New Theory on Primary Mode of Auxin Action 461 



explain the part of the action curve of lAA where there is inhibition. 

 This can be explained by the two attachment point theory. 



Dr. Ray: The crucial point of this theory is one of a specific site 

 involving points at which the auxin combines and brings about ac- 

 tivity. I do not feel that it is fundamentally different from what we 

 have all been thinking about for some years concerning the nature 

 of auxin activity: combination of the auxin molecule with a site of 

 specific shape. I do not think it is at all a question, therefore, of sur- 

 face chemistry or nonspecific adsorption, but as far as the data that 

 this theory is attempting to explain, one primarily of specificity. 



Dr. Fawcett: If one considers the 2,4-dichlorophenoxy structure, 

 then the acetic acid homologue is a highly active compound, although 

 (3-(2,4-dichlorophenoxy)propionic acid, with its somewhat longer side 

 chain, is inactive in, for example, the split pea curvature test. But, if 

 one considers p-(2,3-dichlorophenoxy)propionic acid, this is more ac- 

 tive than 2,3-dichlorophenoxyacetic acid (Fawcett et ah, Proc. Roy. 

 Soc. 150 B: 95. 1959). Are these examples in line with the present 

 hypothesis? 



Dr. Osborne: One small point — if one of the hydrogens on the 

 side chain is replaced with a methyl group in certain of the 2,6-sub- 

 stituted phenoxyacetic acids the biological activity is enhanced al- 

 though, supposedly, the ring would still have difficulty fitting into 

 the hole and the side chain would still be too high for activity. 



Dr. van Overbeek: Yes, but introduction of methyl will improve 

 its partitioning properties so you have more molecules near the site 

 of action and thus a better chance of getting them into position. In 

 addition, the methyl group causes steric hindrance so it brings the 

 side chain over in the effective lateral position. Many things have 

 to be taken into consideration. I disagree with Dr. Ray's comments. 

 This theory is fundamentally different because I am no longer look- 

 ing at a water-soluble system but at an oil-soluble membrane sys- 

 tem; we are not looking at covalent bonding, we are looking at 

 physico-chemical systems. The principle is that of hooking a small 

 polar group into position. What I have tried to do is to show the 

 direction in which we must look for an answer. 



