In angiosperms, a mature male gametophyte is derived
from a pollen mother cell by one meiotic and two mitotic
divisions. In the pollen sac (microsporangium) of the anther,
haploid microspores are formed by meiosis. Mitosis then
follows to produce a two-celled pollen grain with a small
generative cell and a large vegetative cell. This generative
cell will undergo further mitosis to form two male gametes
(nuclei). The pollen tube grows through a spore in the pollen
grain, with the tube (vegetative) nucleus at its tips and the
male nuclei behind.