Sexual Reproduction in Flowering Plants: CBSE Class 12 Biology Chapter 2 Notes

Key Terminology for Exams

Microsporogenesis

The process of formation of microspores from a pollen mother cell (PMC) through meiosis.

Megasporogenesis

The process of formation of megaspores from the megaspore mother cell (MMC) through meiosis.

Pollination

The transfer of pollen grains from the anther to the stigma of a pistil.

Double Fertilization

A unique event in angiosperms where two fusion events, syngamy and triple fusion, occur within the embryo sac.

Syngamy

The fusion of one male gamete with the egg cell to form a diploid zygote.

Triple Fusion

The fusion of the second male gamete with the two polar nuclei (or secondary nucleus) to form a triploid Primary Endosperm Nucleus (PEN).

Apomixis

A form of asexual reproduction that mimics sexual reproduction, leading to the formation of seeds without fertilization. E.g., Grasses.

Polyembryony

The occurrence of more than one embryo in a seed. E.g., Citrus, Mango.

Parthenocarpy

The development of fruit without fertilization. Such fruits are seedless. E.g., Banana.

Sporopollenin

A highly resistant organic polymer that forms the exine (outer layer) of pollen grains, protecting them from environmental stress.

Pre-Fertilisation: Structures and Events

Sexual reproduction in flowering plants begins long before fertilization. The key events are the development of male and female gametophytes. The male reproductive part, the stamen, consists of a filament and an anther. Inside the anther's microsporangia, diploid microspore mother cells (MMCs) undergo meiosis to produce four haploid microspores (microspore tetrad). This process is microsporogenesis. Each microspore then develops into a pollen grain, the male gametophyte. A mature pollen grain has two layers: a tough outer exine made of sporopollenin and a thin inner intine. It contains a large vegetative cell and a smaller generative cell, which later divides to form two male gametes. The female reproductive part is the pistil (or carpel), comprising the stigma, style, and ovary. The ovule (megasporangium) is located inside the ovary. Within the ovule's nucellus, a single diploid megaspore mother cell (MMC) undergoes meiosis (megasporogenesis) to produce four haploid megaspores. Usually, three degenerate, and one functional megaspore develops into the female gametophyte, the embryo sac. Through mitotic divisions, this develops into a mature 7-celled, 8-nucleate structure, ready for fertilization.

The Mechanism of Double Fertilization

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Post-Fertilisation: Fate of Floral Parts

• Zygote (2n) develops into the Embryo.
• Primary Endosperm Nucleus (PEN) (3n) develops into the Endosperm, which provides nourishment to the developing embryo.
• Ovule matures into the Seed.
• Ovary develops into the Fruit.
• Ovary wall develops into the Pericarp (fruit wall).
• Integuments of the ovule develop into the Seed Coat (outer testa and inner tegmen).
• Micropyle persists as a small pore in the seed coat for oxygen and water entry during germination.
• Nucellus may be consumed or may persist as a thin layer called perisperm (e.g., black pepper).
• Sepals, Petals, Stamens, Style, and Stigma usually wither and fall off. In some plants like tomato and brinjal, sepals remain attached to the fruit.

Pollination: Geitonogamy vs. Xenogamy

Common Exam Traps

Ploidy Levels are Crucial: Memorize the ploidy of key structures. A common MCQ will ask for the ploidy of endosperm (3n), zygote (2n), synergids (n), antipodals (n), nucellus (2n), and MMC (2n). Make a chart for quick recall.

Distinguish Key Terms: Be very clear about the difference between:

• Syngamy (fusion to form zygote) vs. Triple Fusion (fusion to form PEN).
• Geitonogamy (functionally cross, genetically self) vs. Xenogamy (true cross).
• Parthenocarpy (seedless fruit) vs. Apomixis (seed without fertilization).

Diagrams: Practice drawing and labeling a L.S. of an anatropous ovule, a mature embryo sac, and a dicot embryo. These are frequently asked in 3 or 5-mark questions.

Quick Revision Check

• What is the ploidy of the Primary Endosperm Nucleus (PEN) and how is it formed? PEN is triploid (3n). It is formed by the fusion of one male gamete (n) with the two polar nuclei (n+n) in the central cell during triple fusion.
• Differentiate between the integument and the testa. The integument is the protective outer layer of the ovule (pre-fertilization). After fertilization, the integument hardens and develops into the testa, which is the outer seed coat.
• Why is sporopollenin important for a pollen grain? Sporopollenin forms the exine and is extremely resistant to chemical and biological degradation. It protects the male gametes inside the pollen grain from harsh environmental conditions like high temperature, desiccation, and enzymatic action.
• What are chasmogamous and cleistogamous flowers? Give an example of a plant that has both. Chasmogamous flowers are open with exposed stamens and stigma to facilitate cross-pollination. Cleistogamous flowers are closed and never open, ensuring self-pollination. Plants like Commelina and Viola (pansy) produce both types of flowers.

Frequently Asked Questions

Frequently Asked Questions