Stomata is a pore, found in the epidermis of leaves, 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. 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.
In plants with floating leaves, stomata may be found only on the upper epidermis and submerged leaves may lack stomata entirely.