Mastering Concepts
19.1
1. What is the evidence that plants evolved from green algae?
Green algae and plants have the following features in common: Photosynthetic pigments
such as chlorophyll a; use of starch as a storage carbohydrate; cell walls made of
cellulose; similar life cycles that feature alternation of generations. DNA sequences
support the hypothesis that plants evolved from green algae as well.
2. Suppose a plant has a mutation that prevents it from closing its stomata. What would
be the consequence?
Stomata are pores that allow plants to absorb CO2 required in photosynthesis, but water
also evaporates from leaf tissues through open stomata. If a plant could not close its
stomata, water would evaporate continuously from its leaves. This loss would not harm
the plant as long as water was available in soil to replace the water lost to evaporation.
But the plant would dry out (and eventually die) if the rate of water loss exceeded the rate
at which water entered the plant through the roots.
3. How does vascular tissue adapt plants to land?
Vascular tissue transports water and nutrients throughout the bodies of plants, an
adaptation that allows for the specialization of roots and shoots. In addition, the lignin in
xylem adds strength that supports vascular plants and enables them to grow tall.
4. Describe the reproductive adaptations of plants.
Nonvascular plants and seedless vascular plants have swimming sperm, so sperm can
reach egg in moist habitats. In seed plants, pollen allows reproduction to occur over great
distances and in the absence of free water, whereas seeds allow for a dormancy stage and
provide food for the developing embryo. In angiosperms, flowers promote efficient
pollination, and fruits aid in seed protection and dispersal.
19.2
1. Describe the three main groups of bryophytes.
Liverworts have flattened, lobed bodies that lack stems. Hornworts have tapering pointed
sporophytes and flat gametophytes. Mosses are small bryophytes with compact bodies
and structures resembling stems and leaves.
2. Why do mosses usually live in moist, shady habitats?
Mosses occur in moist, shady habitats in part because they lack vascular tissue. Without
vascular tissues for support, their bodies remain small; and without an internal system to
transport water within the moss, each cell must acquire water on its own. A second
reason mosses are restricted to moist habitats is that they must be covered by a film of
water for sexual reproduction to occur.
19.3
1. Describe the four groups of seedless vascular plants.
Club and spike mosses have simple leaves and reproductive structures that look like clubs
or spikes. Whisk ferns have rhizomes but lack roots and leaves. Horsetails have branched
rhizomes and stems that bear spores at their tips. True ferns have fronds that grow from
rhizomes and spores on the undersides of the fronds.
2. How do seedless vascular plants reproduce?
Cells in a diploid sporophyte such as a fern frond undergo meiosis, releasing haploid
spores that germinate and develop into gametophytes. The gametophyte produce eggs and
sperm cells by mitosis. Sperm swim to an egg, and fertilization takes place. The fertilized
egg develops into a diploid sporophyte.
3. How are seedless vascular plants similar to and different from bryophytes?
Both are tied to water by swimming sperm that require water to reach eggs. Neither group
has pollen, seeds, flowers, or fruits. Seedless vascular plants, however, have xylem and
phloem, which bryophytes lack.
19.4
1. What are the characteristics of gymnosperms?
Gymnosperms have vascular tissue, pollen, and naked seeds that are not enclosed in
fruits.
2. What are the four groups of gymnosperms?
The four groups of gymnosperms are cycads, ginkgos, conifers, and gnetophytes.
3. What is the role of cones in conifer reproduction?
Meiosis, pollination, and fertilization occur inside the cone. The gametophyte generation
is also produced within cones.
4. What happens during and after pollination?
During pollination, a pollen grain is delivered to a female gametophyte. The pollen grain
germinates, forming a pollen tube that grows into the ovule, toward the egg cell. During
fertilization, a haploid sperm nucleus unites with the haploid egg cell, producing a diploid
zygote (the first cell of the sporophyte generation). The zygote will grow into an embryo,
which is packaged inside a seed.
19.5
1. What are the two largest clades of angiosperms?
The eudicots and monocots are the two largest clades of angiosperms.
2. In what ways are the life cycles of angiosperms similar to and different from those of
conifers?
Similarities include the conspicuous sporophyte stage and the presence of pollen.
However, unlike conifers, the angiosperms produce flowers and fruits. The other major
difference is the production of the endosperm, which provides nutrients for the
developing angiosperm embryo.
3. What is the relationship between flowers and fruits?
The flowers are the structures that produce pollen and egg cells. After fertilization occurs
in the ovary, parts of the flower develop into the fruit, which encloses the seeds.
4. How do animals participate in angiosperm reproduction?
When searching for food, animals pick up pollen from one plant and transfer it to the next
plant they feed on. In this way they pollinate the plants. Animals also disperse fruits and
seeds to new habitats.
19.6
1. Why do researchers collect DNA from permafrost?
The permafrost can contain preserved DNA from ancient plants and animals, allowing
researchers to reconstruct the ancient ecosystem.
2. What are some alternative hypotheses for why the researchers failed to recover any
DNA from sediments that were more than 300,000 to 400,000 years old? How would you
test your hypotheses?
One example of a hypothesis would be that DNA disintegrates after 300,000 to 400,000
years, even frozen in the permafrost. One way to test this would be to go to other areas
known to have life 300,000 to 400,000 years ago, take core samples, and see if DNA
appears in any of the samples.
Write It Out
1. What characteristics do all land plants have in common?
All land plants are photosynthetic multicellular organisms with eukaryotic cells, cellulose
cell walls, starch as a carbohydrate storage material, and a life cycle that features an
alternation of generations. The aboveground surface of the plant also typically has a
cuticle and stomata.
2. Analyze the alternation of generations common to all plants. If you isolated all of the
gametes that one gametophyte produced and analyzed the DNA, would you see variation
among the gametes?
The haploid gametes are the product of mitosis in the haploid gametophyte. Other than
random mutations, you should therefore not see variation among the gametes.
3. How are terrestrial habitats different from aquatic habitats? List the adaptations that
enable plants to obtain resources, transport materials, and reproduce; explain how each
adaptation contributes to reproductive success on land.
In terrestrial environments the nutrients are either in the ground (requiring a root system)
or in the atmosphere (requiring stomata). Also, land plants are in danger of losing water
to their environment; terrestrial habitats therefore have selected for a cuticle. Many plants
contain a vascular system that transports nutrients, water, and sugars between roots and
stems, allowing for large plant size. Pollen allows fertilization to take place without the
aid of water. Seeds and fruits improve reproductive success by protecting the offspring
and providing for their dispersal.
4. List the characteristics that distinguish the four major groups of plants, then provide
an example of a plant within each group.
Bryophytes such as mosses lack vascular tissue and produce swimming sperm. Seedless
vascular plants such as ferns have vascular tissue and produce swimming sperm.
Gymnosperms have vascular tissue, pollen grains, and seeds but do not produce flowers
or fruits. Angiosperms have vascular tissue, pollen grains, flowers, fruits, and seeds.
5. Give at least two explanations for the observation that bryophytes are much smaller
than most vascular plants. How can increased height be adaptive? In what circumstances
is small size adaptive?
One explanation is that bryophytes lack vascular tissue, so they lack the support
necessary for large size. Another is that the sperm must swim from one individual to the
next, selecting for small plants that grow close together. Increased height can be adaptive
in the competition for light. Small size can be adaptive if it allows for rapid development
to reproductive maturity.
6. A fern plant can produce as many as 50 million spores a year. How are these spores
similar to and different from seeds? In a fern population that is neither shrinking nor
growing, approximately what proportion of these spores is likely to survive long enough
to reproduce? What factors might determine whether an individual spore successfully
produces a new fern plant?
(a) A fern’s spores are similar to seeds in that both participate in reproduction and
dispersal. However, a spore is a haploid structure produced by meiosis; a seed contains a
diploid embryo and its food supply packaged inside a tough outer coating. Unlike spores,
seeds may stay dormant for years until conditions are right for germination. (b) In the
stable fern population, only a tiny fraction of the spores survives – just enough to replace
the parents. (c) Factors that influence whether a spore produces a new fern plant include
the substrate where the spore lands, herbivores, sunlight, nutrients, and moisture.
7. How do the adaptations of gymnosperms and angiosperms enable them to live in drier
habitats than bryophytes and seedless vascular plants?
Pollen carried by wind or animals enables plants to reproduce in the absence of free
water; seeds protect the embryo (young sporophyte) until conditions are optimal for
germination and growth.
8. Describe how the petals, ovary, and ovule of flowers participate in reproduction. What
happens to each part after fertilization?
Brightly colored petals attract pollinators; after fertilization, the petals typically wither
and drop off. The ovary contains one or more ovules; after fertilization, the ovary
develops into the fruit. Each ovule contains a female gametophyte; after fertilization, an
ovule develops into a seed.
9. Draw a bar graph showing the number of species in each group of plants. Is there a
correlation between the age of the plant group and the group’s diversity?
Graphs should be based on the data in table 19.1. The angiosperms, which evolved last,
are by far the most diverse plants. The second most diverse group, the nonvascular plants,
evolved first. There is no correlation between the age of the group and its diversity.
10. The immature fruit of the opium poppy produces many chemicals that affect animal
nervous systems. In what way might these chemicals benefit the plant?
If chemicals in the immature fruit produce a negative effect on the animal nervous
system, then animals would avoid the fruit, giving the seeds time to mature. If chemicals
remaining in the mature fruit produce a pleasant effect on the animal nervous system,
then animals would ingest more of the fruit, spreading the seeds over a greater range.
11. Scientists have studied plant populations that have colonized areas with low rates of
herbivory compared to the plant’s previous habitat. After many generations in the new
habitat, the plants produce fewer defense chemicals and grow larger than plants in the
ancestral population. How does natural selection explain these observations? (Hint:
producing defense chemicals uses energy.)
Any plant should invest its resources in ways that promote its reproductive success. Since
producing defense chemicals uses energy, and herbivores pose a minimal threat in the
new habitat, a plant should invest less in defense chemicals and more in additional leaf
area and stem height, allowing the plant to grow taller.
12. In a sentence or two, either support or refute the following statement: The pollen
grains of angiosperms are homologous to the spores of bryophytes.
The statement is incorrect. Pollen grains are male gametophytes; they produce sperm
nuclei by mitosis. The spores of bryophytes are the products of meiosis in the sporophyte.
They give rise to the gametophytes.
13. Some angiosperm species have exclusive relationships with just one species of
pollinator. How would this relationship benefit the plant? What are the risks to the plant?
The relationship is efficient for the plant because it means that many pollen grains will
reach another plant of the same species. The risk is that if the pollinator becomes extinct,
the plant may no longer be able to reproduce.
14. Compare and contrast the life cycles of the four groups of plants. How does each
group represent a variation on the common theme of alternation of generations?
All four groups of plants produce haploid gametophytes and diploid sporophytes, but the
relative sizes of these structures can vary. In bryophytes, the gametophyte is relatively
large and carries out photosynthesis; the sporophyte can be small and depends on the
gametophyte for nutrition. In seedless vascular plants, the gametophyte carries out
photosynthesis, but it is very small and is quickly dwarfed by the growing sporophyte. In
gymnosperms and angiosperms, the gametophytes are extremely small and depend on the
much larger sporophyte for nutrition.
In addition, the gametophytes produce gametes in all groups of plants. In bryophytes and
seedless vascular plants, the sperm cells require water to swim to the egg. In
gymnosperms and angiosperms, the sperm nuclei are packaged into pollen, which is
typically carried to the plant by wind or animals and therefore does not require free water.
15. A slight change in a plant species’ flower structure might favor a different pollinator.
How might such a change in flower structure lead to a new plant species? How does
coevolution between flowering plants and animal pollinators help explain the huge
diversity of angiosperms?
If a change in flower structure attracts a different pollinator to the plant, then the new
plant variety may no longer exchange gametes with the “parent” type. This reproductive
barrier would represent the development of a new species. In this way, over millions of
years, mutations in flowering plants may have selected for new types of pollinators.
Similarly, a mutation in the pollinator might simultaneously select for new flower
structures. Since pollinators and flowers are tied together evolutionarily, mutations in one
affect the other, so evolution occurs quickly.
16. Suppose you and a friend are hiking and you see an unfamiliar plant. What
observations would you make in trying to determine which type of plant it is?
The size of the plant and the presence of true roots and leaves would be two possible
clues. Another strategy would be to look at the reproductive structures, if present. For
example, the presence of flowers or fruits indicates an angiosperm; cones indicate a
gymnosperm; spores produced in capsules or on leaves could indicate a bryophyte or a
seedless vascular plant.
17. People often move plants from one part of the world to another. Sometimes, an
introduced plant species can become invasive, taking over native plant populations. The
U.S. Department of Agriculture manages the National Invasive Species Information
Center, whose website maintains a list of invasive plants. Which plant species are
considered invasive in your home state? Why are those species harmful? Should invasive
plants be eradicated? How?
First part of answer will vary. Invasive plant species are harmful because they compete
for water and soil nutrients, shade out native plants, displace native vegetation, and girdle
or choke out other plants. Answers about eradication will vary.
18. Human activities and natural phenomena can drive plant species to extinction. The
U.S. Department of Agriculture Natural Resources Conservation Service maintains lists
of threatened and endangered plant species. What are some examples of threatened or
endangered species in your area? What are the most important threats to those species?
What are the potential consequences of a plant species’ extinction? What steps should we
take to save threatened and endangered plants?
First part of question may vary. Important threats to endangered species could include
disease, grazing, climate change, wildfires, human activity, invasive species, or
agriculture. Potential consequences of extinction could include loss of genetic diversity,
loss of food for pollinators and herbivores, and loss of habitat for animals. Steps to
prevent extinction could include restricting human activity, reintroducing native plants,
eradicating invasive plants, and implementing laws to limit the import or sale of exotic
and invasive species.
19. What are the pros and cons of pursuing biofuels as alternatives to fossil fuels? In your
opinion, do the pros outweigh the cons, or vice versa? Justify your answer.
Pros include reduced dependence on nonrenewable fossil fuels; reduced greenhouse gas
emissions from burning fossil fuels; and reduced dependence on foreign countries for our
energy supply. Cons include market forces that disrupt the agricultural balance and drive
up the price of corn, making it harder for farmers to feed livestock and making food less
affordable. Biofuel production may also require additional farmland, leading to
deforestation. Answers will vary to opinion question.
Pull It Together
1. Describe an adaptation that arose in each group of vascular plants.
Nonvascular plants were the first to be able to survive on dry land, thanks to a cuticle and
stomata. Seedless vascular plants were the first to have vascular tissue. Gymnosperms
were the first to produce pollen and seeds, and angiosperms are the only plants to produce
flowers and fruits.
2. Circle each plant group that produces spores.
All groups of plants should be circled, since the sporophytes of all plants produce spores
by meiosis.
3. How do bryophytes and seedless vascular plants reproduce if they lack pollen and
seeds?
Bryophytes and seedless vascular plants have gametophytes that produce gametes,
including swimming sperm. The sporophyte generation produces spores, which can be
dispersed to new habitats.
4. What is the relationship between pollen and seeds?
Pollen contains sperm nuclei that fertilize the egg and the polar nuclei. The fertilized
polar nuclei then develop into the endosperm, and the fertilized egg develops into an
embryo. The endosperm and embryo, surrounded by the seed coat, develop into the seed.