Horse-chestnut (Aesculus hippocastanum) pollen grains, 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. The horse-chestnut tree flowers in May. Magnification: x535 when printed at 10 centimetres across.