AbstractA theory for the geotropic movements of theHeliunthus unnuushypocotyl has been given earlier by Johnsson et ul. In the present paper this theory is tested by extensive experiments. The magnitude as well as the rate of geotropic curvature are found to be in agreement with the theoretical predictions.Further, the geotropic reaction time, defined as the time between the start of geotropic stimulation and the start of the geotropic reaction is determined as a function of the stimulation angle with respect to the plumb line. A slight variation found in the reaction time is included in the theory.