Pollen grain of mimosa (Acacia dealbata), computer illustration and photo of mimosa flower. 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.