In flower, the anther of stamen produces pollen grains, which are transferred to the stigma by a variety of agents in a process called pollination. Then it germinates in response to a sugary fluid secreted by the mature stigma. The pollen tube grows out of the pollen grain and travels through the style to reach the ovary. The growing tube carries the two male nuclei and enters into the ovule, through the micropyle. Inside the embryo sac, the tip of the pollen tube ruptures and the 2 male gametes are set free.
One of the 2 male gametes fuses with the egg nucleus and forms a diploid zygote (in a process called syngamy). The other male gamete pushes further into the embryo sac and fuses with the secondary nucleus and gives rise to a triploid nucleus called the primary endosperm nucleus (in a process called triple fusion). This entire phenomenon of fertilisation involving the fusion of one male gamete with the egg, together with the union of the second male gamete with the secondary nucleus or the polar nuclei is called as double fertilisation.