Dear Student,
The pollen grain contains two cells ,the pollen tube cell and the generative cell.After pollen deposits on stigma ,the pollen tube cells grows into a pollen tube through which generative cell travels.As it travels to the embryo sac ,the generative cell splits into two cells and form two sperm cells.As it enters the ovary,one sperm cell fertilize the egg cell forming a diploid zygote and the other sperm cell fuses with the two polar nuclei,forming a triploid cell that develops into the endosperm.As two fertilization is taking place ,this event in angiosperms is known as Double Fertilisation
Regards