Download Answers to Mastering Concepts Questions

Answers to Mastering Concepts Questions
Chapter 26
26.1
1. Three adaptations that contribute to the reproductive success of angiosperms are pollen,
seeds, and flowers that develop into fruits.
2. Some examples of asexual reproduction in plants include suckers, root cuttings, and grafting.
3. Sexual reproduction is adaptive when the environment is variable; asexual reproduction is
adaptive in a more stable environment.
4. A plant life cycle includes multicellular haploid and diploid stages. In contrast, the life cycle of a
typical animal has a multicellular diploid stage, but the haploid life cycle stage is unicellular.
26.2
1. The parts of a flower and their functions are as follows:
Whorl 1: the calyx made up of all sepals, which protect the flower and may attract pollinators;
Whorl 2: the corolla made up of all petals, which often attract pollinators;
Whorl 3: male reproductive parts: stamens composed of anthers and filaments, which produce
pollen;
Whorl 4: female reproductive parts: carpel made up of stigma, style, ovary, and ovule, which
produce egg cells.
2. Sporophytes produce haploid spores by meiosis; these spores give rise to male and female
gametophytes by mitotic cell division. Microspores produce pollen grains (male gametophytes);
megaspores produce the embryo sac (female gametophyte).
3. Pollination transfers pollen (which produces sperm cells) from the male reproductive structures
to the female reproductive structures (which produce egg cells).
4. 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. 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 form the basis of the
human diet.
26.3
1. In monocots, the food stored in the endosperm is transferred to the embryo during germination.
In many eudicots, the embryo’s cotyledons completely absorb the endosperm as they develop.
2. The ovary is the flower part that develops into a fruit. Other parts of the flower may also
develop into fruit parts.
3. Seeds disperse into new habitats by being carried by wind, water, and animals.
26.4
1. The seed expands as it absorbs water, which helps the seed coat to split and exposing 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 photosythesis.
2. In eudicots, cotyledons containing stored food rise out of the ground with the lengthening of the
embryonic shoot. The shoot apical meristem forms at the tip of the stem above the cotyledons.
In monocots, the cotyledon remains belowground, and a coleoptile covers the embryonic leaves.
26.5
1. In animals, a hormone is defined as a biochemical that is made in small quantities in one part
of an organism and is transported throughout the body via the bloodstream. The hormone then
causes a response in target cells. In plants, hormones are signaling molecules, plants lack
dedicate hormone-producing organs. Instead, many plant parts can produce the same hormone,
and plant hormones often act close to where they are produced.
2. The major classes of plant hormones and some of their actions include:
- Auxins: cause seedlings, shoot tips, leaves and embryos to elongate; stimulate growth of roots
from stem cuttings, inhibit growth of lateral buds;
- Cytokinins: stimulate cell division and the growth of lateral buds;
- Gibberellins: cause roots, shoots, and young leaves to elongate; break seed dormancy;
- Ethylene: ripens fruits, 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; promotes fall of leaves, flowers, fruits.
3. 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 apical buds are
pinched off, the plant will become bushy as cytokinin-influenced growth of lateral buds dominates
over growth of apical buds. If apical buds remain in place, the plant will grow tall and leggy
instead of bushy.
26.6
1. Some seeds germinate only in the presence of light. If light is present, phytochrome is in the
Pfr form, which stimulates seed germination. The Pr form inhibits germination. Phytochrome also
controls photoperiodism (the detection of day length). During the day, Pfr is most abundant,
which somehow tells the plant it is day. At night, Pfr is converted back to Pr, which tells the plant it
is night.
2. A seedling grown in the dark is yellowish-white and has a long, spindly shoot. A seedling grown
in the light is green and less spindly.
3. Auxin moves to the shaded side of a shoot, where it causes elongation of shaded cells. As a
result, these cells grow longer than their counterparts on the sunny side of the shoot. By causing
unequal cell size on opposite sides of a shoot, auxins cause the plant to bend toward light.
4. Long-day plants flower only after when days are longer than a critical length, usually 9 to 16
hours. In contrast, short-day plants flower only when light periods are shorter than some critical
length. Long-day plants usually bloom in the spring or early summer; short-day plants flower in
late summer or fall.
5. Experiments showed that interrupting the dark period of a short-day plant inhibits flowering,
indicating that photoperiodism is a response to the duration of the dark period.
6. A circadian rhythm is a physiological cycle that repeats daily. Laboratory experiments that
obliterate environmental cues, such as conditions of constant light or constant darkness, reveal
that circadian rhythms continue (indicating a genetic component). However, if environmental
conditions are reset, a plant’s circadian rhythms also can be reset, but only to a limited extent.
26.7
1. Statoliths are thought to participate in gravitropism by settling into new positions within cells
when a root’s position is altered. This causes the root to re-orient and turn downward. This
response is adaptive because as a root grows, it encounters obstacles in the soil that it must
grow around. Alternately, animals may partially uproot a plant. If the roots can continue to make
their way into soil, the plant can survive these life-threatening situations.
2. Thigmotropism helps some plants to climb because hormones cause differential growth in
shoots, allowing them to encircle physical supports such as branches of other plants, tree limbs,
and trellises.
26.8
1. The following sequence of events occurs when the leaves of a deciduous tree senesce and
fall:
- Chlorophyll, enzymes, and other large molecules are dismantled within leaf cells, revealing
secondary pigments such as carotenoids and anthocyanins;
- ethylene stimulates the production of digestive enzymes that destroy cellulose and pectin in the
cells of the abscission zone at the base of the petiole.
- the leaf is shed.
2. Dormancy is a temporary state of decreased metabolism. Senescence is aging and death, in
which the metabolism eventually ceases completely after the breakdown of essential molecules.