In plants that shed pollens at 3 celled stage , pollen tubes carry 2 male gametes from the beginning. Pollen tube grows and enters the ovule through a small opening called the micropyle . It enters in to one of the synergids through filiform apparatus. Some studies suggests that it is this filiform appartus what guides the entry of pollen tube.
After entering one of the synergids, the two male gametes or nuclei fuses with the egg nucleus.. This is known as syngamy or feltilisation and produces a diploid zygote.
The other male nucleus fuses with the two polar nuclei and forms a triploid nucleus or primary endosperm nucleus ( PEN) . As it involves the fusion of three haploid nuclei, it is known as triple fusion.
Since two sets of fertilisation, syngamy and triple fusion occur in the embryo - sac , it is called double fertilisation. As we know, it is unique to flowering plants.
After the double fertilization,
The diploid zygote divides mitotically to form the embryo of the seed.
The triple nuclues or PEN gives rise to a mass of tissue that develops in to endosperm. It provides nourishment to the growing embryo