The main process involved in the mechanism of stomatal transpiration is the stomatal movement. The stomatal movement is brought about by the several agents which create the osmotic potential in the guard cell and there by controls the movement of stomata.
The stomata are guarded by the kidney shaped guard cells and surrounded by the specialized epidermal cells known as the subsidiary cells. According to a theory the starch synthesis occurs in the guard cell. The starch is converted is converted into sugar during the day time. The guard cells take up the K+ ions from the surrounding cells. The decrease in the water potential, increase in the osmotic pressure and diffusion pressure deficit of the guard cells causes the osmotic diffusion of water from the epidermal cells and the mesophyll cells into the guard cell. Now the guard cells become turgid and swells up which causes the opening of the stomata. Due to the depletion of the osmotically active substances the osmotic pressure and the as well as the Diffusion pressure deficits of the guard cells decreases. When the water is released back into the mesophyll by the process of osmotic diffusion and so that the guard cells become flaccid which results in the closure of stomata.