Double fertilization is the process found in angiosperms in which out of the two male gametes released inside the embryo sac, one fuses with the egg cell (syngamy) and another fuses with secondary nucleus (triple fusion).
Release of Male gametes inside embryo sac
The pollen tube passes through the style and enters the ovule through the micropyle (porogamy). It penetrates the nucellus and enters the embryo sac.
It enters into one synergid through filiform apparatus. The synergid provides moisture to the tip of the pollen tube due to which it bursts open, thereby releasing the male gametes inside the embryo sac.
The synergids direct one male gamete to the egg cell and another male gamete to the secondary nucleus. The synergids degenerate later.
Syngamy (First fertilization)
Out of the two male gametes, one unites with the egg cell(n) to form a diploid zygote(2n). This is called syngamy or first fertilization.
Triple fusion (Second fertilization)
The second male gamete (n) unites with the secondary nucleus (2n) to form a triploid (3n) primary endosperm nucleus (PEN).
Syngamy and triple fusion together represent double fertilization which is a characteristic feature seen in angiosperms.