Pollen grains
A pollen grain contains a sperm cell that fertilizes an egg. If fertilization is successful, a seed is produced. The pollen grains of each species display unique sculpting of the pollen wall, and fossilized pollen serves to identify ancient species. Pollen grains are microscopic in size, ranging in diameter from less than 0.01mm (about 0.0000004 in) to a little over 0.5 mm (about 0.00002 in). Millions of pollen grains waft along in the clouds of pollen seen in the spring, often causing the sneezing and watery eyes associated with pollen allergies. The outer covering of pollen grains, called the pollen wall, may be intricately sculpted with designs that in some instances can be used to distinguish between plant species. A chemical in the wall called sporopollenin makes the wall resistant to decay.