POLLEN

a mass of microspores in a seed plant appearing usually as a fine dust. Each pollen grain is a minute body, of varying shape and structure, formed in the anther, or male apparatus, in seed-bearing plants and transported by various means (wind, water, insects, etc.) to the pistil, or female structure, where fertilization occurs. The pollen grain of flowering plants (angiosperms) consists of three distinct parts. The central cytoplasmic part is the source of nuclei responsible for fertilization. The other parts constituting the wall of the grain are an inner layer, the intine, and an outer layer, the exine. The intine consists, at least in part, of cellulose. 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. The composition of the exine is uncertain; its constituents have been termed sporopollenins. 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 different environments. Because of their remarkably symmetrical structure and surface patterns, pollen grains are readily recognizable under the microscope. The structure of the wall of a pollen grain is oftentimes so characteristic that in some cases species may be identified by pollen grains alone. On the other hand, there are cases in which pollen grains of very like structure occur in quite unrelated plant families. Because of their high resistance to decay, their widespread dispersal by wind and water, and their abundant production by plants, pollen grains are very common constituents of geologic sediments, both recent and ancient. Because of these features pollen grains have provided much information on the origin and geologic history of terrestrial plant life. Pollen is produced in such quantities that it is a significant component of the airborne constituents of the Earth's atmosphere, especially in areas over continents. The proteinaceous substance in many pollen grains (namely, ragweed and many grasses) induces an allergic reaction commonly known as hay fever (q.v.). 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 similar allergies. The study of pollen and spores is known as palynology (q.v.). See also pollination.