The *
microspores of seed plants, which are produced in vast numbers, usually in a *
pollen sac. They are formed as a result of meiosis of somatic *
pollen mother cells. In primitive forms they are formed in relatively unspecialized structures with little protection. In intermediate forms they are produced in specialized *
stamens , which are numerous and develop symmetrically in the flower. In the most advanced forms, pollen is formed in fewer stamens, which are more strategically placed with regard to the pollination mechanism. Pollen structure may be related to methods of pollination. In insect-pollinated species the pollen is often sticky or barbed, whereas wind-pollinated species usually produce light smooth pollen. Pollen grains contain concentrated mitochondria, endoplasmic reticulum, and golgi apparatus. The number of nuclei present in the cell at any given time may be diagnostic. On germination of the pollen grain a pollen tube pushes its way through an aperture in the pollen grain wall and the various nuclei migrate into the pollen tube (see also
generative nuclei ,
vegetative nucleus ). The pollen grain wall consists of a resistant outer *
exine , which may be very highly sculptured, and an inner *
intine , which may protrude through pores in the outer layer. These pores, apart from facilitating germination, may also be instrumental in water regulation and in controlling compatibility systems. Although most pollen is short lived, some pollen may be stored at low temperatures for long periods. See also
palynology .
