Fertilization is the fusion of male and female gametes (sex cells) to produce a zygote (embryo). Following pollination, the pollen grains that contain the male gametes 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 gametes, traveling 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 supply for the developing embryo. Fertilization also initiates other changes: the integument (outer part of ovule) forms a testa (seed coat) around the embryo and endosperm; the petals fall off; the stigma and style wither; and the ovary wall forms a layer (called the pericarp) around the seed. Together, the pericarp and seed form the fruit, which may be succulent or dry.
In some species (e.g., blackberry), 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.
