AbstractThe isolated petioles ofColeusshow a distinct dependence on indoleacetic acid in the development of the epinastic curvature, in aqueous medium. Under the same experimental conditions gibberellic acid shows no effect or an inhibition only.Caffeic acid tested in concentrations between 10−11and 10−3evoked varying responses. In winter‐grown plants the curvaturevs. concentration curve shows two small maxima, which are not evident in the response of summer‐grown plants.The interaction of indoleacetic acid and caffeic acid is tested and discussed in elation with the question of the endogenous auxin in