Double Fertilisation

The pollen tube releases the two male gametes into the cytoplasm of the synergid .One male gametes
fuses with egg cell  which  results in the formation of a diploid cell, the zygote. The other male gamete fuses with the two polar nuclei located in the central cell and  produces a triploid primary endosperm nucleus (PEN) . It is termed triple fusion. Since two types of fusions, syngamy and triple fusion take place in an embryo sac the phenomenon is known as  double fertilisation. The central cell after triple fusion develops into the endosperm while the zygote develops into an embryo.