The pollen grain consists of two male gamates and one vegetative cell.when the pollen grain reaches the stigma, the vegetative cell grows into a tube to reach the ovule in ovary.the two male gamates passes fr this tube to reach the ovule one male gamate fuses with the egg to form embryo and the other male gate fuses with the polar nuclei to.form endosperm.due to the fusion of two male gamates with different things in ovule it is known as double fertilisation.