The stomatal transpiration takes place in three steps. They are;
Osmotic diffusion of water in the leaf from xylem to intercellular spaces through mesophyll cells.
Opening and closing of stomata.
Simple diffusion of the water vapor from inter cellular space to outer atmosphere through the stomata.
In the leaf the mesophyll cells are in close contact with the xylem on one side and the inter cellular space on the other side. When the water is drawn by the mesophyll cells from the xylem it becomes turgid then the diffusion pressure deficit and the osmotic pressure decrease. This releases the water in the form of water vapor into the intercellular space that is close to stomata by the process of osmotic diffusion.
The stomata are guarded by the kidney shaped guard cells and surrounded by the specialized epidermal cells known as the subsidiary cells. The 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.