SolveItClass 12 · NCERT

NCERT Solutions · Class 12 Biology Sexual Reproduction in Flowering Plants

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Exercises 1.11–1.18 (part 2 of 2)

  1. Exercise 1.11

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

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    NCERT_Solution_Class12_Biology_Ch1_Q1-11
    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.

  2. Exercise 1.12

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

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    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.

  3. Exercise 1.13

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

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    (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 coleorrhizaNCERT_Solution_Class12_Biology_Ch1_Q1-13
    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).

  4. Exercise 1.14

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

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    NCERT_Solution_Class12_Biology_Ch1_Q1-14
    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.

  5. Exercise 1.15

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

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    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.

  6. Exercise 1.16

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

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    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.

  7. Exercise 1.17

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

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    NCERT publishes no answers for this textbook, so there is nothing in the book to check this working against. It has also not yet been read through by hand.

    NCERT_Solution_Class12_Biology_Ch1_Q1-17
    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.

  8. Exercise 1.18

    What is apomixis and what is its importance?

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    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.