This is a unique feature of angiosperms. Here one of the two male gametes fertilizes the egg to form zygote or oospore. This act is called fertilization. This is necessarily followed by the fusion of the second male gamete with the product of a fusion of the two polar nuclei, i.e. with the secondary nucleus. This act of fusion of the second male gamete with the secondary nucleus to form the primary endosperm nucleus is called triple fusion and the whole process is referred to as double fertilization.
