Scanning electron micrograph (sem) of a closed stoma on an ash tree leaf (Fraxinus excelsior). 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 specialised cells known as guard cells. These cells swell to close the pore and shrink to open it. Stomata are found mainly on the underside of leaves. Magnification x8490 (x2090 at 10cm wide).
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