FERTILIZATION

FERTILIZATION

Fertilization is the fusion of male and female gametes(sex cells) to produce a zycote(embryo). Following pollination the pollen grains that contains the male gamets are on the stigma, some distance from the female gamete (ovum) inside the ovule. To enable the gametes to meet, the pollen grain germinates and produces a pollen tube, which grows down and enters the embryo sac ( the inner part of the ovule that contains the ovum). Two male gamets, travelling at the tip of the pollen tube, enter the embryo sac. One gamete fuses with the ovum to produce a zygote that will develop into an embryo plant. The other male gamete fuses with two polar nuclei to produce the endosperm, which acts as a food store for the developing embryo. Fertilization also initiates other changes: the integument forms a testa around the embryo and endosperm ; the petals falls off; the stigma and style wither; and the ovary wall forms a layer around the seed. Together the pericap and seed form the fruit, which may be succulent. In some species, apomixis can occur: the seed develops without fertilization of the ovum by a male gamete but endosperm formation and fruit development take place as in other species.