The term pollen refers to a fine powder containing numerous pollen grains that consist of the male gametophytes of seed plants. A pollen grain is composed of two layers: the exine and the intine. The exine is the durable and decay-resistant outer wall of pollen, and the intine is the inner wall of pollen. Pollen allows for sexual reproduction between plants to take place without them needing to be close together or having a water source for the sperm to swim through. Both gymnosperms and angiosperms produce pollen. In gymnosperms, the pollen is produced in the male cones. While in angiosperms, pollen production occurs in the anthers of the stamen. Some species of plants self-pollinate when the pollen moves from the stamen of one flower to the pistil of that same flower. Other species, however, cross-polinate when the pollen travels from the stamen of one flower to the pistil of other flowers. These plants depend on pollinators to disperse their pollen. The primary pollinators are birds, insects, and wind. Once pollen reaches the pistil of a flower, a pollen tube grows until it reaches the ovary. There, the sperm fertilize the ovules.