In botany, a stoma (plural = stomata) is a tiny opening or pore. It is found on plant leaves and stems, and any other green parts of the plant. It is used for gas exchange. Stomata are mostly found on the under-surface of plant leaves. Almost all land plants have stomata.
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 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. To open a guard cell, protons (hydrogen ions, H+) are pumped into the guard cells. Water enters them, the cells get full, and they open.
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Stomata Facts for Kids. Kiddle Encyclopedia.