AbstractThe epinastic curvature of isolated petioles ofColeusshows a dependence upon the concentration of indoleacetic acid in the ambient medium. In low concentrations (between 10−9and 10−7) of indoleacetic acid, a small maximum followed by a small minimum in the curvaturevs. concentration curve can be seen. Between 10−7and 10−5indoleacetic acid there is a concentration dependent increase of the curvature. In higher concentrations the curvature diminishes, and a discolouration of part of the petiole occurs. These phenomena occur in concentrations that promote the formation of adventitious roots.The possibility of a correlation of injurious conditions with the formation of adventitious roots has been investigated, using stem cuttings ofColeus.Exuberant formation of adventitious roots after a treatment with high concentrations of indoleacetic acid was accompanied by necrosis in the lowest part of the cutting.Sulphuric acid, potassium hydroxide and mercuric chloride alone, were shown to cause a promotion of the formation of adventitious roots when applied in concentrations that cause necrosis in the lowest part of the cutting without being lethal to the whole