Carob tree (Ceratonia siliqua) leaf surface, coloured scanning electron micrograph (SEM). The blue/green structures are stomata (leaf pores), the site of gaseous exchange in a plant. The opening and closing of the stomata is controlled by the semi-circular guard cells, which swell and become turgid to close the openings. Magnification: x100 when printed at 10 centimetres wide.