The generative cell of a two-celled pollen divides in the pollen tube but not in a three-celled pollen. Give reasons.

(N/A) In over $66\%$ of angiosperms,pollen grains are shed at the $2$-celled stage,consisting of a vegetative cell and a generative cell.
In these cases,the generative cell undergoes mitotic division within the pollen tube to produce two male gametes after pollination.
In the remaining species,the generative cell divides mitotically to give rise to the two male gametes before the pollen grains are shed. Thus,these pollen grains are already in the $3$-celled stage (one vegetative cell and two male gametes) at the time of release.
Therefore,in a $3$-celled pollen,the division has already occurred prior to pollination,whereas in a $2$-celled pollen,the division occurs later within the pollen tube.