Abstract
The existence of different auxin sensitivities in epidermal and subepidermal tissues (KV Thimann, CL Schneider 1938 Plant Physiol 25: 627-641) suggests a refinement to the Cholodny-Went theory which overcomes some of the difficulties associated with it. A model is presented to account for the inverse tropic responses of shoots and roots through differences in the respective locations of the auxin-sensitive tissues.