Coloured scanning electron micrograph (SEM) of the surface of a flat-leaf parsley (Petroselinum crispum) leaf. The surface is covered in stomata (grey), pores that are the site of gaseous exchange in the plant. The opening and closing of the stomata is controlled by semicircular guard cells. When the guard cells are turgid the stomata are open and when they are flaccid the stomata are closed. Magnification: x50 when printed at 15cm wide.