Phytohormones; structure and physiological activity.

In plants, growth in length by cell elongation is conditioned by certain substances elaborated in the plant and effective in minute amounts, thus having the nature of hormones. A number of simple tests have been worked out which allow of a qualitative and quantitative determination of the growth-promoting properties of various substances. With the aid of these biological tests the native growth substances or auxins have been isolated and a variety of other substances have been shown to possess growth activity. At first sight the large number of substances which are active do not appear to fall within any general classification. Upon further investigation, however, it becomes clear that there are certain common structural characteristics, and an analysis of these provides a first step in attacking the fundamental problem of the relations between chemical structure and physiological activity. This relation has in recent years been attacked in a number of other fields, usually involving physiological processes in animals. On the other hand, it would appear that plants offer an exceptionally favorable field for study in that their structures-and possibly their physiological processes-are somewhat less complicated. Thus, it has been found possible to differentiate between some of the steps in the chain of reactions which lead to cell elongation, so that now it becomes possible to determine the exact structure required in a substance for the regulation of each of these steps. In consideration of these points experiments have already been carried out by several investigators (l-3). Our own program of research has been under way for some time with a twofold object; 763