Stoma

In botany, a stoma (plural "stomata"), also called a stomate (plural "stomates") (from Greek στόμα, "mouth"), is a pore, found in the epidermis of leaves, stems, and other organs, that is used to control gas exchange.

The pore is bordered by a pair of specialized parenchyma cells known as guard cells that are responsible for regulating the size of the opening. The term is also used collectively to refer to an entire stomatal complex, both the pore itself and its accompanying guard cells. Air enters the plant through these openings, and contains carbon dioxide and oxygen, which are used in photosynthesis and respiration, respectively. Oxygen produced as a by-product of photosynthesis diffuses out to the atmosphere through these same openings. Also, water vapor is released into the atmosphere through these pores in a process called transpiration.

Stomata are present in the sporophyte generation of all land plant groups except liverworts. Dicotyledons usually have more stomata on the lower epidermis than the upper epidermis. Monocotyledons, on the other hand, usually have the same number of stomata on the two epidermes. In plants with floating leaves, stomata may be found only on the upper epidermis and submerged leaves may lack stomata entirely.