NCERT · Class 12 · Biology · Sexual Reproduction in Flowering PlantsDescribe the process of double fertilization in angiosperms with a well-labeled diagram description. Explain its significance in the life cycle of flowering plants.

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. Nawashin in 1898 in Lilium and Fritillaria. In this process, out of the two male gametes discharged by a pollen tube into the embryo sac, one fuses with the egg cell and the other fuses with the central cell.

Step-by-Step Process

• Pollen Tube Entry: After the pollen grain lands on the stigma, it germinates and produces a pollen tube. The pollen tube enters the ovule usually through the micropyle and discharges two male gametes into the cytoplasm of the embryo sac (female gametophyte).
• Syngamy (First Fertilization): One of the male gametes moves towards the egg cell and fuses with its nucleus. This fusion of gametes is called syngamy or generative fertilization. It results in the formation of a diploid zygote ($2n$), which subsequently develops into the embryo.
• Triple Fusion (Second Fertilization): The second male gamete moves towards the central cell, which already contains two polar nuclei. It fuses with these two polar nuclei to produce a triploid ($3n$) primary endosperm nucleus (PEN). Since three haploid nuclei are involved in this fusion, it is termed triple fusion.
• Completion of Double Fertilization: Because both syngamy and triple fusion take place within the same embryo sac, the overall process is termed double fertilization. The central cell with the primary endosperm nucleus becomes the primary endosperm cell (PEC) and develops into the endosperm.

Significance of Double Fertilization

• Formation of Endosperm: Triple fusion results in the formation of the endosperm, which provides essential nutrition to the developing embryo during seed germination and early seedling growth.
• Evolutionary Advantage: It ensures that the plant does not expend energy on forming nutritive tissue (endosperm) unless fertilization of the egg cell has successfully occurred, thus conserving metabolic resources.
• Seed Viability: The combined products of double fertilization lead to the formation of a viable seed, ensuring the continuation of the species and successful dispersal mechanisms in diverse ecosystems.