154 



THE CELL AND PROTOPLASM 



finable in chemical terms. Still, the rela- 

 tionship between applied auxin and the re- 

 sulting growth shows that auxin is involved 

 in a reaction with a very high rate of com- 

 bination between auxin and substrate, or 

 carrier, the latter being present in a very 

 small quantity only. This same type of 

 reaction we encounter in enzymatic reac- 

 tions. The substrate, or carrier, seems to 

 be the factor which is transported under 

 the influence of the diffusible auxin. The 

 combination between auxin and its carrier 

 is probably the process which is pH de- 



counterpart of the enzymatic processes, and 

 which is able to account for all available 

 facts. Among these I can mention the fact 

 that although auxin application does not 

 result in an appreciable increase in respira- 

 tion, aerobic respiration is nevertheless es- 

 sential for the action of auxin; and that 

 HCN can inhibit its effect (Bonner 1933). 

 This is understandable if the electron 

 transfer and final oxygen disposal are part 

 of the reaction chain in which auxin takes 

 part; this would be expected if auxin acts 

 as a hydrogen acceptor, and the auxin 



Phenomenon 



Cell elongation 



(with effect on tro 



germination, etc.) 



Bud inhibition 

 Parthenocarpy 

 Root initiation 



Eeaction involved 

 ^ Growth reaction - 



Preparatory reaction ^ 



> Root forming reaction' 



Substances active 

 *^ ^uxin a and b 



Indole acetic acid 

 Indole butyric acid 

 ^aphthalene acetic acid 

 Ciscinnamic acid 

 \ Phenyl acetic acid 

 7 phenyl butyric acid 



*^ 



Ethylene ? 



TABLE I 

 Relation between the Various Growth Phenomena, the Reactions Involved in Each Phenome- 

 non, AND SOME of THE SUBSTANCES ABLE TO TAKE PART IN THESE REACTIONS 



pendent, so that at low pH a greater 

 amount of auxin is combined. This implies 

 that the auxin is fastened to its carrier by 

 means of its carboxyl group. This picture 

 is in agreement with the fact that neither 

 the carboxyl group, nor nitriles, nor esters 

 of growth substances are active by them- 

 selves, which would be expected if the un- 

 dissociated auxin-acid were the effective 

 form. 



"We can complete the picture now by as- 

 suming that the other essential part of the 

 molecule — the double bond — has come into 

 the right spacial relationship with a hydro- 

 gen donator through its fixation by the 

 carboxyl group, and that the double bond 

 acts as a hydrogen acceptor. In this way 

 we form a mental picture of the first step 

 of the growth process, which is a direct 



passes on this hydrogen. Another parallel- 

 ism between respiratory enzymes and auxin 

 action should be pointed out. In enzymes 

 the specificity is not due in the first place 

 to the co-enzyme, but depends upon the 

 protein carrier. In our picture of the 

 auxin action also we had to assume that 

 the type of reaction was not determined in 

 the first place by the auxin, but by the 

 carrier or substrate. In this respect auxin 

 could be compared with adenine in respira- 

 tion: the protein carrier determines which 

 compound adenine nucleotide will be able 

 to oxidize. I present this mental picture of 

 growth not so much for its intrinsic value, 

 as to indicate that perhaps in the near 

 future we shall be able to build up a picture 

 of growth and differentiation along purely 

 physical and chemical lines, similar to the 



