In fertilization when both the male gametes participate in the process of fusion and the fusion takes place twice or at 2 places, the process is called double fertilization.
Double fertilisation is the process occuring in angiosperms (flowering plants) during reproduction, in which two sperm nuclei from each pollen tube fertilise two cells in an ovary. The pollen grain adheres to the stigma of the carpel (female reproductive structure) and grows a pollen tube that penetrates the ovum through a tiny pore called a micropyle. Two sperm cells are released into the ovary through this tube. One of the two sperm cells fertilises the egg cell (at the bottom of the ovule near the micropyle), forming a diploid (2n) zygote. The other sperm cell fuses with two haploid polar nuclei (contained in the central cell) in the centre of the embryo sac (or ovule). The resulting cell is the triploid (3n) primary endosperm nucleus. This triploid cell divides through mitosis and forms the endosperm, a nutrient-rich tissue, inside the seed.