Answers to Review Questions Chapter 15

Answers to review questions – chapter 15

1.(a) What is the male gametophyte of a flowering plant? (pp. 321–322)

Pollen grain.

(b) Where is it located? (pp. 321–322)

The pollen grain is located centrally within the pollen chamber of the anther.

(c) What is the function of the callose wall during microsporogenesis? (pp. 321–322)

During microsporogenesis the microsporocyte undergoes meiosis to give rise to a tetrad of haploid microspores. These are held together within a callose wall and surrounded by the tapetum. The callose wall prevents the passage of molecules larger than sugars to the microspores, thus sealing off microspores from parental influences.

2.(a) What is the female gametophyte of flowering plant? (pp. 322–326)

The embryo sac.

(b) Where is it located? (pp. 322–326)

The embryo sac is located within the ovule, which is contained inside the ovary of the flower.

(c) By means of diagrams, show how it forms by meiotic and mitotic divisions.

See Figure 15.7.

3.What are the special features of the location of the egg and central cells within the embryo sac? Draw a diagram to illustrate your conclusions. (pp. 324–326)

The central cell is located next to the egg cell so that when the sperm nuclei are released from the pollen tube they are positioned well for double fertilisation. Figure 15.9 illustrates the location of these cells and double fertilisation.

4.(a) List the advantages and disadvantages of asexual reproduction and sexual reproduction. (pp. 328–339)

The most obvious advantage of asexual reproduction is the ability to reproduce an individual from a single plant. Asexual reproduction can be very efficient as in the case of apomixis, which allows for rapid multiplication of plants. In agriculture it is of great value in instances where genetically identical offspring are desirable or where propagation from seed is difficult. The major disadvantage of asexual reproduction is the lack of genetic variability in the offspring, reducing the capacity for natural selection in changing environments.

The most obvious advantage of sexual reproduction is the introduction of variation into the gene pool for the species, enabling the process of natural selection (survival of the fittest). In agriculture this variability in offspring allows for selection of the most desirable traits (e.g. productivity, colour, etc.). However, sexual reproduction can be costly to the parents, may have a low success rate and requires both male and female gametes to carry it out.

(b) What processes of sexual reproduction are avoided to produce seeds by apomixis? (pp. 327–328)

Developing diploid eggs do not undergo meiosis but instead begin to divide by mitosis to form an embryo. This also removes the need for pollination and fertilisation from the male gamete (pollen grain).

1. Many bisexual plants prevent self-fertilisation. Describe the various ways by which they do this. (pp. 328–329)

Several mechanisms evolved to reduce self-fertilisation. For example: (1) protandry (male first) or protogyny (female first) flowers go through separate male and female stages, allowing separation of the mature pollen from the receptive stigma of the same plant by time; (2) in dioecious and monoecious species, male and female gametes are not carried in the same flower and thus present a physical separation of gametes, and (3) self-incompatibility, which is a genetically controlled recognition system that stops eggs from being fertilised by pollen of the same plant.

6.(a) What is self-incompatibility? (pp. 329–330)

Self-incompatibility is a genetically controlled recognition system that stops eggs from being fertilised by pollen of the same plant. This involves an exchange of information (often encoded by a single genetic locus, the S locus) between the pollen grain and the pistil. Fertilisation will not occur if the same S allele is present in both pollen and pistil (see Figure 14.13).

(b) By means of a checkerboard, deduce the S alleles of the offspring when a self-incompatible plant, A (S1S2), is crossed reciprocally with another plant, B (S1S3). (Refer to earlier chapters for an explanation of the checkerboard notation and technique.)

Cross 1

Plant A (Pistil)

Cross 2 (reciprocal)

Plant A (Pollen)

S allelles of offspring: S1S3, S2S3 & S1S2

7.By means of labelled diagrams, distinguish between epigeal and hypogeal forms of germination. (pp. 334–335)

See Figure 15.18.

8.(a) In a dicotyledon embryo, what is:

(i) an epicotyl? (pp. 334–335)

The growing meristem of the shoot forms the epicotyl (the part of the axis above the cotyledons that forms leaves and lateral branches).

(ii) a hypocotyl? (pp. 334–335)

The hypocotyl lies immediately below the cotyledons.

(iii) a radicle? (pp. 334–335)

The radicle is the primary root of the seedling and is derived from the root meristem.

(b) What is the scutellum of a monocotyledon embryo? (p. 331)

The scutellum is the single cotyledon of a monocotyledon embryo.

9.Plants grow from meristems. (pp. 335–338)

(a) What organs are derived from the shoot apical meristem? (pp. 335–338)

Cells of the shoot apical meristem divide in a uniform manner to produce an outgrowth that becomes leaf primordium.

(b) How do the root and shoot apical meristems differ in structure? (pp. 335–338)

The root apical meristem lacks a tunica-corpus structure. Rather, the root contains an area of actively dividing cells which give rise to the mature tissues of the root.

(c)What function does the root cap serve? (pp. 335–338)

The root cap is a protective covering for the meristem as it grows through the soil.

10.The ‘ABC’ model describes how the genes regulating floral organ identity interact. (Box 15.3)

(a) What is meant by the term ‘homeotic’?

Mutations in homeotic genes transform one type of organ into another.

(b) What structures do the C region genes determine?

The carpels.