In botany, a "stoma" (also stomate; plural stomata) is a tiny opening or pore, found mostly on the underside of a plant leaf and used for gas exchange. The pore is formed by a pair of specialized parenchyma cells known as guard cells which are responsible for regulating the size of the opening. Air containing carbon dioxide enters the plant through these openings where it's used in photosynthesis and respiration. Oxygen produced by photosynthesis in the spongy layer cells (parenchyma cells with… (More on Stomata)