Thale cress leaf stoma. Coloured scanning electron micrograph (SEM) of a stoma (round, centre) on the surface of a leaf from a thale cress (Arabidopsis thaliana) plant. Stomata are pores that are the site of gaseous exchange in the plant. The opening and closing of the stomata is controlled by semicircular guard cells (dark green). When the guard cells are turgid the stomata are open and when they are flaccid the stomata are closed.
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