Researches on plant growth regulators XIV. The influence of synthetic plant growth substances on lipoid monolayers

In connection with the hypothesis that plant growth substances do not exert their growth promoting activity by a chemical reaction proper (e.g. functioning as redox‐catalysts) but by a more physical type of action on certain structures in the living cell , experiments were made on the influence of these compounds on monolayers of lecithin or oleic acid spread on aqueous solutions. It was found that apparently plant growth substances penetrate into and accumulate in such monolayers, their influence generally consisting in seemingly increasing the molecular area in the film. In some cases (decahydro‐naphthylacetic acid or undecanoic acid in the hypophases) very peculiar curves were obtained. The activity of a number of compounds tested in this way may be understood by considering the balance between the lipophilic part of the molecules and the hydrophilic carboxyl group. Although the type of action proved to be qualitatively as expected, a parallelism between the growth promoting activity of the compounds and their influence on these simple surface layers does not exist. Further detailed studies must provide an answer to the question whether the difference between the action in the model systems and in vivo might be caused by the apparently much too simple and aspecific composition of the films used, or whether, with the ideas put forward in H. Veldstra, Enzymologia 11, 97, 137 (1944). , the physiological action has been simplified too much.