Opening and closing of stomata is due to the:

The guard cells are filled with chloroplasts. The guard cells are rapidly influenced by changes in the turgor pressure that regulate the opening and closing of the stomata. In the case of dicot plants, the wall of the guard cells near the pore is thick and inelastic, whereas, the outer wall is thin, elastic and semi-permeable. When the turgor develops in the guard cells, the outer thin walls expand towards the outer side. This draws the thick inner walls slightly apart and creates a pore between the two guard cells. As a result, the stomata get opened. When the guard cells lose turgor due to the loss of water, the inner walls regain their original shape. The guard cells become flaccid and the stoma closes. Related Theory Usually, the number of stomata is more on the lower surface of a dorsiventral leaf (dicot leaf) while the number of stomata is same on both the surfaces of leaf in isobilateral leaves (monocot leaves).