290 Freed, Reithel, and Remmert 



Shortly after the isolation and identification of indole-3-acetic acid, 

 it was discovered that certain synthetic acids possessed the ability to 

 produce responses from plants similar to that of lAA. Koepfli et al. 

 (17) examined the activity of a number of indole derivatives, and this 

 was followed in turn by the discovery of the activity of 1-naphthalene- 

 acetic acid (36) and the phenoxyacetic acids (37, 38) as well as the 

 biological activity of a number of related substances (33, 38). It was 

 only natural that in the course of such studies attempts would be made 

 to relate the structure of these molecules to their activity and to 

 speculate as to their possible mode of action. It is nearly a requirement 

 that any proposed theory of the relationship of structure to growth 

 regulating activity must be premised on a mechanism of action conso- 

 nant with the molecular structures involved (14). 



A number of interesting theories concerning the mode of action of 

 these substances and the significance of molecular structure relation- 

 ships to activity have been proposed (3,5,9,11,18,19,22.23). Most 

 of the theories, wherein the organic structure of molecules was con- 

 sidered, propose a mechanism of action involving a chemical reaction 

 between an enzyme molecule and the growth regulator (10,11,18). 

 It would probably be naive to assume that the action of these chemi- 

 cals could come about in any way than through their effect on the 

 enzymes of the organism. On the other hand it becomes a more diffi- 

 cult task to obtain conclusive evidence as to the mechanism by which 

 a growth regulator interacts with an enzyme. Various mechanisms 

 have been proposed, ranging from the suggestion that a growth regu- 

 lator serves as a prosthetic group involving a two-point attachment to 

 the enzyme surface (10, 11), and that the growth regulators are chelat- 

 ing agents for essential metals (15). 



The interaction of growth regulating chemicals with the enzymes 

 of the protoplasm by physical rather than chemical forces has been 

 considered in the theory of Veldstra (30) and the work of Brian and 

 Rideal (8). Some support for this theory comes from the detailed con- 

 sideration of dosage-response curves obtained with growth regulating 

 chemicals. Northern's observation of a reduced viscosity of the cyto- 

 plasm upon treatment with a growth regulator was explained on tlie 

 basis of disassociation of the proteins of the cytoplasm (24). This ob- 

 servation would be consonant with the theory of adsorption on the 

 surface of select proteins, weakening the intermolecular bonds and 

 resulting in a loss of viscosity of the gel of the cytoplasm. The in- 

 creased streaming of cytoplasm noted upon treatment witli plant 

 growth regulators would also lend to bear this out (33). 



Consideration of possible physical interaction of the growth regu- 

 lating phenoxyacetic acid with proteins of the organism came about 



