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Pollen grains. Coloured scanning electron micrograph (SEM) of pollen grains (yellow) on the pistil (green) of an unidentified flower. The pistil is the part of a flower consisting of the female reproductive organs. Pollen grains contain the male sex cells (gametes) of a flowering plant. Once they have landed on the pistil, the male gametes travel to the ovaries where they fertilise the female gametes (ovules). Magnification unknown. (KEYSTONE/SCIENCE PHOTO LIBRARY/DR PHILIPPA UWINS, WHISTLER RESEARCH PTY/ SCIENCE PHOTO LIBRARY)