part of a plant. A pore or small hole in the surface a leaf (or other aerial organ) allowing the exchange of gases between tissues and the atmosphere.
Stoma of a leaf under a microscope
Plant stoma guard cells. The chloroplasts look red in this picture

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.[1] Almost all land plants have stomata.

Stomata have two main functions. First is gaseous exchange i.e. intake of carbon dioxide and release of oxygen. The second is the process of transpiration in plants.

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.


  1. Swarthout, Debbie and Hogan, C. Michael 2010. Stomata. Encyclopedia of Earth. National Council for Science and the Environment, Washington DC. [1]