The Auxin Concentration Rule for the Geotropism of Avena Coleoptiles

Experiments with three different auxins viz. indole acetic acid, indole aceto nitrile and naphthyl acetic acid revealed that the magnitude of geotropic curvature of Avena coleoptiles is regulated according to the following rule: A geotropic curvature only appears in auxin concentrations which are suboptimal for coleoptile growth. The largest curvatures occur in the range of solutions where small differences in the concentration cause large differences in the growth of the coleoptile. The natural auxin production in an Avena-coleoptile tip provides for a maximal geotropic reaction and not for maximal growth. The larger the part removed by decapitation the smaller the geotropic curvature of the stump and the nearer the zone of maximal curvature approaches the base. Supplementary observations support the interpretation of the results according to the lateral distribution hypothesis proposed by Cholodny. The stimulating discussions held with Prof. Dr. V. J. Koningsberger and with Dr. H. P. Bottelier on this topic are recognized with pleasure.