Stomata are tiny openings found mostly on the underside of leaves that play a critical role in regulating the exchange of gases, particularly carbon dioxide and oxygen, between the plant and its environment. The two main functions of stomata are to allow for the uptake of carbon dioxide and to limit the loss of water due to evaporation. Stomata regulate gas exchange between the plant and environment and control water loss by changing the size of the stomatal pore. The major function of stomata is to allow sufficient CO2 to enter the leaf in order to maintain optimal photosynthesis under the prevailing environmental conditions. Stomata resemble doughnuts, with a circular pore consisting 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.