In angiosperms; one of the male gamete fuses with the female gamete. Additionally, another male gamete fuses with the secondary nucleus. Fusion between male and female gametes results in formation of the zygote. Apart from that, fusion between another male gamete and the secondary nucleus results in formation of the primary endosperm nucleus (PEN). Since, two instances of fusion of nuclei take place hence it is called double fertilization. Two following figure shows double fertilization: