256 R. M. Miiir and C. HanscJi 



4. Hansch, C, Schmidhaltcr, B., Reiter, F., and Saltonstall, W. Catalytic synthesis 

 of heterocycles. VIII. Deiiydrocyclization of o-ethylbenzenethiols to thiana- 

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5. McRae, D. H., and Bonner, J. Diortho substituted phenoxyacetic acids as anti- 

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6. Muir, R. M., and Hansch, C. The relationship of structure and plant-growth 

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 369-374. 1951. 



7. , and Hansch, C. On the mechanism of action of growth regulators. 



Plant Physiol. 28: 218-232. 1953. 



8. , and Hansch, C. Chemical constitution as related to growtli regulator 



action. Ann. Rev. Plant Physiol. 6: 157-176. 1955. 



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10. Veldstra, H. On form and function of plant growth substances. In: R. L. 

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benzoic acids. Rec. Trav. Chim. Pays-Bas. 71: 318-320. 1952. 



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DISCUSSION 



Dr. Smith: We have studied the effect of dichloro- and dimethyl- 

 substitution of benzoic acids, and I would like to show you some of 

 our results in relation to those obtained by Drs. Muir and Hansch. 

 These assays utilized three test systems (Ann. Appl. Biol. 47: 173. 

 1959), but the data from the pea test are representative of our results. 

 As seen in Table 1, and as stated by Dr. Hansch, the monochloro 

 acids are inactive. However, not only is the 2,6-dichloro- active, but 

 marked activity is also shown by the 2,3-, 2,5- and 2,3,6- derivatives. 

 Similar, but less marked results, were obtaind with mono- and 

 dimethylbenzoic acids. Here again, mono-substitution renders the 

 compound inactive, but some activity is shown by the 2,3-, 2,5-, and 

 2,6- acids. 



Dr. Wain: Dr. Smith has raised an interesting point. In structure 

 activity relationships it is important to study whole series. One finding 

 that must be noted is the quite high activity of the 2,3- compounds 

 which to date has been overlooked. In view of this activity, one has 

 to ask whether a removal of the 2- or 6- substituent is essential in the 

 growth reaction. 1 think a very ingenious hypothesis has been put 

 forward here, but 1 am rather dubious about it myself, especially when 

 one has to accept the elimination of a methyl group from the 2 or 6 

 position, in order to explain the activity of the 2,6-dimethyl- and 

 2,3,6-trimethylbenzoic acids. 



