The object of the investigation was to study the relation between the movements of the guard cells and the osmotic value, as well as the influence of the water deficit of the leaf on these two phenomena. The experimental objects consisted of leaves of Vicia Faba, Rumex acetosa and Rheum rhaponticum. The stomatal aperture, osmotic value and water deficit were measured at fixed intervals during the day. During this time, the leaves were kept in light and saturated humidity at a temperature of 20-22° C. With a suboptimal water deficit, the guard cells are found to open in light and close in darkness. Their osmotic value rises during the opening movement and falls during the closing movement. The course is thus in agreement with the traditional conception of the stomatal movements as a function of the light and of the enzymic metabolism; it is represented by the schema: Osmotic value rises Light → Photoactive opening Photoactive and passive closure ← Darkness Osmotic value falls. Pressure from epiderma cells positive. With a supraoptimal water deficit, the movements of the guard cells are controlled mainly by the water deficit. Three kinds of effect can be distinguished: 1. The deficit initiates stomatal closure. The process has been described earlier and denoted as the hydroactive stomatal closure (Såtlfelt 1929). The hydroactive stomatal closure and the photoactive opening together form the hydrophotic regulator of the water balance. 2. The deficit decreases the osmotic value of the guard cells. It could not however, be determined whether the osmotic value starts to fall before, concurrently with or after initiation of the closing movements of the stomata. 3. The deficit decreases the light sensitivity of the guard cells to the photic opening process. This is because hydroactive closure has an after-effect, which persists for several hours after the deficit has decreased or regressed, and which makes the guard cells indisposed to photic opening. A safety mechanism is inherent in this inhibiting reaction; it protects the leaf from rapidly developing large water deficits which would arise on light, warm days, if the photoactive opening mechanism continued unchanged after the deficit had regressed owing to stomatal closure. Thus, it is found that the photoactive opening reaction is also controlled by a sensitivity regulator, which is initiated by the deficit when it reaches high supraoptimal values. If the safety mechanism is also included, the hydrophotic regulator can be represented by the schema: Osmotic value rises Light → Photoactive opening Hydroactive closure. Decrease in ← Supraoptimal water light sensitivity of guard cells. [ABSTRACT FROM AUTHOR]