NCERT Solutions for Class 12 Biology Chapter 1: Sexual Reproduction in Flowering Plants

• Male gametophyte (pollen grain): develops inside the anther of the stamen — in its four microsporangia (pollen sacs).
• Female gametophyte (embryo sac): develops inside the ovule — within the nucellus of the ovule, which lies in the ovary of the pistil.

Cell division: Meiosis (reduction division) occurs in both, so the spores are haploid (n).

Structures formed: microspore tetrads (which separate into pollen grains) in microsporogenesis; a linear tetrad of megaspores in megasporogenesis.

• Cells of the sporogenous tissue in a young anther act as pollen mother cells (2n).
• Each PMC undergoes meiosis to give a microspore tetrad (n).
• Microspores separate and mature into pollen grains.
• The generative cell of the pollen grain divides mitotically to form two male gametes (before or after pollination).

The ovule (megasporangium) is a small structure attached to the placenta. Its parts are:

• Funicle: the stalk that attaches the ovule to the placenta.
• Hilum: the point where the body of the ovule joins the funicle.
• Integuments: one or two protective envelopes (outer and inner) around the ovule.
• Micropyle: a small opening at the tip left by the integuments; the pollen tube usually enters through it.
• Chalaza: the basal part opposite the micropylar end, where integuments and nucellus merge.
• Nucellus: the mass of cells enclosed by the integuments, rich in reserve food.
• Embryo sac (female gametophyte): located in the nucellus; usually one per ovule, formed from a megaspore.

When the embryo sac develops from only one megaspore, it is called monosporic development.

• The megaspore mother cell divides by meiosis to form four megaspores.
• In most flowering plants one megaspore remains functional and the other three degenerate.
• The single functional megaspore gives rise to the whole embryo sac (female gametophyte).

This is the most common (Polygonum) type of embryo sac development in angiosperms.

How the 8 nuclei arise

• The nucleus of the functional megaspore divides mitotically; the two nuclei move to opposite poles, forming the 2-nucleate embryo sac.
• Two more mitotic divisions give the 4-nucleate and then the 8-nucleate stage. These divisions are free nuclear — no cell wall forms after each division.
• At the 8-nucleate stage, four nuclei lie at the micropylar end and four at the chalazal end.

How the 7 cells are organised

• Micropylar end — egg apparatus (3 cells): 2 synergids + 1 egg cell. Synergids bear the filiform apparatus, which guides the pollen tube.
• Chalazal end — 3 antipodal cells.
• Centre — 1 large central cell with 2 polar nuclei (one from each pole).

Six nuclei are enclosed by cell walls (3 + 3), and the remaining two polar nuclei share the central cell. Hence the mature embryo sac has 3 + 3 + 1 = 7 cells but 8 nuclei.

Chasmogamous flowers are flowers that open normally, exposing their anthers and stigma (e.g. the open flowers of Viola, Oxalis and Commelina).

No, cross-pollination cannot occur in cleistogamous flowers. Reasons:

• Cleistogamous flowers never open, so anthers and stigma remain enclosed.
• Anthers and stigma lie close together; the anthers dehisce inside the closed flower and pollen falls on the stigma of the same flower.
• Pollen from another flower cannot reach the stigma, so only autogamy (self-pollination) is possible.

Such flowers give assured seed set even when pollinators are absent.

1. Non-synchronisation of maturity (dichogamy): pollen is released at a time when the stigma of the same flower is not receptive, or the stigma becomes receptive after the pollen is shed. (Protandry — anthers mature first; protogyny — stigma matures first.)
2. Different positions of anther and stigma (herkogamy): the anther and stigma are placed at different positions/heights, so pollen cannot come into contact with the stigma of the same flower.

Other outbreeding devices: self-incompatibility and production of unisexual flowers (monoecious plants like castor and maize prevent autogamy; dioecious plants like papaya prevent both autogamy and geitonogamy).

Self-incompatibility is a genetic mechanism that prevents pollen from the same flower (or other flowers of the same plant) from fertilising the ovules, by inhibiting pollen germination or pollen-tube growth in the pistil.

Why no seed forms:

• The pistil recognises the pollen as “self” through pollen–pistil interaction and rejects it.
• Either the pollen does not germinate on the stigma, or the pollen tube stops growing in the style.
• The male gametes therefore never reach the embryo sac — no fertilisation, hence no embryo, no endosperm and no seed.

Bagging is covering the emasculated flower (or a female flower) with a bag, usually of butter paper, so that unwanted pollen cannot reach its stigma.

Use in plant breeding (artificial hybridisation):

• It protects the stigma from contamination by foreign pollen.
• When the stigma becomes receptive, pollen collected from the desired male parent is dusted on it and the flower is rebagged.
• This ensures that only the chosen pollen fertilises the ovules, so breeders obtain the intended hybrid with desirable traits.

Triple fusion is the fusion of the second male gamete with the two polar nuclei, producing a triploid (3n) primary endosperm nucleus (PEN).

• Where: in the central cell of the embryo sac.
• How: the pollen tube enters one synergid and releases two male gametes. One fuses with the egg (syngamy → 2n zygote). The other moves to the central cell and fuses with the two polar nuclei (triple fusion). The central cell becomes the primary endosperm cell, which forms the endosperm.
• Nuclei involved: 1 male gamete (n) + 2 polar nuclei (n + n) → PEN (3n).

Syngamy and triple fusion together are called double fertilisation, a feature unique to flowering plants.

• The developing embryo needs a ready supply of food.
• Endosperm development precedes embryo development: the primary endosperm cell divides first to form the endosperm, a nutritive tissue.
• The zygote stays dormant until enough endosperm has formed, so that once it starts dividing the young embryo is assured of nourishment.

Thus the dormancy of the zygote is an adaptation to ensure nutrition for the embryo.

(a) Hypocotyl vs epicotyl

(b) Coleoptile vs coleorrhiza (monocot embryo)

(c) Integument vs testa

(d) Perisperm vs pericarp

• A true fruit develops only from the ovary.
• In apple, the fleshy edible part develops mainly from the thalamus (receptacle), not from the ovary. The ovary forms only the central core containing seeds.
• Because a floral part other than the ovary contributes to the fruit, apple is called a false fruit (so are strawberry and cashew).

Parts forming the fruit: the ovary (ovary wall → pericarp; ovules → seeds) and, in false fruits, the thalamus.

Emasculation is the removal of anthers from a flower bud with forceps before the anthers dehisce.

• When: in artificial hybridisation, if the female parent bears bisexual flowers; it is done at the bud stage, before pollen is released.
• Why: to prevent self-pollination, so that only pollen from the chosen male parent fertilises the ovules. It is followed by bagging.

If the female parent has unisexual flowers, emasculation is not needed — only bagging is done.

Parthenocarpy is the development of fruit without fertilisation, giving seedless fruits. It can be induced by spraying growth hormones such as auxins and gibberellins.

Fruits I would select: banana, grapes, watermelon, orange, lemon, guava and tomato.

Reasons:

• In these fruits the fleshy part is eaten and the seeds are unwanted — seeds are numerous, hard or bitter.
• Seedless fruits are more convenient to eat and are preferred for juice, jam and processing.
• They have higher market value.

The tapetum is the innermost wall layer of the microsporangium. Its cells have dense cytoplasm and generally more than one nucleus. Its roles in forming the pollen wall:

• Nourishes the developing microspores and pollen grains.
• Secretes the enzyme callase, which dissolves the callose wall of the tetrad so that microspores separate.
• Supplies sporopollenin precursors (via Ubisch bodies) that are deposited to form the tough exine — resistant to high temperature, strong acids, alkalis and enzymes.
• Contributes pollenkitt, the sticky, oily coat on pollen of insect-pollinated flowers.

Apomixis is the formation of seeds without fertilisation — a form of asexual reproduction that mimics sexual reproduction (seen in some Asteraceae and grasses).

How it happens: a diploid egg cell forms without reduction division and develops into an embryo; or cells of the nucellus/integument develop directly into embryos (e.g. Citrus, mango).

Importance:

• Hybrid seed industry: seeds from hybrids segregate and lose hybrid traits, so farmers must buy fresh hybrid seed every year. If hybrids are made apomictic, there is no segregation and farmers can reuse the seed year after year.
• Preserves hybrid vigour and desirable traits unchanged.
• Reduces the cost of hybrid seed for farmers.