A stoma (plural stomata) is a pore or opening in the epidermis. They form the only openings in the epidermis of the leaf and it is through the stomata that gases move into and out of the leaf. Stomata are common in the plant kingdom and are found in both angiosperm and gymnosperm leaves. In many plant species stomata tend to be more common on the abaxial surface of the leaf than on the adaxial surface.
A stoma and its surrounding cells are called a stomatal complex. The pore is surrounded by two crescent shaped guard cells which control the open and closing of the stoma. In some plant species, additional cells may be differentiated from the ordinary epidermal cells. These are known as accessory cells.
Unlike other epidermal cells, the guard cells contain chloroplasts. Their cell walls which surround the central pore are much thicker than their outer walls. The cellulose microfibrils which make up their walls radiate out around the circumference of the guard cell as indicated in the diagram (Click on the diagram to learn how a stoma opens and closes).