How does endosperm of angiosperm become triploid ?

Explanation

Endosperm of angisperms develops from the central cell of embryosac where two polar nuclei fuse to form a secondary nucleus. This diploid nucleus of central cell fuses with one of the two male gametes . The two polar nuclei and one male gamete fuses to form a triploid primary endosperm nucleus (3n). Thus, the endosperm is formed this triploid nucleus and becomes triploid.