in Anatomy of Flowering Plants by

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The walls of the guard cells have a peculiar thickening; the wall facing the stomata is thick and outer wall is thin. The guard cells absorbs water due to turgor pressure their by the guard cells are stretched outwards leading to the opening of stoma. After the transpiration water is lost the guard cells become flaccid; the walls becomes straight stoma is closed. The turgor pressure regulates the transpiration through stomata.