Pollen grains on reaching the right stigma become three-celled and begin to germinate. Each pollen grain forms a small tube like structure called pollen tube which emerges through the germ pore. The contents of the pollen grain move into the tube and the tube nucleus occupies the tip of the pollen tube.
Pollen tube grows through the tissues of the stigma and style and finally enters the ovule through the micropyle. Vegetative nucleus or the tube nucleus degenerates and the two sperms (or male gametes), now occupy the tip of the pollen tube.
Tip of pollen tube passes through one of the synergids and bursts to release the two sperms into the embryo sac. One sperm fuses with the egg (syngamy) and forms a diploid zygote. The other sperm fuses with the secondary nucleus to form the primary endosperm nucleus which is triploid in nature.
Since two types of fusion, syngamy and triple fusion take place in an embryo sac, the process is termed as double fertilization. After triple fusion, the triploid primary endosperm cell develops into an endosperm. Endosperm provides food to the developing embryo. The synergids and antipodal cells also degenerate to contribute nutrition to the young embryo.
Events that follow double fertilization are development of endosperm and embryo and maturation of the ovule into seed and ovary into fruit.
Endosperm: The endosperm development begins before embryo development. This is needed to provide the nutritive tissue for the growth of the zygote into an embryo. The primary endosperm cell divides repeatedly and forms an endosperm tissue. There are three ways in which the endosperm may develop.
Nuclear type: The primary endosperm nucleus undergoes repeated mitotic divisions to give rise to free nuclei which arrange themselves at the periphery leaving a large central space. Cell wall formation starts subsequently from periphery towards the centre and endosperm becomes cellular at maturity. This is the most common type of endosperm development and is seen in maize, wheat, and rice.
In Cellular type, each nuclear division of primary endosperm nucleus is followed by cytokinesis, making the endosperm cellular from the beginning.
In Helobial endosperm, the first mitosis of primary endosperm nucleus is followed by cytokinesis and it gives rise to two unequal cells. Subsequently, mitotic divisions in both the cells are free nuclear but ultimately, mature endosperm becomes cellular after cytokinesis.
The zygote divides into two cells, the upper cell (embryonal cell) and lower cell (suspensor cell). The lower cell divides and forms the suspensor. The suspensor pushes the developing embryo into the endosperm to get food.
The embryonal cell divides several times and finally gets differentiated into radicle, plumule and cotyledon. The integuments become hardened and thus form the seed coat which protects the seed. Thus, a seed may be dicotyledonous with two cotyledons (pea, gram) or monocotyledonous with one cotyledon (wheat, rice).