Relation of Environment and of the Physical Properties of Synthetic Growth Substances to the Growth Reaction

It is concluded that the acid curvature in the pea test is due to a greater growth of the cut surface than of the intact surface. This is explained as due to a higher concentration of active auxin on the cut surface, caused by a measurable decrease of the internal pH of the cut surface, when split pea sections are placed in acid buffers. 2. A correlation is found between the dissociation curves and the activities at different internal pHs of cis-cinnamic acid and phenylacetic acid. 3. By correcting for difference in pK, so that only equimolar concentrations of the free acid are compared, it is found that cis-cinnamic acid possesses the same activity as indole (3) acetic acid in the pea test. The activity of phenylacetic acid, although enhanced, was not so high as that of indole (3) acetic acid. 4. By pretreating pea sections with phenylbutyric acid and correcting for difference in pK when possible, the following compounds are found to possess molar activities equal to that of indole (3) acetic acid in the pea test: indole (3) propionic acid, indole (3) butyric acid, napthaleneacetic acid, anthraceneacetic acid, and cis-cinnamic acid. The activities of phenylacetic acid and of indole(3) valeric acid, although increased, are not brought to that of indole (3) acetic acid. 5. It is suggested that compounds possessing the essential molecular structure probably all have the same activity in the pH dependent, and stoichiometric growth reaction, and that the differences in their observed activities are due to differences in activities in secondary processes.