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

In angiosperms the gametophytes are highly reduced and develop inside the reproductive whorls of the flower — the androecium (stamens) and the gynoecium (pistil).

• The microspores formed in the microsporangium develop into pollen grains, which are the male gametophytes.
• The functional megaspore formed in the nucellus develops into the embryo sac, which is the female gametophyte.

How the two gametophytes arise:

• In the anther, cells of the sporogenous tissue act as pollen mother cells; each undergoes meiosis to form a microspore tetrad, and every microspore matures into a pollen grain containing a vegetative cell and a generative cell.
• In the ovule, a single megaspore mother cell undergoes meiosis to form four megaspores; one functional megaspore undergoes three mitotic divisions to form the 7-celled, 8-nucleate embryo sac.

Conclusion: the male gametophyte develops in the microsporangium of the anther and the female gametophyte develops in the nucellus of the ovule — both are thus protected inside the flower.

Microsporogenesis is the formation of microspores from a pollen mother cell by meiosis, while megasporogenesis is the formation of megaspores from the megaspore mother cell by meiosis.

Type of cell division: both events involve meiosis (reduction division) — the diploid (2n) mother cell forms four haploid (n) spores. (The later development of the spores into gametophytes takes place by mitosis.)

Conclusion: both processes are reductional (meiotic) and form haploid spores; microsporogenesis ends in pollen grains, while megasporogenesis ends in a single functional megaspore that forms the embryo sac.

Explanation of each step:

1. Sporogenous tissue: compactly arranged homogenous cells in the centre of each microsporangium of a young anther.
2. Pollen mother cell (PMC): each cell of the sporogenous tissue is a potential PMC (2n) or microspore mother cell.
3. Microspore tetrad: each PMC undergoes meiosis to form a cluster of four haploid microspores.
4. Pollen grain: as the anther matures and dehydrates, the microspores separate and develop into pollen grains (2-celled: a large vegetative cell and a small generative cell).
5. Male gametes: the generative cell divides by mitosis to form two male gametes — before shedding (3-celled pollen, in the remaining ~40% of species) or in the pollen tube after pollination (2-celled pollen — pollen of over 60% of angiosperms is shed at this stage).

Structure of the mature pollen grain formed in this sequence:

• Exine — hard outer layer of sporopollenin; resists high temperature, strong acids and alkalis; with germ pores where sporopollenin is absent (pollen tube emerges through them).
• Intine — thin inner layer of cellulose and pectin.
• Vegetative cell — large, with abundant food reserve and an irregular nucleus; forms the pollen tube.
• Generative cell — small, spindle-shaped; divides to give the two male gametes.

Conclusion: sporogenous tissue → pollen mother cell → microspore tetrad → pollen grain → male gametes; the first change involves meiosis and the last involves mitosis.

The ovule (megasporangium) is a small structure attached to the placenta inside the ovary. After fertilisation it develops into the seed. A typical angiosperm ovule is anatropous (inverted).

Parts of the ovule:

1. Funicle: the stalk by which the ovule is attached to the placenta.
2. Hilum: the region where the body of the ovule fuses with the funicle; it is the junction between ovule and funicle.
3. Integuments: one or two protective envelopes (outer and inner) around the ovule; they later form the seed coat.
4. Micropyle: a small opening left at the tip where the integuments do not cover the nucellus; the pollen tube usually enters through it.
5. Chalaza: the basal part of the ovule, opposite the micropylar end, from where the integuments arise.
6. Nucellus: the mass of cells enclosed by the integuments; its cells have abundant reserve food.
7. Embryo sac (female gametophyte): located in the nucellus; generally one per ovule, formed from a single functional megaspore.

Conclusion: the ovule is the megasporangium; its integuments protect the nucellus, the nucellus houses the embryo sac, and after fertilisation the whole ovule becomes the seed (integuments → seed coat).

When the embryo sac (female gametophyte) develops from a single functional megaspore, the development is called monosporic.

1. The megaspore mother cell (MMC) in the nucellus undergoes meiosis and forms four megaspores (usually a linear tetrad).
2. In a majority of flowering plants, only one megaspore remains functional (usually the chalazal one); the other three degenerate.
3. Only this functional megaspore develops into the embryo sac — hence the name mono (one) sporic (spore). This is the Polygonum type of embryo sac.
4. Its nucleus then divides mitotically three times (free nuclear divisions) to give 8 nuclei, forming the 7-celled, 8-nucleate embryo sac.

Conclusion: in monosporic development all the nuclei of the embryo sac come from one megaspore, so they are genetically identical and haploid.

The mature female gametophyte (embryo sac) of a typical angiosperm is 7-celled and 8-nucleate. It develops as follows:

1. First mitosis: the nucleus of the functional megaspore divides to form 2 nuclei, which move to the opposite poles (2-nucleate embryo sac).
2. Second and third mitoses: the nuclei at each pole divide twice, giving 4 + 4 = 8 nuclei. These are free nuclear divisions — nuclear division is not followed by cell wall formation.
3. Cell formation: after the 8-nucleate stage, cell walls are laid down and the typical embryo sac is organised.

Conclusion: there are 7 cells but 8 nuclei because the central cell contains two polar nuclei. The synergids have special cellular thickenings at the micropylar tip called the filiform apparatus, which guides the pollen tube into the synergid.

Chasmogamous flowers are flowers with exposed anthers and stigma, similar to the flowers of other species. They open normally, so both self- and cross-pollination can occur in them.

Cleistogamous flowers are flowers that do not open at all. Examples: Viola (common pansy), Oxalis and Commelina, which produce both chasmogamous and cleistogamous flowers.

No, cross-pollination cannot occur in cleistogamous flowers, because:

1. The flowers remain closed, so pollen from another flower cannot reach the stigma.
2. The anthers and stigma lie close to each other inside the closed flower.
3. When the anthers dehisce in the flower buds, pollen grains come in contact with the stigma and effect pollination.

Thus cleistogamous flowers are invariably autogamous (self-pollinated). They assure seed set even in the absence of pollinators.

Conclusion: cleistogamous flowers are closed and always self-pollinated, so cross-pollination is not possible in them; chasmogamous flowers of the same plant can be cross-pollinated.

Continued self-pollination causes inbreeding depression. Flowering plants have therefore developed several outbreeding devices to discourage self-pollination and encourage cross-pollination:

1. Non-synchronisation of pollen release and stigma receptivity (dichogamy): in some species the pollen is released before the stigma becomes receptive, or the stigma becomes receptive much before the pollen is released. So the pollen cannot pollinate the stigma of the same flower.
2. Different positions of anther and stigma (herkogamy): in some species the anther and stigma are placed at different positions so that the pollen cannot come in contact with the stigma of the same flower.
3. Self-incompatibility: a genetic mechanism that prevents self-pollen from fertilising the ovules by inhibiting pollen germination or pollen tube growth in the pistil.
4. Production of unisexual flowers: if male and female flowers are on the same plant (monoecious, e.g., castor, maize) autogamy is prevented but geitonogamy is not; if they are on different plants (dioecious, e.g., papaya), both autogamy and geitonogamy are prevented.

Conclusion: dichogamy and herkogamy (the first two devices) mainly prevent autogamy; self-incompatibility and dioecy prevent both autogamy and geitonogamy, thus encouraging cross-pollination and avoiding inbreeding depression.

Self-incompatibility is a genetic mechanism by which a plant prevents its own pollen (from the same flower or other flowers of the same plant) from fertilising its ovules, even though the pollen and ovules are both fertile.

Why self-pollination does not form seeds in such species:

1. The pistil can recognise whether a pollen grain is of the same plant (self/incompatible) or of another plant (compatible). This recognition is mediated by a chemical interaction between the pollen and the pistil.
2. When self-pollen lands on the stigma, the pistil rejects it by inhibiting pollen germination on the stigma, or by inhibiting pollen tube growth in the style.
3. Therefore the pollen tube does not reach the ovule, the male gametes are not delivered to the embryo sac, and no fertilisation takes place.
4. Without fertilisation there is no zygote and no endosperm, so no seed is formed.

Conclusion: self-incompatibility is a genetic outbreeding device; because the pistil rejects self-pollen before it can deliver male gametes, fertilisation and seed formation fail after self-pollination.

Bagging is the technique of covering an emasculated flower (or a flower with only female parts) with a bag made of butter paper or similar material, to prevent contamination of its stigma with unwanted pollen.

Use in plant breeding (artificial hybridisation):

1. In artificial hybridisation, the breeder wants only the desired pollen grains (from the chosen male parent) to pollinate the stigma.
2. If the female parent bears bisexual flowers, the anthers are first removed from the bud before they dehisce (emasculation) and the flower is then bagged.
3. When the stigma becomes receptive, mature pollen collected from the anthers of the male parent is dusted on the stigma, and the flower is rebagged.
4. The bagged flowers are allowed to develop into fruits and seeds.
5. If the female parent bears unisexual flowers, emasculation is not needed; the female flower buds are simply bagged before the flowers open.

Importance: it ensures that fertilisation takes place only by pollen of the desired male parent, so the breeder obtains the desired hybrid with chosen characters.

Conclusion: emasculation and bagging together protect the stigma from unwanted pollen, making artificial hybridisation reliable.

Triple fusion is the fusion of the second male gamete with the two polar nuclei (located in the central cell) to form a triploid primary endosperm nucleus (PEN). Since it involves the fusion of three haploid nuclei, it is called triple fusion.

Where: inside the embryo sac of the ovule — in the central cell.

How it takes place:

1. After compatible pollination, the pollen tube grows through the style, enters the ovule through the micropyle and then enters one of the synergids through the filiform apparatus.
2. The pollen tube releases two male gametes into the cytoplasm of the synergid.
3. One male gamete moves towards the egg cell and fuses with its nucleus — syngamy → diploid zygote.
4. The other male gamete moves towards the two polar nuclei in the central cell and fuses with them — triple fusion → triploid primary endosperm nucleus (PEN).
5. The central cell now becomes the primary endosperm cell (PEC), which develops into the endosperm — the nutritive tissue for the developing embryo.

Nuclei involved: one male gamete nucleus + two polar nuclei. Since syngamy and triple fusion both occur in the same embryo sac, the process is called double fertilisation, an event unique to flowering plants.

Conclusion: triple fusion forms the triploid PEN in the central cell; together with syngamy it constitutes double fertilisation, unique to angiosperms.

The zygote remains dormant for some time after fertilisation because the embryo needs an assured supply of nutrition before it starts developing.

1. After double fertilisation, the primary endosperm cell divides repeatedly to form the endosperm — a triploid tissue rich in reserve food.
2. Endosperm development always precedes embryo development. The zygote waits until a certain amount of endosperm has formed.
3. The endosperm then provides nourishment to the developing embryo; only after that does the zygote divide to form the proembryo and then the globular, heart-shaped and mature embryo.

Conclusion: the dormancy of the zygote is an adaptation that ensures the embryo gets nourishment right from the beginning of its development.

Supporting facts:

• In the most common type, the PEN first forms a free-nuclear endosperm, which later becomes cellular — e.g., the coconut water of a tender coconut is free-nuclear endosperm and the white kernel is cellular endosperm.
• The embryo develops at the micropylar end of the embryo sac, where the zygote lies, and draws food from this endosperm.

The terms in (a) and (b) refer to parts of the embryo; (c) and (d) compare parts of the ovule/seed and the fruit. The embryo diagrams below show (a) and (b):

(a) Hypocotyl and epicotyl

(b) Coleoptile and coleorrhiza (in monocot embryos, e.g., grasses)

(c) Integument and testa

(d) Perisperm and pericarp

Conclusion: epicotyl/plumule and hypocotyl/radicle form the shoot and root; coleoptile and coleorrhiza are protective sheaths of monocot embryos; testa comes from the integument, perisperm from the nucellus and pericarp from the ovary wall.

• A true fruit develops only from the ovary of the flower (e.g., mango, tomato).
• In the apple, the thalamus (receptacle) also contributes to fruit formation — the fleshy edible part of the apple is mainly the thalamus, while the ovary forms only the central core containing seeds.
• Since parts of the flower other than the ovary take part in forming the fruit, the apple is called a false fruit (pseudocarp). Strawberry and cashew are other examples.

Parts forming the apple: thalamus (fleshy edible part) + ovary (core).

In general: after fertilisation the ovary develops into the fruit, the ovary wall into the pericarp, and the ovules into seeds.

Conclusion: the apple is a false fruit because its fleshy edible part is the thalamus, not the ovary.

Emasculation is the removal of anthers from a flower bud before the anthers dehisce, using a pair of forceps.

When it is employed:

• During artificial hybridisation, when the female parent bears bisexual flowers.
• It is done in the bud stage, before the anthers mature and release pollen.

Why it is employed:

1. To prevent self-pollination (autogamy) of the flower by its own pollen.
2. To make sure that only the pollen of the desired male parent fertilises the ovules, so that the breeder gets the desired hybrid combining the useful characters of both parents.

After emasculation the flower is bagged to protect it from unwanted pollen, and later the desired pollen is dusted on the receptive stigma and the flower is rebagged (see Q10).

Conclusion: emasculation is done in bisexual flowers at the bud stage to prevent self-pollination and ensure a planned cross.

Parthenocarpy is the development of fruit without fertilisation; such fruits are seedless. It can be induced by applying growth hormones (e.g., auxins, gibberellins) to the flowers.

Fruits I would select: fruits that are eaten fresh and in which seeds are numerous, hard or unwanted, for example:

• Banana (naturally parthenocarpic), grapes, watermelon, orange / lemon, guava, papaya, tomato and pomegranate.

Reasons:

1. Seedless fruits are easier and more pleasant to eat and have higher consumer and market value.
2. Seedless fruits are preferred for making juices, jams and processed products.
3. In these fruits, the edible part is the fleshy pericarp, so the absence of seeds does not reduce the useful part of the fruit.

Conclusion: growth-substance-induced parthenocarpy is most useful in fleshy fruits eaten fresh or processed, where seeds are undesirable.

The tapetum is the innermost wall layer of the microsporangium, lying just outside the sporogenous tissue. The four wall layers from outside to inside are epidermis, endothecium, middle layers and tapetum.

Features of tapetal cells: they have dense cytoplasm and generally more than one nucleus, showing that they are highly active cells.

Role in pollen-wall formation:

1. The tapetum nourishes the developing pollen grains.
2. Its cells secrete the material that forms the exine (outer wall of the pollen grain), which is made of sporopollenin — one of the most resistant organic materials known. It can withstand high temperatures and strong acids and alkalis, and no enzyme is known that degrades it. That is why pollen grains are well preserved as fossils.
3. It also secretes enzymes (e.g., callase that frees the microspores from the tetrad) and hormones.
4. In many species it provides pollenkitt, an oily, sticky coating on the pollen surface that helps in pollination by insects.

Note: the exine has prominent apertures called germ pores where sporopollenin is absent; the inner wall, intine, is thin and made of cellulose and pectin.

Conclusion: the tapetum is the nutritive and secretory layer of the anther; it supplies sporopollenin for the exine and other substances needed for normal pollen development.

Apomixis is a form of asexual reproduction that mimics sexual reproduction — it is the production of seeds without fertilisation. It is found in some species of Asteraceae and grasses.

Ways in which apomictic seeds develop:

1. A diploid egg cell is formed without reduction division and develops into the embryo without fertilisation.
2. In many species, e.g., citrus and mango, some nucellar cells surrounding the embryo sac start dividing, protrude into the embryo sac and develop into embryos. Such seeds may contain many embryos — polyembryony.

Importance of apomixis:

1. Hybrid seeds without repeated crossing: hybrid seeds must be produced every year because the progeny of hybrids segregate and lose hybrid characters. If hybrids are made apomictic, there is no segregation, so farmers can keep using the same hybrid seed year after year.
2. Preserves hybrid vigour and all desirable characters unchanged, because the embryo is genetically identical to the parent.
3. Reduces the cost of hybrid seed, which is currently very expensive for farmers.
4. Scientists are therefore trying to identify the genes for apomixis and transfer them into hybrid varieties.

Conclusion: apomixis produces seeds without fertilisation; if introduced into hybrids it would let farmers keep using high-yielding hybrid seed every year at low cost.