The opening and closing of stomata is operated by turgor changes in guard cells. When the guard cell absorb water they become turgid and the stomata opens. The opening and closing of stomatal pore is aided by the differential thickening of the guard cell's inner and outer membrane and the orientation of the microfibrils in the cell. 
When the guard cell absorb water then the thin outer walls bulge out and force the inner walls into a crescent shape, thus opening the stomatal pore. The radial orientation of cellulose microfibrils rather than longitudinally enable the stoma to open. When the lose water they lose turgidity , the elastic inner walls regain their original shape, the guard cells become flaccid and the stoma closes.