Physiolog Y: Skoog and Thimann Further Experiments on the Inhibition of the De- Velopment of Lateral Buds by Growth Hormone

In earlier papers (Thimann and Skoog, 1933, 1934) it was shown that the inhibition of lateral buds by the terminal bud in Vicia Faba is due to the production of relatively large quantities of growth hormone by the latter. This substance passes basipetally through the stem, thus reaching the lateral buds, where it inhibits their growth. In these experiments it was shown that a suitable concentration of the hormone, if applied to de-capitated plants, completely inhibits the development of lateral buds. The mechanism whereby a substance whose function is to promote growth may also inhibit it was interpreted in terms of the theory that in the presence of a high concentration of the growth hormone its production by a given tissue is prevented. The present paper is concerned with two further points arising from the previous work. The first is in connection with the mechanism of the inhibition. It has been suggested by Laibach that our explanation that the growth substance inhibits bud development directly, on account of its excess concentration, is incorrect. He considers the inhibition to be only a secondary phenomenon accompanying an increased growth of the stem, which, according to him, takes place when growth substance is applied to the cut surface of the plant. Laibach's experiments are in many ways not strictly comparable with ours. He used living pollen grains as a source of growth hormone, and his paper implies that the plants were decapitated so close to the base that only the cotyledonary buds could develop. Under these circumstances he found that when pollen grains were applied to the decapitated stump a marked increase in growth, particularly in thickness, took place, and the outgrowth of the cotyledonary buds was retarded. In our experiments, on the other hand, a preparation of growth hormone obtained from Rhizopus suinus, and of known purity (about 5%) was used.