Pollen grain of mimosa (Acacia dealbata), computer illustration. The pollen grain contains the male gametes which fertilize the ovules in the ovary of the flowering plant to form a seed. In Acacia, pollen grains regularly associate in multiples of 4 to form compound grains or polyads. The compound grains shown here are typical examples of a 16-grain association in a more or less square arrangement. There are 8 centrally placed grains (4 on top visible here and 4 below not visible) and these are surrounded by 8 peripheral grains.