Explanation

In angiosperms, the functional megaspore is haploid, which undergoes mitotic division and form 7-celled, 8-nucleate embryo sac. Therefore, each nucleus of embryo sac is haploid. At the time of fertilization, one male gamete fused with egg nucleus to form zygote (2$n$), whereas the second male gamete fuses with two polar nucleus(central cell) to form endosperm (3$n$). This type of fertilization is called double fertilization. Double fertilization is unique in angiosperms and discovered by Nawaschin (1898).