The stomata of plant leaves constitute a main regulating system of the transpiration process. Biologists have suggested a turgor mechanism to explain stomatal movement. The functions of the mechanism are quantitatively described by an osmotic water transfer theory and/or the active water transfer theory in the guard cells. Based on the active water transfer theory and the observations of stomatal movement under various conditions of H2O, CO2, and light, a conceptual model of the stomatal control mechanism is proposed. Transfer functions of the mechanisms characterizing the changes in turgor pressure have been derived. The entire system has been organized by the application of adaptive control system theory and has been simulated on an analog computer. The dynamic characteristics have been investigated with consideration of various environmental conditions. (Key words: Computers, analog; plants; soil moisture.)