Plant pores. Coloured scanning electron micrograph (SEM) of stomata (pores) on the pistil of a bellflower (Adenophora sp.). Stomata are the site of gaseous exchange, or respiration. Carbon dioxide enters the plant for use in photosynthesis, and waste oxygen leaves the plant via the stomata. Water also evaporates from the stomata. The highest concentration of stomata is found on the leaves, but they are present all over the plant. The pistil is the female reproductive part of a flower. Magnification: x430 when printed at 10 centimetres wide.