Describe the development of endosperm in angiosperms.

Angiosperms have a unique process called double fertilization in which after the release of male gametes by the pollen tube in the ovary, one of the male gametes fuses with the egg cell and the other male gamete fuses with the diploid secondary nucleus or central cell. The fusion of male gamete with the egg cell is called syngamy and results in the formation of diploid zygote. The fusion of male gamete with the central cell is called triple fusion and results in the formation of a triploid primary endosperm nucleus (PEN) which further develops into endosperm. This simultaneous occurrence of syngamy and triple fusion is called double fertilization and leads to the formation of the zygote and primary endosperm nucleus.