A pollen grain is a tiny, often yellowish structure produced by flowering plants, known as angiosperms. It contains the male gametes, or sperm cells, necessary for fertilization. Pollen grains are typically carried by wind, insects, or animals to the female parts of flowers, where they can fertilize the ovules. Each pollen grain has a protective outer layer called the exine, which helps it survive harsh environmental conditions. The size and shape of pollen grains vary widely among different plant species, making them useful in fields like palynology, the study of pollen, for identifying plants and understanding past climates.