The view of the mechanism of the action of plant growth substances developed elsewhere has led to the assumption that growth substances and inhibitors in plants may be closely related1). The results of some recent investigations10)11), which in our opinion support this view, are discussed, and in the case of cis‐cinnamic acid coumarin (lactone of o‐oxy(cis)cinnamic acid) such a structurally related pair of growth substance and inhibitor is indicated.In the pea test, inhibitor (coumarin) and growth substance (naphthalene acetic acid) prove to be antagonists, while such an influence cannot be established in the blastokolin test. A number of compounds with the structure of inhibitors (unsaturated lactones) are examined in the blastokolin test. Furthermore the importance of these investigations for the natural auxins is outlined. The formation of unsaturated δ‐lactones, being possible in that case, does really occur in the photo‐inactivation of auxin‐a‐lactone (conversion into lumi‐auxone). On account of this the hypothesis is developed that lumi‐auxone (or a closely related compound) may perform the function of an inhibitor.Finally a number of facts from botanical investigations are summarized, which point to the close relationship between growth substance and inhibitor and which, moreover, support the view of the function of lumi‐auxone (o