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

The generative cell in pollen grain divides mitotically to give rise to the two male gametes before pollen grains are shed. This is 3-celled stage i.e., one vegetative cell and two male gametes. And as the pollen grains are already divided this will not divide again in the pollen tube. So the pollen tube carries two male gametes from the beginning.

Pollen grains in 2-celled stage just have 2 cells namely vegetative cell and generative cell. As here division of generative cell did not take place, hence the generative cell of two-celled pollen divides in the pollen tube.