Some Physiological Functions of the Growth Hormone in Higher Plants

The results of the experiments are included in the conclusions at the end of different chapters and in the last chapter of each Part. It is therefore only necessary to summarize the main results. Part I. 1. The inhibition of lateral bud development in Vicia and in Pisum is controlled by the growth hormone produced by the terminal bud. 2. The inhibiting effect produced by the terminal bud can be completely substituted by a continuous application of auxin in relatively high concentrations to the stem of decapitated plants. 3. The inhibiting action of auxin is due to its prevention of synthesis of hormone in the buds and is independent of the action of the hormone in promoting growth of the stem. 4. In Vicia older than seedlings auxin is produced only in the presence of light, but the growth response of the plant to auxin is higher in the dark. 5. Auxin is an essential factor for the growth of the stem. Part II. 1. Auxin is destroyed by x-irradiation both in solution and in the plant. 2. Inactivation of auxin in solution is indirectly through the oxidation by strong oxidizing agents formed by irradiation. 3. In the Avena coleoptile X-rays cause only a temporary decrease in auxin. 4. In green plants (Vicia and Pisum) grown in the light the mechanism of formation of auxin is additionally destroyed so that a gradual permanent decrease in auxin is produced by irradiation. 5. The destruction of auxin and the mechanism of its formation in the plant is a major factor in the immediate inhibition of growth of plants caused by irradiation. 6. The development of lateral buds in irradiated plants is due to the removal of auxin and is not due to a direct stimulation of growth by the X-rays. Part III. 1. A quantitative deseeded Avena test method for small amounts of auxin and precursors of auxin has been described. 2. The presence in the Avena coleoptile of a precursor of auxin transported from the seed has been demonstrated. 3. Tryptophane and β-indol-ethyl amine, which lack auxin activity, have been shown to be changed chemically by the action of the plant into hetero auxin. 4. The physiological behavior of these substances in distinction to auxin has been studied.