<P> 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 facilitates 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 stomatal opening . </P> <P> The term is usually used collectively to refer to the entire stomatal complex, consisting of the paired guard cells and the pore itself, which is referred to as the stomatal aperture . Air enters the plant through these openings by gaseous diffusion, 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 diffuses through the stomata into the atmosphere in a process called transpiration . </P>

Where are stomata usually found on a leaf