Download Answers to End-of-Chapter Questions – Brooker et al ARIS site

Answers to End-of-Chapter Questions – Brooker et al ARIS site
Chapter 39
Test Yourself Questions
1. Where do immature male gametophytes of flowering plants, otherwise known as pollen grains,
develop?
a. in the anthers of a flower
b. in the carpels of a flower
c. while being dispersed by wind, water, or animals
d. within ovules
e. none of the above
Answer: a. Pollen grains develop within the anthers of a flower.
2. Where do mature male gametophytes of flowering plants primarily develop?
a. in the anthers of a flower
b. in the carpels of a flower
c. while being dispersed in wind, water, or by animals
d. within ovules
e. none of the above
Answer: b. Mature male gametophytes produce pollen tubes that grow through a style and into the
ovary of a flower, after germinating on the stigma, the receptive surface of a pistil, which consists of one
or more carpels.
3. Where would you find female gametophytes of a flowering plant?
a. in the anthers of a flower
b. at the stigma of a pistil
c. in the style
d. within ovules in a flower’s ovary
e. none of the above
Answer: d. Female gametophytes are produced within ovules, which occur in flower ovaries.
4. How does double fertilization occur in flowering plants?
a. The two sperm in a pollen tube fertilize the two eggs cells present in each female
gametophyte.
b. One of the two sperm in a pollen tube fertilizes the single egg in a female gametophyte,
while the other fuses with the two nuclei present in the central cell.
c. Two sperm, one contributed by each of two different pollen tubes, fertilize the two egg cells
in a single female gametophyte.
d. Two sperm contributed by separate pollen tubes enter a single female gametophyte; one of
the sperm fertilizes the egg cell, while the other fertilizes the central cell.
e. None of the above
Answer: b. Two sperm are contributed to each female gametophyte by each pollen tube. One of these
sperm cells fertilizes the egg, while the other fuses with the two nuclei of the central cell.
5. A seed is
a. an embryo produced by the fertilization of an egg, which is protected by a seed coat.
b. a structure that germinates to form a seedling under the right conditions.
c. an embryo produced by parthenogenesis, that is enclosed by a seed coat.
d. all of the above.
e. none of the above.
Answer: d. A seed can be produced by fertilization or by parthenogenesis (as in the case of dandelions),
and germinates into a seedling under the right conditions.
6. What is the likely chemical composition of florigen, the long-sought chemical stimulus of flowering?
a. the hormone auxin
b. the protein CO
c. the carbohydrate callose
d. the mineral ion K+
e. none of the above
Answer: e. Recent evidence suggests that florigen is probably messenger RNA that is translated into
the protein FT in the shoot apical meristem.
7. How many whorls of organs occur in complete flowers?
a. two
b. four
c. six
d. eight
e. none of the above
Answer: b. Complete flowers have all four basic flower whorls (organ types): sepals, petals, stamens,
and carpels.
8. If an ovary contains eight ovules, how many seeds could potentially result if pollen tubes reach all
eight ovules?
a. one
b. four
c. eight
d. more than 20
e. none of the above
Answer: c. If the eggs present in all eight ovules were fertilized, eight seeds could be produced.
However, sometimes seeds cannot complete development because of genetic incompatibility between
egg and sperm, or because mutations prevent development.
9. What function(s) does the polysaccharide callose have in the reproduction of flowering plants?
a. Callose forms a coat that isolates young embryos during their early development.
b. Callose forms a coat that isolates groups of four microspores during their early development
into pollen grains.
c. Callose helps to pattern the sculptured sporopollenin walls of pollen grains.
d. Callose forms plugs that concentrate pollen tube cytoplasm at the tips, thereby aiding tip
growth.
e. All the above.
Answer: e. Callose isolates young embryos and microspores, helps to pattern the pollen wall, and aids
tip growth by pollen tubes.
10. From what structure does a fruit pericarp primarily develop?
a. the style
b. a stamen filament
c. the ovary wall
d. a group of fused sepals
e. the stigma
Answer: c. The fruit pericarp primarily develops from the ovary wall.
Conceptual Questions
1. Why are pollen grain walls composed of sporopollenin?
Answer: Pollen grains are vulnerable to mechanical damage and microbial attack during the journey
from the anthers of a flower to a stigma. Sporopollenin is an extremely tough polymer that helps to
protect pollen grains from these dangers. The function of the beautiful sculptured patterns of
sporopollenin on pollen surfaces is unclear.
2. Why are seed coats often tough?
Answer: The embryos within seeds are vulnerable to mechanical damage and microbial attack after they
are dispersed. Seed coats protect embryos from these dangers, and also help to prevent seeds from
germinating until conditions are favorable for seedling survival and growth.
3. Why do flowers occur in such a diversity of shapes and colors?
Answer: Flower diversity is an evolutionary response to diverse pollination circumstances. For example,
plants such as corn that are wind-pollinated produce flowers having a poorly developed perianth. If such
wind-pollinated flowers had large, showy perianths, they would get in the way of pollen dispersal or
acquisition. On the other hand, flowers that are pollinated by animals often have attractive shapes and
colorful petals.
Experimental Questions
1. Why had in vitro plant fertilization been so difficult for plant biologists to accomplish before the work
of Kranz and Lörz?
Answer: Plant gametes, particularly egg cells, are produced deep within flower tissues, and were thus
difficult to isolate.
2. What procedure did Kranz and Lörz use to accomplish fertilization of isolated eggs by sperm cells?
Answer: The investigators used a pulse of electricity to stimulate cell fusion. This process is similar to
electroporation, the use of electrical discharges to cause small pores to appear in the membranes of
cells prior to their transformation during genetic engineering.
3. How did Kranz and Lörz demonstrate that their plants were actually hybrids arising from in vitro
fertilization involving specific genetic parents?
Answer: They obtained egg cells and sperm cells from parents that differed in the color of stigmas and
stigma hairs. They demonstrated that progeny plants were hybrids that had stigmas that were colored
the same as one parent and stigma hairs that were colored the same as the other parent.
Collaborative Questions
1. Observe or think of orchid flowers. Are these flowers bilaterally symmetrical or radially symmetrical,
are these flowers more likely to be wind-pollinated or pollinated by animals, and what gene might be
involved in the production of orchid flower shape?
Answer: Orchid flowers are bilaterally symmetrical because they can be cut by only one plane that
would produce two identical halves. Bilateral symmetry is associated with bee pollination, and in fact
bees do pollinate many orchids, though other pollination processes are involved in some cases. For
example, Chinese botanists recently reported the case of an orchid that inhabits a windless site that is
too dry for animal pollinators. This orchid has adapted to its unusual pollination circumstances by
producing anthers that grow in a near circular path in order to self-pollinate their own stigmas, as a way
to produce seed. Because the gene CYCLOIDEA is involved in the development of bilaterally
symmetrical flowers in at least two distantly related eudicot plants, it is reasonable to think that orchids,
which are monocots, might possess a similar gene.
2. How do plants prevent the production of many offspring expressing deleterious recessive genes?
Answer: If egg cells and sperm cells are genetically similar, they could contribute the same deleterious
recessive genes to the offspring, with the result that offspring would be homozygous for such genes,
which would then be expressed. Humans avoid this process by cultural prohibitions against mating
between close relatives. Plants avoid this problem by using self-incompatibility systems (among other
processes). Self-incompatibility prevents pollen that is too closely related to stigma cells from delivering
sperm cells to eggs in ovules.