MAREK MIS / SCIENCE PHOTO LIBRARY MAREK MIS / SCIENCE PHOTO LIBRARY
Polarised light micrograph of tissue from a tulip (Tulipa sp.) showing three stomata (pores, oval). Stomata are pores that open and close in order to regulate gas exchange in a plant. The aperture of each stoma is controlled by a pair of specialised cells known as guard cells (sausage-shaped). These cells swell to close the pore and shrink to open it. Stomata are found mainly on the underside of leaves. Magnification: x200 when printed 10 centimetres wide.
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