The immediate cause of opening or closing of the stomata is a change in the turgidity of the guard cells. The inner wall of each guard cell towards the pore is thick and elastic. When turgidity increases with in the two guard cells, the thin outer walls bulge out and force the inner walls into a crescent shape, aided by the microfibrils present in the cell walls of guard cells, making it easier for the stoma to open. When the guard cells loose turgor due to water loss, the elastic inner wall regain their original shape, the guard cells become flaccid and the stomata closes.
