(a) When the pollen grains falls on the stigma, the pollen tube enters one of the synergids and releases two male gametes.
(i) One of the male gametes moves towards the egg cell and fuses with it to complete syngamy to form the zygote.
(ii) The other male gamete fuses with the two polar nuclei and forms triploid primary endosperm nucleus (PEN). This is termed as triple fusion.
(iii) Since two kinds of fusion-syngamy and triple fusion-takes place, the process is known as double fertilisation and is chracteristic of flowering plants.