Download Answers to Mastering Concepts Questions

Mastering Concepts
24.1
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. What are some examples of asexual reproduction in plants?
Some examples of asexual reproduction in plants include suckers, root cuttings, and
grafting.
3. 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.
4. Describe alternation of generations in the plant life cycle.
Plants have a haploid gametophyte generation and a diploid sporophyte generation. The
gametophyte generation produces haploid gametes by mitosis. Male and female gametes
fuse in fertilization. The product of fertilization is the zygote, which develops into the
mature sporophyte. Meiosis in the sporophyte produces haploid spores, which develop
into gametophytes.
24.2
1. What is the function of each part of a flower?
Sepals protect the flower and sometimes attract pollinators. Petals often attract
pollinators. Stamens are male reproductive parts. At the tip of each stamen is an anther,
which produces pollen. Carpels are female reproductive parts. The base of a carpel is an
ovary, which contains one or more ovules. Egg cells form inside ovules.
2. 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 from different individuals come into contact with one another.
3. Describe the events of double fertilization.
In double fertilization, one sperm nucleus fertilizes two nuclei in the central cell, forming
triploid endosperm. A second sperm nucleus fertilizes the egg cell, forming the diploid
zygote.
4. What are the components of a seed?
The seed is an embryo and stored food surrounded by a seed coat.
5. How do fruits and seeds disperse to new habitats?
Fruits and seeds can be carried to new habitats by wind, water, or animals.
24.3
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, cells in the embryo’s apical meristems divide 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.
24.4
1. What is a hormone?
A hormone is a biochemical synthesized in small quantities that is 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; and stimulates stomata to
close.
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
and leggy instead of bushy.
24.5
1. What is the function of photoreceptors?
Photoreceptors detect the quality (wavelength) and quantity of light.
2. 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.
3. How does phytochrome help regulate seed germination, circadian rhythms, and
flowering?
Overall, red light causes the Pr form of phytochrome to convert to Pfr, which travels to the
nucleus and influences the expression of key genes. In the case of seed germination, the
activated genes may include those encoding enzymes that digest starch in the endosperm.
Phytochrome also interacts with the genes and proteins that entrain the biological clock,
which controls circadian rhythms. Phytochrome also helps informs the plant of the length
of a night, somehow stimulating the release of the flowering hormone at the appropriate
time.
4. How do long-day plants differ from short-day plants?
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.
24.6
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. How does thigmotropism help some plants climb?
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.
24.7
1. How does seed dormancy promote reproductive success? How does seasonal
dormancy in deciduous trees 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. Seasonal dormancy
allows a plant to withstand the freezing temperatures of winter by reducing water loss,
slowing metabolism, and producing sugars and amino acids that act as an antifreeze.
2. What are some conditions that can release a plant from dormancy?
Longer days, warmer temperatures, and moisture can release a plant from dormancy.
24.8
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 does not hinder dispersal.
2. Summarize the evidence that Tewksbury and Nabhan used to arrive at their conclusion.
In field observations, thrashers ate both chili peppers and hackberries, whereas 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 chewing by mice and rats destroyed chili
seeds.
Write It Out
1. 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.
2. Sketch a flower, indicating the location and function of each of the following parts:
sepal, petal, carpel, stamen, stigma, corolla, calyx, style, and anther.
[Answers will be visual; see figure 24.5]
3. Describe the male and female gametophytes of flowering plants.
The male gametophyte is a two-celled, thick-walled, haploid pollen grain. Female
gametophytes are embryo sacs, each of which contains eight haploid nuclei and seven
cells. The central cell of the embryo sac contains two nuclei. One of the other six cells is
the egg.
4. What is double fertilization? How does it increase the reproductive success of the
parent plant?
One of the two cells in the pollen grain divides, giving rise to two haploid sperm nuclei.
During double fertilization, these two nuclei simultaneously fertilize the nuclei in the
central cell and egg cell of the embryo sac. Fertilization of the nuclei in the central cell
produces the endosperm, whereas fertilization of the egg cell produces the zygote. The
zygote develops into the embryo, which uses the separately developing endosperm as a
food source. Endosperm formation, which is a consequence of double fertilization,
increases the reproductive success of the parent plant because a well-nourished embryo is
more likely to survive long enough to begin photosynthesis on its own.
5. How does an exclusive relationship between a plant and its pollinator benefit each
partner? What are the risks of exclusive partnerships?
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.
6. 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 and seeds?
Roots and leaves ensure that water, minerals, and sugars are available to produce seeds
and fruits. If a plant dies back as it produces fruit and seeds, then more of its energy can
go into reproduction, since energy is no longer needed to maintain leaves.
7. 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 fruits fall at the base of the parent
plant?
Most of the acorns will never develop into new trees. Many will rot or be eaten by
animals, 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.
8. Explain how plants grow toward light. Why is this response adaptive?
Directional light causes movement of auxins within the plant. Auxins are hormones that
accumulate on the shaded side of the plant. Cells with abundant auxins have “stretchier”
cell walls, so they elongate as they take in water. As cells elongate more on the shaded
side of the plant than on the light side of the plant, the stem bends toward the light.
Bending toward the light is adaptive because it maximizes the leaf surface area exposed
to light, so the plant can produce more sugars to use in growth and reproduction.
9. 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.
10. Explain why far-red light inhibits seed germination.
Far red light increases the conversion of the Pfr form of phytochrome to the Pr form.
Since Pfr induces seed germination while Pr does not, far-red light indirectly inhibits seed
germination.
11. How does senescence occur?
Senescence (aging) occurs when a plant stops growing, and its metabolism switches from
synthesis to breakdown. Large organic molecules in the senescing plant part are
dismantled, and essential nutrients move to the stem and roots for storage.
Pull It Together
1. Add the following terms to the concept map: stamen, anther, carpel, ovule, stigma.
“Pollen” connects with the phrase “is produced at the” to “anther,” which in turn
connects with the phrase “is part of a” to “stamen.” “Ovary” connects with the phrase “is
at the base of the” to “carpel.” “Ovary” 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 connects with the word “receives” to “pollen.”
2. Name tissues or cells in an angiosperm that are haploid, diploid, and triploid.
Haploid cells in the pollen grains and embryo sac produce the haploid sperm nuclei and
egg cells. Diploid cells include the zygote and the rest of the sporophyte body. Triploid
cells include the endosperm.