The third (C-type)1 response results from a tonic and a tropistic effect of the irradiation. The Bunsen-Roscoe reciprocity law applies to the tonic as well as to the tropistic component. A simple dependence on the duration of the stimulus does not exist, but is simulated by the time interval required for effectuation of the tonic component. It is concluded that the C-type curvature is identical with the first positive curvature, but is evoked by larger doses of unilateral light because of a desensibilisation of the coleoptiles by the tonic effect of the long unilateral irradiation itself. This desensibilisation process takes a minimum period of 4 resp. 15 minutes to be effective with red-preirradiated or dark-cultivated coleoptiles, respectively.