Double fertilization only occurs in angiosperms. This only occurs when a pollen grain attaches to the stigma for the flower. The pollen grain them falls down the style and lands towards the egg. Once that has happen, one sperm will fertilize the egg, causing the development of diploid zygote (creates the embryo). The other sperm goes and fertilizes the endosperm which contains the nutrients and water for the young embryo when ripen.