398 THIMANN 



been synthesized and tested. These are mainly the acetic, propionic, and 

 butyric acid derivatives of many aromatic nuclei. Besides 2,4-D, perhaps the 

 most widely used is naphthalene- 1 -acetic acid: 



CH2COOH 



Practically all the active compounds consist of a ring which is more or less 

 unsaturated and a side chain bearing an acid group or a group closely related 

 to it (an ester or an amide, for instance, which can be readily converted 

 to the carboxylic acid, though it is not excluded that they could act without 

 such conversion). 



The acid side chains may be linked to the ring nucleus directly or via an 

 atom of oxygen or sulfur as in 2,4-D or in: 



CH2COOH 



In 1953 Jones and his coworkers discovered indoleacetonitrile in cabbage: 



CH2CN 



There is much evidence that this compound, which is highly active as an auxin 

 in a few plants, but of low activity or none at all in many others, is first con- 

 verted to the acid (indoleacetic acid) in order to act. Using chromatographic 

 methods and bioassay, a few amides of indoleacetic acid and the related com- 

 pounds indoleacetaldehyde and indolepyruvic acid have been identified in 

 various plants. The aldehyde, first found by Larsen, in Denmark, appears 

 to be not uncommon in etiolated seedlings. These compounds, too, seem to 

 act primarily by being converted in situ to indoleacetic acid. 



A characteristic of indoleacetic acid is the fact that the carboxyl (acid) 

 group is separated from the ring by a carbon atom. There may be more than 



