Molecular Evolution and Protobiology

Abstract

It is now about twenty years since the carefully controlled experiments of PkAL established finally that growth in plants is under the control of special growth substances or hormones. In the intervening period our knowl-edge has advanced considerably, though mainly only in respect to this one rather well-defined group of hormones, the auxins. These substances, as you know, have been isolated from both plant and animal sources, including both seed plants and fungi, and their chemical nature has been thoroughly eluci-dated. Furthermore, large numbers of synthetic substances have been pre-pared which have similar actions. Their chemistry will be discussed further below; it is enough for the moment to note that they have rather cle^arly defined similarities. The physiology of these growth substances, however, shows much more diversity. The action studied by BOYSEN-JENSEN, PIAL, S6DING, WENT, and the other workers up until the last few years was an action upon cell enlarge-ment, causing growth in the simplest sense of the word. The hormone was envisaged as the mechanism by which the influence of the tip of the plant was exerted on the grnwth of the part below it. However, it was not long before evidence was forthcoming that other quite different influences of one part of the plant upon another were brought about through the action of the same hormones. The first instance of thia was the inhibition of one bud by another. It had been previously indicated that this well-defined inhibiting action was perhaps due to a special inhibiting substance, and we were able to show not only that it is, indeed, but also that the substance is identical with auxin. Thus a substance which promotes growth in some tissues also characteristically inhibits it in others. A more marked instance of the same thing is seen in roots. Here all but