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The Plant Kingdom: Seedless Plants
Moss-covered rocks.
Shown is Elowa Falls, Columbia
River Gorge, Oregon.
A
Darrell Gulin /Getty Images
bout 445 million years ago (mya), the ocean was filled with
vast numbers of fishes, mollusks, and crustaceans as well as
countless microscopic algae, and the water along rocky coastlines
was home to large seaweeds. Occasionally, perhaps, an animal
would crawl out of the water onto land, but it never stayed there
permanently because there was little to eat on land—not a single
blade of grass, no fruit, and no seeds.
During the next 30 million years, a time corresponding roughly
K EY C ONCE P TS
Biologists infer that plants evolved from aquatic green algal
ancestors known as a charophytes.
Adaptations to life on land that have evolved in plants
include a waxy cuticle to prevent water loss; multicellular
gametangia; stomata; and for most plants, vascular tissues
containing lignin.
to the Silurian period (see Table 21-1), plants appeared in abundance and colonized the land. Where did they come from? Although plants living today exhibit great diversity in size, form,
and habitat (see photograph), biologists hypothesize that they all
evolved from a common ancestor that was an ancient green alga.
Recall from Chapter 25 that red algae, green algae, and land plants
are collectively classified as “plants.” Biologists infer this common
Plants undergo an alternation of generations between multicellular gametophyte and sporophyte generations.
ancestor because modern green algae share many biochemical
Mosses and other bryophytes lack vascular tissues and do
not form true roots, stems, or leaves.
plants contain the same photosynthetic pigments: chlorophylls a
In club mosses and ferns, lignin-hardened vascular tissues
that transport water and dissolved substances throughout
the plant body have evolved.
and metabolic traits with modern plants. Both green algae and
and b and carotenoids, including xanthophylls (yellow pigments)
and carotenes (orange pigments). Both store excess carbohydrates
as starch and have cellulose as a major component of their cell
walls. In addition, plants and some green algae share certain details of cell division, including formation of a cell plate during cytokinesis (see Chapter 10).
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Land plants are complex multicellular organisms that range in
plant kingdom consists of hundreds of thousands of species that
size from minute, almost microscopic duckweeds to massive giant
live in varied habitats, from frozen arctic tundra to lush tropical rain
sequoias, some of the largest organisms that have ever lived. The
forests to harsh deserts to moist stream banks.
■
ADAPTATIONS OF PLANTS
Developing
gametes
Learning Objectives
1
2
Discuss some environmental challenges of living on
land, and describe how several plant adaptations meet
these challenges.
Name the green algal group from which plants are hypothesized to have descended, and describe supporting evidence.
What are some features of plants that have let them colonize
so many types of environments? One important difference
(a)
between plants and algae is that a waxy cuticle covers the
aerial portion of a plant. Essential for existence on land, the cuticle helps prevent desiccation, or drying out, of plant tissues by
evaporation. Plants that are adapted to moister habitats may have
a very thin layer of wax, whereas those adapted to drier environments often have a thick, crusty cuticle. (Many desert plants also
have a reduced surface area, particularly of leaves, which minimizes water loss.)
Plants obtain the carbon they need for photosynthesis from
the atmosphere as carbon dioxide (CO2). To be fixed into organic molecules such as sugar, CO2 must first diffuse into the
chloroplasts that are inside green plant cells. Because a waxy cuticle covers the external surfaces of leaves and stems, however,
gas exchange through the cuticle between the atmosphere and
the interior of cells is negligible. Tiny pores called stomata (sing.,
stoma), which dot the surfaces of leaves and stems of almost all
plants, facilitate gas exchange; algae lack stomata.
The sex organs, or gametangia (sing., gametangium), of
most plants are multicellular, whereas the gametangia of algae
are unicellular (❚ Fig. 27-1). Each plant gametangium has a layer
of sterile (nonreproductive) cells that surrounds and protects the
delicate gametes (eggs and sperm cells). In plants, the fertilized
egg develops into a multicellular embryo (young plant) within
the female gametangium. Thus, the embryo is protected during
its development. In algae, the fertilized egg develops away from
its gametangium; in some algae, the gametes are released before
fertilization, whereas in others the fertilized egg is released.
(b)
Unicellular
gametangium
Figure 27-1
and plants
Sterile cells
Multicellular
gametangium
(c)
(d)
Generalized reproductive structures of algae
(a, b) In algae, gametangia are generally unicellular. When the gametes are released, only the wall of the original cell remains. (c, d) In
plants, the gametangia are multicellular, but only the inner cells become gametes. The gametes are surrounded by a protective layer
of sterile (nonreproductive) cells.
Gametophyte
Spore
Sperm
Egg
HAPLOID (n)
GAMETOPHYTE
GENERATION
Meiosis
Fertilization
DIPLOID (2n)
SPOROPHYTE
GENERATION
Zygote
Embryo
Sporophyte
The plant life cycle alternates haploid
and diploid generations
Plants have a clearly defined alternation of generations in which
they spend part of their lives in a multicellular haploid stage and
part in a multicellular diploid stage (❚ Fig. 27-2).1 The haploid
portion of the life cycle is called the gametophyte generation because it gives rise to haploid gametes by mitosis. When two gam1
For convenience we limit our discussion to plants that are not polyploid, although polyploidy is very common in the plant kingdom. We
therefore use the terms diploid and 2n (and haploid and n) interchangeably, although these terms are not actually synonymous.
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Chapter 27
Figure 27-2
The basic plant life cycle
Plants have an alternation of generations, spending part of the cycle in
a haploid gametophyte stage and part in a diploid sporophyte stage.
Depending on the plant group, the haploid or the diploid stage may
be greatly reduced.
etes fuse, the diploid portion of the life cycle, called the sporophyte generation, begins. The sporophyte generation produces
haploid spores by the process of meiosis; these spores represent
the first stage in the gametophyte generation.
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Let us examine alternation of generations more closely. The
haploid gametophytes produce male gametangia, known as antheridia (sing., antheridium), in which sperm cells form, and /or
female gametangia, known as archegonia (sing., archegonium),
each bearing a single egg (❚ Fig. 27-3). Sperm cells reach the female gametangium in a variety of ways, and one sperm cell fertilizes the egg to form a zygote, or fertilized egg.
The diploid zygote is the first stage in the sporophyte generation. The zygote divides by mitosis and develops into a multicellular embryo, the young sporophyte plant. Embryo development
takes place within the archegonium; thus, the embryo is protected as it develops. Eventually the embryo grows into a mature
sporophyte plant. The mature sporophyte has special cells called
sporogenous cells (spore-producing cells, also called spore mother
cells) that divide by meiosis to form haploid spores.
Fertilization of egg by sperm cell ¡ zygote ¡ embryo ¡
mature sporophyte plant ¡ sporogenous cells ¡ meiosis
¡ spores
All plants produce spores by meiosis, in contrast with algae and fungi, which may produce spores by meiosis or mitosis.
The spores represent the first stage in the gametophyte genera-
Developing
sperm cells
Sterile cells
Antheridium
tion. Each spore divides by mitosis to produce a multicellular
gametophyte, and the cycle continues. Plants therefore alternate
between a haploid gametophyte generation and a diploid sporophyte generation.
archegonia ¡ eggs
mature
¡
Spores ¡
gametophyte plants ¡
antheridia ¡ sperm cells
Four major groups of plants evolved
Recent ultrastructural and molecular data indicate that land plants
probably descended from a group of green algae called charophytes or stoneworts (see Fig. 25-16d). Molecular comparisons,
particularly of DNA and RNA sequences, provide compelling
evidence that charophytes are closely allied to plants. These comparisons among plants and various green algae include sequences
of chloroplast DNA, certain nuclear genes, and ribosomal RNA.
In each case, the closest match occurs between charophytes and
plants, indicating that modern charophytes and plants probably
share a recent common ancestor.
The plant kingdom consists of four major groups: bryophytes, seedless vascular plants, and two groups of seeded vascular plants: gymnosperms and angiosperms (flowering plants)
(❚ Fig. 27-4; see also ❚ Table 27-1, which is an overview of plant
phyla). The mosses and other bryophytes are small nonvascular
plants that lack a specialized vascular, or conducting, system to
transport nutrients, water, and essential minerals (inorganic nutrients) throughout the plant body. In the absence of such a system, bryophytes rely on diffusion and osmosis to obtain needed
TABLE 27-1
The Plant Kingdom
(a) Each antheridium, the male gametangium, produces
numerous sperm cells.
Nonvascular plants with a dominant gametophyte generation
(bryophytes)
Phylum Bryophyta (mosses)
Phylum Hepatophyta (liverworts)
Phylum Anthocerophyta (hornworts)
Vascular plants with a dominant sporophyte generation
Seedless plants
Phylum Lycopodiophyta (club mosses)
Egg
Archegonium
Sterile
cells
Phylum Pteridophyta (ferns and their allies, the whisk ferns
and horsetails)
Seed plants
Plants with naked seeds (gymnosperms)
Phylum Coniferophyta (conifers)
Phylum Cycadophyta (cycads)
Phylum Ginkgophyta (ginkgoes)
(b) Each archegonium, the female gametangium, produces
a single egg.
Phylum Gnetophyta (gnetophytes)
Seeds enclosed within a fruit
Phylum Anthophyta (angiosperms or flowering plants)
Figure 27-3
Plant gametangia
Class Eudicotyledones (eudicots)
Class Monocotyledones (monocots)
Shown are generalized moss gametangia.
The Plant Kingdom: Seedless Plants
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Key Point
This cladogram shows hypothetical evolutionary relationships among
living plants, based on current evidence. The four major groups of plants
are bryophytes, seedless vascular plants, and two groups of seed plants—
gymnosperms and angiosperms.
VASCULAR SEED
PLANTS
Angiosperms
Gymnosperms
Ferns
VASCULAR SEEDLESS
PLANTS
Club mosses
Mosses
Liverworts
Hornworts
NONVASCULAR BRYOPHYTES
Evolution
of seeds
Evolution of dominant
sporophyte, vascular
tissue
Evolution of cuticle, multicellular
gametangia, multicellular embryos
Green
algal
ancestor
plant cell wall chemistry, including lignin). The stiffening property of lignin enabled plants to grow tall
(which let them maximize light interception). The
successful occupation of the land by plants, in turn,
made the evolution of terrestrial animals possible by
providing them with both habitat and food.
Club mosses and ferns (which include whisk
ferns and horsetails) are seedless vascular plants
that, like the bryophytes, reproduce and disperse
primarily via spores. Seedless vascular plants arose
and diversified during the Silurian and Devonian
periods of the Paleozoic era, between 444 mya and
359 mya. Club mosses and ferns extend back more
than 420 million years and were of considerable importance as Earth’s dominant plants in past ages.
Fossil evidence indicates that many species of these
plants were the size of immense trees. Most living
representatives of club mosses and ferns are small.
The gymnosperms are vascular plants that reproduce by forming seeds. Gymnosperms produce
seeds borne exposed (unprotected) on a stem or in
a cone. Plants with seeds as their primary means of
reproduction and dispersal first appeared about 359
mya, at the end of the Devonian period. These early
seed plants diversified into many varied species of
gymnosperms.
The most recent plant group to appear is the
flowering plants, or angiosperms, which arose during the early Cretaceous period of the Mesozoic era,
about 130 mya. Like gymnosperms, flowering plants
reproduce by forming seeds. Flowering plants, however, produce seeds enclosed within a fruit.
Review
❚
Figure 27-4
Animated
❚
Plant evolution
Cladograms such as this one represent an emerging consensus that is open to change
as new discoveries are made. Although the arrangement of nonvascular, seedless vascular, and seed plant groupings is widely recognized, the order in which the hornworts,
liverworts, and mosses evolved is not yet resolved.
materials. This reliance means that bryophytes are restricted in
size; if they were much larger, some of their cells could not obtain
enough necessary materials. Bryophytes do not form seeds, the reproductive structures discussed in Chapter 28. Bryophytes reproduce and disperse primarily via haploid spores. Recent molecular
and fossil evidence, discussed later in the chapter, suggests that
bryophytes may have been the earliest plants to colonize land.
The other three groups of plants — seedless vascular plants,
gymnosperms, and flowering plants —have vascular tissues and
are thus known as vascular plants. The two vascular tissues are xylem, for conducting water and dissolved minerals, and phloem,
for conducting dissolved organic molecules such as sugar. A key
step in the evolution of vascular plants was the ability to produce
lignin, a strengthening polymer in the walls of cells that function
for support and conduction (see Chapter 32 for a discussion of
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Chapter 27
❚
❚
What are the most important environmental
challenges that plants face living on land?
What adaptations do plants have to meet these
environmental challenges?
From which group of green algae are plants
hypothesized to have descended?
What is alternation of generations in plants?
BRYOPHYTES
Learning Objectives
3
4
5
Summarize the features that distinguish bryophytes from
other plants.
Name and briefly describe the three phyla of bryophytes.
Describe the life cycle of mosses, and compare their gametophyte and sporophyte generations.
The bryophytes (from the Greek words meaning “moss plant”)
consist of about 16,000 species of mosses, liverworts, and hornworts; bryophytes are the only living nonvascular plants (❚ Table
27-2). Because they have no means for extensive internal transport of water, sugar, and essential minerals, bryophytes are typically quite small. They generally require a moist environment for
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Figure 27-18
Figure 27-19
Reconstruction of Rhynia gwynne-vaughanii
Reconstruction of Aglaophyton major
This leafless
fl
plant, one of Earth’s earliest vascular plants, is now extinct.
It grew about 18 cm (7 in) tall. (Redrawn from D. Edwards, “Evidence
for the Sporophytic Status of the Lower Devonian Plant Rhynia gwynnevaughanii,” Review of Palaeobotany and Palynology, Vol. 29, 1980.)
Recent evidence indicates that this plant, although superficially
fi
similar to other early vascular plants, lacked conducting tissues that are
characteristic of vascular plants. For this reason, it has been reclassified
fi
into a new genus and is no longer considered a rhyniophyte. (Redrawn
from J. D. Mauseth, Botany: An Introduction to Plant Biology, 2nd ed.,
Saunders College Publishing, Philadelphia, 1995.)
is an ongoing enterprise, and over time, existing knowledge is
re-evaluated in light of newly discovered evidence. Aglaophyton
majorr is an excellent example of the self-correcting nature of
science, which is in a perpetually dynamic state and changes in
response to newly available techniques and data.
❚
Review
❚
What adaptations do ferns have that both algae and bryophytes lack?
❚
❚
❚
How does one distinguish between megaphylls and
microphylls?
Which of the following are parts of the sporophyte generation in ferns: frond, sperm cells, egg cell, roots, sorus, sporangium, spores, prothallus, rhizome, antheridium, archegonium, and zygote?
Why are whisk ferns and horsetails now classifi
fied as ferns?
How does heterospory modify the life cycle?
S U M M ARY WI T H K EY TERMS
Learning Objectives
1
Discuss some environmental challenges of living on land, and
describe how several plant adaptations meet these challenges (page 582).
❚ The colonization of land by plants required the evolution
of many anatomical, physiological, and reproductive
adaptations. Plants have a waxy cuticle to protect against
water loss and stomata for gas exchange needed for
photosynthesis.
❚ Plant life cycles have an alternation of generations in
which they spend part of their life cycle in a haploid game-
tophyte generation and part in a diploid sporophyte
generation. The gametophyte plant produces gametes
by mitosis. During fertilization these gametes fuse to form
a zygote, the fi
first stage of the sporophyte generation.
The zygote develops into a multicellular embryo that
the gametophyte protects and nourishes. The mature
sporophyte plant develops from the embryo and produces
sporogenous cells (spore mother cells). These undergo
meiosis to form spores, the first stage in the gametophyte
generation.
The Plant Kingdom: Seedless Plants
597
❚ Most plants have multicellular gametangia with a protective jacket of sterile cells surrounding the gametes.
Antheridia are gametangia that produce sperm cells,
and archegonia are gametangia that produce eggs.
❚ Ferns and other vascular plants have xylem to conduct
water and dissolved minerals and phloem to conduct
dissolved sugar.
2 Name the green algal group from which plants are hypothesized to have descended, and describe supporting evidence
(page 582).
❚ Plants probably arose from a group of green algae called
charophytes. This conclusion is based in part on molecular comparisons of DNA and RNA sequences, which show
the closest match between charophytes and plants.
Explore plant evolution by clicking on the
figure in ThomsonNOW.
Summarize the features that distinguish bryophytes from
other plants (page 584).
❚ Unlike other land plants, bryophytes are nonvascular and
lack xylem and phloem. Bryophytes are the only plants
with a dominant gametophyte generation. Their sporophytes remain permanently attached and nutritionally
dependent on the gametophytes.
4 Name and briefly describe the three phyla of bryophytes
(page 584).
❚ Mosses (phylum Bryophyta) have gametophytes that are
green plants that grow from a filamentous protonema.
❚ Many liverworts (phylum Hepatophyta) have gametophytes that are flattened, lobelike thalli; others are leafy.
❚ Hornworts (phylum Anthocerophyta) have thalloid
gametophytes.
5 Describe the life cycle of mosses, and compare their gametophyte and sporophyte generations (page 584).
❚ The green moss gametophyte bears archegonia and/or
antheridia at the top of the plant. During fertilization, a
sperm cell fuses with an egg cell in the archegonium. The
zygote grows into an embryo that develops into a moss
sporophyte, which is attached to the gametophyte. Meiosis occurs within the capsule of the sporophyte to produce
spores. When a spore germinates, it grows into a protonema that forms buds that develop into gametophytes.
3
Watch the life cycles of the mosses and liverworts by clicking on the figures in ThomsonNOW.
Discuss the features that distinguish seedless vascular plants
from algae and bryophytes (page 589).
❚ Seedless vascular plants have several adaptations that
algae and bryophytes lack, including vascular tissues and
a dominant sporophyte generation. As in bryophytes,
reproduction in seedless vascular plants depends on water
as a transport medium for motile sperm cells.
7 Name and briefly describe the two phyla of seedless vascular
plants (page 589).
❚ Sporophytes of club mosses (phylum Lycopodiophyta)
consist of roots, rhizomes, erect branches, and leaves that
are microphylls.
❚ Ferns (phylum Pteridophyta) are the largest and most
diverse group of seedless vascular plants. The fern sporophyte consists of a rhizome that bears fronds and true
roots. Phylum Pteridophyta also includes whisk ferns and
horsetails. Sporophytes of whisk ferns have dichotomously branching rhizomes and erect stems; they lack
true roots and leaves. Horsetail sporophytes have roots,
rhizomes, aerial stems that are hollow and jointed, and
leaves that are reduced megaphylls.
8 Describe the life cycle of ferns, and compare their sporophyte and gametophyte generations (page 589).
❚ Fern sporophytes have roots, rhizomes, and leaves that
are megaphylls. Their leaves, or fronds, bear sporangia in
clusters called sori. Meiosis in sporangia produces haploid
spores. The fern gametophyte, called a prothallus, develops from a haploid spore and bears both archegonia and
antheridia.
6
Watch the life cycle of the ferns by clicking on
the figure in ThomsonNOW.
9
Compare the generalized life cycles of homosporous and
heterosporous plants (page 589).
❚ Homospory, the production of one kind of spore, is
characteristic of bryophytes, most club mosses, and most
ferns, including whisk ferns and horsetails. In homospory,
spores give rise to gametophyte plants that produce both
egg cells and sperm cells.
❚ Heterospory, the production of two kinds of spores (microspores and megaspores), occurs in certain club mosses,
certain ferns, and all seed plants. Microspores give rise
to male gametophytes that produce sperm cells. Megaspores give rise to female gametophytes that produce
eggs. The evolution of heterospory was an essential step
in the evolution of seeds.
T E S T Y O U R U N DE R S TAND ING
1. The bryophytes (a) include mosses, liverworts, and hornworts
(b) include whisk ferns, horsetails, and club mosses (c) are
small plants that lack a vascular system (d) a and c (e) b and c
2. The waxy layer that covers aerial parts of plants is the (a) cuticle (b) archegonium (c) protonema (d) stoma (e) thallus
3. A strengthening compound found in cell walls of vascular
plants is (a) xanthophyll (b) lignin (c) cutin (d) cellulose
(e) carotenoid
4. Stomata (a) help prevent desiccation of plant tissues
(b) transport water and minerals through plant tissues
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Chapter 27
(c) allow gas exchange for photosynthesis (d) strengthen cell
walls (e) produce male gametes
5. The female gametangium, or
, produces an
egg; the male gametangium, or
, produces
sperm cells. (a) antheridium; archegonium (b) archegonium;
megaphyll (c) megasporangium; antheridium (d) archegonium; antheridium (e) megasporangium; megaphyll
6. Liverworts and hornworts share life-cycle similarities with
(a) ferns (b) mosses (c) horsetails (d) club mosses (e) whisk
ferns
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7. The green, gametangia-bearing moss plant (a) is the haploid
gametophyte generation (b) is the diploid sporophyte generation (c) is called a protonema (d) contains cells with single
large chloroplasts (e) b and c
to conduct
8. Seedless vascular plants have
water and minerals and
to conduct sugar.
(a) cuticle; xylem (b) phloem; stomata (c) phloem; xylem
(d) stomata; cuticle (e) xylem; phloem
9. A(an)
is a leaf that arose from a branch
system. (a) antheridium (b) microphyll (c) megaphyll
(d) sorus (e) microspore
10. These plants have vascularized stems but lack true roots and
leaves. (a) mosses (b) club mosses (c) horsetails (d) whisk
ferns (e) hornworts
11. These plants have hollow, jointed stems that are impregnated
with silica. (a) mosses (b) club mosses (c) horsetails (d) whisk
ferns (e) hornworts
12. Which of the following statements about ferns is not true?
(a) ferns have motile sperm cells that swim through water to
the egg-containing archegonium (b) ferns are vascular plants
(c) ferns are the most economically important group of bryophytes (d) the fern sporophyte consists of a rhizome, roots,
and fronds (e) the diversity of ferns is greatest in the tropics
13. Plants probably descended from a group of green algae called
(a) rhyniophytes (b) Calamites (c) epiphytes (d) charophytes
(e) club mosses
14. Which of the following is not a characteristic of plants? (a) cuticle (b) unicellular gametangia (c) stomata (d) multicellular
embryo (e) alternation of generations
15. In plant life cycles (a) the first products of meiosis are gametes (b) spores are part of the diploid sporophyte generation
(c) the embryo gives rise to a zygote (d) the first stage in the
diploid sporophyte generation is the zygote (e) the first stage
in the haploid gametophyte generation is the prothallus
C R I TI C AL TH I N KI NG
1. Which group probably colonized the land first, plants or
animals? Explain.
2. What adaptations do bryophytes have that algae lack?
3. Evolution Link. How may the following trends in plant
evolution be adaptive to living on land?
a. dependence on water for fertilization ¡ no need for
water as a transport medium
b. homospory ¡ heterospory
4. Analyzing Data. According to the cladogram in Figure 27-4,
which plants evolved first: nonvascular bryophytes, seedless
vascular plants, or seed plants? Which clade would be considered the outgroup: hornworts, liverworts, or mosses?
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The Plant Kingdom: Seedless Plants
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