Explanation

Pollen grain is the mother cell of male gametophyte. Development of male gametophyte begins inside the micro sporangium. The microspore nucleus divide mitotically to form a smaller generative cell and a much large, vegetative cell (tube cell) the generative cell produces two male gametes, whereas, the vegetative cell form pollen tube after pollination. Pollen grain contains two cells, $i.e,$ tube cell and generative cell at the time of pollination.