Double fertilisation :-

•  After entering one of the synergids, the pollen tube releases the two male gametes into the cytoplasm of the synergid. One male gamete moves toward the egg cell and fuses with its nucleus (syngamy). This forms the zygote (a diploid cell).
• The other male gamete moves towards the two polar nuclei located in the central cell and fuses with them to produce a triploid primary endosperm nucleus (PEN).
• As this involves the fusion of three haploid nuclei it is called triple fusion.
• Since 2 types of fusions (syngamy & triple fusion) take place in an embryo sac it is called double fertilisation. It is an event unique to flowering plants.
• The central cell after triple fusion becomes the primary endosperm cell (PEC) and develops into the endosperm while the zygote develops into an embryo.