What is triple fusion? Where and how does it take place? Name the nuclei involved in triple fusion.

Explanation

Fusion of second male gamete with secondary nucleus (fusion of product of two polar nuclei) is called Triple fusion.
It occurs in the embryosac. Pollen tube with two male gametes enters into embryosac by destroying one of the synergids.
The tip of the pollen tube dissolves and releases two male gametes in the vicinity of the egg. In tripple fusion one male gamete and secondary nucleus (two polar nuclei) are involved.