Question

describe the mechanism opening and closing of stomata

Answer 1

Mechanism of opening and closing of stomata
Opening of stomata: Solutes from neighbouring epidermal and mesophyll cells enter the guard cells lowering its osmotic potential and water potential. This lowered water potential and osmotic potential will allow movement of water into guard cells from neighbouring cells. Guard cells become turgid due to water accumulation in them which results in the opening of the guard cells. 
Closing of stomata: As the somata open the solute concentration is reduced. This makes the water from the guard cells to move away into neighbouring cells. Now, guard cells becom flaccid with no water. They collapse against each other and result in the closing of stomata. 

Answer 2

The opening and closing of stomata is guided by the guard cells present around the stomata.They close and open to allow passage of carbon dioxide and oxygen as well as help in transpiration.In light, guard cells take up water by osmosis and become turgid. Because their inner walls are rigid they are pulled apart, opening the pore. In darkness water is lost and the inner walls move together closing the pore.