The Synthetic Auxins: Relation Between 

 Structure and Activity 



KENNETH V. THIMANN 



THE discovery of the auxins in the early 1930's was accompanied by 

 a great increase in physiological experimentation. This broadened 

 the basis of our knowledge of auxins, for it brought to light the fact 

 that the auxins control many activities of the hfe of plants. In addition 

 to promoting cell elongation of stems and coleoptiles, which was the 

 first known function, the following processes (reviewed in 10) are all 

 controlled or promoted by auxins: i) the initiation of roots on stems; 

 2) the initiation of roots on roots (but only to a hmited extent) ; 3) cell 

 division both in the cambium and other stem tissues; 4) growth of the 

 ovary into a fruit (involving both cell division and enlargement); 5) 

 growth of parenchymatous tissues into tumors; 6) streaming of proto- 

 plasm (studied only to a hmited degree). However, the development 

 of buds and the elongation of roots are inhibited by auxin in all but the 

 very lowest concentrations. 



The first widely held view of auxin action was that it acted in some 

 way to increase the plasticity of the cell wall, thus allowing elongation 

 to take place under the cell's own osmotic force. However, the participa- 

 tion of auxin in so many processes, involving also cell division and the 

 inhibition of enlargement, makes it clear that this concept is insufficient 

 and that some much more profound action or group of actions must 

 take place. This leads at once to the idea that auxin probably acts in 

 some process of metabolism common to nearly all plant cells and capable 

 of leading to varied results depending on the influence of additional 

 factors on the cell. Such a process of metaboHsm would of course be 

 enzymatic. Since the amount of auxin needed is far too small for it to 



