Answers to Mastering Concepts Questions
22.1
1. What adaptations contribute to the reproductive success of angiosperms?
Three adaptations that contribute to the reproductive success of angiosperms are pollen,
seeds, and flowers that develop into fruits.
2. When are sexual and asexual reproduction each adaptive?
Sexual reproduction is adaptive when the environment is variable; asexual reproduction
is adaptive in a more stable environment.
3. What are some examples of asexual reproduction in plants?
Some examples of asexual reproduction in plants include suckers, root cuttings, and
grafting.
22.2
1. What is the function of each part of a flower?
Sepals protect the flower and may attract pollinators. Petals often attract pollinators.
Stamens are male reproductive parts, which produce pollen. Carpels are female
reproductive parts that produce egg cells.
2. Describe the male and female gametophytes in angiosperms.
The male gametophyte consists of two cells, one of which divides by mitosis to produce
two sperm nuclei. The female gametophyte consists of an embryo sac containing eight
nuclei in seven cells.
3. How does pollen move from one flower to another, and why is this process essential
for sexual reproduction?
Animals or wind carry pollen. This process is essential to ensure that male and female
gametes come into contact with one another.
4. Describe the events of double fertilization.
In double fertilization, one sperm nucleus fertilizes two polar nuclei to form triploid
endosperm. A second sperm nucleus fertilizes the egg cell, forming the diploid zygote.
5. How is endosperm important to plants? To humans?
Endosperm nourishes the young plant until it begins producing its own food by
photosynthesis. Carbohydrates stored in the endosperm of seeds of various cereal crops
support the human diet.
6. What are the components of a seed?
The seed is an embryo and stored food surrounded by a seed coat.
7. Which flower parts develop into a fruit?
The ovary is the flower part that develops into a fruit. Other parts of the flower,
including the receptacle, may also develop into fruit parts.
8. What are the two main functions of fruits?
Fruits protect developing seeds and promote their dispersal once they are mature.
9. How do fruits and seeds disperse to new habitats?
Fruits and seeds can be carried to new habitats by wind, water, or animals.
22.3
1. Why must seeds absorb water before germinating?
The seed expands as it absorbs water, which helps the seed coat to split and exposes the
embryo to oxygen. In addition, absorption of water causes the embryo to release
hormones that trigger the production of enzymes that digest stored starch, which will
nourish the young seedling until it begins to produce its own food by photosynthesis.
2. What are the events of early seedling development?
After seed germination, the embryo divides at the apical meristems to form shoots and
roots. Gravity stimulates the downward growth of roots. At first, the only source of
energy is stored food from the seed, but the seedling produces its own food by
photosynthesis once leaves form on the seedling’s stem.
3. How does natural selection influence seed size?
Seed size reflects a tradeoff between nutrient storage and dispersal. Small seeds can
disperse far away from the parent plant, but they contain little energy to sustain early
seedling growth. Conversely, large seeds contain abundant energy but may not disperse
easily to new habitats.
22.4
1. What is a hormone?
A hormone is a biochemical synthesized in small quantities that is often, but not always,
transported to other locations in the organism, where it stimulates or inhibits a response
in the target cells.
2. How does a plant hormone exert its effects?
Plant hormones bind to a receptor protein on or in a target cell, stimulating chemical
reactions that alter the expression of one or more genes.
3. List the major classes of plant hormones and name some of their functions.
Auxins cause seedlings, shoot tips, leaves and embryos to elongate; stimulate growth of
roots from stem cuttings; and inhibit growth of lateral buds. Cytokinins stimulate cell
division and the growth of lateral buds. Gibberellins cause roots, shoots, and young
leaves to elongate, and they break seed dormancy. Ethylene ripens fruits and stimulates
leaves and flowers to age and drop from plant. Abscisic acid inhibits shoot growth; keeps
buds and seeds dormant; keeps proteins in storage in seeds; stimulates stomata to close;
and stimulates leaves, flowers, and fruits to fall from the plant.
4. Give an example of how plant hormones interact.
Auxins are most concentrated in shoots, whereas cytokinins are most concentrated in
roots. Cytokinins move upward in the plant and stimulate lateral buds to sprout. In
contrast, auxins move downward in a plant shoot, suppressing the growth of lateral buds.
If the terminal buds are pinched off, the plant will become bushy, as cytokinin-influenced
growth of lateral buds occurs. If terminal buds remain in place, the plant will grow tall
instead of bushy.
22.5
1. What is auxin’s role in phototropism?
Auxin moves to the shaded side of a shoot, where it causes elongation of shaded cells. By
causing unequal cell growth on opposite sides of a shoot, auxins cause the plant to bend
toward light.
2. How does light help regulate flowering time?
Long-day plants flower only when days grow longer than a critical length, usually 9 to 16
hours. In contrast, short-day plants flower only when light periods become shorter than
some critical length. Long-day plants usually bloom in the spring or early summer; shortday plants flower in late summer or fall.
22.6
1. How do auxins participate in gravitropism?
In roots, auxin accumulation inhibits cell elongation. If a plant is turned on its side, auxin
accumulates on the lower side. The root turns downward as cells on the upper side of the
root elongate more rapidly than those on the lower side.
2. What is thigmotropism?
Thigmotropism is a response to touch in a specialized structure such as a tendril.
Hormones cause differential growth in the tendril, allowing it to encircle physical
supports such as a trellis or the branches of another plant.
22.7
1. What did Tewksbury and Nabhan conclude about the function of capsaicin in mature
chili fruits?
Capsaicin in chili fruits deters small mammals that chew and destroy seeds. The capsaicin
has no effect on birds, however, and so it does not hinder dispersal.
2. Summarize the evidence that Tewksbury and Nabhan used to arrive at their conclusion.
In field observations during daylight hours, thrashers ate both chili peppers and
hackberries; at night, small mammals ate only the hackberries. Laboratory studies
supported the field observations, showing that thrashers would eat hackberries and
pungent chilies, whereas mammals would eat only hackberries and chilies lacking
capsaicin. Furthermore, seeds from chilies eaten by thrashers remained viable, whereas
mice and rats destroyed chili seeds.
Answers to Write It Out Questions
1. Give an example of asexual reproduction in a plant.
Two examples of asexual reproduction in a plant are taking cuttings or grafting.
2. Explain how flowers, fruits, and seeds contribute to the reproductive success of
angiosperms.
Flowers bring together the egg and sperm. The fruit protects the seed and helps ensure its
dispersal. The seed protects the dormant embryo and nourishes the young seedling after
germination.
3. Sketch a flower, indicating the location and function of each of the following parts:
sepal, petal, carpel, stamen, stigma, corolla, calyx, style, anther.
[Answers will be visual; see figure 22.3.]
4. Name tissues or cells in an angiosperm that are haploid, diploid, and triploid.
Haploid cells are the pollen grains and embryo sac, which produce the haploid sperm and
egg cells. Diploid cells include the zygote and the rest of the sporophyte body. Triploid
cells include the endosperm.
5. What happens to the two sperm nuclei that form inside a pollen tube?
One sperm nucleus fertilizes the egg cell to yield the diploid zygote; the other fertilizes
the polar nuclei to yield the triploid endosperm.
6. How does an exclusive relationship between a plant and its pollinator benefit each
partner? What are the risks of exclusivity?
The benefit to the pollinator is that it is assured a specific nutrient source that will meet
its needs and that will be relatively inaccessible to competitors. The plant benefits as its
pollen is specifically delivered to plants within its species, cutting down on the number of
sperm that need to be produced. Both partners are at risk, however, if the other goes
extinct. If the remaining partner is too specialized, it may not be able to survive.
7. If fruit production is a measure of fitness, why wouldn’t a plant spend all of its energy
producing fruits instead of roots and leaves? Why do you think some annual plants die
back as they produce fruits?
Plants must invest energy in roots, which acquire water and minerals that the rest of the
plant uses. Plants also must invest energy in leaves, which use sunlight to produce sugars.
Without roots and leaves, a plant wouldn't be able to produce fruits at all.
Annual plants only live for one growing season. As an annual plant dies back, it might
maximize its reproductive success by diverting sugars and minerals from its other plant
parts to its fruits.
8. An oak tree may produce thousands of acorns, which squirrels bury or eat. Why does
the tree make so many acorns? Why might a plant whose seeds disperse far from the
parent have better reproductive success than one whose seeds fall at the base of the parent
plant?
Most of the acorns will never develop into new trees. Squirrels eat many of the acorns,
many will rot, others will be eaten by insects, some may not be viable, and many will
land in unsuitable habitats. Dispersing seeds far from a parent plant reduces competition
for light, water, and nutrients between a plant and its offspring.
9. How does seed dormancy promote reproductive success?
Seed dormancy allows a seed to germinate in favorable conditions, rather than in the cold
winter months when sunlight is scarce and water is frozen in the soil.
10. List the major plant hormones and describe some of their actions.
Auxins promote cell elongation; cytokinins stimulate cell division in seeds, roots and
fruits. Gibberellins stimulate stem elongation and seed germination. Ethylene hastens
fruit ripening. Abscisic acid induces and maintains seed dormancy.
11. How does pruning stimulate the growth of new branches on a shrub?
Removal of a shoot tip decreases the concentration of auxins and increases the
concentrations of cytokinins. This hormonal signal triggers new branch growth.
12. Explain why one rotten orange promotes decay of all of the others.
Ethylene, a gaseous hormone, stimulates fruit ripening. An overripe fruit can release so
much of the hormone that nearby fruits also ripen too quickly and rot.
13. What is the function of photoreceptors?
Photoreceptors detect the quality and quantity of light.
14. Describe three tropisms. Which hormone is common to all three?
Three tropisms are phototropism (growth toward light), gravitropism (growth toward or
away from gravity), and thigmotropism (growth toward or away from a mechanical
stimulus). Auxins are hormones that participate in all three.
15. Develop a hypothesis that explains why it might be adaptive for a plant to flower in
response to photoperiod rather than temperature.
Answers will vary. One possible hypothesis is that day length is a more reliable indicator
of the time of year than is temperature. Accurate detection of season can affect
reproductive success; pollinators may not be available if a plant flowers too early or too
late.
16. How is gravitropism adaptive?
Gravitropism ensures that roots will grow downward in the direction of water and
nutrients in the soil.
Answers to Pull It Together Questions
1. Add the following terms to the concept map: stamen, anther, carpel, ovule, stigma.
"Pollen" connects with the phrase "is produced at" to "Anther", which connects with the
phrase "is part of a" to "Stamen". "Carpel" connects with the phrase "forms the central
female part of a" to "Flower". "Carpel" also connects with the phrase "contains one or
more" to "Ovules". "Stigma" connects with the phrase "is at the top of the" to "Carpel",
and "Stigma" also connects with the phrase "receives" to "Pollen".
2. What event leads to the formation of the endosperm?
When a sperm nucleus fertilizes the polar nuclei, the resulting triploid cell develops into
the endosperm.
3. What roles do animals play in plant life cycles?
Animals carry pollen from flower to flower. Animals also scatter seeds in their feces, or
seeds can be carried in animal fur.
4. What hormones are involved in plant development?
[See Table 22.2 for a complete answer.] Auxins help stems grow toward light and help
roots grow downward. Cytokinins promote cell division in many plant parts. Ethylene
triggers fruit to ripen and flowers to senesce. Abscisic acid readies a plant for dormancy,
and gibberellins trigger seed germination.
5. List examples of environmental conditions that influence plant development?
Light has the largest influence on plant development, because the amount of light
ultimately limits energy production. Water availability also affects root growth and the
adaptations of the stem and leaves. Temperature and moisture both affect seed
germination. Wind, salt, and other physical conditions also affect plant height, branching,
and other features.