The endosperm is formed by a fusion between the sperm nucleus and two haploid polar nuclei.
Endosperm is a tissue surrounding the embryo of flowering plant seeds, which provides nutrition to the developing embryo. Endosperm formation involves double fertilization. There is fertilization of one sperm nucleus to produce a zygote, while the other sperm nucleus fuses with two haploid polar nuclei to form an endosperm cell. Endosperm is triploid because three cells are fused together.