Several minute openings or stomata are found on the epidermis of all the green aerial parts of plants, but are abundant on the lower surface on the leaves as they regulate the process of transpiration. A large number of stomata occur on the upper surface of leaves of aquatic plants. Each stomata is surrounded by two cells known as the guard cells. In the dicotyledons plants these are bean shaped, but in sedges and grasses these are dumb-bell shaped. The guard cell are living. Their outer walls are thin where as inner ones surrounding the aperture are highly thickened. Due to this variation in the thickening, the guard cell may becomes turgid and flaccid, depending upon the supply of water in them, which makes the opening and closing of stomata possible. Some times a few neighbouring epidermal cells in vicinity of guard cells become specialized in their shape and size and contents. These are known as subsidiary cells. The stomatal aperture, guard cells and the surrounding subsidiary cell are together called stomatal apparatus.