Pollen

Mass of microscopic spores in a seed plant that appears usually as a fine dust. Each pollen grain is tiny, varies in shape and structure, is formed in the stamens in seed plants, and is transported by various means (&see; pollination) to the pistil, where fertilization occurs. The outer layer of a pollen grain is very resistant to disintegration; treatment with intense heat, strong acids, or strong bases has little effect on it.

Because the grains often are very distinctive, some plant species may be identified by their pollen grains alone. Common components of both recent and ancient geologic sediments, pollen grains have provided much information on the origin and geologic history of plant life on land. Pollen is produced in such quantities that it is a significant part of the airborne components of earth's atmosphere. The protein-containing substance in many pollen grains (e.g., ragweed and many grasses) causes the allergic reaction commonly known as hay fever.