Write in detail, the structure of pollen grain.

Pollen grains in Angiosperms are two celled structures. Each pollen grain contains two haploid cells, the generative cell and the vegetative cell.It is non-motile and normally round structure. It is protected by a two-layered wall. The outer layer is thick, stiff and protective and is called exine. The inner layer is thin and elastic and is made up of cellulose and pectin. There are a few thin areas in the exine called germ pores.