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.