224 B. A berg 



differences in regulator uptake and translocation, by effects on auxin 

 metabolism, on other metabolic processes, and so on, but species dif- 

 ferences in the auxin system may also be at work (4, 11). Assuming 

 an attachment to a protein receptor by many weak bonds as the ini- 

 tial phase in auxin action, the additional hypothesis of slight differ- 

 ences in the structure of the receptor in different species and organs 

 seems fairly natural. It would also be in good keeping with known 

 facts about organ and species specificity of isodynamic enzymes (15). 

 Minor differences in the adsorption areas of enzymes of different 

 origin may result in catalysis of the same reaction of the same sub- 

 strate with very different rates, and may bring about a different rela- 

 tive, or even absolute, specificity toward some substrates. The assump- 

 tion of a cooperation of several similar, but not identical, auxin re- 

 ceptors in the same system (17) is a further step in the same direc- 

 tion that may facilitate the interpretation of the synergistic phenom- 

 ena and also of the behavior of the intermediate regulators. 



In the way now outlined, the basic uniformity of auxin regula- 

 tion of growth is understood as well as the existence of many devi- 

 ations from the common scheme. The connection between the pri- 

 mary auxin action and the growth responses of different type remains 

 enigmatical, however. 



THE INTERMEDIATE REGULATORS 



The existence of intermediate regulators can be inferred already 

 from their interactions with typical auxins and antiauxins (1, 3), but 

 further substantial support comes from their composite concentra- 

 tion-response curves which are particularly instructive in the case of 

 oat coleoptile cylinders. AV'ith increasing concentration the growth is 

 at first inhibited, but then the inhibition subsides and may give 

 place to a quite considerable stimulation (Figure 2). The antiauxin 

 nature of the initial inhibition is indicated by the occurrence of a 

 corresponding initial stimulation of wheat root growth, and also by 

 its disappearance in the presence of lAA. 



It is very interesting to observe the occurrence of intermediate 

 regulators in series of substances, in which a gradual shift in the 

 chemical structure causes a shift from auxin to antiatixin character. 

 One series of this type is represented by the phenoxyacetic acids ^\'it]l 

 pflra-substituents of growing size (6). In 4-BrPOA the intermediate 

 character is quite pionounced, in 4-MePOA, 4-IPOA, and 1-EtPOA 

 the auxin component is gradually lessened, and, with 4-iP-, 1-Ni-, and 

 4-tB-POA substances with pine or almost pine antiauxin activity are 

 reached (Figure 3). Another example is given by the homologous 

 (-|-)-a-phenoxyalkylcarboxylic acids (Table 1, I-'igure 3). The pro- 



