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Grass pollen grain, coloured scanning electron micrograph (SEM). Pollen grains, containing the male sex cells (gametes), are reproductive structures produced by the male parts of flowering plants. The structure shown is the hard outer casing (exine) that protects the gametes during transit between flowers. (KEYSTONE/SCIENCE PHOTO LIBRARY/Kevin Mackenzie / UNIVERSITY OF ABERDEEN / SCIENCE PHOTO LIBRARY)