K. V. THIMANN 



carboxyl group must be separated from the ring by at least one carbon 

 (or oxygen) atom. The situation in which a number of chemically 

 different substances exert the same biological activity has occurred 

 also in the field of vitamins, though there, perhaps, the divergence 

 between the natural and synthetic active substances is not so great. 

 For synthetic materials with vitamin activity the term "vitamer" has 

 been proposed (1). On the same basis, synthetic substances with 

 growth-promoting activity in the standard auxin tests could be called 

 "auximers," but at present there seems little need for this term. 



It is not proposed to discuss in this essay the chemistry of the 

 auxins but rather to consider the mechanism of their action, with par- 

 ticular reference to the nature of plant growth. 



Historically, the first proof that the auxins influence more than 

 one type of growth came from the study of bud inhibition. It is typical 

 of shoot growth that, when one bud is vigorously developing, others 

 below it on the same stem are inhibited from doing so. The apical 

 bud thus "dominates" the lateral buds. Only when it is removed, 

 as in pruning, can the lateral buds develop. After evidence had been 

 brought forward, mainly by Snow, that this inhibition is exerted by 

 means of a substance diffusing out of the growing bud toward the 

 tissues below it, it was finally shown that pure auxin b, or also indole- 

 acetic acid, in quantities comparable to those of the auxin produced 

 by the growing bud, can duplicate the effect. Paradoxically, then, a 

 growth-promoting substance is here acting as a growth inhibitor (6) . It 

 should be noted that the movement of auxin in living tissue, as out of a 

 bud into the stem, is invariably in the direction from apex to base. 

 This movement is strictly polar, even taking place against an auxin 

 gradient. Transport occurs in the reverse direction only in dead 

 tissues, or when unphysiologically high auxin concentrations are 

 applied. 



Very shortly afterward, two other important cases were brought 

 to light. It was demonstrated, by Went and Bouillenne, that a root- 

 forming hormone diffuses downward out of buds and leaves to stimulate 

 the formation of roots at the base of a cutting. In the purification of 

 this hormone. Went and Thimann found that root-forming activity was 

 concentrated parallel with growth-promoting activity (?. e., auxin), 

 and finally that numerous auxins — synthetic and otherwise — acted as 

 root-forming hormones. At about the saine time. Snow showed that, 



