NCERT · Class 12 · Biology · Sexual Reproduction in Flowering PlantsDescribe the process of double fertilization in angiosperms with the help of a neat and well-labeled diagram. Also, 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. Nawaschin 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 diploid secondary nucleus.

Step-by-Step Process

• Pollen Tube Entry: After the pollen grain lands on a compatible stigma, it germinates to form a pollen tube. The pollen tube enters the ovule through the micropyle and penetrates one of the synergids, where it discharges two male gametes.
• Syngamy (First Fertilization): One of the male gametes moves towards the egg cell and fuses with its nucleus. This process is known as syngamy or true fertilization, resulting 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 and fuses with the two polar nuclei (or secondary nucleus) located there. Since this involves the fusion of three haploid nuclei, it is termed triple fusion, resulting in a triploid ($3n$) primary endosperm nucleus (PEN).

Significance of Double Fertilization

• Formation of Endosperm: Triple fusion leads to the formation of the primary endosperm nucleus, which develops into the endosperm. The endosperm provides essential nutritive tissue for the developing embryo during seed germination.
• Restoration of Diploidy: Syngamy restores the diploid condition ($2n$) in the life cycle, ensuring genetic stability across generations through the formation of a viable zygote.
• Prevention of Wasteful Resource Allocation: Plant resources are not wasted on unfertilized ovules, as endosperm development is typically triggered only after successful fertilization takes place.
• Evolutionary Advantage: It ensures that the nutritive tissue is formed only when an embryo has been successfully created, making reproductive efficiency remarkably high in angiosperms.