The mass of grains containing the male gametes of seed plants, which are produced in large numbers in the pollen sacs. The pollen grains of insect-pollinated plants may be spiny or pitted and are usually larger than those of wind-pollinated plants, which are usually smooth and light. The pollen grain contains the male gametophyte generation; it contains two male nuclei: a generative nucleus and a tube nucleus. The wall of the mature pollen grain consists of the tough outer wall (exine) and the more delicate narrower intine. The latter gives rise to the pollen tube. See also pollination.