Pollen grains are microscopic structures produced by the male reproductive organs of flowering plants and trees. They consist of microgametophytes, which produce male gametes or sperm cells. Each pollen grain is a single cell containing two male gametes. Pollen grains have a hard outer wall made largely of sporopollenin, which is highly resistant to environmental damage and may even survive in sedimentary rocks for many millions of years. Pollen grains come in a wide variety of shapes, sizes, and surface markings characteristic of the species. They are transported by various means such as wind, water, and insects to the female reproductive organs of plants, where fertilization occurs. Pollen grains are important for the study of plant life and evolution, as they are highly resistant to decay and are abundant in geologic sediments. They are also useful as taxonomic tools, as the structure of a pollen grain is oftentimes so distinctive that in some cases species may be identified by pollen grains alone.