Sexual Reproduction in Flowering Plants Class 12 Notes

Structure of a Flower and Reproductive Organs

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.

Microsporogenesis and the Male Gametophyte

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.

Megasporogenesis and the Female 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.

Pollination and Its Agents

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.

Double Fertilization

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.

Post-Fertilization Changes

• 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).

Apomixis, Polyembryony and Pollen Application

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.

Solved Examples

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.