Exercise practice: Sexual reproduction in flowering plants

6.1 Exercise practice: the chapter's eighteen questions

The chapter closes with one exercise section of eighteen questions. Each one is answered below in my own words, with the reasoning that earns marks rather than just the final phrase. Try the question first, then read the check.

6.1.1 E1 Which parts hold the male and the female gametophyte?

Question. Identify the flower structures that house the male gametophyte and the female gametophyte.

Answer. The male gametophyte is the pollen grain, and it forms in the microsporangia of the anther. The female gametophyte is the embryo sac, and it forms in the ovule inside the ovary.

6.1.2 E2 Microsporogenesis versus megasporogenesis

Question. Set out the differences between microsporogenesis and megasporogenesis. Which kind of cell division do they use? What structures do they end with?

Answer. Microsporogenesis takes place in the anther, starts from a pollen mother cell and ends with pollen grains. Megasporogenesis takes place in the ovule, starts from a megaspore mother cell and ends with an embryo sac. Both depend on meiosis, so both begin from a diploid cell and produce haploid products. One difference in outcome matters: all four microspores of a tetrad become pollen grains, while only one of the four megaspores builds the embryo sac.

6.1.3 E3 Put the stages in order

Question. Put these five terms in the order in which they arise while a pollen grain develops: male gametes, microspore tetrad, pollen grain, sporogenous tissue, pollen mother cell.

Answer. Start with the sporogenous tissue. Its cells produce the pollen mother cells, and meiosis gives a tetrad of microspores. The tetrad separates into individual pollen grains, and each grain later gives the male gametes. So the order is sporogenous tissue, pollen mother cell, microspore tetrad, pollen grain, male gametes.

6.1.4 E4 A labelled ovule

Question. Draw a labelled diagram of a typical angiosperm ovule and describe its parts.

Answer. A stalk called the funicle fixes the ovule to the placenta. Where the body meets the stalk is the hilum. One or two integuments surround the body and leave a gap, the micropyle, at the top. The base on the opposite side is the chalaza. Food-rich nucellus fills the space inside the integuments, and the embryo sac lies within the nucellus. A good diagram marks funicle, hilum, integuments, micropyle, chalaza, nucellus and embryo sac.

6.1.5 E5 What is monosporic development?

Question. Explain what monosporic development of the female gametophyte means.

Answer. The megaspore mother cell undergoes meiosis and produces four haploid megaspores. Three of them degenerate in most species, leaving one functional megaspore. The entire embryo sac is built from that single survivor, and a gametophyte that comes from one spore is described as monosporic.

6.1.6 E6 Seven cells, eight nuclei

Question. With the help of a neat diagram, explain why the mature female gametophyte has seven cells but eight nuclei.

Answer. Three free nuclear mitotic rounds raise the nucleus count to eight. Walls form afterwards and separate the nuclei into cells. At the micropylar pole, two synergids and one egg make the egg apparatus. Three antipodal cells occupy the chalazal pole. The last two nuclei are the polar nuclei, and both sit in the large central cell. The central cell therefore carries two nuclei in one cell, which gives seven cells in total but eight nuclei.

6.1.7 E7 Chasmogamous flowers

Question. What is a chasmogamous flower? Is cross-pollination possible in a cleistogamous flower? Justify your answer.

Answer. A chasmogamous flower opens normally and displays its anthers and stigma. Cleistogamous flowers never open, so their anthers and stigma stay together inside the bud and pollen is transferred within the same flower. Cross-pollination is impossible there, because no foreign pollen can get in. The advantage is that seed is set without any pollinator.

6.1.8 E8 Avoiding self-pollen

Question. Describe two devices by which a flower prevents self-pollination.

Answer. One device is timing: the anther and the stigma become mature at different moments, so both are never ready together. Another is spacing: the anther and the stigma sit at different places in the flower, so the pollen cannot fall on the stigma of the same flower. Self-incompatibility and single-sex flowers are further devices covered in the chapter.

6.1.9 E9 Self-incompatibility

Question. Define self-incompatibility and explain why seed does not form in self-incompatible species.

Answer. Self-incompatibility is a genetic system in which the pistil refuses pollen from the same plant. It acts by stopping the pollen from germinating on the stigma or by halting the growth of its tube in the style. Without a tube, the male gametes never reach the egg, so there is no syngamy and no seed.

6.1.10 E10 Bagging

Question. What does bagging mean, and why is it important for a plant breeder?

Answer. Bagging means enclosing a flower, or a flower bud after its anthers have been removed, in a bag of butter paper or similar material. The bag keeps unwanted pollen away from the stigma. A breeder needs it during artificial hybridisation, where only the chosen pollen should reach the stigma.

6.1.11 E11 Triple fusion

Question. What is triple fusion? Give its location, the nuclei involved and the product.

Answer. Triple fusion happens in the central cell of the embryo sac, after the pollen tube has discharged its two male gametes into a synergid. One male gamete fuses with both polar nuclei there. The three nuclei are haploid, and the product is the triploid primary endosperm nucleus. The central cell then becomes the primary endosperm cell.

6.1.12 E12 A waiting zygote

Question. Why does the zygote stay inactive for a period inside a fertilised ovule?

Answer. The embryo needs a food reserve, and the endosperm supplies it. If the zygote divided at once, the embryo would start growing before its food was ready. The pause lets endosperm accumulate first, so the delay protects the developing embryo.

6.1.13 E13 Four pairs to distinguish

Question. Distinguish between these pairs: hypocotyl and epicotyl, coleoptile and coleorhiza, integument and testa, perisperm and pericarp.

Answer. The hypocotyl is the axis below the cotyledons and ends in the radicle, while the epicotyl is the axis above the cotyledons and ends in the plumule. The coleoptile is the sheath covering the shoot apex and leaf primordia of a grass embryo, while the coleorhiza covers the radicle and root cap. An integument is one of the protective layers of the ovule, while the testa is the seed coat that those layers become. Perisperm is the nucellus that survives inside a seed, while pericarp is the wall of a fruit.

6.1.14 E14 Why apple is a false fruit

Question. Why is an apple called a false fruit, and which parts of the flower build the fruit?

Answer. In an apple the thalamus becomes part of the fruit along with the ovary, and a fruit built from more than the ovary is called false. In a true fruit only the ovary takes part, and its wall becomes the pericarp.

6.1.15 E15 Emasculation

Question. What is emasculation, and when does a breeder use it?

Answer. Emasculation is the removal of anthers from a bisexual flower bud before the anthers open, done with forceps. A breeder uses it when the plant chosen as the female parent has bisexual flowers and must not pollinate itself, so that only the pollen selected by the breeder reaches the stigma.

6.1.16 E16 Parthenocarpy for which fruits?

Question. If parthenocarpy can be induced with growth substances, which fruits would you choose and why?

Answer. The sensible targets are fruits whose consumers prefer them seedless, such as banana, grape and citrus. A parthenocarpic fruit develops without fertilisation, so it carries no seeds, and that is an advantage in fruit eaten fresh.

6.1.17 E17 The tapetum and the pollen wall

Question. What does the tapetum do while the pollen grain wall is being formed?

Answer. The tapetum lines the microsporangium and feeds the pollen as it develops. Its cells secrete the material for the tough outer exine, including sporopollenin, and they support the formation of the intine and of the germ pores. Pollen from an anther with a damaged tapetum has a faulty wall.

6.1.18 E18 Apomixis

Question. Define apomixis and explain its importance.

Answer. Apomixis is the production of seed without fertilisation, reported in some Asteraceae and in grasses. An embryo can form from a diploid egg cell that skipped meiosis, or from nucellar cells. The seedlings are genetically identical to the parent. Breeders value this because an apomictic hybrid would not segregate, and a farmer could then save seed from a hybrid crop instead of buying fresh hybrid seed each year.