Double fertilization or double fertilisation (see spelling differences) is a complex fertilization mechanism of angiosperms. This process involves the fusion of a female gametophyte or megagametophyte, also called the embryonic sac, with two male gametes (sperm). It begins when a pollen grain adheres to the stigmatic surface of the carpel, the female reproductive structure of angiosperm flowers. The pollen grain begins to germinate (unless a type of self-incompatibility that acts in the stigma occurs in that particular species and is activated), forming a pollen tube that penetrates and extends down through the style toward the ovary as it follows chemical signals released by the egg. The tip of the pollen tube then enters the ovary by penetrating the micropyle opening in the ovule, and releases two sperm into the embryonic sac (megagametophyte).
The mature embryonic sac of an unfertilized ovule is 7-cellular and 8-nucleate. It is arranged in the form of 3+1+3 (from top to bottom) i.e. 3 antipodal cells, 1 central cell (binucleate), 2 synergids & 1 egg cell. One sperm fertilizes the egg cell and the other sperm fuses with the two polar nuclei of the large central cell of the megagametophyte. The haploid sperm and haploid egg fuse to form a diploid zygote, the process being called syngamy, while the other sperm and the diploid central cell fuse to form a triploid primary endosperm cell (triple fusion). Some plants may form polyploid nuclei. The large cell of the gametophyte will then develop into the endosperm, a nutrient-rich tissue which nourishes the developing embryo. The ovary, surrounding the ovules, develops into the fruit, which protects the seeds and may function to disperse them.
