Double fertilization is a phenomenon unique to angiosperms. Each pollen grain produces two sperm; Of the two sperm cells, one sperm fertilizes the egg cell, forming a diploid zygote (syngamy); the other sperm fuses with the two polar nuclei, forming a triploid cell that develops into the endosperm (triple fusion) Together, these two fertilization events in angiosperms are known as double fertilization.