Syngamy

The pollen tube grows to a very limited extent in the synergid. It releases the contents either through a terminal or a subterminal pore. The contents include the two male gametes along with the accompanying cytoplasm, some reserve nutrients and perhaps vegetative nucleus. One of the sperms enters the egg and the other to the central cell. The distance that the male gamete has to travel to come in contact with the egg or secondary nucleus is insignificant. The male gamete comes in contact with the plasma membrane of the egg that forms a bridge through which the male gamete enters.

Nuclear fusion is initiated by the joining of the outer membrane of the two nuclei. The inner membrane also fuses at localized areas forming a small bridge between the two nuclei. Enlargement of bridge causes eventual fusion of the male and egg nuclei (syngamy). Depending upon the stage of the male gamete at the time of nuclear fusion, three types have been recognized. (a) in pre-mitotic type-the male nucleus fuses with the egg before reaching the mitotic inter phase, (b) in post-mitotic type the male nucleus undergoes inter phase while in contact with the egg and the fusion is postponed until the initiation of the first mitosis in both, and (c) in intermediate cases, the fusion occurs when male nucleus is still at interphase. Generally, the male cytoplasm does not take part in fertilization. However, there are plants in which biparental inheritance of plastids has been demonstrated.