Stomata are minute pores of elliptical shape surrounded by two specialised epidermal cells known as guard cells. The guard cells are kidney shaped in dicots. The portion of guard cell wall lying close to stomatal aperture is thick and inelastic.

The remain led of the cell wall is thin, elastic and permeable. Each guard cell has a cytoplasm lining and a central vacuole containing cell sap. Its cytoplasm contains a nucleus and a number of chloroplasts.

The epidermal cells surrounding guard cells arc specialised and are known as subsidiary cells. They support the movement of guard cells. The size, shape and number of stomata of guard cells vary from plant to plant.

The opening and closing of stomata are brought about by changes in the volume and shape of the guard cells.

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The expansion and contraction of the guard cells is caused due to the attainment of turgid and flaccid condition respectively. When the guard cells happen to absorb water from the surrounding cells, they become turgid. When turgidity increases, the outer thin walls of guard cells stretch outward.

The outward bulging causes the pulling apart of the inelastic inner thick walls which become concave resulting in the opening of the stomata. When the guard cells lose turgidity, the thick walls revert to their original position, narrowing the stoma and ultimately closing it.