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In botany, stomata (sing. stoma or stomate) are tiny pores, mostly on the undersurface of leavess, used for gas exchange. Air containing carbon dioxide and oxygen enters the plant through these openings for photosynthesis and respiration. Waste oxygen produced by photosynthesis in the chlorenchyma cells of the leaf interior exits through these same openings. Also, excess water is released into the atmosphere through these pores in a process called transpiration. The opening and closing of a stoma is controlled by guard cells that surround the opening and involves sodium-potassium pumps.

Stomata can be seen under a microscope by taking nail varnish impressions of the leaf.

In medicine, a stoma is a surgically created opening into the body. The best known form of a stoma is the opening created by a colostomy to let feces out of the body.