The union of one sperm with the egg is known as fertilization. However, several developmental processes in the vegetative and germinative cells prepare the two sperms for a process known as double fertilization. A mitotic division of the germinative cell generates the sperm cells. This process that can take place on the growing pollen tube or inside the pollen grain.
In a growing pollen tube, the vegetative nucleus disintegrates and the sperm cells will take the lead and enter the embryo sac for successful fertilization. Usually, the interactions between the pollen grain and the pistil ensure that the sperm cells will often reach the micropyle of the ovule.
Once the sperm reach the micropyle, the growth of other tubes stops. In the embryo sac (female gametophyte), four cells are located at the micropylar side. Of those four, the first pair that the sperm cells will encounter are the synergids.
One of these is always bigger than the other and carries the filiform apparatus, a structure resembling hairs that degenerates after pollination and before fertilization. The synergids act as chemical attractants to the pollen tube, which penetrates the synergids via the filiform apparatus and then releases the two sperm cells.
One of the sperm cells will fuse with the egg (haploid), producing the zygote, the future embryo ; the other sperm cell will fuse with the primary endosperm nucleus (diploid) , generating the endosperm, which is tissue that serves as a food reserve . This event is said to be double fertilization which produce the diploid zygote and the triploid endosperm. The remaining cells of the female gametophyte are the antipodals; they usually degenerate after fertilization has taken place.