Formation of pollen grains (microsporogenesis)

In a young anther each of the four pollen sacs contains many large diploid (2N) spore mother cells (pollen mother cells).

1. Each spore mother cell divides by meiosis to form 4 haploid (N) microspores.
2. Each microspore's nucleus divides mitotically into two nuclei: the tube nucleus (large) and the generative nucleus (small). The cell is now a pollen grain with a thick, protective wall (an outer exine and inner intine).
3. The wall between adjacent pollen sacs breaks down; the anther opens (dehisces) and the pollen grains are released, ready to be dispersed.