Plants form an intermediate distributor, since a large difference in water potential between the moist soil and the normally dry atmosphere is very common. They close when too much water is lost, or when not enough supply exists. The osmotic pressure of the stomata is far larger in the guard cells than in the subsidiary cells. This ratio shifts in favour of the subsidiary cells when the stomata are closed. The stomata of most plant species are closed in darkness.
Light stimulates opening.
The action spectrum is similar to that of photosynthesis. Blue light is especially effective. The stomata of CAM-plants , like Crassulaceans, are opened during the night. They depend on the accumulation of carbon dioxide during the night. Opened stomata would cause intolerable transpiration losses in the areas that CAM-plants live in.