Abundant evidence demonstrates that roots bend in response to gravity due to a regulated movement of the plant hormone auxin known as polar auxin transport. This was described in the 1920s in the Cholodny-Went model. The model was independently proposed by the Russian scientist N. Cholodny of the University of Kiev in 1927 and by Frits Went of the California Institute of Technology in 1928, both based on work they had done in 1926. Auxin exists in nearly every organ and tissue of a plant, but it has been reoriented in the gravity field, can initiate differential growth resulting in root curvature.