Sexual Reproduction in Flowering Plants - CLASS 12 NOTES

Sexual Reproduction in Flowering Plants - Sexual reproduction in flowering plants is a fascinating process that involves the formation of male and female gametes, pollination, fertilization, and the development of seeds and fruits. This guide explores all aspects of sexual reproduction in flowering plants, focusing on key processes and their biological significance.

Structure of a Flower

A flower is the reproductive structure of angiosperms (flowering plants). It comprises male and female reproductive parts.

1. Male Reproductive Part (Androecium)

Stamen: The male reproductive organ consists of:

• Filament: The stalk that supports the anther.
• Anther: A bilobed structure that contains pollen sacs, each lobe having two thecae (dithecous structure).

2. Female Reproductive Part (Gynoecium)

Pistil: The female reproductive organ consists of:

• Stigma: The part that receives pollen grains.
• Style: The tube connecting the stigma to the ovary.
• Ovary: The enlarged basal part containing ovules, which develop into seeds after fertilization.

Microsporangium and Microsporogenesis

Microsporangium

• The anther contains microsporangia, which develop into pollen sacs during the mature stage.

Structure:

• Epidermis: Outer protective layer.
• Endothecium: Provides structural support.
• Middle Layers: 3–4 layers of parenchyma cells for protection.
• Tapetum: Innermost layer, providing nourishment to developing pollen grains.

Microsporogenesis

• The process of formation of microspores (pollen grains) from microspore mother cells (MMCs) via meiosis.

Steps:

1. MMC undergoes meiosis I and meiosis II.
2. Results in a tetrad of haploid microspores.
3. The microspores separate to form individual pollen grains.

Structure of Pollen Grain

• Exine: Outer tough layer made of sporopollenin.
• Intine: Inner layer made of cellulose and pectin.
• Germ Pores: Openings in the exine for germination.

Contents:

• Vegetative Cell: Larger cell that helps in pollen tube formation.
• Generative Cell: Divides to form two male gametes.

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Megasporangium and Megasporogenesis

Megasporangium (Ovule)

• The ovule is the female gametophyte contained in the ovary.

Structure:

• Funicle: Stalk attaching the ovule to the placenta.
• Hilum: Junction between ovule and funicle.
• Integuments: Outer layers protecting the nucellus.
• Micropyle: Opening in the integuments for pollen tube entry.
• Nucellus: Nutrient-rich tissue.
• Embryo Sac: Female gametophyte.

Megasporogenesis

• Formation of a megaspore from the megaspore mother cell (MMC) through meiosis.

Steps:

1. MMC undergoes meiosis, producing four haploid megaspores.
2. Three degenerate, leaving one functional megaspore.
3. The functional megaspore undergoes mitosis to form the embryo sac (7-celled, 8-nucleate structure).

Pollination

Pollination is the transfer of pollen grains from the anther to the stigma.

Types of Pollination

Autogamy (Self-Pollination):

• Transfer of pollen within the same flower.
• Requires synchrony in the maturation of anther and stigma.

Geitonogamy:

• Transfer of pollen between different flowers of the same plant.

Xenogamy (Cross-Pollination):

• Transfer of pollen between flowers of different plants.

Agents of Pollination

Abiotic Agents:

• Anemophily: Pollination by wind (e.g., grasses, corn).
• Hydrophily: Pollination by water (e.g., Vallisneria, seagrasses).

Biotic Agents:

• Entomophily: By insects (e.g., Amorphophallus).
• Ornithophily: By birds (e.g., Bombax).
• Chiropterophily: By bats (e.g., bananas).
• Myrmecophily: By ants (e.g., Rubiaceae).

Fertilization

Double Fertilization

• A unique feature of angiosperms.

Steps:

1. One male gamete fuses with the egg cell to form a diploid zygote (syngamy).
2. The other male gamete fuses with the two polar nuclei to form a triploid primary endosperm nucleus (triple fusion).

Post-Fertilization Events

Endosperm Development:

• Provides nutrition to the developing embryo.
• Types: Nuclear, cellular, or helobial.

Embryo Development:

• The zygote develops into the embryo.
• Parts: Plumule (shoot), radicle (root), and cotyledons.

Seed Formation:

• Ovules mature into seeds.

Types:

• Albuminous: Endosperm persists (e.g., coconut, wheat).
• Exalbuminous: No endosperm (e.g., pea, groundnut).

Fruit Formation:

• Ovary matures into the fruit.
• True Fruit: Develops from the ovary.
• False Fruit: Develops from other parts of the flower (e.g., apple, strawberry).

Special Mechanisms in Flowering Plants

Parthenocarpy:

• Development of fruits without fertilization (e.g., bananas).

Apomixis:

• Seed formation without fertilization (e.g., mango).

Polyembryony:

• Presence of multiple embryos in a seed (e.g., citrus).

FAQs About Sexual Reproduction in Flowering Plants

1. What is microsporogenesis?

Microsporogenesis is the formation of haploid microspores from diploid microspore mother cells through meiosis.

2. What is double fertilization?

Double fertilization is a process in angiosperms where one male gamete forms a zygote, and the other forms a triploid endosperm.

3. What are the types of pollination?

Pollination can be classified into autogamy (self-pollination), geitonogamy (within the same plant), and xenogamy (cross-pollination).

4. What is the role of the tapetum?

The tapetum provides nourishment to developing pollen grains and secretes enzymes like callase for tetrad separation.

5. What is the function of the micropyle?

The micropyle is an opening in the ovule that allows entry of the pollen tube for fertilization.

6. What are the types of fruits?

• True fruits develop from the ovary.
• False fruits develop from other floral parts (e.g., apple, cashew).