Exercises
Name the parts of an angiosperm flower in which development of male and female gametophyte take place.
Answer
The male gametophyte (pollen grain) develops inside the microsporangia (pollen sacs) of the anther, which is part of the stamen. The female gametophyte (embryo sac) develops inside the nucellus of the ovule, which lies within the ovary of the pistil.
Differentiate between microsporogenesis and megasporogenesis. Which type of cell division occurs during these events? Name the structures formed at the end of these two events.
Answer
Microsporogenesis: formation of microspores from a pollen mother cell (microspore mother cell) inside the microsporangium of the anther. Many pollen mother cells in each anther divide, and each gives four microspores arranged as a tetrad.
Megasporogenesis: formation of megaspores from the megaspore mother cell (MMC), a single cell in the micropylar region of the nucellus of the ovule. It forms four megaspores, usually in a linear row, of which three degenerate and only one stays functional.
Type of division: meiosis (reduction division) in both, so the spores are haploid.
Structures formed: microsporogenesis ends in microspore tetrads (which separate to become pollen grains); megasporogenesis ends in four megaspores, of which one functional megaspore develops into the embryo sac.
Arrange the following terms in the correct developmental sequence: Pollen grain, sporogenous tissue, microspore tetrad, pollen mother cell, male gametes.
Answer
Sporogenous tissue
⇒ pollen mother cell
⇒ microspore tetrad
⇒ pollen grain
⇒ male gametes.
With a neat, labelled diagram, describe the parts of a typical angiosperm ovule.
Answer
(Diagram: an anatropous ovule showing funicle, hilum, integuments, micropyle, nucellus, embryo sac and chalaza.) The ovule (megasporangium) is attached to the placenta by a stalk called the funicle. The point where the funicle joins the body of the ovule is the hilum. The ovule is covered by one or two protective layers called integuments, which leave a small opening at the tip called the micropyle. The basal end, opposite the micropyle, is the chalaza. Inside the integuments is a mass of cells called the nucellus, rich in reserve food. Lying within the nucellus is the embryo sac (female gametophyte), usually one per ovule, formed from a functional megaspore.
What is meant by monosporic development of female gametophyte?
Answer
In most flowering plants the megaspore mother cell undergoes meiosis to form four megaspores, but only one of them remains functional while the other three degenerate. The embryo sac develops from this single functional megaspore. Development of the female gametophyte from just one megaspore is called monosporic development.
With a neat diagram explain the 7-celled, 8-nucleate nature of the female gametophyte.
Answer
(Diagram: mature embryo sac with egg apparatus at micropylar end, central cell with two polar nuclei, and three antipodals at chalazal end.)
The nucleus of the functional megaspore divides mitotically to give two nuclei that move to opposite poles. Two more rounds of free nuclear mitosis give 4 nuclei at each pole, i.e. 8 nuclei in total (2
⇒ 4
⇒ 8 nucleate stage). Cell walls then form, organising the embryo sac:
At the micropylar end: three cells form the egg apparatus – one egg cell and two synergids (synergids bear a filiform apparatus that guides the pollen tube).
At the chalazal end: three cells called antipodals.
In the centre: one large central cell containing two polar nuclei (one from each pole).
Thus there are 3 + 3 + 1 = 7 cells but 3 + 3 + 2 = 8 nuclei, making the embryo sac 7-celled and 8-nucleate.
What are chasmogamous flowers? Can cross-pollination occur in cleistogamous flowers? Give reasons for your answer.
Answer
Chasmogamous flowers have exposed anthers and stigma, similar to flowers of other species, and open normally. Cross-pollination cannot occur in cleistogamous flowers, because these flowers never open; the anthers and stigma lie close together inside the closed flower, so when the anthers dehisce, pollen falls directly on the stigma of the same flower. Foreign pollen cannot reach the stigma, so they are invariably self-pollinated (autogamous).
Mention two strategies evolved to prevent self-pollination in flowers.
Answer
(i) Pollen release and stigma receptivity are not synchronised: either pollen is shed before the stigma becomes receptive or the stigma becomes receptive well before pollen is released. (ii) The anther and stigma are placed at different positions in the flower, so pollen cannot easily reach the stigma of the same flower. (Other devices: self-incompatibility, and production of unisexual flowers, e.g. dioecy in papaya.)
What is self-incompatibility? Why does self-pollination not lead to seed formation in self-incompatible species?
Answer
Self-incompatibility is a genetic mechanism that prevents pollen from fertilising ovules of the same flower or of other flowers on the same plant. In such species the pistil recognises its own pollen as incompatible and blocks it, either by preventing pollen germination on the stigma or by stopping pollen-tube growth in the style. Since the pollen tube never reaches the ovule, the male gametes are not delivered, fertilisation does not occur, and no seed is formed.
What is bagging technique? How is it useful in a plant breeding programme?
Answer
Bagging is covering an emasculated flower (or a female flower) with a bag, usually made of butter paper, to keep unwanted pollen away from its stigma. In artificial hybridisation the breeder wants only pollen of the chosen male parent to reach the stigma. Bagging prevents contamination by foreign pollen; when the stigma becomes receptive, the desired pollen is dusted on it and the flower is re-bagged so that the fruits and seeds obtained come from the intended cross only.
What is triple fusion? Where and how does it take place? Name the nuclei involved in triple fusion.
Answer
Triple fusion is the fusion of one male gamete with the two polar nuclei, i.e. fusion of three haploid nuclei. It takes place in the central cell of the embryo sac. The pollen tube enters a synergid and releases two male gametes; one fuses with the egg (syngamy), and the other moves to the central cell and fuses with the two polar nuclei, forming a triploid primary endosperm nucleus (PEN). The central cell becomes the primary endosperm cell, which develops into the endosperm. Nuclei involved: one male gamete nucleus and two polar nuclei.
Why do you think the zygote is dormant for sometime in a fertilised ovule?
Answer
The endosperm develops before the embryo. The zygote stays dormant until a certain amount of endosperm is formed, because the endosperm provides nourishment to the developing embryo. This ensures that a food supply is ready before the embryo starts dividing and growing.
Differentiate between: (a) hypocotyl and epicotyl; (b) coleoptile and coleorrhiza; (c) integument and testa; (d) perisperm and pericarp.
Answer
(a) Hypocotyl is the part of the embryonal axis below the level of attachment of the cotyledons; it ends in the radicle (root tip). Epicotyl is the part of the embryonal axis above the cotyledons; it ends in the plumule (shoot tip).
(b) Coleoptile is the hollow foliar sheath enclosing the shoot apex and a few leaf primordia in a monocot (grass) embryo. Coleorrhiza is the undifferentiated sheath enclosing the radicle and root cap in the monocot embryo.
(c) Integument is the protective covering of the ovule (before fertilisation). Testa is the outer seed coat formed from the outer integument after fertilisation.
(d) Perisperm is the residual, persistent nucellus found in some seeds (e.g. black pepper, beet); it is part of the seed. Pericarp is the wall of the fruit, developed from the ovary wall.
Why is apple called a false fruit? Which part(s) of the flower forms the fruit?
Answer
A true fruit develops only from the ovary. In apple, the fleshy edible part develops mainly from the thalamus (receptacle), and not just the ovary, so it is called a false fruit. The ovary forms only the central core containing the seeds, while the swollen thalamus forms the bulk of the fruit. (Strawberry and cashew are similar examples.)
What is meant by emasculation? When and why does a plant breeder employ this technique?
Answer
Emasculation is removal of anthers from a bisexual flower bud with forceps before the anthers dehisce. A breeder uses it during artificial hybridisation when the female parent bears bisexual flowers. It is done at the bud stage, before the anthers mature, to prevent self-pollination so that only pollen from the chosen male parent fertilises the flower. The emasculated flower is then bagged.
If one can induce parthenocarpy through the application of growth substances, which fruits would you select to induce parthenocarpy and why?
Answer
Parthenocarpic fruits develop without fertilisation and are seedless. I would choose fruits in which seeds are unwanted or troublesome when eating or processing, e.g. grapes, watermelon, orange, lemon, guava, pomegranate, papaya and banana (banana is already naturally parthenocarpic). Seedless fruits are easier to eat and to process into juices, jams and pulp, and therefore have higher commercial value.
Explain the role of tapetum in the formation of pollen-grain wall.
Answer
The tapetum is the innermost wall layer of the microsporangium. Its cells have dense cytoplasm and usually more than one nucleus. It nourishes the developing pollen grains. It also supplies materials for the pollen wall: it secretes precursors of sporopollenin, which forms the hard outer exine, as well as enzymes and the sticky pollenkitt that coats the pollen surface. Thus the tapetum contributes to the formation and finishing of the pollen-grain wall.
What is apomixis and what is its importance?
Answer
Apomixis is the production of seeds without fertilisation; it is a form of asexual reproduction that mimics sexual reproduction (seen in some Asteraceae and grasses). The embryo may form from a diploid egg cell formed without reduction division, or from nucellar cells. Importance: since there is no segregation of characters, the hybrid character is maintained in apomictic seeds. If hybrids are made apomictic, farmers can reuse their seeds year after year instead of buying fresh hybrid seed each season, lowering the cost of cultivation. Hence apomixis is valuable in agriculture and horticulture.
Important Extra Questions
What is double fertilisation?
Answer
In angiosperms two fusions occur in one embryo sac: syngamy (male gamete + egg
⇒ diploid zygote) and triple fusion (male gamete + two polar nuclei
⇒ triploid primary endosperm nucleus). This is called double fertilisation.
Why are pollen grains well preserved as fossils?
Answer
Their exine is made of sporopollenin, one of the most resistant organic materials known; it withstands high temperatures, strong acids and alkalis, and no enzyme is known to degrade it.
What is polyembryony? Give an example.
Answer
Occurrence of more than one embryo in a seed. In many Citrus varieties and mango, some nucellar cells develop into extra embryos.
Distinguish between geitonogamy and xenogamy.
Answer
Geitonogamy is transfer of pollen to the stigma of another flower on the same plant (genetically like autogamy). Xenogamy is transfer of pollen to the stigma of a flower on a different plant, bringing genetically different pollen.
Name the two cells of a mature pollen grain and state their fate.
Answer
The large vegetative cell, which forms the pollen tube, and the small generative cell, which divides mitotically to form two male gametes.