Stomata are structures present in the epidermis of leaves. Each stoma is composed of two
bean-shaped cells called guard cells, which enclose stomatal pore. In grasses, guard cells are dumb-bell shaped. The guard cells possess chloroplasts and regulate the opening and closing of stomata. Sometimes, a few epidermal cells, in the vicinity of the guard cells become specialised in their shape and size known as subsidiary cells. The stomatal aperture, guard cells and the surrounding subsidiary cells are together called stomatal apparatus.