The phenomenon of double fertilization is unique and important in the life history of angiosperms. In angiosperms, both the male gametes are functional and take part in nuclear fusion during fertilization. One of them fuses with the egg nucleus to form the diploid oospore. It represents the first cell of the future sporophytic generation. This process of gametic fusion is called true fertilization or syngamy. At about the same time, the other male gamete fuses with the secodnary nucleus resulting in the formation of a triploid (3n) primary endosperm nucleus. This process is called the triple fusion.
The above phenomenon of fertilization involving the fusion of one male gamete with the egg together with the fusion of the second male gamete with the secondary nucleus or polar nuclei is called double fertilization. It is characteristic of all angiosperms and is essential for the production of viable seeds.
Significance of Double Fertilization:
Double fertilization was discovered by Nawaschin in 1898. As a result of fertilization, the second male gamete fuses with the secondary nucleus to produce endosperm. The endosperm produced as a result of double fertilization provides food material to the developing embryo.
According to Brink and Cooper (1940), the endosperm is a tissue developed to compensate for the extreme reduction of female gametophyte in angiosperms.