Evening primrose pollen grains. Coloured scanning electron micrograph (SEM) of pollen grains from an evening primrose (Oenothera sp.) flower, showing the characteristic threads secreted by the pollen grains to hold them together in packages and probably to aid in attachment to insect pollinators. Pollen grains are the male sex cells of a flowering plant. Magnification: x200 when printed at 10 centimetres wide.