Structure of a stoma:
The stoma is a minute pore or opening in the epidermis of the leaf. The opening is usually surrounded by two elliptical (crescent shaped) cells called guard cells. These cells have a dense granular cytoplasm and some chloroplasts. The size of the stomatal aperture varies. It is 4 nun wide and 26 mm long in Corn. 3X7 mm in Phaseotus ‘Pulgaris, 8 X 38 mm in Avena spp. etc.
The cell walls of guard cells are unevenly thick and this plays an important role in the opening and closing of the stomata. Usually the wall facing the pore is thicker than the one away from the pore. The wall away from the pore is elastic and semipermeable. Each guard cell has a cytoplasmic lining and a central vacuole filled with cell sap. Guard cells are uninucleate.
Close to the guard cells are present a few specialized epidermal cells called subsidiary cells. These subsidiary cells may or may not have the origin from the same (meristemoid) cell giving rise to the guard cells. Stomata are classified into various categories based on their location on the leaf surface, on development, on the nature and number of subsidiary cells associated with it and also on the periodicity of opening and closing.