In majority of the plants, the single micropylar nucleus lying in the micropylar end and also the polar nucleus from the chalazal end moves towards the centre. At the centre of the embryo sac both of the nucleus fuses to form the secondary nucleus which is diploid. During the process of double fertilization, one male nucleus fuses with the secondary nucleus and it forms the triploid endosperm nucleus. This type of fusion of three nuclei is called triple fusion. The double fertilization and triple fusion is the characteristics of the angiosperms.