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.
When a stoma opens, the inner (pore surrounding) walls of the two guard cells move apart. This is caused by two aspects of the specialised anatomy of the guard cells:
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1. The inner wall of the guard cell which surrounds the pore is thicker than the outer walls.
2. The cellulose microfibrils which make up the cell wall of the guard cell, radiate out around their circumference.
As water moves into the vacuoles of the guard cells, their content is increased and so is the pressure of their cytoplasm against their cell walls. The cell walls begin to stretch. The arrangement of the cellulose microfibrils and the difference in thickness of the wall causes the outer wall to stretch more than the inner. The outer walls thus pull the inner walls away from each other causing the pore to open.