The immediate cause of the opening and 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 whereas the outerwall is thin. When turgidity increases within the two guard cells flanking each stomatal pore, the thin outer walls bulge out and force the inner walls into a crescent shape. The opening and closing of the stoma is also aided due to the orientation of the microfibrils in the cell walls of the guard cells. Cellulose microfibrils are oriented radially rather than longitudinals making it easier for the stoma to open.
