Chemical structure and biological activity 



quoted because they show the pattern of the kind of investigations that can 

 be made, and are being made in this laboratory, rather than that they 

 necessarily represent the ultimate and final conclusion for this type of study. 

 From this preliminary work, a given substituted aryl nucleus (2-methyl- 

 4-chloro-) was selected so that a series of phenoxy acids covdd be studied to 

 determine the effect of side chain length, following the type of investigation 

 carried out by Wain and Wightman (1954) using entirely different kinds of 

 assay systems. It must be confessed that there was some prior thought that 

 again it might be found that there were parallels between the potato tuber 

 system and the assay systems of Wain and Wightman. However, as the data 

 show, marked ability of this series of substituted phenoxy acids to function 

 in the potato tuber system by acting synergistically with coco-nut milk was 

 only revealed by the first member of the series. Subsequent members of the 

 series (see Figure 5) were but slightly active or actually toxic in their effects. 

 Even here, however, there is some indication that the compounds with 4 and 

 6-carbon side chains were more effective than those with 3, 5, or 7 carbon 

 atoms. The much greater effectiveness of the substituted caproic acid, with 

 5 methylene groups, finds a parallel with some recent work at Wye 

 (Wightman, 1955). 



Average final fresh wt. of cultures 

 Tuber A Tuber B 



Figure 5. Effect of side chain length on the ability of various oj-{A:-chloro-2-methylphenoxy)alkyl- 

 carboxylic acids to induce growth in potato tuber explants cultured in a medium containing coco-nut milk. 



This type of investigation clearly needs to be extended. As this 

 communication is being written, certain other examples of substituted 

 phenoxy compounds are being tried to obtain further cases in which the 

 2 position is occupied and the 4 position is vacant. Similarly, investigations 

 are being carried out with a considerable number of substituted aryloxy 

 ethanols. The results of these investigations, which will be recorded in terms 

 of fresh weight, cell number, and cell size, are not yet available but when they 

 are they will add to the evidence documented above. The main point to be 



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