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In botany, a stoma (also stomate; plural stomata) is a tiny opening or pore, found mostly on the undersurface of a plant leaf, and used for gas exchange. Air containing carbon dioxide and oxygen enters the plant through these openings where it gets used in 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 cellular sodium-potassium pumps.

The easiest way to view stomata on a leaf is to take a nail varnish impression of it. (1) Paint about one square centimeter of the underside of the leaf with transparent nail varnish. (2) Allow to dry out thoroughly (takes about 30 minutes). (3) Peel off and place on a microscope slide. The stomata leave clearly visible impressions in the nail varnish. If you use a graticule slide you can count how many stomata (per unit area) are on the leaf surface, a characteristic of physiological significance. Some technicians report equal success using a thin layer of polyvinyl acrylate glue instead of varnish.

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.