The small male reproductive bodies produced in the pollen sacs of seed plants (gymnosperms and angiosperms). On maturation in the pollen sac, a pollen grain may reach 0.00007 mg as in spruce, or less than 1/20 of this weight. A grain usually has two waxy, durable outer walls, the exine, and an inner fragile wall, the intine. These walls surround the contents with their nuclei and reserves of starch and oil. Pollen identification depends on interpretation of morphological features. Exine and aperture patterns are especially varied in the more highly evolved dicots, so that recognition at family, genus, or even species level may be possible despite the small surface area available on a grain. Since the morphological characters are conservative in the extreme, usually changing very slowly through geologic time, studies of fine detail serve to establish the lineal descent of many plants living today. Extreme variations in size may occur within a family, but pollen grains range mainly from 24 to 50 micrometers, with the dicot range being from 2 μm in Myosotis to 250 μm in Mirabilis; the monocots range from about 15 to 150 μm or more in the ginger family, with eelgrass (Zostera) having pollen measuring 2550 × 3.7 μm in a class of its own; living gymnosperms range from 15 μm in Gnetum to about 180 μm in Abies (including sacs), while fossil types range from about 11 μm (one-furrowed) to 300 μm. Most grains are free (monads) though often loosely grouped because of the spines or sticky oils and viscin threads. Compound grains (polyads) are richly developed in some angiosperm families, commonly occurring in four (tetrads), or in multiples of four up to 64 or more. In most microspores (pollen and spore) the polar axis runs from the inner (proximal) face to the outer (distal) face, as oriented during tetrad formation. The equator crosses it at right angles. Bilateral grains dominate in the gymnosperms and monocots, the polar axis usually being the shorter one, with the single aperture on the distal side. On the other hand, almost all dicot pollen is symmetrical around the polar axis (usually the long axis), with shapes ranging mainly from spheroidal to ellipsoidal, with rounded equatorial outlines, and sometimes “waisted” as in the Umbelliferae. Three-pored grains often have strikingly triangular outlines in polar view. Rarely lacking, the flexible membranes of apertures are sometimes covered only by endexine. They allow for sudden volume changes, as for the emergence of the germ tube. They are classified as furrows, with elongate outlines, and pores usually more or less circular in shape. Short slitlike intermediate forms occur. A few families have no apertures, some as a result of reduction of exine as an adaptation to wind- or water-pollination. Gymnosperms may also lack openings, or have one small papilla; most have one furrow, or a long weak area. Few pollen grains are completely smooth (psilate) at ordinary magnifications; most have sculpture both on the surface and the structure below it. Flower Pollination Reproduction (plant) | 
