The osmotic value (incipient plasmolysis) of the epidermal cells ofVicia Faba rises with a water deficit, if it is of several days' duration, and sometimes leads to transient wilting. The stomatal cells are an exception, because their osmotic value undergoes little change. Consequently, the osmotic potential of the stomatal cells is strongly negative in relation to that of the epidermal cells. This potential decreases and finally disappears after the plant has been watered, since the osmotic value of the epidermal cells falls; it reaches that of the guard cells after 12–14 hours. Owing to the negative osmotic potential of the guard cells, stomatal opening is prevented as long as the deficit lasts, as well as during the time required for restoring the deficit. Even if it has been restored, the impediment to opening persists for a certain time, because of the after-effect exerted by the water deficit on hydroactive closure. The expenses of the investigation were defrayed by a grant from the Science Research Council of Sweden. Valuable help in carrying out the investigation has been given by Fil. kand. Gösta Stenbeck.