Stoma is a pore found in the epidermis of leaves, stems, and other organs of plants that controls the rate of gas exchange between the plant and the environment. Stomata are present in the sporophyte generation of all land plant groups except liverworts. In vascular plants, the number, size, and distribution of stomata varies widely. Dicotyledons usually have more stomata on the lower surface of the leaves than the upper surface. Monocotyledons such as onion, oat, and maize may have about the same number of stomata on both leaf surfaces.

Stomata are critical for photosynthesis, as they regulate gas exchange for photosynthesis, as well as moisture levels in tissues. They are tiny, microscopic, and resemble doughnuts - a circular pore with a hole in the middle for gas to enter or leave the plant. The pore consists of two cells, each known as a guard cell. They can swell or shrink to open or close the pore, which is critical for regulating gas exchange for photosynthesis, as well as moisture levels in tissues.

Stomata play a critical role in regulating the exchange of gases, particularly carbon dioxide and oxygen, between the plant and its environment. When stomata are open, they allow carbon dioxide to enter the leaf, which is then used by the plant to produce glucose food through photosynthesis.