Inside the embryo sac, one male gamete fuses with egg cells to form a zygote (2n) and this is called syngamy or true act of fertilisation. This result of syngamy, i.e., zygote (2n) ultimately develops into an embryo.

The second male gamete fuses with 2 polar nuclei or secondary nucleus to form triploid primary endosperm nucleus and this is called triple fusion. The result of triple fusion, i.e., primary endosperm nucleus (3n) ultimately develops into a nutritive tissue for developing embryo called endosperm.

The nuclei involved in this triple fusion are the two polar nuclei or secondary nucleus and the second male gamete.