VIKTOR SYKORA / SCIENCE PHOTO LIBRARY VIKTOR SYKORA / SCIENCE PHOTO LIBRARY
Grape leaf stoma. Coloured scanning electron micrograph (SEM) of a closed stoma (centre-right) on a leaf from a grape (Vitis sp.) vine. Stomata are pores that open and close in order to regulate gas exchange in a plant. A stoma comprises a pore, the aperture of which is controlled by a pair of specialized cells known as guard cells (red). When the guard cells are turgid the stomata are open and when they are flaccid the stomata are closed. Stomata are found mainly on the underside of leaves. Magnification: x200 when printed 10 cm wide.
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