In angiosperms (flowering plants), two sets of fertilisation take place within the embryo sac (female gametophyte), that includes:
(i) Syngamy - fusion of male gamete with the egg cell.
(ii) Triple fusion - Fusion of another male gamete with the two polar nuclei

The process of double fertilisation in angiosperms can be explained as follows:

• After pollination, the pollen grain germinates on the stigma and the intine of the pollen grain emerges out as thin popllen tube.
• The pollen tube grows through the stigma and style through the ovary.
• Each pollen tube contains two nuclei - one vegetative and one generative nucleus.
• The vegetative nucleus degenerates and the generative nucleus divides to give rise to two male gametes.
• Pollen tube enters the ovule through the micropyle and enters the embryo sac through one of the synergids of the egg apparatus, where it gets reptured to release the two male gametes.
• One male gamete fuses with the egg to form a diploid zygote (Syngamy)
• The other male gamete fuses with two polar nuclei to form a triploid endosperm nucleus. (Triple fusion)