32 EVOLUTION OF PLANTS
CHAPTER OUTLINE
Plants (p. 672)
32.1
32.2
Adapting to Terrestrial Living (p. 672; Figs. 32.1, 32.2, 32.3)
A. Plants are multicellular, terrestrial autotrophs that occur almost exclusively on land and derive
energy through photosynthesis.
B. The green algae gave rise to the diversity of land plants, which have now evolved into nearly
300,000 different species.
C. Three major challenges had to be overcome for living on land: organisms had to absorb
minerals, conserve water, and develop a new mode of reproduction suitable for land.
D. Absorbing Minerals
1. Many of the early plants had symbiotic fungi, such as the present-day mycorrhizae, to aid
in the uptake of minerals and nutrients from rocky shores devoid of soil.
E. Conserving Water
1. A waxy cuticle evolved to help retard water loss.
2. Gases could be exchanged with the atmosphere through tiny openings called stomata that
could also be closed to further reduce water loss.
F. Reproducing on Land
1. The development of spores was an evolutionary trend that protected reproductive cells
from drying.
2. Transferring gametes from one plant to the next during sexual reproduction posed a
problem that has been answered by the development of spores in many plant lines.
3. Haploid spores are produced by sporophytes, the diploid generation that alternates with
the haploid gamete-producing gametophyte generation.
4. This alternation of generations solved many problems for plants reproducing sexually on
land.
Plant Evolution (p. 674; Fig. 32.4; Table 32.1)
A. As plants evolved greater specializations on land, below-ground root systems developed,
along with vascular tissue to connect above- and below-ground portions of plants.
B. Four key innovations highlight the evolutionary trends in plants: an alternation of generations,
vascular tissue, seeds, and flowers and fruits.
Seedless Plants (p. 676)
32.3
32.4
Nonvascular Plants (p. 676; Fig. 32.5)
A. Liverworts and Hornworts
1. The liverworts and hornworts are two groups of primitive plants that completely lack any
kind of vascular system.
2. These small plants can only survive in moist habitats.
B. Primitive Conducting Systems: Mosses
1. Mosses, a third type of fairly primitive plant, managed to evolve a simple, soft-sided tube
capable of conducting water over short distances.
2. In the liverworts, hornworts, and mosses, the main plant body is the gametophyte, with
the sporophyte being borne on the gametophyte.
The Evolution of Vascular Tissue (p. 677; Figs. 32.6, 32.7)
A. Vascular plants have specialized cells that can conduct materials and fluids.
B. Seven phyla of plants have such vascular tissue.
C. The fossil record indicates that vascular plants have been around for 430 million years.
D. Vascular tissue consists of strands of specialized cells the form a network throughout the
plant.
32.5
E. Two types of vascular tissue can be found in vascular plants.
1. Xylem conducts water and minerals from the roots upward to the rest of the plant
2. Phloem conducts carbohydrates throughout the plant.
F. How vascular plants grow has changed somewhat over the course of evolution.
1. Primitive vascular plants exhibited primary growth only, which involves cell division at
the tips of the stem and roots.
2. Around 380 million years ago, vascular plants developed secondary growth, the ability to
grow in width, in addition to primary growth.
3. Secondary growth produces wood.
Seedless Vascular Plants (p. 678; Figs. 32.8, 32.9)
A. Two phyla of modern vascular plants lack seeds and rely on the presence of water to enable
sperm to swim to their destination for fertilization.
B. Ferns are among the most abundant of the seedless vascular plants, and are especially
numerous in the tropics.
C. The Life of a Fern
1. Vascular seedless plants have independent gametophyte and sporophyte individuals, but
the sporophyte generation is dominant.
2. Spores are produced on the fronds and fall to the earth to develop into gametophytes.
3. On the small heart-shaped gametophytes, gametes are produced, fertilization occurs, and
a new sporophyte grows.
The Advent of Seeds (p. 680)
32.6
32.7
Evolution of Seed Plants (p. 680; Figs. 32.10, 32.11, 32.12)
A. The evolution of seeds has greatly enhanced the ability of plants to survive on land.
B. A seed protects the developing embryo when it is most vulnerable to drying out or predation.
C. In seed plants, the gametophyte generation is even smaller than in seedless vascular plants,
and gametophytes exist entirely within the sporophyte plant.
1. Pollen grains, or male gametophytes, develop from microspores and are transported by
various means, depending upon the particular adaptations of a plant.
2. The female gametophyte develops from a megaspore produced within an ovule; the egg
is fertilized only after pollination.
D. The first seed plants were the gymnosperms with incompletely enclosed ovules.
E. Angiosperms evolved later, with their ovules completely enclosed within a vessel called a
carpel.
F. The Structure of a Seed
1. A seed is made up of a sporophyte embryo surrounded by a hard, protective seed coat.
2. Nourishing endosperm is also housed within the seed coat to provide a food source until
germination.
3. Seeds are adapted to life on land because they enable better dispersal, or colonization of
new areas or better habitats, and because they protect the embryo during a period of
dormancy when the embryo is waiting for better conditions before germination.
4. Seeds are also tuned into the appropriate environmental cues that indicate when moisture
and temperature are optimal before germination and growth of the embryo out of the
seed.
Gymnosperms (p. 682; Figs. 32.13, 32.14)
A. Gymnosperms are represented on earth today by four different phyla: conifers, cycads,
gnetophytes, and ginkgos.
B. We are most familiar with the conifers whose needlelike leaves help resist water loss.
C. The Life of a Gymnosperm
1. Conifers form two kinds of cones: seed cones that bear female gametophytes and pollen
cones that produce male gametophytes.
2. Conifers rely on the wind to transfer pollen to the seed cone.
3. Once the sperm cell inside the pollen fuses with the egg cell, a new zygote is formed that
can eventually develop into a sporophyte.
The Evolution of Flowers (p. 684)
32.8
32.9
32.10
32.11
Rise of the Angiosperms (p. 684)
A. Angiosperms comprise about 250,000 species or 90% of all living plants.
B. Angiosperms evolved special structures, called flowers, to attract and reward animal
pollinators, which has led to their success.
C. The Flower
1. Flowers are structured into four concentric whorls.
2. The outermost whorl, made up of sepals, protects the flower while it is still a bud.
3. The second whorl is made up of the colorful petals that attract pollinators.
4. The third whorl is the male portion of the flower, made up of stamens that bear a pollenproducing swollen tip called the anther.
5. The innermost whorl is female in nature, and contains the carpel that houses the ovary
and ovule, a long stalk called the style leading to the ovules, and a stigma at the tip that
receives pollen.
6. When pollen arrives at the stigma, a pollen tube grows down through the style to reach
the ovule.
Why Are There Different Kinds of Flowers? (p. 685; Figs. 32.15, 32.16)
A. Some flower adaptations that have coevolved to attract and satisfy the needs of the pollinator
include unique petal arrangements and colors, patterns of color, odor, and nectar rewards.
B. Angiosperms that rely on wind pollination, like grasses, oaks, and birches, do not have large,
showy, fragrant flowers.
Double Fertilization (p. 686; Figs. 32.17, 32.18)
A. Angiosperm seeds have a greatly improved food source for the growing embryo.
B. Endosperm is a triploid nutritive source for the growing embryo, and forms as a result of
double fertilization.
C. One sperm nucleus fertilizes the ovule while another fuses with the polar nuclei to form the
triploid endosperm.
D. Endosperm divides very rapidly to ensure an adequate food supply for the embryo.
E. In some angiosperms, food is stored in swollen, fleshy leaves called cotyledons.
1. Dicots are plants with two cotyledons, like oak and maple trees.
2. Monocots have only one cotyledon and other characteristics different from dicots.
3. Grasses are among the most successful monocots.
Fruits (p. 687; Fig. 32.19)
A. Fruits are a means of dispersing seeds.
B. A fruit is a ripened, fleshy ovary containing the seeds.
C. Animals eat fruits and pass the seeds in feces.
D. Many specialized types of fruit can be found among the angiosperms.
KEY TERMS
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autotroph (p. 672)
cuticle (p. 672) The cuticle is a waxy, waterproof substance that prevents water loss to the air.
stoma (p. 672) In leaves and in some green stems, carbon dioxide enters through these openings that
can be closed to prevent water loss.
alternation of generations (p. 673)
gametophyte (p. 673) Haploid generation; forms haploid gametes by mitosis.
sporophyte (p. 673) Diploid generation; forms haploid spores by meiosis.
liverwort (p. 676)
hornwort (p. 676)
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moss (p. 676) People often confuse mosses with algae, calling growth on stream rocks “moss” when it
is algae.
vascular tissue (p. 677)
primary growth (p. 677) This involves cell division at the tips of the shoots and roots.
secondary growth (p. 677) Secondary growth is growth in girth.
ferns (p. 679) A group of seedless vascular plants with about 11,000 living species.
fronds (p. 679)
seed (p. 680) A seed is an embryo with a watertight cover.
gymnosperm (p. 680)
angiosperm (p. 680) Angiosperms have an ovule that is completely enclosed by the sporophyte when
fertilized.
pollination (p. 680)
pollen (p. 680)
endosperm (p. 681) Nutritious triploid tissue within the seed.
cotyledon (p. 681)
conifers (p. 683) Conifers are one type of gymnosperm that produces its seeds in cones.
flower (p. 684) Flowers are the reproductive organs of angiosperms.
double fertilization (p. 686) Fertilization of both the zygote and endosperm is double fertilization.
dicot (p. 687)
monocot (p. 687)
fruit (p. 687)
LECTURE SUGGESTIONS AND ENRICHMENT TIPS
1.
Seedless Vascular Plants. Few people give much notice to the seedless vascular plants, partly
because they are small, and partly because of lack of knowledge about them. Do a “show and tell”
demonstration of the seedless vascular plants, allowing students to examine them individually. Show
them examples of Marchantia with its archegoniophores and antheriodiophores, and Mnium or similar
moss species with its archegonial heads. Demonstration specimens of a variety of ferns and prepared
slides of fern prothalli are useful in illustrating the stages of a fern life cycle.
2.
3.
Endangered Plants. Students are fairly well aware of the idea of endangered species, although many
people automatically think of mammals and birds when they hear that phrase. What we don't often
realize is the importance of endangered plants, and the effect of plant species extinctions on
ecosystems. The fossil record indicates that five or six large bouts of extinction have occurred over the
past 500 million years, and most of the organisms going extinct at once were animals. We tend to
notice when animals in our vicinity become less numerous, but we are less apt to notice, or perhaps
care about, the plants. However, plants are more important from an ecological standpoint because most
insect and other animal species depend on plants in one way or another to stay alive. Probably 10% of
the world's plant species are now facing extinction, and in a few short years, as many as 25% of the
world's plant species can be expected to be threatened with extinction. Questions to consider as you
discuss this with your students are: What can be done to slow down extinction rates of plants? Where
are many extinctions occurring? (In the tropics, but examples can be found everywhere.) What will
happen to the animals when the plants on which they depend become extinct? What can each of us do
to help solve this problem?
Plants During the Time of the Dinosaur. What types of plants dominated the landscape during the day
of the dinosaur? Illustrate your presentation with drawings of the types of plants present during various
geologic periods. Bring in fossil specimens, or photographs of same, as available. During the Silurian
period of the Paleozoic era (around 410 million years ago), the low-lying primitive vascular land plants
first appeared. Seed ferns evolved during the Devonian period, approximately 390 million years ago.
During the Carboniferous period, 286–360 million years ago, the great coal-forming forests dominated
the earth. Ferns, horsetails, and club mosses were at their most numerous. Conifers appeared in the
fossil record next, some 246–286 million years ago during the Permian period. At the beginning of the
Mesozoic era, during the dawn of the dinosaurs, forests of ferns and gymnosperms covered the
landscape, and cycads and ginkgoes first appeared in the fossil record. When dinosaurs reached
immense sizes and numbers in the Jurassic period 150–200 million years ago, cycads and other
gymnosperms flourished. From 70 to 150 million years ago, the flowering plants appeared, placental
mammals evolved, and dinosaurs went extinct in massive numbers at the close of the Cretaceous
period. From then on, the angiosperms and mammals have been the dominant organisms, along with
modern insect groups and, most recently, humans.
CRITICAL THINKING QUESTIONS
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3.
Describe the evidence that suggests land plants evolved from green algae.
Explain why the evolution of the seed was a critical step in the domination of land by plants.
In what geographical areas are gymnosperms, rather than angiosperms, dominant and why?