Question 1

Name the parts of an angiosperm flower in which development of male and female gametophyte take place.

Accepted Answer

- The male gametophyte develops in the anther and the female gametophyte develops in the ovule. - In the anther: the microspores formed inside each microsporangium (pollen sac) mature into pollen grains, and the pollen grain is the male gametophyte. - In the ovule: the functional megaspore inside the nucellus develops into the embryo sac, and the embryo sac is the female gametophyte. - The anther is part of a stamen (androecium); the ovules arise from the placenta inside the ovary of the pistil (gynoecium).

Question 2

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.

Accepted Answer

- Microsporogenesis is the formation of microspores from a pollen mother cell (PMC); megasporogenesis is the formation of megaspores from the megaspore mother cell (MMC). - Site: microsporogenesis occurs in the sporogenous tissue at the centre of a microsporangium of the anther; megasporogenesis occurs in the micropylar region of the nucellus of the ovule. - Cell division: meiosis in both events. - Structures formed at the end of microsporogenesis: a microspore tetrad — a cluster of four haploid microspores, all of which normally separate and mature into pollen grains. - Structures formed at the end of megasporogenesis: four megaspores (a megaspore tetrad); in a majority of flowering plants only one is functional and the other three degenerate.

Question 3

Arrange the following terms in the correct developmental sequence: Pollen grain, sporogenous tissue, microspore tetrad, pollen mother cell, male gametes.

Accepted Answer

- The correct developmental sequence is: sporogenous tissue → pollen mother cell → microspore tetrad → pollen grain → male gametes. - Cells of the sporogenous tissue lie in the centre of a young microsporangium; each is a potential pollen (microspore) mother cell. - The pollen mother cell undergoes meiosis (microsporogenesis) to give the microspore tetrad. - As the anther matures and dehydrates, the microspores dissociate and develop into pollen grains. - The generative cell of the pollen grain divides mitotically to form the two male gametes — before shedding in about 40 per cent of species, otherwise during pollen tube growth.

Question 4

With a neat, labelled diagram, describe the parts of a typical angiosperm ovule.

Accepted Answer

- A typical angiosperm ovule is the megasporangium, a small body attached to the placenta inside the ovary; the standard figure is of an anatropous (inverted) ovule. - Funicle — the stalk by which the ovule is attached to the placenta. - Hilum — the region where the body of the ovule fuses with the funicle; it is the junction between ovule and funicle. - Integuments — one or two protective envelopes that encircle the nucellus. - Micropyle — the small opening organised at the tip, where the integuments do not close over the nucellus. - Chalaza — the basal part of the ovule, lying opposite the micropylar end. - Nucellus — the mass of cells enclosed within the integuments, with abundant reserve food materials. - Embryo sac (female gametophyte) — located in the nucellus; an ovule generally has a single embryo sac formed from one megaspore. What your diagram must show: - Draw an oval body turned back on itself, joined along a curved funicle; mark hilum where funicle meets the body. - Put the micropyle at the end lying next to the funicle, and the chalaza at the opposite (far) end, with a straight line through the two marking the ovule's axis. - Show two concentric layers around the body as the integuments, leaving a narrow gap at the micropylar tip. - Inside the integuments shade the nucellus, and within it draw the oval embryo sac. - Inside the embryo sac place the egg apparatus (one egg cell flanked by two synergids) at the micropylar end, the three antipodals at the chalazal end, and the two polar nuclei in the large central cell. - Label every part with a leader line: funicle, hilum, integuments, micropyle, chalaza, nucellus, embryo sac, egg cell, synergids, antipodals, polar nuclei.

Question 5

What is meant by monosporic development of female gametophyte?

Accepted Answer

- Monosporic development means the embryo sac is formed from a single megaspore. - The megaspore mother cell divides meiotically and produces four megaspores. - In a majority of flowering plants only one megaspore is functional and the other three degenerate. - Only that functional megaspore divides and develops into the female gametophyte (embryo sac) — hence the term monosporic.

Question 6

With a neat diagram explain the 7-celled, 8-nucleate nature of the female HUMAN REPRODUCTION

Accepted Answer

- A mature angiosperm embryo sac has eight nuclei but only seven cells, because two of the eight nuclei stay together, uncelled, in one large central cell. How it comes about: - The nucleus of the functional megaspore divides mitotically into two nuclei that move to opposite poles — the 2-nucleate embryo sac. - Two more sequential mitotic divisions give the 4-nucleate and then the 8-nucleate stages. - These divisions are strictly free nuclear — no cell wall follows each nuclear division. - Only after the 8-nucleate stage are cell walls laid down. Six of the eight nuclei get walls and become cells; the remaining two, the polar nuclei, remain free in the large central cell. The seven cells: - Egg apparatus at the micropylar end — one egg cell and two synergids (the synergids bear the filiform apparatus at their micropylar tip, which guides the pollen tube in). - Three antipodals at the chalazal end. - One large central cell containing the two polar nuclei. - Count: 3 + 3 + 1 = 7 cells; 3 + 3 + 2 = 8 nuclei. What your diagram must show: - Draw an oval embryo sac with the micropylar end at the bottom (or left) and the chalazal end opposite. - At the micropylar end draw three cells: a large central pear-shaped egg cell with two synergids flanking it, and show the filiform apparatus as thickenings at the synergid tips. - At the chalazal end draw three small cells, the antipodals. - Fill the whole middle with one large central cell and place the two polar nuclei in it, just above the egg apparatus. - Label all seven cells and both polar nuclei, and write "7-celled, 8-nucleate" beside the figure.

Question 7

What are chasmogamous flowers? Can cross-pollination occur in cleistogamous flowers? Give reasons for your answer.

Accepted Answer

- Chasmogamous flowers are flowers that open and expose their anthers and stigma, like the flowers of most other species. - No — cross-pollination cannot occur in cleistogamous flowers. - Reason: cleistogamous flowers do not open at all, so pollen from another flower can never reach their stigma. - Inside the closed bud the anthers and stigma lie close together; when the anthers dehisce, the pollen grains come into direct contact with the stigma of the same flower. - Cleistogamous flowers are therefore invariably autogamous (self-pollinated), and they give assured seed-set even when no pollinators are present. - Plants such as Viola (common pansy), Oxalis and Commelina produce both kinds of flower.

Question 8

Mention two strategies evolved to prevent self-pollination in flowers.

Accepted Answer

- Asynchrony of pollen release and stigma receptivity — in some species the pollen is shed before the stigma of the same flower becomes receptive, or the stigma becomes receptive well before the pollen is released, so self-pollen cannot function. - Different positions of anther and stigma — in other species the anther and stigma are placed at different levels within the flower, so pollen cannot come into contact with the stigma of the same flower. - Both these devices prevent autogamy. - (The chapter names two further devices: self-incompatibility, a genetic mechanism, and the production of unisexual flowers — monoecy as in castor and maize, dioecy as in papaya.)

Question 9

What is self-incompatibility? Why does self-pollination not lead to seed formation in self-incompatible species?

Accepted Answer

- Self-incompatibility is a genetic mechanism that prevents self-pollen — from the same flower or from another flower of the same plant — from fertilising the ovules. - It works by inhibiting pollen germination on the stigma or pollen tube growth in the style. - Self-pollination therefore does not lead to seed formation because the pistil recognises the pollen as of the wrong type (incompatible) and rejects it. - With no pollen tube reaching the ovule, the two male gametes are never discharged into a synergid, so syngamy and triple fusion cannot occur. - No fertilisation means no zygote and no endosperm — so the ovule never matures into a seed.

Question 10

What is bagging technique? How is it useful in a plant breeding programme?

Accepted Answer

- Bagging is covering an emasculated flower with a bag of suitable size, generally made of butter paper, to prevent contamination of its stigma with unwanted pollen. - Use in plant breeding: in artificial hybridisation the breeder must be sure that only the desired pollen fertilises the female parent. - Sequence followed: emasculate the bisexual flower bud → bag it → when the stigma of the bagged flower becomes receptive, dust it with mature pollen collected from the chosen male parent → rebag the flower and let the fruit develop. - If the female parent bears unisexual flowers, emasculation is not needed — the female flower buds are simply bagged before the flowers open, pollinated when the stigma is receptive, and rebagged. - Value: it keeps the cross genetically pure, so the hybrid seed obtained is the intended combination and not the product of stray pollen.

Question 11

What is triple fusion? Where and how does it take place? Name the nuclei involved in triple fusion.

Accepted Answer

- Triple fusion is the fusion of one male gamete with the two polar nuclei of the central cell, producing the triploid primary endosperm nucleus (PEN). - It is called triple fusion because three haploid nuclei fuse. - Where: inside the central cell of the embryo sac of the ovule. - How: the pollen tube enters one of the synergids through the filiform apparatus and releases its two male gametes into the synergid cytoplasm. One male gamete fuses with the egg (syngamy); the other male gamete moves towards the two polar nuclei in the central cell and fuses with them. - Nuclei involved: one male gamete nucleus + two polar nuclei. - The central cell then becomes the primary endosperm cell (PEC) and develops into the endosperm. Since syngamy and triple fusion both occur in the same embryo sac, the event is called double fertilisation — unique to flowering plants.

Question 12

Why do you think the zygote is dormant for sometime in a fertilised ovule?

Accepted Answer

- Because the embryo must have food ready before it starts growing — most zygotes divide only after a certain amount of endosperm has been formed. - Endosperm development therefore precedes embryo development: the primary endosperm cell divides repeatedly to form the triploid endosperm tissue filled with reserve food. - The waiting period is an adaptation to provide assured nutrition to the developing embryo, whose cells are used up in growth and cannot feed themselves. - Once enough endosperm is laid down, the zygote at the micropylar end divides and passes through the proembryo, globular and heart-shaped stages to the mature embryo.

Question 13

Differentiate between: (a) hypocotyl and epicotyl; (b) coleoptile and coleorrhiza; (c) integument and testa; (d) perisperm and pericarp.

Accepted Answer

(a) Hypocotyl and epicotyl - Hypocotyl is the cylindrical portion of the embryonal axis below the level of the cotyledons; it terminates at its lower end in the radicle (root tip), which is covered by a root cap. - Epicotyl is the portion of the embryonal axis above the level of the cotyledons; it terminates in the plumule (stem tip). (b) Coleoptile and coleorrhiza - Both are sheaths in the embryo of a grass (monocot). Coleoptile is the hollow foliar structure enclosing the shoot apex and a few leaf primordia at the upper end of the embryonal axis. - Coleorrhiza is the undifferentiated sheath enclosing the radicle and root cap at the lower end of the embryonal axis. (c) Integument and testa - Integuments are the one or two protective envelopes of the ovule, before fertilisation; they encircle the nucellus and leave the micropyle open. - Testa is the tough protective seed coat of the mature seed — the integument of the ovule after it has hardened; the micropyle survives in it as a small pore that lets oxygen and water in during germination. (d) Perisperm and pericarp - Perisperm is the residual, persistent nucellus left in some seeds, e.g., black pepper and beet. - Pericarp is the wall of the fruit, formed from the wall of the ovary; it may be fleshy (guava, orange, mango) or dry (groundnut, mustard).

Question 14

Why is apple called a false fruit? Which part(s) of the flower forms the fruit?

Accepted Answer

- Apple is a false fruit because it does not develop from the ovary alone — the thalamus also contributes to fruit formation. - In most plants the other floral parts degenerate and fall off as the ovary develops into the fruit; such ovary-only fruits are true fruits. - In apple the thalamus grows and becomes the fleshy edible part, so the fruit is made of thalamus + ovary. - Strawberry and cashew are the chapter's other examples of false fruits.

Question 15

What is meant by emasculation? When and why does a plant breeder employ this technique?

Accepted Answer

- Emasculation is the removal of the anthers from a flower bud, before the anther dehisces, using a pair of forceps. - When — at the bud stage. It is done on the flower chosen as the female parent, while it is still a bud, so that its anthers are taken out before they split open and shed pollen. - When — only if the female parent bears bisexual flowers. If the female parent produces unisexual flowers there is no need for emasculation; its female flower buds are simply bagged before the flowers open. - Why — it is a step in artificial hybridisation, one of the major approaches of crop improvement. The breeder crosses different species, and often genera, to combine desirable characters and produce commercially 'superior' varieties. - In such crossing experiments it must be certain that only the desired pollen grains are used for pollination and that the stigma is protected from contamination by unwanted pollen. Taking the anthers out of a bisexual flower before they dehisce removes the flower's own pollen, so it cannot pollinate itself. - Emasculation is always paired with bagging: the emasculated flower is covered with a bag of suitable size, generally of butter paper, to prevent contamination of its stigma. - When the stigma of the bagged flower attains receptivity, mature pollen grains collected from the anthers of the chosen male parent are dusted on the stigma, the flower is rebagged, and the fruits are allowed to develop.

Question 16

If one can induce parthenocarpy through the application of growth substances, which fruits would you select to induce parthenocarpy and why?

Accepted Answer

- Choose fruits whose edible part is the fleshy pericarp and whose seeds are a nuisance to the eater — for example banana, orange, watermelon, papaya, guava and grapes. - Reason: parthenocarpic fruits develop without fertilisation and are therefore seedless, so the whole fruit becomes edible pulp with nothing to spit out or remove. - Banana is the chapter's natural example of a parthenocarpic fruit, which shows the trait is workable in a fleshy fruit. - Fruits with many ovules per ovary, such as watermelon and papaya, gain the most, because these otherwise carry a very large number of seeds. - Fruits grown for their seeds — groundnut, mustard, cereals, pulses — must never be chosen, since a seedless crop would destroy the yield itself.

Question 17

Explain the role of tapetum in the formation of pollen-grain wall.

Accepted Answer

- The role the chapter gives the tapetum is that it nourishes the developing pollen grains — it feeds the microspores while they mature into pollen grains and build up their wall. - It is the innermost of the four wall layers that surround a microsporangium — epidermis, endothecium, middle layers and tapetum. The outer three protect the sporangium and help in dehiscence of the anther to release the pollen; the nourishing role belongs to the tapetum alone. - Its cells are built for that supply role: they possess dense cytoplasm and generally have more than one nucleus. - The wall so formed on the pollen grain is prominently two-layered — the hard outer exine made of sporopollenin, interrupted by germ pores where sporopollenin is absent, and the thin, continuous inner intine of cellulose and pectin. - Sporopollenin is one of the most resistant organic materials known: it withstands high temperatures and strong acids and alkalis, no enzyme that degrades it is so far known, and its presence is why pollen grains are so well preserved as fossils. - Note the limit of the book's claim: this chapter credits the tapetum with nourishment, and does not say that the tapetum itself secretes the sporopollenin of the exine.

Question 18

What is apomixis and what is its importance?

Accepted Answer

- Apomixis is the production of seeds without fertilisation — a form of asexual reproduction that mimics sexual reproduction. - It has evolved in a few flowering plants, particularly some species of Asteraceae and grasses. - Ways it happens: in some species the diploid egg cell is formed without reduction division and develops into an embryo without fertilisation; more often, as in many Citrus and mango varieties, some nucellar cells surrounding the embryo sac divide, protrude into the embryo sac and develop into embryos — which is why such ovules contain many embryos ( polyembryony ). Importance: - Hybrid varieties have hugely raised productivity, but hybrid seed must be produced afresh every year: seed collected from hybrids segregates in the progeny and does not keep the hybrid characters. - Producing hybrid seed is costly, so the seed is too expensive for many farmers. - If hybrids are made into apomicts, there is no segregation of characters in the progeny, and the farmer can keep using seed from his own crop year after year instead of buying fresh hybrid seed. - Because of this, active research is going on in laboratories around the world to understand the genetics of apomixis and to transfer apomictic genes into hybrid varieties.

NCERT Solutions · Class 12 Biology Sexual Reproduction in Flowering Plants

Exercises 1.1–1.10 (part 1 of 2)

Exercise 1.1

Exercise 1.2

Exercise 1.3

Exercise 1.4

Exercise 1.5

Exercise 1.6

Exercise 1.7

Exercise 1.8

Exercise 1.9

Exercise 1.10

Exercises 1.11–1.18 (part 2 of 2)

Exercise 1.11

Exercise 1.12

Exercise 1.13

Exercise 1.14

Exercise 1.15

Exercise 1.16

Exercise 1.17

Exercise 1.18