A pollen grain is a small structure that contains the male reproductive cells of flowering and cone-bearing plants. It is formed in the male structures of seed-bearing plants and is transported by various means, such as wind, water, or insects, to the female structures where fertilization occurs. The male part of flowering plants is the stamen, and the pollen grain is formed in the anther of the stamen. A pollen grain comprises a tough outer wall that is largely made of an unusual substance known as sporopollenin, which is highly resistant to environmental damage and may even survive in sedimentary rocks for many millions of years. The outer wall of the pollen grain is often elaborately decorated and may be punctuated with small openings called apertures. Pollen grains display remarkable variation in size, shape, number of apertures, and surface texture, which make it possible to identify and classify a plant at the family and genus level, and often even species level. The following characteristics are useful in the identification of pollen types: size, shape, number of apertures, and surface texture. Pollen grains consist of three parts: the central cytoplasmic part, which is the source of nuclei responsible for fertilization, the intine, which is the inner layer of the cell wall comprising cellulose and pectin, and the exine, which is the outer and most durable layer of the cell wall and is very resistant to disintegration.