Mastering Concepts
18.1
1. How have plants changed the landscape, and how are they vital to life today?
Plants have changed the landscape on Earth by settling on land, providing the base of
most food webs, and providing habitats for animals, fungi, and other organisms. Their
decomposing tissues provide nutrients for organisms that live in leaf litter and enrich
watery habitats like lakes and streams. Plants also changed the atmospheric levels of
carbon dioxide and O2. Plants are vital to life today for all of the same reasons.
2. What evidence suggests that plants evolved from green algae?
Photosynthetic pigments such as chlorophyll a, use of starch as a storage carbohydrate,
cell walls made of cellulose, and similar life cycles that feature alternation of generations
are the features shared by green algae and plants.
3. What are the functions of the cuticle and stomata?
The cuticle is a waxy coating that protects the plant from drying out. The stomata are
pores in the leaf that allow for gas exchange.
4. How does vascular tissue adapt plants to land?
Vascular tissue transports water and nutrients throughout the bodies of plants. Vascular
tissue allows plants to grow tall and distributes water and nutrients throughout the plant
body.
5. Describe the reproductive adaptations of plants.
There are several reproductive adaptations in plants. The first is the ability to alternate
life cycle stages between a diploid (sporophyte) and multicellular haploid stage
(gametophyte). Related to this adaptation is the evolutionary trend to increase the size
and independence of the sporophyte. Pollen is an adaptation that allows reproduction to
occur over great distances and in the absence of free water. Seeds allow for a dormancy
stage and provide food for the developing embryo. Flowers promote efficient pollination,
and fruits aid in seed protection and dispersal.
6. What features differentiate the four major groups of plants?
The presence or absence of swimming sperm, vascular tissue, pollen, seeds, flowers, and
fruits differentiate the four major groups of plants.
18.2
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. Explain the observation that mosses occur more frequently in moist, shady habitats
than in hot, sunny places.
Mosses occur more frequently in moist, shady habitats because they lack vascular tissue
and because they must be covered by a film of water for sexual reproduction to occur.
3. How do bryophytes reproduce asexually and sexually?
Bryophytes reproduce asexually by forming gemmae, small pieces of tissue that detach
and grow into new plants. Bryophytes reproduce sexually by forming swimming sperm
and egg cells within tissues of separate haploid male and female gametophytes.
Conveyed in a droplet of water, sperm swim to the egg cells. The fertilized egg develops
into the diploid sporophyte, which grows a sporangium in which spore mother cells
undergo meiosis to produce haploid spores. These grow into haploid gametophytes that
will differentiate into male and female plants.
18.3
1. Describe the four groups of seedless vascular plants.
The four groups of seedless vascular plants are:
- club and spike mosses, which have reproductive structures that look like clubs or
spikes;
- whisk ferns, which have rhizomes but lack roots and leaves;
- horsetails, which have branched rhizomes and stems that bear spores at their tips;
- true ferns, which have fronds that grow from rhizomes and spores on the undersides of
the fronds.
2. How do seedless vascular plants reproduce?
Seedless vascular plants reproduce sexually. A sporophyte such as a fern frond releases
haploid spores that germinate and develop into gametophytes. Different places on the
gametophyte have egg- and sperm-producing structures. 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 their 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.
18.4
1. What are the characteristics of gymnosperms?
The characteristics of gymnosperms include 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 in gymnosperms?
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 zygote
that will grow into the sporophyte generation.
18.5
1. What are the two largest clades of angiosperms?
The eudicots and monocots are the two largest clades.
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 fruit. The other major
difference is the production of the endosperm, which provides nutrients for the
developing 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 cross fertilize the plants. Animals also disperse fruits
and seeds to new habitats.
18.6
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 even in the permafrost DNA erodes after
300,000 to 400,000 years. 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, and starch as a carbohydrate storage material.
2. As described in chapter 17, the term algae encompasses many separate lineages of
organisms. What evidence did biologists use to determine which types of algae are most
closely related to land plants?
Evidence includes similar photosynthetic pigments, cell walls with cellulose, use of
starch as a nutrient reserve, and similar DNA sequences.
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 since the plants aren't surrounded by water
they 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. A few plant species, such as duckweed and water lilies, either float on or are
submerged in water. These aquatic plants apparently evolved from terrestrial plants.
Which adaptations do you expect to be minimized in these aquatic plants?
You would expect these plants to lack lignin in their cell walls. They may also lack a
thick cuticle.
5. What are the four major groups of plants and the characteristics that distinguish them?
Bryophytes – lack vascular tissue, produce swimming sperm; seedless vascular plants –
have vascular tissue, produce swimming sperm; gymnosperms – have vascular tissue,
pollen grains, and seeds but no flowers or fruits; angiosperms – have vascular tissue,
pollen grains, flowers, fruits, and seeds.
6. 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. Increased height can be adaptive in the competition for light. Small size can be
adaptive if it allows for rapid development to reproductive maturity.
7. Which adaptations enabled seedless vascular plants to invade new habitats?
Vascular tissue allowed the plants to grow taller and outcompete non-vascular plants for
available sunlight. These plants also contain a strong root system, which allows them to
absorb nutrients and water in dry habitats.
8. 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 serve in both 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, predators, sunlight, nutrients, and proper moisture.
9. Some websites for gardeners warn against the “foul-smelling fruits” that female ginkgo
trees produce. In what way is this statement incorrect?
The “fruit” of a ginkgo is not a true fruit; ginkgos are gymnosperms, not angiosperms.
10. How do the adaptations of gymnosperms and angiosperms enable them to live in drier
habitats than bryophytes and seedless vascular plants?
Pollen enables plants to reproduce in the absence of free water; seeds protect the embryo
(young sporophyte) until conditions are optimal for germination and growth.
11. How do angiosperms differ from gymnosperms? How are the two groups of plants
similar?
Angiosperms have flowers that produce pollen and egg cells; in gymnosperms, cones
produce pollen and egg cells. Another difference is that angiosperms produce their seeds
in fruits, whereas gymnosperms produce “naked” seeds. Similarities include vascular
tissue, pollen, seeds, and the dominant sporophyte generation.
12. Some viroids (infectious RNA molecules that cause diseases in plants) move to new
plant hosts in pollen. Which of the four main groups of plants could “catch” viroids in
this way? How could you test the hypothesis that a particular viroid spreads in pollen and
not via an insect vector or direct plant-to-plant contact?
Both the angiosperms and gymnosperms use either wind or animal pollination and so
would be susceptible to viroids that travel in pollen. You could test the pollen
transmission hypothesis by inserting viroids in pollen, insects, and other plants. You
could then expose different groups of plants to one of these vectors and look to see which
group of plants becomes infected.
13. 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 the fruit produces a pleasant effect on the animal nervous system, then it would cause
the animals to ingest more of the fruit, spreading the seeds over a greater range for
possible germination. If the fruit produces a negative effect on the animal nervous system
then it would cause the animals to ignore the immature fruit, giving it time to fully
develop.
14. Scientists hypothesize that a geological catastrophe, such as an asteroid impact, killed
the dinosaurs. If this is true, how might angiosperms have survived such a catastrophe?
Seeds can protect embryos over a long period of time. They could have remained
dormant until conditions once again allowed for plant growth.
15. 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.
16. 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
carried to the plant by wind or animals and therefore does not require free water.
17. Use the Internet to learn more about a well-studied plant species other than
Arabidopsis thaliana and corn. How has the plant you chose contributed to knowledge of
general biology and plant biology?
[Answers may vary]
18. Make a list of everything you own or use that comes from plants or plant parts.
Which of the four main groups of plants are represented in your list?
[Answers may vary]
19. 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.
20. 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. Reasons species are harmful include: competing for soil
nutrients, shading out native plants, displacing native vegetation, girdling or choking
other plants. Answers about eradication will vary.
21. 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 could include: pests, grazing, climate
change, wildfires, human activity, invasive species, or agriculture. Potential
consequences to extinction could include: loss of gene banks, loss of food for pollinators
or herbivores, loss of habitat for animals. Steps to take could include: restricting human
activity, reintroducing native plants, eradicating invasive plants, implementing laws to
limit the import or sale of exotic and invasive species.
22. 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: reduce dependence on fossil fuels, reduce greenhouse gas emission from burning
fossil fuels, and reduce dependence on foreign countries for our energy supply.
Cons: May cause agricultural focus to shift heavily towards corn, disrupting the
agricultural balance; may drive up the price of corn, making it harder for farmers to feed
livestock and making food less affordable; may require increased farmland production,
leading to deforestation
Answers will vary to opinion question.
Pull It Together
1. What are the main groups of plants within the bryophytes, seedless vascular plants,
gymnosperms, and angiosperms?
Bryophytes include mosses, liverworts and hornworts. Seedless vascular plants include
club mosses, whisk ferns, true ferns and horsetails. Gymnosperms include conifers,
ginkgo trees, gnetophytes and cycads. The two largest groups of angiosperms are
currently monocots and eudicots.
2. For each of the four main groups of plants, describe the gametophyte and sporophyte
generations.
Bryophytes
Gametophyte
Supplies all nutrition for
plant. Various shapes.
Sporophyte
Various shapes. Needs
gametophyte for nutrition.
Seedless vascular plants
(e.g. ferns)
Gymnosperms
Angiosperms
Small, heart-shaped.
Large (fronds).
Microscopic, depends on
sporophyte for nutrition.
Microscopic, depends on
sporophyte for nutrition.
Large, makes cones.
Large, makes flowers.
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.