When the pollen grain through pollen tube reaches ovule , two male gametes are released. in the ovule, there are 8 cells present , 3 at top (supporting cells) , 3 at bottom out of which 1 is egg cell (present in bottom) and two in the middle of ovule called plar nuclei . Out of the 2 male gamete fuses one fuses with egg cell and the other fuses with the polar nuclei present in middle.
The fusion of male gamete with middle egg leads to formaion of endosperm and the one fuses with egg cell leads to formation of zygoat.
Since the overall process includes two fertilization, therefore it is called double fertilization.
