Solution

Opening of stomatal pore is regulated by guard cells. When guard cells gain water, they become turgid. In turgid guard cells the thin outer wall is pushed outside, which pulls the thick inner wall outside due to radial micellation of cellulose fibrils. This causes opening of stomata. When guard cells lose water, they become flaccid and inner wall of guard cells moves back towards the stomatal pore and pore closes.
According to the most widely accepted view, the change in turgidity of guard cell is brought about by movement of potassium ions. Movement of potassium ions into guard cells lowers their water potential, thus triggering movement of water into the guard cells making them turgid, while movement of potassium ions from guard cells to outside, reverses the lowering of water potential, thus triggering outflow of water making them flaccid.