Sexual Reproduction in Flowering Plants Class 12 Notes

A typical flower has four whorls: calyx (sepals), corolla (petals), androecium (stamens) and gynoecium (pistils). The stamen is the male reproductive unit consisting of a slender filament topped by a bilobed anther. Each anther has two thecae, each containing two microsporangia (pollen sacs) — four in total per anther.

Inside each microsporangium, a mass of diploid microsporocytes (pollen mother cells) undergoes meiosis to produce tetrads of haploid microspores — this process is called microsporogenesis. Each microspore matures into a pollen grain, which is the immature male gametophyte.

In the ovule, a single diploid megaspore mother cell (2n) located near the micropylar end undergoes meiosis to produce four haploid megaspores. Three of these degenerate, and the one closest to the chalaza survives. This functional megaspore undergoes three successive mitotic divisions to form the 8-nucleate, 7-celled embryo sac (female gametophyte).

• Three cells at the micropylar end form the egg apparatus — two synergids and one egg cell. The synergids have the filiform apparatus that guides the pollen tube.
• Three antipodal cells at the chalaza end; they usually degenerate before or after fertilization.
• The large central cell contains two polar nuclei that fuse with one sperm to form the triploid primary endosperm nucleus.

Adaptations for Wind Pollination

• Light, dry, smooth pollen grains and large, often feathery stigmas.
• Excessive pollen production (e.g. grasses produce millions).
• Flowers are small, dull-coloured, without nectar or scent.
• Exposed stamens that sway in the breeze.

Adaptations for Insect Pollination (Entomophily)

• Brightly coloured petals, fragrance, and nectar guides.
• Pollen grains are sticky or spiny and often held together by pollenkitt.
• Trap-door mechanisms in flowers like Aristolochia ensure insects pick up pollen.

Water pollination (hydrophily) is rare, seen in Vallisneria and Hydrilla. Pollen is released underwater and carried to the stigma on the water surface. Sponge-like stigmas are adapted to catch floating pollen.

When a compatible pollen grain lands on the stigma, it germinates to form a pollen tube that grows down the style, guided by the filiform apparatus of the synergids, and enters the ovule through the micropyle. The generative cell divides to form two sperm cells.

• The zygote undergoes repeated mitotic divisions to form the embryo. In dicots, the embryo has two cotyledons and a radicle-plumule axis (heart-shaped torpedo stage). In monocots, there is a single cotyledon (scutellum) and a protective sheath called the coleoptile over the plumule and coleorhiza over the radicle.
• Integuments harden into the seed coat. The ovary wall becomes the fruit wall (pericarp). A fruit may be fleshy (mango — drupe) or dry (wheat — caryopsis).
• Some ovules abort; some flowers develop parthenocarpic fruits without fertilization (banana, pineapple).

Pollen grains are rich in nutrients and are used as food supplements (pollen pills for athletes). They can also be stored in liquid nitrogen at −196°C for years (cryopreservation) and used in pollen banking for crop improvement programmes. Pollen viability varies: wheat and rice pollen lose viability within minutes, while some species retain it for months.

Example: Explain why the embryo sac is described as 8-nucleate but 7-celled. How many sperm cells are needed for complete fertilization in angiosperms?

Solution: The functional megaspore undergoes three mitotic divisions to produce 8 nuclei. These are distributed as 3 in the egg apparatus, 3 antipodals, and 2 polar nuclei in the central cell. Since the central cell contains 2 nuclei sharing one cell, there are only 7 cells total. Two sperm cells are needed — one for syngamy with the egg, one for triple fusion with the polar nuclei — hence double fertilization.

Example: A plant has feathery stigmas, produces millions of lightweight pollen grains per flower, and the flowers lack nectar and scent. Identify the pollination agent and explain how the plant is adapted.

Solution: This is wind pollination (anemophily). Feathery stigmas maximize the surface area to catch wind-borne pollen. Excessive pollen production compensates for the random, inefficient mode of transfer. Absence of nectar and scent means there is no need to attract pollinators. Examples include grasses, wheat, maize and many gymnosperms.

Exercises (NCERT)

1. 1
   Name the parts of an angiosperm flower in which development of male and female gametophytes takes place.
2. 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.
3. 3
   Arrange the following terms in the correct developmental sequence: Pollen grain, sporogenous tissue, microspore tetrad, pollen mother cell, male gametes.
4. 4
   With a neat, labelled diagram, describe the parts of a typical angiosperm ovule.
5. 5
   What is meant by monosporic development of the female gametophyte?
6. 6
   With a neat diagram explain the 7-celled, 8-nucleate nature of the female gametophyte.
7. 7
   What are chasmogamous flowers? Can cross-pollination occur in cleistogamous flowers? Give reasons.
8. 8
   Mention two strategies evolved to prevent self-pollination in flowers.
9. 9
   What is self-incompatibility? Why does self-pollination not lead to seed formation in self-incompatible species?
10. 10
    What is the bagging technique? How is it useful in a plant breeding programme?
11. 11
    What is triple fusion? Where and how does it take place? Name the nuclei involved in triple fusion.
12. 12
    Why do you think the zygote is dormant for some time in a fertilised ovule?
13. 13
    Differentiate between: (a) hypocotyl and epicotyl (b) coleoptile and coleorhiza (c) integument and testa (d) perisperm and pericarp.
14. 14
    Why is apple called a false fruit? Which part(s) of the flower forms the fruit?
15. 15
    What is meant by emasculation? When and why does a plant breeder employ this technique?
16. 16
    If one can induce parthenocarpy through the application of growth substances, which fruits would you select and why?
17. 17
    Explain the role of tapetum in the formation of the pollen-grain wall.
18. 18
    What is apomixis and what is its importance?

Answers at a glance

1. 1
   Male gametophyte (pollen grain) develops in the anther/microsporangium; female gametophyte (embryo sac) develops in the ovule (nucellus inside the megasporangium).
2. 2
   Microsporogenesis forms microspores (pollen grains) from a pollen mother cell; megasporogenesis forms megaspores from a megaspore mother cell. Both involve meiosis. Structures formed: the microspore tetrad (→ pollen grains) and the megaspore tetrad (one functional megaspore → embryo sac).
3. 3
   Sporogenous tissue → pollen mother cell (PMC) → microspore tetrad → pollen grain → male gametes.
4. 4
   Ovule parts: funicle (stalk), hilum (junction of funicle and body), integuments (protective layers), micropyle (pore at the tip), chalaza (base), nucellus (nutritive tissue) and the embryo sac (female gametophyte).
5. 5
   Monosporic development: the embryo sac develops from a single functional megaspore out of the four formed by meiosis in the megaspore mother cell.
6. 6
   The mature embryo sac is 8-nucleate and 7-celled: 3 antipodal cells, 2 synergids, 1 egg cell (3 cells at the micropylar end + 3 antipodals + 1 central cell with 2 polar nuclei).
7. 7
   Chasmogamous flowers are open flowers with exposed reproductive parts. Cross-pollination generally cannot occur in cleistogamous flowers because they never open — the anthers and stigma lie close together and only self-pollination (autogamy) occurs.
8. 8
   Dichogamy (anthers and stigma mature at different times), herkogamy (a physical barrier between anther and stigma) and self-incompatibility.
9. 9
   Self-incompatibility is a genetic mechanism that prevents pollen of the same flower/plant from fertilising the ovule; hence no viable zygote or seed is formed.
10. 10
    Bagging: covering an emasculated flower with a paper/butter-paper bag before anthesis to prevent unwanted pollen. It prevents contamination by foreign pollen and enables controlled cross-pollination in breeding programmes.
11. 11
    Triple fusion is the fusion of one male gamete with the two polar nuclei of the central cell, forming the triploid primary endosperm nucleus (PEN). It occurs in the embryo sac; nuclei involved: one male gamete and the two polar nuclei.
12. 12
    The zygote is dormant because the endosperm must first develop to provide nutrition for the developing embryo.
13. 13
    (a) Hypocotyl is the part below the cotyledons, epicotyl above them; (b) coleoptile is the sheath enclosing the plumule, coleorhiza the sheath over the radicle; (c) integuments are the ovule coverings that become the seed coat, testa is the outer seed coat; (d) perisperm is the residual nucellus, pericarp is the wall of the fruit.
14. 14
    Apple is a false fruit because the fleshy edible part develops from the thalamus and not from the ovary. The ovary forms the core of the fruit.
15. 15
    Emasculation is the removal of anthers from a bisexual flower bud before dehiscence. It is done to prevent self-pollination so that controlled cross-pollination (hybridisation) can be carried out.
16. 16
    Seedless fruits such as banana, grapes and papaya are selected, as parthenocarpy (fruit development without fertilisation) gives seedless varieties preferred by consumers.
17. 17
    The tapetum is the innermost layer of the microsporangium; it nourishes the developing pollen grains and provides sporopollenin and other materials for the formation of the exine (pollen wall).
18. 18
    Apomixis is the production of seeds without fertilisation. Its importance: it produces genetically uniform progeny and preserves hybrid vigour, so desirable hybrids can be maintained year after year without losing characters.

MCQ Questions

2 Mark Questions

1. 1
   What is the importance of sporopollenin in pollen grains?
2. 2
   Differentiate between autogamy and geitonogamy.
3. 3
   What is the role of endothecium in anther dehiscence?
4. 4
   Mention any two advantages of cross-pollination.
5. 5
   Define parthenocarpy and give one example.

3 Mark Questions

1. 1
   Describe the structure of a mature pollen grain.
2. 2
   Explain the process of megasporogenesis with a suitable description.
3. 3
   What is pollination? Explain any two types with examples.
4. 4
   Describe the process of double fertilisation in angiosperms.
5. 5
   Differentiate between albuminous and non-albuminous seeds with examples.

5 Mark Questions

1. 1
   Describe the development of the female gametophyte (embryo sac) in angiosperms (Polygonum type).
2. 2
   Explain the post-fertilisation changes leading to the formation of seed and fruit in angiosperms.
3. 3
   What is apomixis? How is it different from polyembryony? Discuss their significance in plant breeding.