pollen, minute grains, usually yellow in color but occasionally white, brown, red, or purple, borne in the anther sac at the tip of the slender filament of the stamen of a flowering plant or in the male cone of a conifer. The pollen grain is actually the male gametophyte generation of seed plants (see reproduction). Inside the anther, pollen mother cells divide by meiosis to form pollen grains whose nuclei contain half the number of chromosomes characteristic of the parent plant. Each pollen grain contains two sperm nuclei and one tube nucleus. After successful pollination, the pollen germinates on the surface of the stigma of the pistil and produces a tube that grows down through the style to an ovule inside the ovary at the base of the pistil. The sperm nuclei are then discharged into the ovule; one fuses with the egg nucleus (see fertilization) and the other fuses with the polar nuclei to form endosperm (food-storage tissue) that in many cases nourishes the developing embryo in the seed. This process is basically similar in the conifers, except that in conifers there is no double fertilization and there may be a season's lapse between pollination and fertilization (see cone). Pollen grains, like sperms, are always produced in much greater quantities than are actually used, particularly by those plants that rely on the wind for pollination (e.g., grasses and conifers). Often clouds of dustlike pollen can be seen floating from wind-pollinated trees. Plants pollinated by insects and birds usually have sticky pollen and conspicuous flowers with colorful petals that often secrete perfume or nectar or both to attract the agents. Although pollen grains are microscopic in size and are thus visible to the human eye only in quantity, they are so diversified in appearance that plants are often identifiable by their pollen alone, e.g., by pathology. The waxy outer covering (which contains proteins and sugar—an additional attraction to pollen-gathering insects) is marked by characteristic patterns of ridges, spines, and knobs and is capable of expanding and contracting in the presence of moisture or dryness. Pollen grains are also remarkable for the length of the tubes some must produce: corn pollen tubes may grow 8 or 10 in. (20.3–25.4 cm) from the stigmas through the filamentous styles (commonly called "silk") to the ovaries. The life span of pollen may be less than two hours; its ability to produce the allergic reaction of hay fever continues indefinitely.
The Columbia Electronic Encyclopedia, 6th ed. Copyright © 2012, Columbia University Press. All rights reserved.