CBSE · Class 12 · Biology · Sexual Reproduction in Flowering PlantsDescribe the process of double fertilization in angiosperms with the help of a well-labeled diagram concept. Also, explain the post-fertilization events that lead to the development of seed and fruit.

Introduction to Double Fertilization\nDouble fertilization is a unique and characteristic event that occurs in all angiosperms (flowering plants). It was discovered by S.G. Nawin in Lilium and Fritillaria. In this process, out of the two male gametes released by a pollen tube into the embryo sac, one fuses with the egg cell and the other fuses with the central cell.

Mechanism of Double Fertilization

• Entry of Pollen Tube: After reaching the ovary, the pollen tube enters the ovule usually through the micropyle (porogamy). Upon entering, the tip of the pollen tube ruptures, releasing two male gametes into the cytoplasm of the embryo sac (female gametophyte).
• Syngamy (Generative Fertilization): One of the male gametes moves towards the egg cell and fuses with its nucleus. This fusion of gametes is known as syngamy or generative fertilization, which results in the formation of a diploid zygote ($2n$). The zygote eventually develops into an embryo.
• Triple Fusion (Vegetative Fertilization): The second male gamete moves towards the central cell and fuses with the two polar nuclei (or secondary nucleus if they have already fused). Since three haploid nuclei are involved in this fusion, it is termed triple fusion. This results in the formation of a triploid ($3n$) primary endosperm nucleus (PEN).

Significance of Double Fertilization

• It ensures that the endosperm is formed only when the egg is fertilized, thus preventing energy wastage in unfertilized ovules.
• It provides nutritional tissue (endosperm) to the developing embryo.

Post-Fertilization Events\nFollowing double fertilization, several structural and physiological changes occur in the flower leading to seed and fruit formation:

• Endosperm Development: The primary endosperm nucleus divides repeatedly to form a triploid endosperm tissue, which stores food materials to nourish the developing embryo.
• Embryogenesis: The diploid zygote remains dormant for a period and then divides to form the embryo (proembryo, globular, heart-shaped, and mature embryo consisting of epicotyl, hypocotyl, and cotyledons).
• Seed Formation: The ovule transforms into a seed. The integuments of the ovule harden to form the protective seed coat (testa and tegmen), and the micropyle remains as a small pore for oxygen and water entry during germination.
• Fruit Formation: The ovary develops into a fruit, and the ovary wall transforms into the pericarp (fruit wall). Other floral parts like sepals, petals, and stamens usually wither and fall off, though in some fruits like apple and strawberry, the thalamus also contributes to fruit formation (false fruits).