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Stoma of a leaf under a microscope

In botany, a stoma (also stomate; plural stomata) is a tiny opening or pore that is used for gas exchange. They are mostly found on the under-surface of plant leaves.

Air enters the plant through these openings. The carbon dioxide is used in photosynthesis. Some of the oxygen produced is used in respiration. Surplus oxygen exits through these same openings. Also, water vapor goes into the atmosphere through these pores in a process called transpiration.

The pore is formed by a pair of cells known as guard cells. These adjust the size of the opening by opening or closing. Stomata have a pair of guard cells for each stoma.