptive value to an an-
imal that travels through its environment, while radiality
is of value to an animal whose environment meets it on
all sides. Echinoderms attached to the sea bottom by a
central stalk were once common, but only about 80 such
species survive today.
Echinoderms are a unique, exclusively marine group of
organisms in which deuterostome development and an
endoskeleton are seen for the first time.
936 Part XII Animal Diversity
47.2 Echinoderms are deuterostomes with an endoskeleton.
Sponges
Cnidarians
Flatworms
Nematodes
Mollusks Annelids
Arthropods
Echinoderms
Chordates
(a) (b) (c)
FIGURE 47.3
Diversity in echinoderms. (a) Sea star, Oreaster occidentalis (class Asteroidea), in the Gulf of California, Mexico. (b) Warty sea cucumber,
Parastichopus parvimensis (class Holothuroidea), Philippines. (c) Sea urchin (class Echinoidea).
Chapter 47 Echinoderms 937
Each tube foot has
a water-filled sac at
its base; when the sac
contracts, the tube foot
extends — as when you
squeeze a balloon.
Sea stars have
a delicate skin
stretched over a
calcium-rich
endoskeleton of
spiny plates.
Sea stars walk using a water
vascular system. Hundreds
of tube feet extend from the
bottom of each arm. When
suckers on the bottom of
the feet attach to the sea
floor, the animal's muscles
can pull against them to haul
itself along.
Tube
feet
Madreporite
Water
vascular
system
Ampulla
Radial
canal
Digestive
glands
Stomach
Gonad
Mouth
Anus
Sea stars often drop arms
when under attack and
rapidly grow new ones.
Amazingly, an arm can
sometimes regenerate a
whole animal!
PHYLUM ECHINODERMATA: Deuterostome development
and endoskeleton
Skeletal plates
Echinoderms have
deuterostome development
and are bilaterally symmetrical
as larvae. Adults have five-part
radial symmetry. They often
have five arms, or multiples
of five.
Sea stars reproduce
sexually. The gonads
lie in the ventral region
of each arm.
FIGURE 47.4
The evolution of deuterostome development and an endoskeleton. Echinoderms, such as sea stars (phylum Echinodermata), are
coelomates with a deuterostome pattern of development. A delicate skin stretches over an endoskeleton made of calcium-rich plates, often
fused into a continuous, tough, spiny layer.
Echinoderm Body Plan
The body plan of echinoderms undergoes a fundamental
shift during development. All echinoderms have secondary
radial symmetry, that is, they are bilaterally symmetrical
during larval development but become radially symmetrical
as adults. Because of their radially symmetrical bodies, the
usual terms used to describe an animal’s body are not ap-
plicable: dorsal, ventral, anterior, and posterior have no
meaning without a head or tail. Instead, the body structure
of echinoderms is discussed in reference to their mouths
which are located on the oral surface. Most echinoderms
crawl along on their oral surfaces, although in sea cucum-
bers and some other echinoderms, the animal’s axis lies hor-
izontally and they crawl with the oral surface in front.
Echinoderms have a five-part body plan corresponding to
the arms of a sea star or the design on the “shell” of a sand
dollar. These animals have no head or brain. Their nervous
systems consist of a central nerve ring from which branches
arise. The animals are capable of complex response patterns,
but there is no centralization of function.
Endoskeleton
Echinoderms have a delicate epidermis, containing thou-
sands of neurosensory cells, stretched over an endoskeleton
composed of either movable or fixed calcium-rich (calcite)
plates called ossicles. The animals grow more or less contin-
uously, but their growth slows down with age. When the
plates first form, they are enclosed in living tissue. In some
echinoderms, such as asteroids and holothuroids, the ossi-
cles are widely scattered and the body wall is flexible. In
others, especially the echinoids, the ossicles become fused
and form a rigid shell. In many cases, these plates bear
spines. In nearly all species of echinoderms, the entire skele-
ton, even the long spines of sea urchins, is covered by a layer
of skin. Another important feature of this phylum is the
presence of mutable collagenous tissue which can range
from being tough and rubbery to weak and fluid. This
amazing tissue accounts for many of the special attributes of
echinoderms, such as the ability to rapidly autotomize body
parts. The plates in certain portions of the body of some
echinoderms are perforated by pores. Through these pores
extend tube feet, part of the water-vascular system that is a
unique feature of this phylum.
The Water-Vascular System
The water-vascular system of an echinoderm radiates from a
ring canal that encircles the animal’s esophagus. Five radial
canals, their positions determined early in the development
of the embryo, extend into each of the five parts of the body
and determine its basic symmetry (figure 47.5). Water en-
ters the water-vascular system through a madreporite, a
sievelike plate on the animal’s surface, and flows to the ring
canal through a tube, or stone canal, so named because of
938 Part XII Animal Diversity
Madreporite
Stone
canal
Tube
feet
Ring canal
Radial canal
Lateral canal
Ampulla
Radial nerve
Tube foot
Radial canal
Ampulla
Gonad
Digestive gland
Skeletal plate
Papula (gill)
FIGURE 47.5
The water-vascular system of an echinoderm. Radial canals allow water to flow into the tube feet. As the ampulla in each tube foot
contracts, the tube extends and can attach to the substrate. Subsequently, muscles in the tube feet contract bending the tube foot and
pulling the animal forward.
the surrounding rings of calcium carbonate. The five radial
canals in turn extend out through short side branches into
the hollow tube feet (figure 47.6). In some echinoderms,
each tube foot has a sucker at its end; in others, suckers are
absent. At the base of each tube foot is a muscular sac, the
ampulla, which contains fluid. When the ampulla contracts,
the fluid is prevented from entering the radial canal by a
one-way valve and is forced into the tube foot, thus extend-
ing it. When extended, the foot can attach itself to the sub-
strate. Longitudinal muscles in the tube foot wall then con-
tract causing the tube feet to bend. Relaxation of the
muscles in the ampulla allows the fluid to flow back into the
ampulla which moves the foot. By the concerted action of a
very large number of small individually weak tube feet, the
animal can move across the sea floor.
Sea cucumbers (see figure 47.3b) usually have five rows of
tube feet on the body surface that are used in locomotion.
They also have modified tube feet around their mouth cav-
ity that are used in feeding. In sea lilies, the tube feet arise
from the branches of the arms, which extend from the mar-
gins of an upward-directed cup. With these tube feet, the
animals take food from the surrounding water. In brittle
stars (see figure 47.1), the tube feet are pointed and special-
ized for feeding.
Body Cavity
In echinoderms, the coelom, which is proportionately large,
connects with a complicated system of tubes and helps pro-
vide circulation and respiration. In many echinoderms, res-
piration and waste removal occurs across the skin through
small, fingerlike extensions of the coelom called papulae (see
figure 47.5). They are covered with a thin layer of skin and
protrude through the body wall to function as gills.
Reproduction
Many echinoderms are able to regenerate lost parts, and
some, especially sea stars and brittle stars, drop various parts
when under attack. In a few echinoderms, asexual reproduc-
tion takes place by splitting, and the broken parts of sea stars
can sometimes regenerate whole animals. Some of the
smaller brittle stars, especially tropical species, regularly re-
produce by breaking into two equal parts; each half then re-
generates a whole animal.
Despite the ability of many echinoderms to break into
parts and regenerate new animals from them, most repro-
duction in the phylum is sexual and external. The sexes in
most echinoderms are separate, although there are few ex-
ternal differences. Fertilized eggs of echinoderms usually
develop into free-swimming, bilaterally symmetrical larvae
(figure 47.7), which differ from the trochophore larvae of
mollusks and annelids. These larvae form a part of the
plankton until they metamorphose through a series of stages
into the more sedentary adults.
Echinoderms are characterized by a secondary radial
symmetry and a five-part body plan. They have
characteristic calcium-rich plates called ossicles and a
unique water-vascular system that includes hollow tube
feet.
Chapter 47 Echinoderms 939
FIGURE 47.6
Tube feet. The nonsuckered tube feet of the sea star, Ludia
magnifica, are extended.
Anus
Cilia
Mouth
Gut
Developing coelom and
water vascular system
FIGURE 47.7
The free-swimming larva of an echinoderm. The bands of cilia
by which the larva moves are prominent in this drawing. Such
bilaterally symmetrical larvae suggest that the ancestors of the
echinoderms were not radially symmetrical, like the living
members of the phylum.
There are more than 20 extinct classes
of echinoderms and an additional 6 with
living members: (1) Crinoidea, sea lilies
and feather stars; (2) Asteroidea, sea
stars, or starfish; (3) Ophiuroidea, brittle
stars; (4) Echinoidea, sea urchins and
sand dollars; (5) Holothuroidea, sea cu-
cumbers, and (6) Concentricycloidea,
sea daisies. Sea daisies were recently dis-
covered living on submerged wood in
the deep sea.
Class Crinoidea: The
Sea Lilies and Feather
Stars
Sea lilies and feather stars, or crinoids
(class Crinoidea) differ from all other
living echinoderms in that the mouth
and anus are located on their upper
surface in an open disc. The two struc-
tures are connected by a simple gut.
These animals have simple excretory
and reproductive systems and an exten-
sive water-vascular system. The arms,
which are the food-gathering struc-
tures of crinoids, are located around
the margins of the disc. Different
species of crinoids may have from 5 to
more than 200 arms extending upward
from their bodies, with smaller struc-
tures called pinnules branching from
the arms. In all crinoids, the number of
arms is initially small. Species with
more than 10 arms add additional arms
progressively during growth. Crinoids
are filter feeders, capturing the micro-
scopic organisms on which they feed by
means of the mucus that coats their
tube feet, which are abundant on the
animals’ pinnules.
Scientists that study echinoderms
believe that the common ancestors of
this phylum were sessile, sedentary, ra-
dially symmetrical animals that resem-
bled crinoids. Crinoids were abundant
in ancient seas, and were present when
the Burgess Shale was deposited about
515 million years ago. More than 6000
fossil species of this class are known, in
comparison with the approximately 600
living species.
Sea Lilies
There are two basic crinoid body
plans. In sea lilies, the flower-shaped
body is attached to its substrate by a
stalk that is from 15 to 30 cm long, al-
though in some species the stalk may
be as much as a meter long (figure
47.8). Some fossil species had stalks up
to 20 meters long. If they are detached
from the substrate, some sea lilies can
move slowly by means of their feather-
like arms. All of the approximately 80
living species of sea lilies are found
below a depth of 100 meters in the
ocean. Sea lilies are the only living
echinoderms that are fully sessile.
Feather Stars
In the second group of crinoids, the
520 or so species of feather stars, the
disc detaches from the stalk at an
early stage of development (figure
47.9). Adult feather stars have long,
many-branched arms and usually an-
chor themselves to their substrate by
claw-like structures. However, some
feather stars are able to swim for
short distances, and many of them
can move along the substrate. Feather
stars range into shallower water than
do sea lilies, and only a few species of
either group are found at depths
greater than 500 meters. Along with
sea cucumbers, crinoids are the most
abundant and conspicuous large in-
vertebrates in the warm waters and
among the coral reefs of the western
Pacific Ocean. They have separate
sexes, with the sex organs simple
masses of cells in special cavities of
the arms and pinnules. Fertilization is
usually external, with the male and fe-
male gametes shed into the water, but
brooding—in which the female shel-
ters the young—occurs occasionally.
Crinoids, the sea lilies and feather
stars, were once far more
numerous. Crinoids are the only
echinoderms attached for much of
their lives to the sea bottom.
940 Part XII Animal Diversity
47.3 The six classes of echinoderms are all radially symmetrical as adults.
FIGURE 47.8
Sea lilies, Cenocrinus asterius. Two
specimens showing a typical parabola of
arms forming a “feeding net.” The water
current is flowing from right to left,
carrying small organisms to the stalked
crinoid’s arms. Prey, when captured, are
passed down the arms to the central mouth.
This photograph was taken at a depth of
about 400 meters in the Bahamas from the
Johnson-Sea-Link Submersible of the
Harbor Branch Foundation, Inc.
FIGURE 47.9
Feather star. This feather star is on the
Great Barrier Reef in Australia.
Class Asteroidea: The Sea Stars
Sea stars, or starfish (class Asteroidea; figure 47.10), are
perhaps the most familiar echinoderms. Among the most
important predators in many marine ecosystems, they
range in size from a centimeter to a meter across. They are
abundant in the intertidal zone, but they also occur at
depths as great as 10,000 meters. Around 1500 species of
sea stars occur throughout the world.
The body of a sea star is composed of a central disc that
merges gradually with the arms. Although most sea stars
have five arms, the basic symmetry of the phylum, mem-
bers of some families have many more, typically in multi-
ples of five. The body is somewhat flattened, flexible, and
covered with a pigmented epidermis.
Endoskeleton
Beneath the epidermis is an endoskeleton of small calcium-
rich plates called ossicles, bound together with connective
tissue. From these ossicles project spines that make up the
spiny upper surface. Around the base of the spines are
minute, pincerlike pedicellariae, bearing tiny jaws manipu-
lated by muscles. These keep the body surface free of de-
bris and may aid in food capture.
The Water-Vascular System
A deep groove runs along the oral (bottom) surface of each
arm from the central mouth out to the tip of the arm. This
groove is bordered by rows of tube feet, which the animal
uses to move about. Within each arm, there is a radial canal
that connects the tube feet to a ring canal in the central
body. This system of piping is used by sea stars to power a
unique hydraulic system. Contraction of small chambers
called ampullae attached to the tube feet forces water into
the podium of the feet, extending them. Conversely, con-
traction of muscles in the tube foot retracts the podium,
forcing fluid back into the ampulla. Small muscles at the
end of each tube foot can raise the center of the disclike
end, creating suction when the foot is pressed against a
substrate. Hundreds of tube feet, moving in unison, pull
the arm along the surface.
Feeding
The mouth of a sea star is located in the center of its oral
surface. Some sea stars have an extraordinary way of feed-
ing on bivalve mollusks. They can open a small gape be-
tween the shells of bivalves by exerting a muscular pull on
the shells (figure 47.11). Eventually, muscular fatigue in the
bivalve results in a very narrow gape, sufficient enough for
the sea star to insert its stomach out through its mouth into
the bivalve. Within the mollusk, the sea star secretes its di-
gestive enzymes and digests the soft tissues of its prey, re-
tracting its stomach when the process is complete.
Reproduction
Most sea stars have separate sexes, with a pair of gonads
lying in the ventral region inside each arm. Eggs and sperm
are shed into the water so that fertilization is external. In
some species, fertilized eggs are brooded in special cavities
or simply under the animal. They mature into larvae that
swim by means of conspicuous bands of cilia.
Sea stars, also called starfish, are five-armed, mobile
predators.
Chapter 47 Echinoderms 941
FIGURE 47.10
Class Asteroidea. This class includes the familiar starfish, or sea
stars.
FIGURE 47.11
A sea star attacking a clam. The tube feet, each of which ends in
a suction cup, are located along grooves on the underside of the
arms.
Class Ophiuroidea:
The Brittle Stars
Brittle stars (class Ophiuroidea; figure
47.12) are the largest class of echino-
derms in numbers of species (about
2000) and they are probably the most
abundant also. Secretive, they avoid
light and are more active at night.
Brittle stars have slender, branched
arms. The most mobile of echino-
derms, brittle stars move by pulling
themselves along, “rowing” over the
substrate by moving their arms, often
in pairs or groups, from side to side.
Some brittle stars use their arms to
swim, a very unusual habit among
echinoderms.
Brittle stars feed by capturing sus-
pended microplankton and organic de-
tritus with their tube feet, climbing
over objects on the ocean floor. In ad-
dition, the tube feet are important sen-
sory organs and assist in directing food
into the mouth once the animal has
captured it. As implied by their com-
mon name, the arms of brittle stars de-
tach easily, a characteristic that helps to
protect the brittle stars from their
predators.
Like sea stars, brittle stars have five
arms. More closely related to the sea
stars than to the other classes of the
phylum, on closer inspection they are
surprisingly different. They have no
pedicellariae, as sea stars have, and the
groove running down the length of
each arm is closed over and covered
with ossicles. Their tube feet lack am-
pullae, have no suckers, and are used
for feeding, not locomotion.
Brittle stars usually have separate
sexes, with the male and female ga-
metes in most species being released
into the water and fusing there. De-
velopment takes place in the plankton
and the larvae swim and feed using
elaborate bands of cilia. Some species
brood their young in special cavities
and fully developed juvenile brittle
stars emerge at the end of
development.
Brittle stars, very secretive, pull
themselves along with their arms.
Class Echinoidea:
The Sea Urchins
and Sand Dollars
The members of the class Echinoidea,
sand dollars and sea urchins, lack dis-
tinct arms but have the same five-part
body plan as all other echinoderms
(figure 47.13). Five rows of tube feet
protrude through the plates of the
calcareous skeleton, and there are also
openings for the mouth and anus.
These different openings can be seen
in the globular skeletons of sea
urchins and in the flat skeletons of
sand dollars. Both types of endoskele-
ton, often common along the
seashore, consist of fused calcareous
plates. About 950 living species con-
stitute the class Echinoidea.
Echinoids walk by means of their
tube feet or their movable spines,
which are hinged to the skeleton by a
joint that makes free rotation possible.
Sea urchins and sand dollars move
along the sea bottom, feeding on algae
and small fragments of organic mater-
ial. They scrape these off the substrate
with the large, triangular teeth that
ring their mouths. The gonads of sea
urchins are considered a great delicacy
by people in different parts of the
world. Because of their calcareous
plates, sea urchins and sand dollars are
well preserved in the fossil record,
with more than 5000 additional species
described.
As with most other echinoderms,
the sexes of sea urchins and sand dol-
lars are separate. The eggs and sperm
are shed separately into the water,
where they fuse. Some brood their
young, and others have free-
swimming larvae, with bands of cilia
extending onto their long, graceful
arms.
Sand dollars and sea urchins lack
arms but have a five-part symmetry.
942 Part XII Animal Diversity
FIGURE 47.12
Class Ophiuroidea. Brittle stars crawl
actively across their marine substrates.
FIGURE 47.13
Class Echinoidea. (a) Sand dollar,
Echinarachnius parma. (b) Giant red sea
urchin, Strongylocentrotus franciscanus.
(a)
(b)
Classes Holothuroidea and
Concentricycloidea: Sea Cucumbers
and Sea Daisies
Sea Cucumbers
Sea cucumbers (class Holothuroidea) are shaped somewhat
like their plant namesakes. They differ from the preceding
classes in that they are soft, sluglike organisms, often with
a tough, leathery outside skin (figure 47.14). The class
consists of about 1500 species found worldwide. Except for
a few forms that swim, sea cucumbers lie on their sides at
the bottom of the ocean. Their mouth is located at one
end and is surrounded by eight to 30 modified tube feet
called tentacles; the anus is at the other end. The tentacles
around the mouth may secrete mucus, used to capture the
small planktonic organisms on which the animals feed.
Each tentacle is periodically wiped off within the esopha-
gus and then brought out again, covered with a new supply
of mucus.
Sea cucumbers are soft because their calcareous skele-
tons are reduced to widely separated microscopic plates.
These animals have extensive internal branching systems,
called respiratory trees, which arise from the cloaca, or
anal cavity. Water is pulled into and expelled from the
respiratory tree by contractions of the cloaca; gas ex-
change takes place as this process occurs. The sexes of
most cucumbers are separate, but some of them are
hermaphroditic.
Most kinds of sea cucumbers have tube feet on the body
in addition to tentacles. These additional tube feet, which
might be restricted to five radial grooves or scattered over
the surface of the body, may enable the animals to move
about slowly. On the other hand, sea cucumbers may sim-
ply wriggle along whether or not they have additional tube
feet. Most sea cucumbers are quite sluggish, but some, es-
pecially among the deep-sea forms, swim actively. Sea cu-
cumbers, when irritated, sometimes eject a portion of their
intestines by a strong muscular contraction that may send
the intestinal fragments through the anus or even rupture
the body wall.
Sea Daisies
The most recently described class of echinoderms (1986),
sea daisies are strange little disc-shaped animals, less than 1
cm in diameter, discovered in waters over 1000 m deep off
New Zealand (figure 47.15). Only two species are known so
far. They have five-part radial symmetry, but no arms.
Their tube feet are located around the periphery of the
disc, rather than along radial lines, as in other echinoderms.
One species has a shallow, saclike stomach but no intestine
or anus; the other species has no digestive tract at all—the
surface of its mouth is covered by a membrane through
which it apparently absorbs nutrients.
Sea cucumbers are soft-bodied, sluglike animals
without arms. The newly discovered sea daisies are the
most mysterious echinoderms. Tiny and simple in form,
they live at great depths, absorbing food from their
surroundings.
Chapter 47 Echinoderms 943
FIGURE 47.14
Class Holothuroidea. Sea cucumber.
FIGURE 47.15
Class Concentricycloidea. Sea daisy.
944 Part XII Animal Diversity
Chapter 47
Summary Questions Media Resources
47.1 The embryos of deuterostomes develop quite differently from those of protostomes.
? The two major evolutionary lines of coelomate
animals—the protostomes and the deuterostomes—
are both represented among the oldest known fossils
of multicellular animals, dating back some 650
million years.
? In the protostomes, the mouth develops from or near
the blastopore, and the early divisions of the embryo
are spiral. At early stages of development, the fate of
the individual cells is already determined, and they
cannot develop individually into a whole animal.
? In the deuterostomes, the anus develops from or near
the blastopore, and the mouth forms subsequently on
another part of the gastrula. The early divisions of
the embryo are radial. At early stages of development,
each cell of the embryo can differentiate into a whole
animal.
1. What patterns of embryonic
development related to cleavage
and the blastopore occur in
protostome coelomates? What
patterns occur in deuterostome
coelomates?
2. Which major coelomate phyla
are protostomes and which are
deuterostomes? How does the
early developmental fate of cells
differ between the two groups?
How is the development of the
coelom from mesodermal tissue
different between them?
? Echinoderms are exclusively marine deuterostomes
that are radially symmetrical as adults.
? The epidermis of an echinoderm stretches over an
endoskeleton made of separate or fused calcium-rich
plates.
? Echinoderms use a unique water-vascular system that
includes tube feet for locomotion and feeding.
3. What type of symmetry and
body plan do adult echinoderms
exhibit?
4. What is the composition and
location of the echinoderm
skeleton?
5. How do echinoderms respire?
How developed is their digestive
system?
47.2 Echinoderms are deuterostomes with an endoskeleton.
? Crinoids are sessile for some or all of their lives and
have a mouth and anus located on the upper surface
of the animal.
? Sea stars are active predators that move about on
their tube feet.
? Brittle stars use their tube feet for feeding and move
about using two arms at a time.
? The endoskeletons of sea urchins and sand dollars
consist of fused calcareous plates that have been well
preserved in the fossil record.
? The endoskeletons of sea cucumbers are drastically
reduced and separated, making them soft-bodied.
? Sea daisies are a newly described class of echinoderms
with disc-shaped bodies.
6. In what two ways do members
of the phylum Echinodermata
reproduce? What type of larva
do they possess?
7. How do sea cucumbers
superficially differ from other
echinoderms? How are some of
their tube feet specially
modified? What is the extent of
their skeleton? What is the
function of their unique
respiratory tree? How is their
reproduction different from that
of other echinoderms?
47.3 The six classes of echinoderms are all radially symmetrical as adults.
www.mhhe.com/raven6e www.biocourse.com
? Echinoderms
945
48
Vertebrates
Concept Outline
48.1 Attaching muscles to an internal framework
greatly improves movement.
The Chordates. Chordates have an internal flexible rod,
the first stage in the evolution of a truly internal skeleton.
48.2 Nonvertebrate chordates have a notochord but
no backbone.
The Nonvertebrate Chordates. Lancelets are thought
to resemble the ancestors of vertebrates.
48.3 The vertebrates have an interior framework of
bone.
Characteristics of Vertebrate. Vertebrates have a true,
usually bony endoskeleton, with a backbone encasing the
spinal column, and a skull-encased brain.
48.4 The evolution of vertebrates involves invasions of
sea, land, and air.
Fishes. Over half of all vertebrate species are fishes,
which include the group from which all other vertebrates
evolved.
History of the Fishes. Swim bladders have made bony
fishes a particularly successful group.
Amphibians. The key innovation that made life on land
possible for vertebrates was the pulmonary vein.
History of the Amphibians. In the past, amphibians
were far more diverse, and included many large, armored
terrestrial forms.
Reptiles. Reptiles were the first vertebrates to completely
master the challenge of living on dry land.
The Rise and Fall of Dominant Reptile Groups. Now-
extinct forms of reptiles dominated life on land for 250
million years. Four orders survive today.
Birds. Birds are much like reptiles, but with feathers.
History of the Birds. Birds are thought to have evolved
from dinosaurs with adaptations of feathers and flight.
Mammals. Mammals are the only vertebrates that possess
hair and milk glands.
History of the Mammals. Mammals evolved at the same
time as dinosaurs, but only became common when
dinosaurs disappeared.
M
embers of the phylum Chordata (figure 48.1) exhibit
great improvements in the endoskeleton over what is
seen in echinoderms. As we saw in the previous chapter,
the endoskeleton of echinoderms is functionally similar to
the exoskeleton of arthropods; it is a hard shell that encases
the body, with muscles attached to its inner surface. Chor-
dates employ a very different kind of endoskeleton, one
that is truly internal. Members of the phylum Chordata are
characterized by a flexible rod that develops along the back
of the embryo. Muscles attached to this rod allowed early
chordates to swing their backs from side to side, swimming
through the water. This key evolutionary advance, attach-
ing muscles to an internal element, started chordates along
an evolutionary path that led to the vertebrates—and, for
the first time, to truly large animals.
FIGURE 48.1
A typical vertebrate. Today mammals, like this snow leopard,
Panthera uncia, dominate vertebrate life on land, but for over 200
million years in the past they were a minor group in a world
dominated by reptiles.
946 Part XII Animal Diversity
The Chordates
Chordates (phylum Chordata) are
deuterostome coelomates whose near-
est relations in the animal kingdom
are the echinoderms, the only other
deuterostomes. However, unlike
echinoderms, chordates are character-
ized by a notochord, jointed appendages,
and segmentation. There are some
43,000 species of chordates, a phylum
that includes birds, reptiles, amphib-
ians, fishes, and mammals.
Four features characterize the chor-
dates and have played an important
role in the evolution of the phylum
(figure 48.2):
1. A single, hollow nerve cord
runs just beneath the dorsal sur-
face of the animal. In verte-
brates, the dorsal nerve cord differentiates into the
brain and spinal cord.
2. A flexible rod, the notochord, forms on the dorsal
side of the primitive gut in the early embryo and is
present at some developmental stage in all chor-
dates. The notochord is located just below the
nerve cord. The notochord may persist throughout
the life cycle of some chordates or be displaced dur-
ing embryological development as in most verte-
brates by the vertebral column that forms around
the nerve cord.
3. Pharyngeal slits connect the pharynx, a muscular
tube that links the mouth cavity and the esophagus,
with the outside. In terrestrial vertebrates, the slits do
not actually connect to the outside and are better
termed pharyngeal pouches. Pharyngeal pouches are
present in the embryos of all vertebrates. They be-
come slits, open to the outside in animals with gills,
but disappear in those lacking gills. The presence of
these structures in all vertebrate embryos provides ev-
idence to their aquatic ancestry.
4. Chordates have a postanal tail that extends beyond
the anus, at least during their embryonic develop-
ment. Nearly all other animals have a terminal anus.
All chordates have all four of these characteristics at
some time in their lives. For example, humans have pha-
ryngeal slits, a dorsal nerve cord, and a notochord as em-
bryos. As adults, the nerve cord remains while the noto-
chord is replaced by the vertebral column and all but one
pair of pharyngeal slits are lost. This remaining pair
forms the Eustachian tubes that connect the throat to the
middle ear.
48.1 Attaching muscles to an internal framework greatly improves movement.
Sponges
Cnidarians
Flatworms
Nematodes
Mollusks Annelids
Arthropods
Echinoderms
Chordates
Postanal
tail
Notochord
Hollow dorsal
nerve cord
Pharyngeal
pouches
FIGURE 48.2
Some of the principal features of the
chordates, as shown in a generalized
embryo.
FIGURE 48.3
A mouse embryo. At 11.5 days of development, the mesoderm is
already divided into segments called somites (stained dark in this
photo), reflecting the fundamentally segmented nature of all
chordates.
A number of other characteristics also distinguish the
chordates fundamentally from other animals. Chordates’
muscles are arranged in segmented blocks that affect the
basic organization of the chordate body and can often be
clearly seen in embryos of this phylum (figure 48.3).
Most chordates have an internal skeleton against which
the muscles work. Either this internal skeleton or the no-
tochord (figure 48.4) makes possible the extraordinary
powers of locomotion that characterize the members of
this group.
Chordates are characterized by a hollow dorsal nerve
cord, a notochord, pharyngeal gill slits, and a postanal
tail at some point in their development. The flexible
notochord anchors internal muscles and allows rapid,
versatile movement.
Chapter 48 Vertebrates 947
PHYLUM CHORDATA: Notochord
In a lancelet, the simplest chordate, the
flexible notochord persists throughout
life and aids swimming by giving muscles
something to pull against. In the lancelet
these muscles form a series of discrete
blocks that can easily be seen. More
advanced chordates have jointed appendages.
Lancelets are filter-feeders
with highly reduced
sensory systems. The
animal has no head, eyes,
ears, or nose. Instead,
sensory cells that detect
chemicals line the oral
tentacles.
Lancelets feed on microscopic
protists caught by filtering them
through cilia and gills on the
pharyngeal slits. As the cilia that
line the front end of the
gut passage beat, they draw water
through the mouth, through the
pharynx, and out the slits.
Unlike that of vertebrates, the
skin of a lancelet has only a
single layer of cells.
Lancelets lack pigment in their
skins, and so are transparent.
Notochord
Water
Oral hood
with tentacles
Gill slits
in pharynx
Atrium
Atriopore
Anus
Intestine
Dorsal nerve
cord
FIGURE 48.4
Evolution of a notochord. Vertebrates, tunicates, and lancelets are chordates (phylum Chordata), coelomate animals with a flexible rod,
the notochord, that provides resistance to muscle contraction and permits rapid lateral body movements. Chordates also possess
pharyngeal slits (reflecting their aquatic ancestry and present habitat in some) and a hollow dorsal nerve cord. In vertebrates, the
notochord is replaced during embryonic development by the vertebral column.
The Nonvertebrate Chordates
Tunicates
The tunicates (subphylum Urochordata) are a group of
about 1250 species of marine animals. Most of them are
sessile as adults (figure 48.5a,b), with only the larvae hav-
ing a notochord and nerve cord. As adults, they exhibit
neither a major body cavity nor visible signs of segmenta-
tion. Most species occur in shallow waters, but some are
found at great depths. In some tunicates, adults are colo-
nial, living in masses on the ocean floor. The pharynx is
lined with numerous cilia, and the animals obtain their
food by ciliary action. The cilia beat, drawing a stream of
water into the pharynx, where microscopic food particles
are trapped in a mucous sheet secreted from a structure
called an endostyle.
The tadpolelike larvae of tunicates plainly exhibit all of
the basic characteristics of chordates and mark the tuni-
cates as having the most primitive combination of features
found in any chordate (figure 48.5c). The larvae do not
feed and have a poorly developed gut. They remain free-
swimming for only a few days before settling to the bot-
tom and attaching themselves to a suitable substrate by
means of a sucker.
Tunicates change so much as they mature and adjust
developmentally to a sessile, filter-feeding existence that
it would be difficult to discern their evolutionary rela-
tionships by examining an adult. Many adult tunicates se-
crete a tunic, a tough sac composed mainly of cellulose.
The tunic surrounds the animal and gives the subphylum
its name. Cellulose is a substance frequently found in the
cell walls of plants and algae but is rarely found in ani-
948 Part XII Animal Diversity
48.2 Nonvertebrate chordates have a notochord but no backbone.
Heart
Pharynx
Endostyle
Gill slit
Tunic
Gonad
Incurrent siphon
Excurrent
Stomach
Stomach
Genital duct
Intestine
Nerve ganglion
Hypophyseal duct
siphon
Heart
Pharynx
with gill slits
Notochord
Dorsal nerve cord
Atriopore
(excurrent siphon)
Mouth
(incurrent siphon)
(b)
(c)
(a)
FIGURE 48.5
Tunicates (phylum Chordata, subphylum Urochordata).
(a) The sea peach, Halocynthia auranthium. (b) Diagram of the
structure of an adult tunicate. (c) Diagram of the structure of a
larval tunicate, showing the characteristic tadpolelike form. Larval
tunicates resemble the postulated common ancestor of the
chordates.
mals. In colonial tunicates, there may
be a common sac and a common
opening to the outside. There is a
group of Urochordates, the Larvacea,
which retains the tail and notochord
into adulthood. One theory of verte-
brate origins involves a larval form,
perhaps that of a tunicate, which ac-
quires the ability to reproduce.
Lancelets
Lancelets are scaleless, fishlike marine
chordates a few centimeters long that
occur widely in shallow water
throughout the oceans of the world.
Lancelets (subphylum Cephalochor-
data) were given their English name
because they resemble a lancet—a
small, two-edged surgical knife. There
are about 23 species of this subphy-
lum. Most of them belong to the
genus Branchiostoma, formerly called
Amphioxus, a name still used widely. In
lancelets, the notochord runs the en-
tire length of the dorsal nerve cord
and persists throughout the animal’s
life.
Lancelets spend most of their time
partly buried in sandy or muddy substrates, with
only their anterior ends protruding (figure 48.6).
They can swim, although they rarely do so. Their
muscles can easily be seen as a series of discrete
blocks. Lancelets have many more pharyngeal gill
slits than fishes, which they resemble in overall
shape. They lack pigment in their skin, which has
only a single layer of cells, unlike the multilayered
skin of vertebrates. The lancelet body is pointed at
both ends. There is no distinguishable head or
sensory structures other than pigmented light re-
ceptors.
Lancelets feed on microscopic plankton, using a
current created by beating cilia that lines the oral
hood, pharynx, and gill slits (figure 48.7). The gill
slits provide an exit for the water and are an adaptation for
filter feeding. The oral hood projects beyond the mouth
and bears sensory tentacles, which also ring the mouth.
Males and females are separate, but no obvious external dif-
ferences exist between them.
Biologists are not sure whether lancelets are primitive or
are actually degenerate fishes whose structural features
have been reduced and simplified during the course of evo-
lution. The fact that lancelets feed by means of cilia and
have a single-layered skin, coupled with distinctive features
of their excretory systems, suggest that this is an ancient
group of chordates. The recent discovery of fossil forms
similar to living lancelets in rocks 550 million years old—
well before the appearance of any fishes—also argues for
the antiquity of this group. Recent studies by molecular
systematists further support the hypothesis that lancelets
are vertebrates’ closest ancestors.
Nonvertebrate chordates, including tunicates and
lancelets, have notochords but not vertebrae. They are
the closest relatives of vertebrates.
Chapter 48 Vertebrates 949
FIGURE 48.6
Lancelets. Two lancelets, Branchiostoma lanceolatum (phylum Chordata, subphylum
Cephalochordata), partly buried in shell gravel, with their anterior ends protruding. The
muscle segments are clearly visible; the square objects along the side of the body are
gonads, indicating that these are male lancelets.
Atrium AtrioporeGill slits
in pharynx
Oral hood
with tentacles
Notochord
Intestine
Dorsal
nerve cord
Anus
Gonad
FIGURE 48.7
The structure of a lancelet. This diagram shows the path through which
the lancelet’s cilia pull water.
Characteristics of Vertebrates
Vertebrates (subphylum Vertebrata) are chordates with a
spinal column. The name vertebrate comes from the indi-
vidual bony segments called vertebrae that make up the
spine. Vertebrates differ from the tunicates and lancelets in
two important respects:
1. Vertebral column. In vertebrates, the notochord is
replaced during the course of embryonic develop-
ment by a bony vertebral column. The column is a
series of bones that encloses and protects the dorsal
nerve cord like a sleeve (figure 48.8).
2. Head. In all vertebrates but the earliest fishes, there
is a distinct and well-differentiated head, with a skull
and brain. For this reason, the vertebrates are some-
times called the craniate chordates (Greek kranion,
“skull”).
In addition to these two key characteristics, vertebrates
differ from other chordates in other important respects:
1. Neural crest. A unique group of embryonic cells
called the neural crest contributes to the development
of many vertebrate structures. These cells develop on
the crest of the neural tube as it forms by an invagina-
tion and pinching together of the neural plate (see
chapter 60 for a detailed account). Neural crest cells
then migrate to various locations in the developing
embryo, where they participate in the development of
a variety of structures.
2. Internal organs. Among the internal organs of ver-
tebrates, livers, kidneys, and endocrine glands are
characteristic of the group. The ductless endocrine
glands secrete hormones that help regulate many of
the body’s functions. All vertebrates have a heart and
a closed circulatory system. In both their circulatory
and their excretory functions, vertebrates differ
markedly from other animals.
3. Endoskeleton. The endoskeleton of most verte-
brates is made of cartilage or bone. Cartilage and
bone are specialized tissue containing fibers of the
protein collagen compacted together. Bone also
contains crystals of a calcium phosphate salt. Bone
forms in two stages. First, collagen is laid down in a
matrix of fibers along stress lines to provide flexibil-
ity, and then calcium minerals infiltrate the fibers,
providing rigidity. The great advantage of bone
over chitin as a structural material is that bone is
strong without being brittle. The vertebrate en-
doskeleton makes possible the great size and extra-
ordinary powers of movement that characterize this
group.
Overview of the Evolution of Vertebrates
The first vertebrates evolved in the oceans about 470 mil-
lion years ago. They were jawless fishes with a single caudal
fin. Many of them looked like a flat hot dog, with a hole at
one end and a fin at the other. The appearance of a hinged
jaw was a major advancement, opening up new food op-
tions, and jawed fishes became the dominant creatures in
the sea. Their descendants, the amphibians, invaded the
land. Salamander-like amphibians and other, much larger
now-extinct amphibians were the first vertebrates to live
successfully on land. Amphibians, in turn, gave rise to the
first reptiles about 300 million years ago. Within 50 million
years, reptiles, better suited than amphibians to living out
of water, replaced them as the dominant land vertebrates.
With the success of reptiles, vertebrates truly came to
dominate the surface of the earth. Many kinds of reptiles
evolved, ranging in size from smaller than a chicken to big-
950 Part XII Animal Diversity
48.3 The vertebrates have an interior framework of bone.
Ectoderm
Vertebral
body developing
around notochord
Neural tube
Notochord
Rib
Neural arch
Centrum
Forming
neural arch
Blood vessels
FIGURE 48.8
Embryonic development of a vertebra. During the course of
evolution of animal development, the flexible notochord is
surrounded and eventually replaced by a cartilaginous or bony
covering, the centrum. The neural tube is protected by an arch
above the centrum, and the vertebra may also have a hemal arch,
which protects major blood vessels below the centrum. The
vertebral column functions as a strong, flexible rod that the
muscles pull against when the animal swims or moves.
ger than a truck. Some flew, and others swam. Among
them evolved reptiles that gave rise to the two remaining
great lines of terrestrial vertebrates, birds (descendants of
the dinosaurs) and mammals. Dinosaurs and mammals ap-
pear at about the same time in the fossil record, 220 million
years ago. For over 150 million years, dinosaurs dominated
the face of the earth. Over all these centuries (think of it—
over a million centuries!) the largest mammal was no bigger
than a cat. Then, about 65 million years ago, the dinosaurs
abruptly disappeared, for reasons that are still hotly de-
bated. In their absence, mammals and birds quickly took
their place, becoming in turn abundant and diverse.
The history of vertebrates has been a series of evolution-
ary advances that have allowed vertebrates to first invade
the sea and then the land. In this chapter, we will examine
the key evolutionary advances that permitted vertebrates to
invade the land successfully. As you will see, this invasion
was a staggering evolutionary achievement, involving fun-
damental changes in many body systems.
Vertebrates are a diverse group, containing members
adapted to life in aquatic habitats, on land, and in the air.
There are eight principal classes of living vertebrates
(figure 48.9). Four of the classes are fishes that live in the
water, and four are land-dwelling tetrapods, animals
with four limbs. (The name tetrapod comes from two
Greek words meaning “four-footed.”) The extant classes
of fishes are the superclass Agnatha (the jawless fishes),
which includes the class Myxini, the hagfish, and the class
Cephalaspidomorphi, the lampreys; Chondrichthyes, the
cartilaginous fishes, sharks, skates, and rays; and Oste-
ichthyes, the bony fishes that are dominant today. The
four classes of tetrapods are Amphibia, the amphibians;
Reptilia, the reptiles; Aves, the birds; and Mammalia, the
mammals.
Vertebrates, the principal chordate group, are
characterized by a vertebral column and a distinct head.
Chapter 48 Vertebrates 951
500
400
300
200
100
0
Ordovician
(505–438)
Silurian
(438–408)
Devonian
(408–360)
Carboniferous
(360–280)
Permian
(280–248)
Triassic
(248–213)
Jurassic
(213–144)
Cretaceous
(144–65)
Tertiary
(65–2)
Quaternary
(2–Present)
T
ime
(millions of years ago)
Jawless
fishes
(two classes)
Amphibians
Mammals
Birds
Reptiles
Cartilaginous
fishes
Modern bony
fishes
Placoderms
(extinct)
Primitive amphibians
(extinct)
Primitive reptiles
(extinct)
Ostracoderms
(extinct)
Chordate ancestor
Acanthodians
(extinct)
FIGURE 48.9
Vertebrate family tree. Two classes of vertebrates comprise the Agnatha, or jawless fishes. Primitive amphibians arose from fish.
Primitive reptiles arose from amphibians and gave rise to mammals and to dinosaurs, which survive today as birds.
Fishes
Over half of all vertebrates are fishes. The most diverse and
successful vertebrate group (figure 48.10), they provided
the evolutionary base for invasion of land by amphibians.
In many ways, amphibians, the first terrestrial vertebrates,
can be viewed as transitional—fish out of water. In fact,
fishes and amphibians share many similar features, among
the host of obvious differences. First, let us look at the
fishes (table 48.1).
The story of vertebrate evolution started in the ancient
seas of the Cambrian Period (570 to 505 million years ago),
when the first backboned animals appeared (figure 48.11).
Wriggling through the water, jawless and toothless, these
first fishes sucked up small food particles from the ocean
floor like miniature vacuum cleaners. Most were less than a
foot long, respired with gills, and had no paired fins—just a
primitive tail to push them through the water. For 50 mil-
lion years, during the Ordovician Period (505 to 438 mil-
lion years ago), these simple fishes were the only verte-
brates. By the end of this period, fish had developed
primitive fins to help them swim and massive shields of
bone for protection. Jawed fishes first appeared during the
Silurian Period (438 to 408 million years ago) and along
with them came a new mode of feeding. Later, both the
cartilaginous and bony fishes appeared.
952 Part XII Animal Diversity
48.4 The evolution of vertebrates involves invasions of sea, land, and air.
Jawed fishes with heavily armored heads;
often quite large
Fishes with jaws; all now extinct; paired
fins supported by sharp spines
Most diverse group of vertebrates; swim
bladders and bony skeletons; paired fins
supported by bony rays
Largely extinct group of bony fishes;
ancestral to amphibians; paired lobed fins
Streamlined hunters; cartilaginous
skeletons; no swim bladders; internal
fertilization
Jawless fishes with no paired appendages;
scavengers; mostly blind, but a well-
developed sense of smell
Largely extinct group of jawless fishes
with no paired appendages; parasitic and
nonparasitic types; all breed in fresh water
Table 48.1 Major Classes of Fishes
Approximate
Typical Number of
Class Examples Key Characteristics Living Species
Placodermi
Acanthodii
Osteichthyes
Chondrichthyes
Myxini
Cephalaspidomorphi
FIGURE 48.10
Fish are diverse and include more species than all other kinds
of vertebrates combined.
Armored fishes
Spiny fishes
Ray-finned fishes
Lobe-finned fishes
Sharks, skates, rays
Hagfishes
Lampreys
Extinct
Extinct
20,000
7
850
43
17
Characteristics of Fishes
From whale sharks that are 18 meters long to tiny cich-
lids no larger than your fingernail, fishes vary consider-
ably in size, shape, color, and appearance. Some live in
freezing Arctic seas, others in warm freshwater lakes, and
still others spend a lot of time out of water entirely.
However varied, all fishes have important characteristics
in common:
1. Gills. Fishes are water-dwelling creatures and must
extract oxygen dissolved in the water around them.
They do this by directing a flow of water through
their mouths and across their gills. The gills are com-
posed of fine filaments of tissue that are rich in blood
vessels. They are located at the back of the pharynx
and are supported by arches of cartilage. Blood moves
through the gills in the opposite direction to the flow
of water in order to maximize the efficiency of oxygen
absorption.
2. Vertebral column. All fishes have an internal
skeleton with a spine surrounding the dorsal nerve
cord, although it may not necessarily be made of
bone. The brain is fully encased within a protective
box, the skull or cranium, made of bone or cartilage.
3. Single-loop blood circulation. Blood is pumped
from the heart to the gills. From the gills, the oxy-
genated blood passes to the rest of the body, then re-
turns to the heart. The heart is a muscular tube-pump
made of four chambers that contract in sequence.
4. Nutritional deficiencies. Fishes are unable to syn-
thesize the aromatic amino acids and must consume
them in their diet. This inability has been inherited
by all their vertebrate descendants.
Fishes were the first vertebrates to make their
appearance, and today they are still the largest
vertebrate class. They are the vertebrate group from
which all other vertebrates evolved.
Chapter 48 Vertebrates 953
550
500
450
400
350
300
250
200
150
100
50
0
Agnathans
Lamprey
Amphibians
Frog
Chondrichthyes
Shark
Acanthodians
(extinct)
Spiny fishes
Placoderms
(extinct)
Armored fishes
Ostracoderms
(extinct)
Shell-skinned
fishes
Osteichthyes
(lobe-finned fishes)
Coelacanth
Osteichthyes
(ray-finned fishes)
Perch
Cambrian
(570–505)
Ordovician
(505–438)
Silurian
(438–408)
Devonian
(408–360)
Carboniferous
(360–280)
Permian
(280–248)
Triassic
(248–213)
Jurassic
(213–144)
Cretaceous
(144–65)
Tertiary
(65–2)
Quaternary
(2–Present)
Time
(millions of years ago)
FIGURE 48.11
Evolution of the fishes. The evolutionary relationships among the different groups of fishes as well as between fishes and amphibians is
shown. The spiny and armored fishes that dominated the early seas are now extinct.
History of the Fishes
The First Fishes
The first fishes were members of
the five Ostracoderm orders (the
word means “shell-skinned”). Only
their head-shields were made of
bone; their elaborate internal skele-
tons were constructed of cartilage.
Many ostracoderms were bottom
dwellers, with a jawless mouth un-
derneath a flat head, and eyes on
the upper surface. Ostracoderms
thrived in the Ordovician Period
and in the period which followed,
the Silurian Period (438 to 408
million years ago), only to become
almost completely extinct at the
close of the following Devonian
Period (408 to 360 million years
ago). One group, the jawless Ag-
natha, survive today as hagfish and
parasitic lampreys (figure 48.12).
A fundamentally important evolutionary advance oc-
curred in the late Silurian Period, 410 million years ago—
the development of jaws. Jaws evolved from the most ante-
rior of a series of arch-supports made of cartilage that were
used to reinforce the tissue between gill slits, holding the
slits open (figure 48.13). This transformation was not as
radical as it might at first appear. Each gill arch was formed
by a series of several cartilages (later to become bones)
arranged somewhat in the shape of a V turned on its side,
with the point directed outward. Imagine the fusion of the
front pair of arches at top and bottom, with hinges at the
points, and you have the primitive vertebrate jaw. The top
half of the jaw is not attached to the skull directly except at
the rear. Teeth developed on the jaws from modified scales
on the skin that lined the mouth.
Armored fishes called placoderms and spiny fishes called
acanthodians both had jaws. Spiny fishes were very com-
mon during the early Devonian,
largely replacing ostracoderms, but
became extinct themselves at the close
of the Permian. Like ostracoderms,
they had internal skeletons made of
cartilage, but their scales contained
small plates of bone, foreshadowing
the much larger role bone would play
in the future of vertebrates. Spiny
fishes were predators and far better
swimmers than ostracoderms, with as
many as seven fins to aid them swim-
ming. All of these fins were reinforced
with strong spines, giving these fishes
their name. No spiny fishes survive
today.
By the mid-Devonian, the heavily armored placoderms
became common. A very diverse and successful group,
seven orders of placoderms dominated the seas of the late
Devonian, only to become extinct at the end of that period.
The front of the placoderm body was more heavily ar-
mored than the rear. The placoderm jaw was much im-
proved from the primitive jaw of spiny fishes, with the
upper jaw fused to the skull and the skull hinged on the
shoulder. Many of the placoderms grew to enormous sizes,
some over 30 feet long, with two-foot skulls that had an
enormous bite.
954 Part XII Animal Diversity
Cartilaginous fishes
Bony fishes
Reptiles
Birds
Mammals
Amphibians
Jawless fishes
FIGURE 48.12
Specialized mouth of a lamprey.
Lampreys use their suckerlike mouths to
attach themselves to the fishes on which
they prey. When they have done so, they
bore a hole in the fish with their teeth and
feed on its blood.
Skull
Gill slits
Anterior gill arches
FIGURE 48.13
Evolution of the jaw. Jaws evolved from the anterior gill arches of ancient, jawless fishes.
The Rise of Active Swimmers
At the end of the Devonian, essen-
tially all of these pioneer vertebrates
disappeared, replaced by sharks and
bony fishes. Sharks and bony fishes
first evolved in the early Devonian,
400 million years ago. In these
fishes, the jaw was improved even
further, with the first gill arch be-
hind the jaws being transformed
into a supporting strut or prop, join-
ing the rear of the lower jaw to the
rear of the skull. This allowed the
mouth to open very wide, into al-
most a full circle. In a great white
shark, this wide-open mouth can be
a very efficient weapon.
The major factor responsible for
the replacement of primitive fishes
by sharks and bony fishes was that
they had a superior design for swimming. The typical shark
and bony fish is streamlined. The head of the fish acts as a
wedge to cleave through the water, and the body tapers
back to the tail, allowing the fish to slip through the water
with a minimum amount of turbulence.
In addition, sharks and bony fishes have an array of mo-
bile fins that greatly aid swimming. First, there is a propul-
sion fin: a large and efficient tail (caudal) fin that helps
drive the fish through the water when it is swept side-to-
side, pushing against the water and thrusting the fish for-
ward. Second, there are stabilizing fins: one (or sometimes
two) dorsal fins on the back that act as a stabilizer to pre-
vent rolling as the fish swims through the water, while an-
other ventral fin acts as a keel to prevent side-slip. Third,
there are the paired fins at shoulder and hip (“A fin at each
corner”), consisting of a front (pectoral) pair and a rear
(pelvic) pair. These fins act like the elevator flaps of an air-
plane to assist the fish in going up or down through the
water, as rudders to help it turn sharply left or right, and as
brakes to help it stop quickly.
Sharks Become Top Predators
In the period following the Devonian, the Carboniferous
Period (360 to 280 million years ago), sharks became the
dominant predator in the sea. Sharks (class Chon-
drichythes) have a skeleton made of cartilage, like primitive
fishes, but it is “calcified,” strengthened by granules of cal-
cium carbonate deposited in the outer layers of cartilage.
The result is a very light and strong skeleton. Streamlined,
with paired fins and a light, flexible skeleton, sharks are su-
perior swimmers (figure 48.14). Their pectoral fins are par-
ticularly large, jutting out stiffly like airplane wings—and
that is how they function, adding lift to compensate for the
downward thrust of the tail fin. Very aggressive predators,
some sharks reached enormous size.
Sharks were among the first vertebrates to develop
teeth. These teeth evolved from rough scales on the skin
and are not set into the jaw, as yours are, but rather sit
atop it. The teeth are not firmly anchored and are easily
lost. In a shark’s mouth, the teeth are arrayed in up to 20
rows, the teeth in front doing the biting and cutting,
while behind them other teeth grow and await their turn.
When a tooth breaks or is worn down, a replacement
from the next row moves forward. One shark may even-
tually use more than 20,000 teeth. This programmed loss
of teeth offers a great advantage: the teeth in use are al-
ways new and sharp. The skin is covered with tiny teeth-
like scales, giving it a rough “sandpaper” texture. Like
the teeth, these scales are constantly replaced throughout
the shark’s life.
Reproduction among the Chondrichythes is the most
advanced of any fishes. Shark eggs are fertilized internally.
During mating, the male grasps the female with modified
fins called claspers. Sperm run from the male into the fe-
male through grooves in the claspers. Although a few
species lay fertilized eggs, the eggs of most species develop
within the female’s body, and the pups are born alive.
Many of the early evolutionary lines of sharks died out
during the great extinction at the end of the Permian Pe-
riod (280 to 248 million years ago). The survivors thrived
and underwent a burst of diversification during the Meso-
zoic era, when most of the modern groups of sharks ap-
peared. Skates and rays (flattened sharks that are bottom-
dwellers) evolved at this time, some 200 million years
after the sharks first appeared. Sharks competed success-
fully with the marine reptiles of that time and are still the
dominant predators of the sea. Today there are 275
species of sharks, more kinds than existed in the
Carboniferous.
Chapter 48 Vertebrates 955
Jawless fishes
Bony fishes
Reptiles
Birds
Mammals
Amphibians
Cartilaginous fishes
FIGURE 48.14
Chondrichthyes. Members of the class
Chondrichthyes, such as this bull shark, are
mainly predators or scavengers and spend
most of their time in graceful motion. As
they move, they create a flow of water past
their gills, extracting oxygen from the
water.
Bony Fishes Dominate the
Water
Bony fishes (members of the class Os-
teichthyes, figure 48.15) evolved at the
same time as sharks, some 400 million
years ago, but took quite a different
evolutionary road. Instead of gaining
speed through lightness, as sharks did,
bony fishes adopted a heavy internal
skeleton made completely of bone.
Such an internal skeleton is very
strong, providing a base against which
very strong muscles could pull. The
process of ossification (the evolutionary
replacement of cartilage by bone) hap-
pened suddenly in evolutionary terms,
completing a process started by sharks,
who lay down a thin film of bone over
their cartilage. Not only is the internal
skeleton ossified, but also the external
skeleton, the outer covering of plates and scales. Many scien-
tists believe bony fishes evolved from spiny sharks, which
also had bony plates set in their skin. Bony fishes are the
most successful of all fishes, indeed of all vertebrates. There
are several dozen orders containing more than 20,000 living
species.
Unlike sharks, bony fishes evolved in fresh water. The
most ancient fossils of bony fishes are found in freshwater
lake beds from the middle Devonian. These first bony
fishes were small and possessed paired air sacs connected to
the back of the throat. These sacs could be inflated with air
to buoy the fish up or deflated to sink it down in the water.
Most bony fishes have highly mobile fins, very thin
scales, and completely symmetrical tails (which keep the
fish on a straight course as it swims through the water).
This is a very successful design for a fish. Two great groups
arose from these pioneers: the lobe-finned fishes, ancestors
of the first tetrapods, and the ray-finned fishes, which in-
clude the vast majority of today’s fishes.
The characteristic feature of all ray-finned fishes is an in-
ternal skeleton of parallel bony rays that support and stiffen
each fin. There are no muscles within the fins; they are
moved by muscles within the body. In ray-finned fishes, the
primitive air sacs are transformed into an air pouch, which
provides a remarkable degree of control over buoyancy.
Important Adaptations of Bony Fishes
The remarkable success of the bony fishes has resulted
from a series of significant adaptations that have enabled
them to dominate life in the water. These include the swim
bladder, lateral line system, and gill cover.
Swim Bladder. Although bones are heavier than carti-
laginous skeletons, bony fishes are still buoyant because
they possess a swim bladder, a gas-filled
sac that allows them to regulate their
buoyant density and so remain sus-
pended at any depth in the water effort-
lessly (figure 48.16). Sharks, by contrast,
must move through the water or sink, as
their bodies are denser than water. In
primitive bony fishes, the swim bladder
is a ventral outpocketing of the pharynx
behind the throat, and these species fill
the swim bladder by simply gulping air
at the surface of the water. In most of
today’s bony fishes, the swim bladder is
an independent organ that is filled and
drained of gases, mostly nitrogen and
oxygen, internally. How do bony fishes
manage this remarkable trick? It turns
out that the gases are released from their
blood. Gas exchange occurs across the
wall of the swim bladder and the blood
vessels located near the swim bladder. A variety of physio-
logical factors controls the exchange of gases between the
blood stream and the swim bladder.
Lateral Line System. Although precursors are found in
sharks, bony fishes possess a fully developed lateral line sys-
tem. The lateral line system consists of a series of sensory
organs that project into a canal beneath the surface of the
skin. The canal runs the length of the fish’s body and is
open to the exterior through a series of sunken pits. Move-
956 Part XII Animal Diversity
Jawless fishes
Cartilaginous fishes
Reptiles
Birds
Mammals
AmphibiansBony fishes
FIGURE 48.15
Bony fishes. The bony fishes (class Osteichthyes) are extremely
diverse. This Korean angelfish in Fiji is one of the many striking
fishes that live around coral reefs in tropical seas.
ment of water past the fish forces water through the canal.
The sensory organs consist of clusters of cells with hairlike
projections called cilia, embedded in a gelatinous cap. The
hairs are deflected by the slightest movement of water over
them. The pits are oriented so that some are stimulated no
matter what direction the water moves (see chapter 55).
Nerve impulses from these sensory organs permit the fish
to assess its rate of movement through water, sensing the
movement as pressure waves against its lateral line. This is
how a trout orients itself with its head upstream.
The lateral line system also enables a fish to detect mo-
tionless objects at a distance by the movement of water re-
flected off the object. In a very real sense, this is the fish
equivalent of hearing. The basic mechanism of cilia deflec-
tion by pressure waves is very similar to what happens in
human ears (see chapter 55).
Gill Cover. Most bony fishes have a hard plate called the
operculum that covers the gills on each side of the head.
Flexing the operculum permits bony fishes to pump water
over their gills. The gills are suspended in the pharyngeal
slits that form a passageway between the pharynx and the
outside of the fish’s body. When the operculum is closed, it
seals off the exit. When the mouth is open, closing the op-
erculum increases the volume of the mouth cavity, so that
water is drawn into the mouth. When the mouth is closed,
opening the operculum decreases the volume of the mouth
cavity, forcing water past the gills to the outside. Using this
very efficient bellows, bony fishes can pass water over the
gills while stationary in the water. That is what a goldfish is
doing when it seems to be gulping in a fish tank.
The Path to Land
Lobe-finned fishes (figure 48.17) evolved 390 million years
ago, shortly after the first bony fishes appeared. Only seven
species survive today, a single species of coelacanth and six
species of lungfish. Lobe-finned fishes have paired fins that
consist of a long fleshy muscular lobe (hence their name),
supported by a central core of bones that form fully articu-
lated joints with one another. There are bony rays only at
the tips of each lobed fin. Muscles within each lobe can
move the fin rays independently of one another, a feat no
ray-finned fish could match. Although rare today, lobe-
finned fishes played an important part in the evolutionary
story of vertebrates. Amphibians almost certainly evolved
from the lobe-finned fishes.
Fishes are characterized by gills and a simple, single-
loop circulatory system. Cartilaginous fishes, such as
sharks, are fast swimmers, while the very successful
bony fishes have unique characteristics such as swim
bladders and lateral line systems.
Chapter 48 Vertebrates 957
Primitive fish
Swim
bladder
Swim bladder
Pharynx
Modern bony
fish
FIGURE 48.16
Diagram of a swim bladder. The bony fishes use this structure,
which evolved as a ventral outpocketing of the pharynx, to control
their buoyancy in water.
FIGURE 48.17
The living coelacanth, Latimeria chalumnae. Discovered in the
western Indian Ocean in 1938, this coelacanth represents a group
of fishes that had been thought to be extinct for about 70 million
years. Scientists who studied living individuals in their natural
habitat at depths of 100 to 200 meters observed them drifting in
the current and hunting other fishes at night. Some individuals are
nearly 3 meters long; they have a slender, fat-filled swim bladder.
Latimeria is a strange animal, and its discovery was a complete
surprise.
Amphibians
Frogs, salamanders, and caecilians, the
damp-skinned vertebrates, are direct
descendants of fishes. They are the
sole survivors of a very successful
group, the amphibians, the first verte-
brates to walk on land. Most present-
day amphibians are small and live
largely unnoticed by humans. Am-
phibians are among the most numer-
ous of terrestrial animals; there are
more species of amphibians than of
mammals. Throughout the world am-
phibians play key roles in terrestrial
food chains.
Characteristics of Living
Amphibians
Biologists have classified living
species of amphibians into three orders (table 48.2): 3680
species of frogs and toads in 22 families make up the order
Anura (“without a tail”); 369 species of salamanders and
newts in 9 families make up the order Urodela or Caudata
(“visible tail”); and 168 species (6 families) of wormlike,
nearly blind organisms called caecilians that live in the
tropics make up the order Apoda or Gymnophiona (“with-
out legs”). They have key characteristics in common:
1. Legs. Frogs and salamanders have four legs and can
move about on land quite well. Legs were one of the
key adaptations to life on land. Caecilians have lost
their legs during the course of adapting to a burrow-
ing existence.
2. Cutaneous respiration. Frogs, salamanders, and
caecilians all supplement the use of lungs by respiring
directly across their skin, which is kept moist and
provides an extensive surface area. This mode of res-
piration is only efficient for a high surface-to-volume
ratio in an animal.
3. Lungs. Most amphibians possess a
pair of lungs, although the internal sur-
faces are poorly developed, with much
less surface area than reptilian or mam-
malian lungs. Amphibians still breathe
by lowering the floor of the mouth to
suck air in, then raising it back to force
the air down into the lungs.
4. Pulmonary veins. After blood is
pumped through the lungs, two large
veins called pulmonary veins return the
aerated blood to the heart for repump-
ing. This allows the aerated blood to be
pumped to the tissues at a much higher
pressure than when it leaves the lungs.
5. Partially divided heart. The initial
chamber of the fish heart is absent in
amphibians, and the second and last
chambers are separated by a dividing
wall that helps prevent aerated blood
from the lungs from mixing with non-
aerated blood being returned to the heart from the
rest of the body. This separates the blood circulation
into two separate paths, pulmonary and systemic.
The separation is imperfect; the third chamber has
no dividing wall.
Several other specialized characteristics are shared by all
present-day amphibians. In all three orders, there is a zone
of weakness between the base and the crown of the teeth.
They also have a peculiar type of sensory rod cell in the
retina of the eye called a “green rod.” The exact function of
this rod is unknown.
Amphibians, with legs and more efficient blood
circulation than fishes, were the first vertebrates to
walk on land.
958 Part XII Animal Diversity
Table 48.2 Orders of Amphibians
Approximate
Typical Number of
Order Examples Key Characteristics Living Species
Anura
Caudata
Apoda
(Gymnophiona)
Frogs, toads
Salamanders, newts
Caecilians
3680
369
168
Compact tailless body; large head fused
to the trunk; rear limbs specialized for
jumping
Slender body; long tail and limbs set out at
right angles to the body
Tropical group with a snakelike body; no
limbs; little or no tail
Jawless fishes
Cartilaginous fishes
Bony fishes
Reptiles
Birds
Mammals
Amphibians
History of the Amphibians
The word amphibia (a Greek word meaning “both lives”)
nicely describes the essential quality of modern day am-
phibians, referring to their ability to live in two worlds: the
aquatic world of their fish ancestors and in the terrestrial
world that they first invaded. In this section, we will review
the checkered history of this group, almost all of whose
members have been extinct for the last 200 million years.
Then, in the following section, we will examine in more
detail what the few kinds of surviving amphibians are like.
Origin of Amphibians
Paleontologists (scientists who study fossils) agree that am-
phibians must have evolved from the lobe-finned fishes, al-
though for some years there has been considerable dis-
agreement about whether the direct ancestors were
coelacanths, lungfish, or the extinct rhipidistian fishes.
Good arguments can be made for each. Many details of
amphibian internal anatomy resemble those of the coela-
canth. Lungfish and rhipidistians have openings in the tops
of their mouths similar to the internal nostrils of amphib-
ians. In addition, lungfish have paired lungs, like those of
amphibians. Recent DNA analysis indicates lungfish are in
fact far more closely related to amphibians than are coela-
canths. Most paleontologists consider that amphibians
evolved from rhipidistian fishes, rather than lungfish, be-
cause the pattern of bones in the early amphibian skull and
limbs bears a remarkable resemblance to the rhipidistians.
They also share a particular tooth structure.
They successful invasion of land by vertebrates involved
a number of major adaptations:
1. Legs were necessary to support the body’s weight as
well as to allow movement from place to place (figure
48.18).
2. Lungs were necessary to extract oxygen from air.
Even though there is far more oxygen available to
gills in air than in water, the delicate structure of fish
gills requires the buoyancy of water to support them
and they will not function in air.
3. The heart had to be redesigned to make full use of
new respiratory systems and to deliver the greater
amounts of oxygen required by walking muscles.
4. Reproduction had to be carried out in water until
methods evolved to prevent eggs from drying out.
5. Most importantly, a system had to be developed to
prevent the body itself from drying out.
Chapter 48 Vertebrates 959
Tibia
Tibia
Femur
Femur
Pelvis
Pelvis
Fibula
Fibula
(a) Lobe-finned fish
(b) Early amphibian
Humerus
Humerus
Shoulder
Shoulder
Radius
Radius
Ulna
Ulna
FIGURE 48.18
A comparison between the limbs of a lobe-finned fish and those of a primitive amphibian. (a) A lobe-finned fish. Some of these
animals could probably move onto land. (b) A primitive amphibian. As illustrated by their skeletal structure, the legs of such an animal
could clearly function on land much better than the fins of the lobe-finned fish.
The First Amphibian
Amphibians solved these problems only partially, but their
solutions worked well enough that amphibians have sur-
vived for 350 million years. Evolution does not insist on
perfect solutions, only workable ones.
Ichthyostega, the earliest amphibian fossil (figure 48.19)
was found in a 370-million-year-old rock in Greenland. At
that time, Greenland was part of the North American con-
tinent and lay near the equator. For the next 100 million
years, all amphibian fossils are found in North America.
Only when Asia and the southern continents all merged
with North America to form the supercontinent Pangaea
did amphibians spread throughout the world.
Ichthyostega was a strongly built animal, with four sturdy
legs well supported by hip and shoulder bones. The shoul-
der bones no longer attached to the skull as in fish. The
hipbones were braced against the backbone unlike in fish,
so the limbs could support the animal’s weight. To
strengthen the backbone further, long, broad ribs that
overlap each other formed a solid cage for the lungs and
heart. The rib cage was so solid that it probably couldn’t
expand and contract for breathing. Instead, Ichthyostega ob-
tained oxygen somewhat as a fish does, by lowering the
floor of the mouth to draw air in, then raising it to push air
down the windpipe into the lungs.
The Rise and Fall of Amphibians
Amphibians first became common during the Carbonifer-
ous Period (360 to 280 million years ago). Fourteen fami-
lies of amphibians are known from the early Carboniferous,
nearly all aquatic or semiaquatic, like Ichthyostega. By the
late Carboniferous, much of North America was covered
by low-lying tropical swamplands, and 34 families of am-
phibians thrived in this wet terrestrial environment, sharing
it with pelycosaurs and other early reptiles. In the early
Permian Period that followed (280 to 248 million years
ago), a remarkable change occurred among amphibians—
they began to leave the marshes for dry uplands. Many of
these terrestrial amphibians had bony plates and armor
covering their bodies and grew to be very large, some as
big as a pony (figure 48.20). Both their large size and the
complete covering of their bodies indicate that these am-
phibians did not use the skin respiratory system of present-
day amphibians, but rather had an impermeable leathery
skin to prevent water loss. By the mid-Permian, there were
40 families of amphibians. Only 25% of them were still
semiaquatic like Ichthyostega; 60% of the amphibians were
fully terrestrial, 15% were semiterrestrial.
This was the peak of amphibian success. By the end of
the Permian, a reptile called a therapsid had become com-
mon, ousting the amphibians from their newly acquired
niche on land. Following the mass extinction event at the
end of the Permian, therapsids were the dominant land ver-
tebrate and most amphibians were aquatic. This trend con-
tinued in the following Triassic Period (248 to 213 million
years ago), which saw the virtual extinction of amphibians
from land. By the end of the Triassic, there were only 15
families of amphibians (including the first frog), and almost
without exception they were aquatic. Some of these grew to
great size; one was 3 meters long. Only two groups of am-
phibians are known from the following Jurassic Period (213
to 144 million years ago), the anurans (frogs and toads) and
the urodeles (salamanders and newts). The Age of Amphib-
ians was over.
960 Part XII Animal Diversity
FIGURE 48.19
Amphibians were the first vertebrates to walk on land.
Reconstruction of Ichthyostega, one of the first amphibians with
efficient limbs for crawling on land, an improved olfactory sense
associated with a lengthened snout, and a relatively advanced ear
structure for picking up airborne sounds. Despite these features,
Ichthyostega, which lived about 350 million years ago, was still
quite fishlike in overall appearance and represents a very early
amphibian.
FIGURE 48.20
A terrestrial amphibian of the Permian. Cacops, a large, extinct
amphibian, had extensive body armor.
Amphibians Today
All of today’s amphibians descended from the two families
of amphibians that survived the Age of the Dinosaurs. Dur-
ing the Tertiary Period (65 to 2 million years ago), these
moist-skinned amphibians underwent a highly successful
invasion of wet habitats all over the world, and today there
are over 4200 species of amphibians in 37 different families.
Anura. Frogs and toads, amphibians without tails, live in
a variety of environments from deserts and mountains to
ponds and puddles (figure 48.21a). Frogs have smooth,
moist skin, a broad body, and long hind legs that make
them excellent jumpers. Most frogs live in or near water,
although some tropical species live in trees. Unlike frogs,
toads have a dry, bumpy skin, short legs, and are well
adapted to dry environments. All adult anurans are carni-
vores, eating a wide variety of invertebrates.
Most frogs and toads return to water to reproduce, lay-
ing their eggs directly in water. Their eggs lack water-tight
external membranes and would dry out quickly out of the
water. Eggs are fertilized externally and hatch into swim-
ming larval forms called tadpoles. Tadpoles live in the
water, where they generally feed on minute algae. After
considerable growth, the body of the tadpole gradually
changes into that of an adult frog. This process of abrupt
change in body form is called metamorphosis.
Urodela (Caudata). Salamanders have elongated bodies,
long tails, and smooth moist skin (figure 48.21b). They typ-
ically range in length from a few inches to a foot, although
giant Asiatic salamanders of the genus Andrias are as much
as 1.5 meters long and weigh up to 33 kilograms. Most
salamanders live in moist places, such as under stones or
logs, or among the leaves of tropical plants. Some salaman-
ders live entirely in water.
Salamanders lay their eggs in water or in moist places.
Fertilization is usually external, although a few species
practice a type of internal fertilization in which the female
picks up sperm packets deposited by the male. Unlike anu-
rans, the young that hatch from salamander eggs do not
undergo profound metamorphosis, but are born looking
like small adults and are carnivorous.
Apoda (Gymnophiona). Caecilians, members of the
order Apoda (Gymnophiona), are a highly specialized
group of tropical burrowing amphibians (figure 48.21c).
These legless, wormlike creatures average about 30 cen-
timeters long, but can be up to 1.3 meters long. They have
very small eyes and are often blind. They resemble worms
but have jaws with teeth. They eat worms and other soil in-
vertebrates. The caecilian male deposits sperm directly into
the female, and the female usually bears live young. Mud
eels, small amphibians with tiny forelimbs and no hind
limbs that live in the eastern United States, are not apo-
dans, but highly specialized urodelians.
Amphibians ventured onto land some 370 million years
ago. They are characterized by moist skin, legs
(secondarily lost in some species), lungs (usually), and a
more complex and divided circulatory system. They are
still tied to water for reproduction.
Chapter 48 Vertebrates 961
(a)
(b)
(c)
FIGURE 48.21
Class Amphibia. (a) Red-eyed tree frog, Agalychnis callidryas
(order Anura). (b) An adult barred tiger salamander, Ambystoma
tigrinum (order Caudata). (c) A XXXXXXX caecilian,
XXXXXXXX xxxxxxxxx (order Gymnophiona).
Reptiles
If one thinks of amphibians as a
first draft of a manuscript about
survival on land, then reptiles are
the finished book. For each of the
five key challenges of living on
land, reptiles improved on the in-
novations first seen in amphibians.
Legs were arranged to support the
body’s weight more effectively, al-
lowing reptile bodies to be bigger
and to run. Lungs and heart were
altered to make them more effi-
cient. The skin was covered with
dry plates or scales to minimize
water loss, and eggs were encased
in watertight covers (figure 48.22).
Reptiles were the first truly terres-
trial vertebrates.
Over 7000 species of reptiles
(class Reptilia) now live on earth (table 48.3). They are a
highly successful group in today’s world, more common
than mammals. There are three reptile species for every
two mammal species. While it is traditional to think of
reptiles as more primitive than mammals, the great major-
ity of reptiles that live today evolved from lines that ap-
peared after therapsids did (the line that leads directly to
mammals).
Key Characteristics of Reptiles
All living reptiles share certain fundamental characteristics,
features they retain from the time when they replaced am-
phibians as the dominant terrestrial vertebrates. Among the
most important are:
1. Amniotic egg. Amphibians never
succeeded in becoming fully ter-
restrial because amphibian eggs
must be laid in water to avoid dry-
ing out. Most reptiles lay water-
tight eggs that contain a food
source (the yolk) and a series of
four membranes—the yolk sac, the
amnion, the allantois, and the
chorion (figure 48.22). Each mem-
brane plays a role in making the
egg an independent life-support
system. The outermost membrane
of the egg is the chorion, which
lies just beneath the porous shell.
It allows respiratory gases to pass
through, but retains water within
the egg. Within, the amnion en-
cases the developing embryo
within a fluid-filled cavity. The
yolk sac provides food from the yolk
for the embryo via blood vessels con-
necting to the embryo’s gut. The al-
lantois surrounds a cavity into which
waste products from the embryo are
excreted. All modern reptiles (as well
as birds and mammals) show exactly
this same pattern of membranes
within the egg. These three classes are
called amniotes.
2. Dry skin. Living amphibians have
a moist skin and must remain in
moist places to avoid drying out.
Reptiles have dry, watertight skin. A
layer of scales or armor covers their
bodies, preventing water loss. These
scales develop as surface cells fill
with keratin, the same protein that
forms claws, fingernails, hair, and
bird feathers.
3. Thoracic breathing. Amphibians
breathe by squeezing their throat to pump air into
their lungs; this limits their breathing capacity to the
volume of their mouth. Reptiles developed pul-
monary breathing, expanding and contracting the rib
cage to suck air into the lungs and then force it out.
The capacity of this system is limited only by the vol-
ume of the lungs.
Reptiles were the first vertebrates to completely master
the challenge of living on dry land.
962 Part XII Animal Diversity
Jawless fishes
Cartilaginous fishes
Bony fishes
Birds
Mammals
Amphibians
Reptiles
Embryo
Leathery
shell
Chorion
Allantois
Yolk sac
Amnion
FIGURE 48.22
The watertight egg. The amniotic egg is perhaps the most important feature that allows
reptiles to live in a wide variety of terrestrial habitats.
Chapter 48 Vertebrates 963
Stegosaur
Tyrannosaur
Pterosaur
Plesiosaur
Ichthyosaur
Lizards
Snakes
Turtles, tortoises,
sea turtles
Crocodiles,
alligators,
gavials, caimans
Tuataras
Table 48.3 Major Orders of Reptiles
Approximate
Typical Number of
Order Examples Key Characteristics Living Species
Ornithischia
Saurischia
Pterosauria
Plesiosaura
Ichthyosauria
Squamata,
suborder
Sauria
Squamata,
suborder
Serpentes
Chelonia
Crocodylia
Rhynchocephalia
Dinosaurs with two pelvic bones facing
backward, like a bird’s pelvis; herbivores,
with turtlelike upper beak; legs under body
Dinosaurs with one pelvic bone facing
forward, the other back, like a lizard’s
pelvis; both plant- and flesh-eaters; legs
under body
Flying reptiles; wings were made of skin
stretched between fourth fingers and body;
wingspans of early forms typically 60
centimeters, later forms nearly 8 meters
Barrel-shaped marine reptiles with sharp
teeth and large, paddle-shaped fins; some
had snakelike necks twice as long as
their bodies
Streamlined marine reptiles with many body
similarities to sharks and modern fishes
Lizards; limbs set at right angles to body;
anus is in transverse (sideways) slit; most are
terrestrial
Snakes; no legs; move by slithering; scaly
skin is shed periodically; most are terrestrial
Ancient armored reptiles with shell of bony
plates to which vertebrae and ribs are fused;
sharp, horny beak without teeth
Advanced reptiles with four-chambered heart
and socketed teeth; anus is a longitudinal
(lengthwise) slit; closest living relatives to
birds
Sole survivors of a once successful group
that largely disappeared before dinosaurs;
fused, wedgelike, socketless teeth; primitive
third eye under skin of forehead
Extinct
Extinct
Extinct
Extinct
Extinct
3800
3000
250
25
2
The Rise and Fall of Dominant
Reptile Groups
During the 250 million years that reptiles were the domi-
nant large terrestrial vertebrates, four major forms of rep-
tiles took turns as the dominant type: pelycosaurs, therap-
sids, thecodonts, and dinosaurs.
Pelycosaurs: Becoming a Better Predator
Early reptiles like pelycosaurs were better adapted to life on
dry land than amphibians because they evolved watertight
eggs. They had powerful jaws because of an innovation in
skull design and muscle arrangement. Pelycosaurs were
synapsids, meaning that their skulls had a pair of temporal
holes behind the openings for the eyes. An important fea-
ture of reptile classification is the presence and number of
openings behind the eyes (see figure 48.27). Their jaw
muscles were anchored to these holes, which allowed them
to bite more powerfully. An individual pelycosaur weighed
about 200 kilograms. With long, sharp, “steak knife” teeth,
pelycosaurs were the first land vertebrates to kill beasts
their own size (figure 48.23). Dominant for 50 million
years, pelycosaurs once made up 70% of all land verte-
brates. They died out about 250 million years ago, replaced
by their direct descendants—the therapsids.
Therapsids: Speeding Up Metabolism
Therapsids (figure 48.24) ate ten times more frequently than
their pelycosaur ancestors (figure 48.24). There is evidence
that they may have been endotherms, able to regulate their
own body temperature. The extra food consumption would
have been necessary to produce body heat. This would
have permitted therapsids to be far more active than other
vertebrates of that time, when winters were cold and long.
For 20 million years, therapsids (also called “mammallike
reptiles”) were the dominant land vertebrate, until largely
replaced 230 million years ago by a cold-blooded, or ec-
tothermic, reptile line—the thecodonts. Therapsids be-
came extinct 170 million years ago, but not before giving
rise to their descendants—the mammals.
Thecodonts: Wasting Less Energy
Thecodonts were diapsids, their skulls having two pairs of
temporal holes, and like amphibians and early reptiles, they
were ectotherms (figure 48.25). Thecodonts largely re-
placed therapsids when the world’s climate warmed 230
million years ago. In the warm climate, the therapsid’s en-
dothermy no longer offered a competitive advantage, and
ectothermic thecodonts needed only a tenth as much food.
Thecodonts were the first land vertebrates to be bipedal—
to stand and walk on two feet. They were dominant
through the Triassic and survived for 15 million years, until
replaced by their direct descendants—the dinosaurs.
964 Part XII Animal Diversity
FIGURE 48.23
A pelycosaur. Dimetrodon, a carnivorous pelycosaur, had a dorsal
sail that is thought to have been used to dissipate body heat or
gain it by basking.
FIGURE 48.24
A therapsid. This small weaslelike cynodont therapsid,
Megazostrodon, may have had fur. From the late Triassic, it is so
similar to modern mammals that some paleontologists consider it
the first mammal.
FIGURE 48.25
A thecodont. Euparkeria, a thecodont, had rows of bony plates
along the sides of the backbone, as seen in modern crocodiles and
alligators.
Dinosaurs: Learning to Run Upright
Dinosaurs evolved from thecodonts about 220 million years
ago. Unlike the thecodonts, their legs were positioned di-
rectly underneath their bodies, a significant improvement
in body design (figure 48.26). This design placed the
weight of the body directly over the legs, which allowed di-
nosaurs to run with great speed and agility. A dinosaur fos-
sil can be distinguished from a thecodont fossil by the pres-
ence of a hole in the side of the hip socket. Because the
dinosaur leg is positioned underneath the socket, the force
is directed upward, not inward, so there was no need for
bone on the side of the socket. Dinosaurs went on to be-
come the most successful of all land vertebrates, dominat-
ing for 150 million years. All dinosaurs became extinct
rather abruptly 65 million years ago, apparently as a result
of an asteroid’s impact.
Figures 48.27 and 48.28 summarize the evolutionary re-
lationships among the extinct and living reptiles.
Chapter 48 Vertebrates 965
FIGURE 48.26
The largest mounted dinosaur in the world. This
145-million-year-old Brachiosaurus, a plant-eating
sauropod over 80 feet long, lived in East Africa.
Pelycosaur Turtle
Lizards and
snakes Thecodont Dinosaur Crocodilians Birds
Lateral
temporal
opening
Synapsid skull
Orbit
Orbit
Anapsid skull
Dorsal
temporal
opening
Orbit
Lateral
temporal
opening
Diapsid skull
Synapsids: skull with
single pair of lateral
temporal openings
Chelonia: solid-roofed anapsid skull,
plastron, and carapace derived from
dermal bone and fused to part of axial
skeleton
Squamata: fusion of snout
bones, characteristics of
palate, skull roof, vertebrae,
ribs, pectoral girdle, humerus
Archosauria: presence of opening
anterior to eye, orbit shaped like
inverted triangle, teeth laterally
compressed
Diapsids: diapsid skull
with 2 pairs of temporal openings
Turtle-diapsid clade (Sauropsida)
characteristics of skull
and appendages
Amniotes: extraembryonic
membranes of amnion,
chorion, and allantois
FIGURE 48.27
Cladogram of amniotes.
966 Part XII Animal Diversity
350
300
200
250
100
150
50
0
Crocodiles MammalsBirdsTuatarasSnakesLizardsTurtles
Early reptiles
(extinct)
Therapsids
(extinct)
Dinosaurs
(extinct)
Pelycosaurs
(extinct)
Thecodonts
(extinct)
Carboniferous
(360–280)
Permian
(280–248)
Triassic
(248–213)
Jurassic
(213–144)
Cretaceous
(144–65)
Tertiary
(65–2)
Quaternary
(2–Present)
T
ime
(millions of years ago)
FIGURE 48.28
Evolutionary relationships among the reptiles. There are four orders of living reptiles: turtles, lizards and snakes, tuataras, and
crocodiles. This phylogenetic tree shows how these four orders are related to one another and to dinosaurs, birds, and mammals.
Today’s Reptiles
Most of the major reptile orders are now extinct. Of the 16
orders of reptiles that have existed, only 4 survive.
Turtles. The most ancient surviving lineage of rep-
tiles is that of turtles. Turtles have anapsid skulls much
like those of the first reptiles. Turtles have changed little
in the past 200 million years.
Lizards and snakes. Most reptiles living today belong
to the second lineage to evolve, the lizards and snakes.
Lizards and snakes are descended from an ancient lin-
eage of lizardlike reptiles that branched off the main line
of reptile evolution in the late Permian, 250 million
years ago, before the thecodonts appeared (figure 48.28).
Throughout the Mesozoic era, during the dominance of
the dinosaurs, these reptiles survived as minor elements
of the landscape, much as mammals did. Like mammals,
lizards and snakes became diverse and common only
after the dinosaurs disappeared.
Tuataras. The third lineage of surviving reptiles to
evolve were the Rhynchocephalonts, small diapsid rep-
tiles that appeared shortly before the dinosaurs. They
lived throughout the time of the dinosaurs and were
common in the Jurassic. They began to decline in the
Cretaceous, apparently unable to compete with lizards,
and were already rare by the time dinosaurs disappeared.
Today only two species of the order Rhynchocephalia
survive, both tuataras living on small islands near New
Zealand.
Crocodiles. The fourth lineage of living reptile, croc-
odiles, appeared on the evolutionary scene much later
than other living reptiles. Crocodiles are descended
from the same line of thecodonts that gave rise to the di-
nosaurs and resemble dinosaurs in many ways. They
have changed very little in over 200 million years. Croc-
odiles, pterosaurs, thecodonts, and dinosaurs together
make up a group called archosaurs (“ruling reptiles”).
Other Important Characteristics
As you might imagine from the structure of the amniotic
egg, reptiles and other amniotes do not practice external
fertilization as most amphibians do. There would be no
way for a sperm to penetrate the membrane barriers pro-
tecting the egg. Instead, the male places sperm inside the
female, where they fertilize the egg before the membranes
are formed. This is called internal fertilization.
The circulatory system of reptiles is improved over that
of fish and amphibians, providing oxygen to the body more
efficiently (figure 48.29). The improvement is achieved by
extending the septum within the heart from the atrium
partway across the ventricle. This septum creates a partial
wall that tends to lessen mixing of oxygen-poor blood with
oxygen-rich blood within the ventricle. In crocodiles, the
septum completely divides the ventricle, creating a four-
chambered heart, just as it does in birds and mammals (and
probably in dinosaurs).
All living reptiles are ectothermic, obtaining their
heat from external sources. In contrast, endothermic an-
imals are able to generate their heat internally. In addi-
tion, homeothermic animals have a constant body tem-
perature, and poikilothermic animals have a body
temperature that fluctuates with ambient temperature.
Thus, a deep-sea fish may be an ectothermic
homeotherm because its heat comes from an external
source, but its body temperature is constant. Reptiles are
largely ectothermic poikilotherms; their body tempera-
ture is largely determined by their surroundings. Reptiles
also regulate their temperature through behavior. They
may bask in the sun to warm up or seek shade to prevent
overheating. The thecodont ancestors of crocodiles were
ectothermic, as crocodiles are today. The later dinosaurs
from which birds evolved were endothermic. Crocodiles
and birds differ in this one important respect. Ec-
tothermy is a principal reason why crocodiles have been
grouped among the reptiles.
Chapter 48 Vertebrates 967
Heart
Lungs
Body
Lung Lung
Systemic
capillaries
Dorsal
aorta
Ventricle Atrium
Heart
Gills Body
Ventral
aorta
Gills
Systemic
capillaries
(a) (b)
FIGURE 48.29
A comparison of
reptile and fish
circulation. (a) In
reptiles such as this
turtle, blood is
repumped after leaving
the lungs, and
circulation to the rest
of the body remains
vigorous. (b) The blood
in fishes flows from the
gills directly to the rest
of the body, resulting
in slower circulation.
Kinds of Living Reptiles
The four surviving orders of reptiles contain about 7000
species. Reptiles occur worldwide except in the coldest re-
gions, where it is impossible for ectotherms to survive.
Reptiles are among the most numerous and diverse of ter-
restrial vertebrates. The four living orders of the class Rep-
tilia are Chelonia, Rhynchocephalia, Squamata, and Croco-
dilia.
Order Chelonia: Turtles and Tortoises. The order
Chelonia consists of about 250 species of turtles (most of
which are aquatic; figure 48.30) and tortoises (which are
terrestrial). They differ from all other reptiles because their
bodies are encased within a protective shell. Many of them
can pull their head and legs into the shell as well, for total
protection from predators. Turtles and tortoises lack teeth
but have sharp beaks.
Today’s turtles and tortoises have changed very little
since the first turtles appeared 200 million years ago. Tur-
tles are anapsid—they lack the temporal openings in the
skull characteristic of other living reptiles, which are diap-
sid. This evolutionary stability of turtles may reflect the
continuous benefit of their basic design—a body covered
with a shell. In some species, the shell is made of hard
plates; in other species, it is a covering of tough, leathery
skin. In either case, the shell consists of two basic parts.
The carapace is the dorsal covering, while the plastron is
the ventral portion. In a fundamental commitment to this
shell architecture, the vertebrae and ribs of most turtle and
tortoise species are fused to the inside of the carapace. All
of the support for muscle attachment comes from the shell.
While most tortoises have a domed-shaped shell into
which they can retract their head and limbs, water-dwelling
turtles have a streamlined, disc-shaped shell that permits
rapid turning in water. Freshwater turtles have webbed
toes, and in marine turtles, the forelimbs have evolved into
flippers. Although marine turtles spend their lives at sea,
they must return to land to lay their eggs. Many species mi-
grate long distances to do this. Atlantic green turtles mi-
grate from their feeding grounds off the coast of Brazil to
Ascension Island in the middle of the South Atlantic—a
distance of more than 2000 kilometers—to lay their eggs
on the same beaches where they hatched.
Order Rhynchocephalia: Tuatara. The order Rhyn-
chocephalia contains only two species today, the tuataras,
large, lizardlike animals about half a meter long. The only
place in the world where these endangered species are
found is on a cluster of small islands off the coast of New
Zealand. The native Maoris of New Zealand named the tu-
atara for the conspicuous spiny crest running down its
back.
An unusual feature of the tuatara (and some lizards) is
the inconspicuous “third eye” on the top of its head, called
a parietal eye. Concealed under a thin layer of scales, the
eye has a lens and retina and is connected by nerves to the
brain. Why have an eye, if it is covered up? The parietal
eye may function to alert the tuatara when it has been ex-
posed to too much sun, protecting it against overheating.
Unlike most reptiles, tuataras are most active at low tem-
peratures. They burrow during the day and feed at night on
insects, worms, and other small animals.
Order Squamata: Lizards and Snakes. The order Squa-
mata (figure 48.31) consists of three suborders: Sauria,
some 3800 species of lizards, Amphisbaenia, about 135
species of worm lizards, and Serpentes, about 3000 species
of snakes. The distinguishing characteristics of this order
are the presence of paired copulatory organs in the male
and a lower jaw that is not joined directly to the skull. A
movable hinge with five joints (your jaw has only one) al-
lows great flexibility in the movements of the jaw. In addi-
tion, the loss of the lower arch of bone below the lower
opening in the skull of lizards makes room for large mus-
cles to operate their jaws. Most lizards and snakes are car-
nivores, preying on insects and small animals, and these im-
provements in jaw design have made a major contribution
to their evolutionary success.
The chief difference between lizards and snakes is that
most lizards have limbs and snakes do not. Snakes also lack
movable eyelids and external ears. Lizards are a more an-
cient group than modern snakes, which evolved only 20
million years ago. Common lizards include iguanas,
chameleons, geckos, and anoles. Most are small, measuring
less than a foot in length. The largest lizards belong to the
monitor family. The largest of all monitors is the Komodo
dragon of Indonesia, which reaches 3 meters in length and
weighs up to 100 kilograms. Snakes also vary in size from
only a few inches long to those that reach nearly 10 meters
in length.
Lizards and snakes rely on agility and speed to catch
prey and elude predators. Only two species of lizard are
venomous, the Gila monster of the southwestern United
States and the beaded lizard of western Mexico. Similarly,
most species of snakes are nonvenomous. Of the 13 families
of snakes, only 4 are venomous: the elapids (cobras, kraits,
968 Part XII Animal Diversity
FIGURE 48.30
Red-bellied turtles, Pseudemys rubriventris. This turtle is
common in the northeastern United States.
and coral snakes); the sea snakes; the vipers (adders, bush-
masters, rattlesnakes, water moccasins, and copperheads);
and some colubrids (African boomslang and twig snake).
Many lizards, including skinks and geckos, have the abil-
ity to lose their tails and then regenerate a new one. This
apparently allows these lizards to escape from predators.
Order Crocodilia: Crocodiles and Alligators. The
order Crocodilia is composed of 25 species of large, pri-
marily aquatic, primitive-looking reptiles (figure 48.32). In
addition to crocodiles and alligators, the order includes two
less familiar animals: the caimans and gavials. Crocodilians
have remained relatively unchanged since they first
evolved.
Crocodiles are largely nocturnal animals that live in or
near water in tropical or subtropical regions of Africa, Asia,
and South America. The American crocodile is found in
southern Florida and Cuba to Columbia and Ecuador. Nile
crocodiles and estuarine crocodiles can grow to enormous
size and are responsible for many human fatalities each
year. There are only two species of alligators: one living in
the southern United States and the other a rare endangered
species living in China. Caimans, which resemble alligators,
are native to Central America. Gavials are a group of fish-
eating crocodilians with long, slender snouts that live only
in India and Burma.
All crocodilians are carnivores. They generally hunt by
stealth, waiting in ambush for prey, then attacking fero-
ciously. Their bodies are well adapted for this form of
hunting: their eyes are on top of their heads and their
nostrils on top of their snouts, so they can see and breathe
while lying quietly submerged in water. They have enor-
mous mouths, studded with sharp teeth, and very strong
necks. A valve in the back of the mouth prevents water
from entering the air passage when a crocodilian feeds
underwater.
Crocodiles resemble birds far more than they do other
living reptiles. Alone among living reptiles, crocodiles
care for their young (a trait they share with at least some
dinosaurs) and have a four-chambered heart, as birds do.
There are also many other points of anatomy in which
crocodiles differ from all living reptiles and resemble
birds. Why are crocodiles more similar to birds than to
other living reptiles? Most biologists now believe that
birds are in fact the direct descendants of dinosaurs. Both
crocodiles and birds are more closely related to di-
nosaurs, and each other, than they are related to lizards
and snakes.
Many major reptile groups that dominated life on land
for 250 million years are now extinct. The four living
orders of reptiles include the turtles, lizards and snakes,
tuataras, and crocodiles.
Chapter 48 Vertebrates 969
FIGURE 48.31
Representatives from the order Squamata. (a) An Australian
skink, Sphenomorophus. Some burrowing lizards lack legs, and the
snakes evolved from one line of legless lizards. (b) A smooth green
snake, Liochlorophis vernalis.
(a)
(b)
FIGURE 48.32
River crocodile, Crocodilus acutus. Most crocodiles resemble
birds and mammals in having four-chambered hearts; all other
living reptiles have three-chambered hearts. Crocodiles, like birds,
are more closely related to dinosaurs than to any of the other
living reptiles.
Birds
Only four groups of animals have
evolved the ability to fly—insects,
pterosaurs, birds, and bats. Pterosaurs,
flying reptiles, evolved from gliding
reptiles and flew for 130 million years
before becoming extinct with the di-
nosaurs. There are startling similarities
in how these very different animals
meet the challenges of flight. Like
water running downhill through similar
gullies, evolution tends to seek out sim-
ilar adaptations. There are major dif-
ferences as well. The success of birds
lies in the development of a structure
unique in the animal world—the
feather. Developed from reptilian
scales, feathers are the ideal adaptation
for flight—lightweight airfoils that are
easily replaced if damaged (unlike the
vulnerable skin wings of pterosaurs and bats). Today, birds
(class Aves) are the most successful and diverse of all terres-
trial vertebrates, with 28 orders containing a total of 166
families and about 8800 species (table 48.4).
Key Characteristics of Birds
Modern birds lack teeth and have only vestigial tails, but
they still retain many reptilian characteristics. For instance,
birds lay amniotic eggs, although the shells of bird eggs are
hard rather than leathery. Also, reptilian scales are present
on the feet and lower legs of birds. What makes birds
unique? What distinguishes them from living reptiles?
1. Feathers. Feathers are modified
reptilian scales that serve two func-
tions: providing lift for flight and
conserving heat. The structure of
feathers combines maximum flexi-
bility and strength with minimum
weight (figure 48.33). Feathers de-
velop from tiny pits in the skin
called follicles. In a typical flight
feather, a shaft emerges from the
follicle, and pairs of vanes develop
from its opposite sides. At maturity,
each vane has many branches called
barbs. The barbs, in turn, have
many projections called barbules
that are equipped with microscopic
hooks. These hooks link the barbs to
one another, giving the feather a
continuous surface and a sturdy but
flexible shape. Like scales, feathers
can be replaced. Feathers are unique
to birds among living animals. Recent fossil finds sug-
gest that some dinosaurs may have had feathers.
2. Flight skeleton. The bones of birds are thin and
hollow. Many of the bones are fused, making the bird
skeleton more rigid than a reptilian skeleton. The
fused sections of backbone and of the shoulder and
hip girdles form a sturdy frame that anchors muscles
during flight. The power for active flight comes from
large breast muscles that can make up 30% of a bird’s
total body weight. They stretch down from the wing
and attach to the breastbone, which is greatly en-
larged and bears a prominent keel for muscle attach-
ment. They also attach to the fused collarbones that
form the so-called “wishbone.” No other living verte-
brates have a fused collarbone or a keeled breastbone.
Birds are the most diverse of all
terrestrial vertebrates. They are closely
related to reptiles, but unlike reptiles or
any other animals, birds have feathers.
970 Part XII Animal Diversity
Jawless fishes
Cartilaginous fishes
Bony fishes
Reptiles
Mammals
Amphibians
Birds
Shaft
Quill
Shaft
Barbules
Hooks
Barb
FIGURE 48.33
A feather. This enlargement shows how the
vanes, secondary branches and barbs, are linked
together by microscopic barbules.
Chapter 48 Vertebrates 971
Table 48.4 Major Orders of Birds
Approximate
Typical Number of
Order Examples Key Characteristics Living Species
Passeriformes
Apodiformes
Piciformes
Psittaciformes
Charadriiformes
Columbiformes
Falconiformes
Galliformes
Gruiformes
Anseriformes
Strigiformes
Ciconiiformes
Procellariformes
Sphenisciformes
Dinornithiformes
Struthioniformes
Crows, mockingbirds,
robins, sparrows,
starlings, warblers
Hummingbirds,
swifts
Honeyguides,
toucans,
woodpeckers
Cockatoos, parrots
Auks, gulls, plovers,
sandpipers, terns
Doves, pigeons
Eagles, falcons,
hawks, vultures
Chickens, grouse,
pheasants, quail
Bitterns, coots,
cranes, rails
Ducks, geese, swans
Barn owls, screech
owls
Herons, ibises, storks
Albatrosses, petrels
Emperor penguins,
crested penguins
Kiwis
Ostriches
Songbirds
Well-developed vocal organs; perching
feet; dependent young
Fast fliers
Short legs; small bodies; rapid wing beat
Woodpeckers or toucans
Grasping feet; chisel-like, sharp bills
can break down wood
Parrots
Large, powerful bills for crushing seeds;
well-developed vocal organs
Shorebirds
Long, stiltlike legs; slender probing bills
Pigeons
Perching feet; rounded, stout bodies
Birds of prey
Carnivorous; keen vision; sharp, pointed
beaks for tearing flesh; active during the day
Gamebirds
Often limited flying ability; rounded bodies
Marsh birds
Long, stiltlike legs; diverse body shapes;
marsh-dwellers
Waterfowl
Webbed toes; broad bill with filtering
ridges
Owls
Nocturnal birds of prey; strong beaks;
powerful feet
Waders
Long-legged; large bodies
Seabirds
Tube-shaped bills; capable of flying for long
periods of time
Penguins
Marine; modified wings for swimming;
flightless; found only in southern
hemisphere; thick coats of insulating
feathers
Kiwis
Flightless; small; primitive; confined to
New Zealand
Ostriches
Powerful running legs; flightless; only two
toes; very large
5276
(largest of all bird
orders; contains over
60% of all species)
428
383
340
331
303
288
268
209
150
146
114
104
18
2
1
History of the Birds
A 150-million-year-old fossil of the first known bird, Ar-
chaeopteryx (figure 48.34)—pronounced “archie-op-ter-ichs”—
was found in 1862 in a limestone quarry in Bavaria, the
impression of its feathers stamped clearly into the rocks.
Birds Are Descended from Dinosaurs
The skeleton of Archaeopteryx shares many features with
small theropod dinosaurs. About the size of a crow, its skull
has teeth, and very few of its bones are fused to one an-
other—dinosaurian features, not avian. Its bones are solid,
not hollow like a bird’s. Also, it has a long reptilian tail, and
no enlarged breastbone such as modern birds use to anchor
flight muscles. Finally, it has the forelimbs of a dinosaur.
Because of its many dinosaur features, several Archaeopteryx
fossils were originally classified as the coelurosaur Compsog-
nathus, a small theropod dinosaur of similar size—until
feathers were discovered on the fossils. What makes Ar-
chaeopteryx distinctly avian is the presence of feathers on its
wings and tail. It also has other birdlike features, notably
the presence of a wishbone. Dinosaurs lack a wishbone, al-
though thecodonts had them.
The remarkable similarity of Archaeopteryx to Compsog-
nathus has led almost all paleontologists to conclude that
Archaeopteryx is the direct descendant of dinosaurs—in-
deed, that today’s birds are “feathered dinosaurs.” Some
even speak flippantly of “carving the dinosaur” at Thanks-
giving dinner. The recent discovery of feathered dinosaurs
in China lends strong support to this inference. The di-
nosaur Caudipteryx, for example, is clearly intermediate be-
tween Archaeopteryx and dinosaurs, having large feathers on
its tail and arms but also many features of velociraptor di-
nosaurs (figure 48.35). Because the arms of Caudipteryx
were too short to use as wings, feathers probably didn’t
evolve for flight. Instead, they probably served as insula-
tion, much as fur does for animals. Flight is something that
certain kinds of dinosaurs achieved as they evolved longer
arms. We call these dinosaurs birds.
Despite their close affinity to dinosaurs, biologists con-
tinue to classify birds as Aves, a separate class, because of
the key evolutionary novelties of birds: feathers, hollow
bones, and physiological mechanisms such as supereffi-
cient lungs that permit sustained, powered flight. It is be-
cause of their unique adaptations and great diversity that
972 Part XII Animal Diversity
FIGURE 48.34
Archaeopteryx. An artist’s reconstruction of Archaeopteryx, an early
bird about the size of a crow. Closely related to its ancestors
among the bipedal dinosaurs, Archaeopteryx lived in the forests of
central Europe 150 million years ago. The true feather colors of
Archaeopteryx are not known.
FIGURE 48.35
The evolutionary path to the birds. Almost all paleontologists now accept the theory that birds are the direct descendents of theropod
dinosaurs.
birds are assigned to a separate class.
This practical judgment should not
conceal the basic agreement among al-
most all biologists that birds are the
direct descendants of theropod di-
nosaurs, as closely related to
coelurosaurs as are other theropods
(see figure 48.35).
By the early Cretaceous, only a few
million years after Archaeopteryx, a di-
verse array of birds had evolved, with
many of the features of modern birds.
Fossils in Mongolia, Spain, and China
discovered within the last few years
reveal a diverse collection of toothed
birds with the hollow bones and
breastbones necessary for sustained
flight. Other fossils reveal highly spe-
cialized, flightless diving birds. The
diverse birds of the Cretaceous shared
the skies with pterosaurs for 70 mil-
lion years.
Because the impression of feathers
is rarely fossilized and modern birds have hollow, delicate
bones, the fossil record of birds is incomplete. Relation-
ships among the 166 families of modern birds are mostly
inferred from studies of the degree of DNA similarity
among living birds. These studies suggest that the most
ancient living birds are the flightless birds, like the os-
trich. Ducks, geese, and other waterfowl evolved next, in
the early Cretaceous, followed by a diverse group of
woodpeckers, parrots, swifts, and owls. The largest of the
bird orders, Passeriformes, or songbirds (60% of all
species of birds today), evolved in the mid-Cretaceous.
The more specialized orders of birds, such as shorebirds,
birds of prey, flamingos, and penguins, did not appear
until the late Cretaceous. All but a few of the modern or-
ders of toothless birds are thought to have arisen before
the disappearance of the pterosaurs and dinosaurs at the
end of the Cretaceous 65 million years ago.
Birds Today
You can tell a great deal about the habits and food of a bird
by examining its beak and feet. For instance, carnivorous
birds such as owls have curved talons for seizing prey and
sharp beaks for tearing apart their meal. The beaks of
ducks are flat for shoveling through mud, while the beaks
of finches are short, thick seed-crushers. There are 28 or-
ders of birds, the largest consisting of over 5000 species
(figure 48.36).
Many adaptations enabled birds to cope with the heavy
energy demands of flight:
1. Efficient respiration. Flight muscles consume an
enormous amount of oxygen during active flight.
The reptilian lung has a limited internal surface
area, not nearly enough to ab-
sorb all the oxygen needed.
Mammalian lungs have a greater
surface area, but as we will see in
chapter 53, bird lungs satisfy this
challenge with a radical re-
design. When a bird inhales, the
air goes past the lungs to a series
of air sacs located near and
within the hollow bones of the
back; from there the air travels
to the lungs and then to a set of
anterior air sacs before being ex-
haled. Because air always passes
through the lungs in the same
direction, and blood flows past
the lung at right angles to the
airflow, gas exchange is highly
efficient.
2. Efficient circulation. The
revved-up metabolism needed to
power active flight also requires
very efficient blood circulation, so
that the oxygen captured by the lungs can be deliv-
ered to the flight muscles quickly. In the heart of
most living reptiles, oxygen-rich blood coming from
the lungs mixes with oxygen-poor blood returning
from the body because the wall dividing the ventricle
into two chambers is not complete. In birds, the wall
dividing the ventricle is complete, and the two blood
circulations do not mix, so flight muscles receive fully
oxygenated blood.
In comparison with reptiles and most other verte-
brates, birds have a rapid heartbeat. A hummingbird’s
heart beats about 600 times a minute. An active
chickadee’s heart beats 1000 times a minute. In con-
trast, the heart of the large, flightless ostrich averages
70 beats per minute—the same rate as the human
heart.
3. Endothermy. Birds, like mammals, are endother-
mic. Many paleontologists believe the dinosaurs that
birds evolved from were endothermic as well. Birds
maintain body temperatures significantly higher than
most mammals, ranging from 40° to 42°C (your body
temperature is 37°C). Feathers provide excellent in-
sulation, helping to conserve body heat. The high
temperatures maintained by endothermy permit me-
tabolism in the bird’s flight muscles to proceed at a
rapid pace, to provide the ATP necessary to drive
rapid muscle contraction.
The class Aves probably debuted 150 million years ago
with Archaeopteryx. Modern birds are characterized by
feathers, scales, a thin, hollow skeleton, auxiliary air
sacs, and a four-chambered heart. Birds lay amniotic
eggs and are endothermic.
Chapter 48 Vertebrates 973
FIGURE 48.36
Class Aves. This Western tanager, Piranga
ludoviciana, is a member of the largest order
of birds, the Passeriformes, with over 5000
species.
Mammals
There are about 4100 living species of
mammals (class Mammalia), the small-
est number of species in any of the five
classes of vertebrates. Most large,
land-dwelling vertebrates are mam-
mals (figure 48.37), and they tend to
dominate terrestrial communities, as
did the dinosaurs that they replaced.
When you look out over an African
plain, you see the big mammals, the
lions, zebras, gazelles, and antelope.
Your eye does not as readily pick out
the many birds, lizards, and frogs that
live in the grassland community with
them. But the typical mammal is not
all that large. Of the 4100 species of
mammals, 3200 are rodents, bats,
shrews, or moles (table 48.5).
Key Mammalian Characteristics
Mammals are distinguished from all other classes of verte-
brates by two fundamental characteristics that are unique
to mammals:
1. Hair. All mammals have hair. Even apparently
naked whales and dolphins grow sensitive bristles on
their snouts. Evolution of fur and the ability to regu-
late body temperature enabled mammals to invade
colder climates that ectothermic reptiles could not in-
habit, and the insulation fur provided may have en-
sured the survival of mammals when the dinosaurs
perished.
Unlike feathers, which evolved from modified
reptilian scales, mammalian hair is a completely dif-
ferent form of skin structure. An individual mam-
malian hair is a long, protein-rich filament that ex-
tends like a stiff thread from a bulblike foundation
beneath the skin known as a hair follicle. The fila-
ment is composed mainly of dead cells filled with the
fibrous protein keratin.
One of the most important functions of hair is in-
sulation against heat loss. Mammals are endothermic
animals, and typically maintain body temperatures
higher than the temperature of their surroundings.
The dense undercoat of many mammals reduces the
amount of body heat that escapes.
Another function of hair is camouflage. The col-
oration and pattern of a mammal’s coat usually matches
its background. A little brown mouse is practically in-
visible against the brown leaf litter of a forest floor,
while the orange and black stripes of a Bengal tiger dis-
appear against the orange-brown color of the tall grass
in which it hunts. Hairs also function as sensory struc-
tures. The whiskers of cats and dogs are stiff hairs that
are very sensitive to touch. Mam-
mals that are active at night or
live underground often rely on
their whiskers to locate prey or to
avoid colliding with objects. Hair
can also serve as a defense
weapon. Porcupines and hedge-
hogs protect themselves with
long, sharp, stiff hairs called
quills.
2. Mammary glands. All female
mammals possess mammary
glands that secrete milk. New-
born mammals, born without
teeth, suckle this milk. Even
baby whales are nursed by their
mother’s milk. Milk is a fluid
rich in fat, sugar, and protein. A
liter of human milk contains 11
grams of protein, 49 grams of
fat, 70 grams of carbohydrate
(chiefly the sugar lactose), and 2 grams of minerals
critical to early growth, such as calcium. About 95%
of the volume is water, critical to avoid dehydration.
Milk is a very high calorie food (human milk has 750
kcal per liter), important because of the high energy
needs of a rapidly growing newborn mammal. About
50% of the energy in the milk comes from fat.
Mammals first appeared 220 million years ago, evolving
to their present position of dominance in modern
terrestrial ecosystems. Mammals are the only
vertebrates that possess hair and milk glands.
974 Part XII Animal Diversity
Jawless fishes
Cartilaginous fishes
Bony fishes
Reptiles
Birds
Amphibians
Mammals
FIGURE 48.37
Mammals. African elephants, Loxodonta africana, at a water hole
(order Proboscidea).
Chapter 48 Vertebrates 975
1814
986
390
280
240
233
211
79
69
34
30
17
2
Table 48.5 Major Orders of Mammals
Approximate
Typical Number of
Order Examples Key Characteristics Living Species
Rodentia
Chiroptera
Insectivora
Marsupialia
Carnivora
Primates
Artiodactyla
Cetacea
Lagomorpha
Pinnipedia
Edentata
Perissodactyla
Proboscidea
Small plant-eaters
Chisel-like incisor teeth
Flying mammals
Primarily fruit- or insect-eaters; elongated
fingers; thin wing membrane; nocturnal;
navigate by sonar
Small, burrowing mammals
Insect-eaters; most primitive placental
mammals; spend most of their time
underground
Pouched mammals
Young develop in abdominal pouch
Carnivorous predators
Teeth adapted for shearing flesh; no native
families in Australia
Tree-dwellers
Large brain size; binocular vision; opposable
thumb; end product of a line that branched off
early from other mammals
Hoofed mammals
With two or four toes; mostly herbivores
Fully marine mammals
Streamlined bodies; front limbs modified into
flippers; no hind limbs; blowholes on top of
head; no hair except on muzzle
Rodentlike jumpers
Four upper incisors (rather than the two seen in
rodents); hind legs often longer than forelegs;
an adaptation for jumping
Marine carnivores
Feed mainly on fish; limbs modified for
swimming
Toothless insect-eaters
Many are toothless, but some have degenerate,
peglike teeth
Hoofed mammals with one or three toes
Herbivorous teeth adapted for chewing
Long-trunked herbivores
Two upper incisors elongated as tusks; largest
living land animal
Beavers, mice,
porcupines, rats
Bats
Moles, shrews
Kangaroos, koalas
Bears, cats, raccoons,
weasels, dogs
Apes, humans,
lemurs, monkeys
Cattle, deer,
giraffes, pigs
Dolphins, porpoises,
whales
Rabbits, hares, pikas
Sea lions, seals,
walruses
Anteaters,
armadillos, sloths
Horses,
rhinoceroses, zebras
Elephants
History of the Mammals
Mammals have been around since the
time of the dinosaurs, although they
were never common until the dinosaurs
disappeared. We have learned a lot
about the evolutionary history of mam-
mals from their fossils.
Origin of Mammals
The first mammals arose from therap-
sids in the mid-Triassic about 220
million years ago, just as the first di-
nosaurs evolved from thecodonts.
Tiny, shrewlike creatures that lived in
trees eating insects, mammals were
only a minor element in a land that
quickly came to be dominated by di-
nosaurs. Fossils reveal that these early
mammals had large eye sockets, evi-
dence that they may have been active
at night. Early mammals had a single
lower jawbone. Therapsid fossils show
a change from the reptile lower jaw
with several bones to a jaw closer to
the mammalian-type jaw. Two of the
bones forming the therapsid jaw joint
retreated into the middle ear of mam-
mals, linking with a bone already
there producing a three-bone struc-
ture that amplifies sound better than
the reptilian ear.
Early Divergence in Mammals
For 155 million years, while the dinosaurs flourished,
mammals were a minor group of small insectivores and
herbivores. Only five orders of mammals arose in that
time, and their fossils are scarce, indicating that mammals
were not abundant. However, the two groups to which
present-day mammals belong did appear. The most prim-
itive mammals, direct descendents of therapsids, were
members of the subclass Prototheria. Most prototherians
were small and resembled modern shrews. All prototheri-
ans laid eggs, as did their therapsid ancestors. The only
prototherians surviving today are the monotremes—the
duckbill platypus and the echidnas, or spiny anteaters.
The other major mammalian group is the subclass Theria.
All of the mammals you are familiar with, including hu-
mans, are therians. Therians are viviparous (that is, their
young are born alive). The two major living therian
groups are marsupials, or pouched mammals, and placen-
tal mammals. Kangaroos, opossums, and koalas are mar-
supials. Dogs, cats, humans, horses, and most other mam-
mals are placentals.
The Age of Mammals
At the end of the Cretaceous Period 65 million years ago,
the dinosaurs and numerous other land and marine animals
became extinct, but mammals survived, possibly because of
the insulation their fur provided. In the Tertiary Period
(lasting from 65 million years to 2 million years ago), mam-
mals rapidly diversified, taking over many of the ecological
roles once dominated by dinosaurs (table 48.6). Mammals
reached their maximum diversity late in the Tertiary Pe-
riod, about 15 million years ago. At that time, tropical con-
ditions existed over much of the world. During the last 15
million years, world climates have deteriorated, and the
area covered by tropical habitats has decreased, causing a
decline in the total number of mammalian species. There
are now 19 orders of mammals.
976 Part XII Animal Diversity
Table 48.6 Some Groups of Extinct Mammals
Group Description
Cave bears
Irish elk
Mammoths
Giant ground
sloths
Sabertooth
cats
Numerous in the ice ages; this enormous
vegetarian bear slept
through the winter in large groups.
Neither Irish nor an elk (it is a kind
of deer), Megaloceros was the largest
deer that ever lived, with horns spanning
12 feet. Seen in French
cave paintings, they became
extinct about 2500 years ago.
Although only two species of elephants
survive today, the elephant family
was far more diverse during the
late Tertiary. Many were cold-
adapted mammoths with fur.
Megatherium was a giant
20-foot ground sloth that
weighed three tons and was as
large as a modern elephant.
The jaws of these large,
lionlike cats opened
an incredible 120 degrees
to allow the animal to drive its
huge upper pair of saber teeth into prey.
Characteristics of Modern Mammals
Endothermy. Mammals are endothermic, a crucial adap-
tation that has allowed mammals to be active at any time of
the day or night and to colonize severe environments, from
deserts to ice fields. Many characteristics, such as hair that
provides insulation, played important roles in making en-
dothermy possible. Also, the more efficient blood circula-
tion provided by the four-chambered heart and the more
efficient respiration provided by the diaphragm (a special
sheet of muscles below the rib cage that aids breathing)
make possible the higher metabolic rate upon which en-
dothermy depends.
Placenta. In most mammal species, females carry their
young in a uterus during development, nourishing them
through a placenta, and give birth to live young. The pla-
centa is a specialized organ within the uterus of the preg-
nant mother that brings the bloodstream of the fetus into
close contact with the bloodstream of the mother (figure
48.38). Food, water, and oxygen can pass across from
mother to child, and wastes can pass over to the mother’s
blood and be carried away.
Teeth. Reptiles have homodont dentition: their teeth are
all the same. However, mammals have heterodont denti-
tion, with different types of teeth that are highly specialized
to match particular eating habits (figure 48.39). It is usually
possible to determine a mammal’s diet simply by examining
its teeth. Compare the skull of a dog (a carnivore) and a
deer (an herbivore). The dog’s long canine teeth are well
suited for biting and holding prey, and some of its premo-
lar and molar teeth are triangular and sharp for ripping off
chunks of flesh. In contrast, canine teeth are absent in deer;
instead the deer clips off mouthfuls of plants with flat,
chisel-like incisors on its lower jaw. The deer’s molars are
large and covered with ridges to effectively grind and break
up tough plant tissues. Rodents, such as beavers, are gnaw-
ers and have long incisors for chewing through branches or
stems. These incisors are ever-growing; that is, the ends
wear down, but new incisor growth maintains the length.
Chapter 48 Vertebrates 977
Embryo Umbilical cord
Chorion
Placenta
Uterus
Amnion
Yolk
sac
FIGURE 48.38
The placenta. The placenta is characteristic of the largest group
of mammals, the placental mammals. It evolved from membranes
in the amniotic egg. The umbilical cord evolved from the
allantois. The chorion, or outermost part of the amniotic egg,
forms most of the placenta itself. The placenta serves as the
provisional lungs, intestine, and kidneys of the embryo, without
ever mixing maternal and fetal blood.
Dog
Deer
Beaver
Elephant Human
Grinding teeth
Ripping teeth
Chiseling teeth
Incisors Canine Premolars and molars
FIGURE 48.39
Mammals have different types of specialized teeth. While
reptiles have all the same kind of teeth, mammals have different
types of teeth specialized for different feeding habits. Carnivores
such as dogs, have canine teeth that are able to rip food; some of the
premolars and molars in dogs are also ripping teeth. Herbivores, such
as deer, have incisors to chisel off vegetation and molars designed to
grind up the plant material. In the beaver, the chiseling incisors
dominate. In the elephant, the incisors have become specialized
weapons, and molars grind up vegetation. Humans are omnivores;
we have ripping, chiseling, and grinding teeth.
Digesting Plants. Most mammals
are herbivores, eating mostly or only
plants. Cellulose, the major component
of plant cell walls, forms the bulk of a
plant’s body and is a major source of
food for mammalian herbivores. The
cellulose molecule has the structure of
a pearl necklace, with each pearl a glu-
cose sugar molecule. Mammals do not
have enzymes that can break the links
between the pearls to release the glu-
cose elements for use as food. Herbivo-
rous mammals rely on a mutualistic
partnership with bacteria that have the
necessary cellulose-splitting enzymes
to digest cellulose into sugar for them.
Mammals such as cows, buffalo,
antelopes, goats, deer, and giraffes
have huge, four-chambered stomachs
that function as storage and fermenta-
tion vats. The first chamber is the
largest and holds a dense population
of cellulose-digesting bacteria.
Chewed plant material passes into
this chamber, where the bacteria at-
tack the cellulose. The material is then digested further
in the rest of the stomach.
Rodents, horses, rabbits, and elephants are herbivores
that employ mutualistic bacteria to digest cellulose in a dif-
ferent way. They have relatively small stomachs, and in-
stead digest plant material in their large intestine, like a
termite. The bacteria that actually carry out the digestion
of the cellulose live in a pouch called the cecum that
branches from the end of the small intestine.
Even with these complex adaptations for digesting cellu-
lose, a mouthful of plant is less nutritious than a mouthful
of flesh. Herbivores must consume large amounts of plant
material to gain sufficient nutrition. An elephant eats 135
to 150 kg (300 to 400 pounds) each day.
Horns and Hooves. Keratin, the protein of hair, is also
the structural building material in claws, fingernails, and
hooves. Hooves are specialized keratin pads on the toes of
horses, cows, sheep, antelopes, and other running mam-
mals. The pads are hard and horny, protecting the toe and
cushioning it from impact.
The horns of cattle and sheep are composed of a core of
bone surrounded by a sheath of keratin. The bony core is
attached to the skull, and the horn is not shed. The horn
that you see is the outer sheath, made of hairlike fibers of
keratin compacted into a very hard structure. Deer antlers
are made not of keratin but of bone. Male deer grow and
shed a set of antlers each year. While growing during the
summer, antlers are covered by a thin layer of skin known
as velvet. A third type of horn, the rhinoceros horn, is com-
posed only of keratinized fibers with no bony core.
Flying Mammals. Bats are the only mammals capable of
powered flight (figure 48.40). Like the wings of birds, bat
wings are modified forelimbs. The bat wing is a leathery
membrane of skin and muscle stretched over the bones of
four fingers. The edges of the membrane attach to the side
of the body and to the hind leg. When resting, most bats
prefer to hang upside down by their toe claws. Bats are the
second largest order of mammals, after rodents. They have
been a particularly successful group because many species
have been able to utilize a food resource that most birds do
not have access to—night-flying insects.
How do bats navigate in the dark? Late in the eigh-
teenth century, the Italian biologist Lazzaro Spallanzani
showed that a blinded bat could fly without crashing into
things and still capture insects. Clearly another sense other
than vision was being used by bats to navigate in the dark.
When Spallanzani plugged the ears of a bat, it was unable
to navigate and collided with objects. Spallanzani con-
cluded that bats “hear” their way through the night world.
We now know that bats have evolved a sonar system
that functions much like the sonar devices used by ships
and submarines to locate underwater objects. As a bat
flies, it emits a very rapid series of extremely high-
pitched “clicking” sounds well above our range of human
hearing. The high-frequency pulses are emitted either
through the mouth or, in some cases, through the nose.
The soundwaves bounce off obstacles or flying insects,
and the bat hears the echo. Through sophisticated pro-
cessing of this echo within its brain, a bat can determine
not only the direction of an object but also the distance
to the object.
978 Part XII Animal Diversity
FIGURE 48.40
Greater horseshoe bat, Rhinolophus ferrumequinum. The bat is the only mammal
capable of true flight.
The Orders of Mammals
There are 19 orders of mammals. Seventeen of them (con-
taining 94% of the species) are placental. The other two
are the primitive monotremes and the marsupials.
Monotremes: Egg-laying Mammals. The duck-billed
platypus and two species of echidna, or spiny anteater, are
the only living monotremes (figure 48.41a). Among living
mammals, only monotremes lay shelled eggs. The structure
of their shoulder and pelvis is more similar to that of the
early reptiles than to any other living mammal. Also like
reptiles, monotremes have a cloaca, a single opening
through which feces, urine, and reproductive products
leave the body. Monotremes are more closely related to
early mammals than are any other living mammal.
In addition to many reptilian features, monotremes
have both defining mammalian features: fur and function-
ing mammary glands. Young monotremes drink their
mother’s milk after they hatch from eggs. Females lack
well-developed nipples so the babies cannot suckle. In-
stead, the milk oozes onto the mother’s fur, and the babies
lap it off with their tongues.
The platypus, found only in Australia, lives much of its
life in the water and is a good swimmer. It uses its bill
much as a duck does, rooting in the mud for worms and
other soft-bodied animals. Echidnas of Australia and
New Guinea have very strong, sharp claws, which they
use for burrowing and digging. The echidna probes with
its long, beaklike snout for insects, especially ants and
termites.
Marsupials: Pouched Mammals. The major difference
between marsupials (figure 48.41b) and other mammals is
their pattern of embryonic development. In marsupials, a
fertilized egg is surrounded by chorion and amniotic mem-
branes, but no shell forms around the egg as it does in
monotremes. During most of its early development, the
marsupial embryo is nourished by an abundant yolk within
the egg. Shortly before birth, a short-lived placenta forms
from the chorion membrane. Soon after, sometimes within
eight days of fertilization, the embryonic marsupial is born.
It emerges tiny and hairless, and crawls into the marsupial
pouch, where it latches onto a nipple and continues its de-
velopment.
Marsupials evolved shortly before placental mammals,
about 100 million years ago. Today, most species of marsu-
pials live in Australia and South America, areas that have
been historically isolated. Marsupials in Australia and New
Guinea have diversified to fill ecological positions occupied
by placental mammals elsewhere in the world. For example,
kangaroos are the Australian grazers, playing the role ante-
lope, horses, and buffalo perform elsewhere. The placental
mammals in Australia and New Guinea today arrived rela-
tively recently and include some introduced by humans.
The only marsupial found in North America is the Virginia
opossum.
Placental Mammals. Mammals that produce a true pla-
centa that nourishes the embryo throughout its entire de-
velopment are called placental mammals (figure 48.41c).
Most species of mammals living today, including humans,
are in this group. Of the 19 orders of living mammals, 17
are placental mammals. They are a very diverse group,
ranging in size from 1.5 g pygmy shrews to 100,000 kg
whales.
Early in the course of embryonic development, the pla-
centa forms. Both fetal and maternal blood vessels are
abundant in the placenta, and substances can be exchanged
efficiently between the bloodstreams of mother and off-
spring. The fetal placenta is formed from the membranes
of the chorion and allantois. The maternal side of the pla-
centa is part of the wall of the uterus, the organ in which
the young develop. In placental mammals, unlike marsupi-
als, the young undergo a considerable period of develop-
ment before they are born.
Mammals were not a major group until the dinosaurs
disappeared. Mammal specializations include the
placenta, a tooth design suited to diet, and specialized
sensory systems.
Chapter 48 Vertebrates 979
(a) (b)
(c)
FIGURE 48.41
Three types of mammals. (a) This echidna, Tachyglossus
aculeatus, is a monotreme. (b) Marsupials include kangaroos, like
this adult with young in its pouch. (c) This female African lion,
Panthera leo (order Carnivora), is a placental mammal.
980 Part XII Animal Diversity
Chapter 48
Summary Questions Media Resources
48.1 Attaching muscles to an internal framework greatly improves movement.
? The chordates are characterized by a dorsal nerve
cord and by the presence, at least early in
development, of a notochord, pharyngeal slits, and a
postanal tail. In vertebrates, a bony endoskeleton
provides attachment sites for skeletal muscle.
1. What are the four primary
characteristics of the chordates?
? Tunicates and the lancelets seem to represent ancient
evolutionary Chordate offshoots.
2. What are the three subphyla
of the chordates? Give an
example of each.
48.2 Nonvertebrate chordates have a notochord but no backbone.
? Vertebrates differ from other chordates in that they
possess a vertebral column, a distinct and well-
differentiated head, and a bony skeleton.
3. What is the relationship
between the notochord and the
vertebral column in vertebrates?
48.3 The vertebrates have an interior framework of bone.
? Members of the group Agnatha differ from other
vertebrates because they lack jaws.
? Jawed fishes constitute more than half of the
estimated 42,500 species of vertebrates and are
dominant in fresh and salt water everywhere.
? The first land vertebrates were the amphibians.
Amphibians are dependent on water and lay their
eggs in moist places.
? Reptiles were the first vertebrates fully adapted to
terrestrial habitats. Scales and amniotic eggs
represented significant adaptations to the dry
conditions on land.
? Birds and mammals were derived from reptiles and
are now among the dominant groups of animals on
land. The members of these two classes have
independently become endothermic, capable of
regulating their own body temperatures; all other
living animals are ectothermic, their temperatures set
by external conditions.
? The living mammals are divided into three major
groups: (1) the monotremes, or egg-laying mammals,
consisting only of the echidnas and the duck-billed
platypus; (2) the marsupials, in which the young are
born at a very early stage of development and
complete their development in a pouch; and (3) the
placental mammals, which lack pouches and suckle
their young.
4. What is one advantage of
possessing jaws? From what
existing structures did jaws
evolve?
5. What is the primary
disadvantage of a bony skeleton
compared to one made of
cartilage?
6. What is the lateral line
system in fishes? How does it
function?
7. The successful invasion of
land by amphibians involved five
major innovations. What were
they, and why was each
important?
8. How does the embryo obtain
nutrients and excrete wastes
while contained within the egg?
9. From what reptilian
structure are feathers derived?
10. How do amphibian, reptile,
and mammal legs differ?
11. Exactly how would you
distinguish a cat from a dog? (be
specific)
48.4 The evolution of vertebrates involves successful invasions of sea, land, and air.
BIOLOGY
RAVEN
JOHNSON
SIX TH
EDITION
www.mhhe.com/raven6ch/resource28.mhtml
? Chordates
? Introduction to
Vertebrates
? Enhancement Chapter:
Dinosaurs, Sections 6
and 7
? Activity: Lamprey
? Activity: Fin Fish
? Fish
? Amphibians
? Reptiles
? Birds
? Mammals
? Enhancement Chapter:
Dinosaurs, Sections 5
? Book Review: The
Pope’s Rhinoceros by
Norfolk
? Student Research:
Phylogeny of Hylid
Frogs
? Student Research:
Metamorphosis in
Flatfish
? Evolution of Fish
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