Pollen grains are tiny structures used by plants as part of reproduction. Plants that use pollen typically release huge numbers of pollen grains, but only a few will reach plants of the same species. Pollen grains can vary greatly in size and structure from one species to another, but all share certain characteristics.
Pollen develops on the anther, a stem-like structure that constitutes the male part of a flower. It has to make a journey to another plant and land on the female stigma, a ball-like structure on the end of a long stalk called the style. Different plants use different means of distributing pollen. In some plants, the individual grains of pollen are very light and are attached to parachute-like structures that carry them into the air. In other plants, bright flowers, strong smells or the nutritional value of the pollen itself draw animals called pollinators. Some of the pollen sticks to the bodies of pollinators and is then distributed as the pollinators feed on other plants of the same species.
Pollen grains are protected by two walls: the outer wall called the exine and the inner wall called the intine. When pollen grains are released, either via animals pollinators or through the air, they gradually loose moisture. To help slow the loss of moisture and preserve the reproductive cells inside, pollen grains fold over on themselves as they dehydrate along creases called apertures. This folding decreases the surface area of the pollen, which helps to slow water loss and keep the pollen cell viable.
Pollen grains have three cells that are involved in fertilization: one tube nucleus and two sperm nuclei. When a pollen grain lands on the stigma of a flower, the tube nucleus forms into a long tube, which penetrates the stigma and descends through the style to the ovum and the polar nuclei at the bottom, which holds the seed ovary. The two sperm nuclei use this tube to fertilize the plant. One fertilizes the ovum, which develops into the embryo -- the part that actually grows into a new plant. The other fertilizes the polar nuclei. This forms the endosperm, a source of stored food that will feed the seed as it germinates.