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What cause the opening and closing of guard cells of stomata during transpiration ?

Answer

The change in the turgidity of guard cells results in closing or opening of stomata. The inner and outer walls of the guard cells differ in thickness, the inner walls are thick while the outer are thin. When the guard cells absorb water their turgidity increases, thus the thin outer walls bulge out and force the inner walls into a crescent shape. The change in shape is aided due to the radial orientation of the microfibrils in the cell walls of the guard cells. These microfibrills help the guard cells to bulge out thus opening the stomata. During tranpiration the loss of water makes the guard cells lose turgidity, the elastic inner walls regain their original shape, the guard cells become flaccid and the stomata closes.