pollen, a mass of microspores in aseed plant appearing usually as a fine dust. Each pollen grain is a minute body, of varying shape and structure, formed in the male structures of seed-bearing plants and transported by various means (wind, water, insects, etc.) to the female structures, wherefertilization occurs. Inangiosperms, pollen is produced by theanthers of thestamens inflowers. Ingymnosperms, it is formed in themicrosporophylls of the microstrobili (male pollencones). Pollen consists of one or more vegetative cells and a reproductive cell. A pollen grain itself is not the malegamete. In angiosperms and certain gymnosperms, the vegetative cell forms thepollen tube that grows to meet the unfertilized ovules, and the reproductive cell is the source of thesperm. (See alsopollination.)

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Most pollen grains consist of three distinct parts. The central cytoplasmic part is the source of nuclei responsible for fertilization. The other partsconstituting the wall of the grain are an inner layer, theintine, and an outer layer, theexine. The intine consists, at least in part, ofcellulose orhemicellulose. The outer and most durable layer, the exine, is very resistant to disintegration; treatment with intense heat, strong acids, or strong bases has little effect upon it. Theconstituents of the exine have been termedsporopollenins. The internal parts of the pollen grain are easily broken down, whereas the exine layer, and thus the general form of the pollen grain, is easily preserved in various kinds of sediments; the quality of preservation may vary with differentenvironments.

Because of their high resistance to decay, their widespread dispersal by wind and water, and their abundant production byplants, pollen grains are very common constituents of geologicsediments, both recent and ancient. Because of these features, pollen grains have provided much information on the origin and geologic history of terrestrialplant life. Additionally, given their remarkably symmetrical structure and surface patterns, fresh and preserved pollen grains are readily recognizable under themicroscope.Characteristics such as the exine sculpturing and the size and number of apertures through which the pollen tubes grow are useful as taxonomic tools. The structure of a pollen grain is oftentimes so distinctive that in some cases species may be identified by pollen grains alone. Nearly allangiosperm andgymnosperm plant families and many genera can be identified solely by their pollen grains. The study of pollen andspores is known aspalynology.

Pollen is produced in such quantities that it is a significant component of the airborne constituents of Earth’satmosphere, especially in areas overcontinents. The proteinaceous substance in many pollen grains (namely,ragweed and manygrasses) induces an allergic reaction commonly known ashay fever. Frequently local governmental authorities publish pollen counts, estimates of the concentration of pollen grains in the air, for the purpose of indicating the relative discomfort that may be experienced by sufferers from hay fever and similarallergies.