Double fertilization in Angiosperms

Double fertilization is a unique reproductive mechanism exhibited only by angiosperms (flowering plants). It involves the fusion of two male gametes with two female gametes resulting in the formation of embryo and endosperm.

Male gametophyte formation

The male gametophyte, also known as the pollen grain, is formed within the anthers of the flower. The anther consists of four microsporangia, each containing numerous microsporocytes that undergo meiosis to produce haploid microspores. Each microspore undergoes mitosis to produce a two-celled pollen grain comprising a vegetative cell and a generative cell.

Female gametophyte formation

The female gametophyte, also known as the embryo sac, is formed within the ovule of the flower. The ovule contains a megasporangium that undergoes meiosis to produce a single functional megaspore. The megaspore undergoes mitosis three times to produce eight haploid nuclei, which become enclosed by a cell wall to form the embryo sac. The embryo sac has seven cells and eight nuclei in total.

Double fertilization

Double fertilization occurs when a mature pollen grain lands on the stigma of a compatible flower. The pollen grain germinates and the vegetative cell forms a pollen tube that grows down the style towards the ovary. The generative cell divides to produce two sperm cells.

One sperm cell fuses with the egg cell (syngamy) to form a diploid zygote, which develops into the embryo. The other sperm cell fuses with the two polar nuclei (triple fusion) to form a triploid nucleus, which develops into the endosperm. The endosperm provides nourishment to the developing embryo.

Significance

Double fertilization is significant because it ensures that the endosperm is only formed if the egg is fertilized. This prevents wastage of resources on unfertilized ovules. The endosperm also provides nourishment to the developing embryo, enabling it to grow into a seed. This unique reproductive mechanism has contributed to the success and diversity of angiosperms.