The grains of pollen produced by different species of plants have a remarkable range of different forms. Produced in groups of four, each grain is effectively a single cell (with two nuclei) between a few and few hundred microns across. At an overall level most grains seem to have regular polyhedral shapes, though often with bulges or dents. Perhaps such forms arise through grains effectively being made with small numbers of roughly spherical elements being either as tightly or loosely packed as possible. The outer walls of pollen grains are often covered with a certain density of tiny columns that can form spikes, or can have plates on top that can form cross-linkages and can join together to appear as patches.