222 B. Aberg 



tion: (-)-)2-lPOP belongs to the l series (13, 14, 16, and personal com- 

 munication from A. Fredga). 



Some growth regulating chemicals, not belonging to the phenoxy 

 group, will be abbreviated as follows: lAA: indole-3-acetic acid, 1- 

 NMSP: a-(l-naphthylmethylthio)propionic acid. l-CioHj-CHo-S-CH- 

 (CH3)-COOH, 2-NMSeA: 2-naphthylmethylselenoacetic acid, 1- and 

 2-NOA: 1- and 2-naphthoxyacetic acid, respectively. 



GENERAL CONSIDERATIONS 



At present we are far from a detailed understanding of the auxin 

 action (or actions). We know a large number of substances, all of 

 which affect various growth processes in the same manner as does 

 lAA, and this has led to the assumption of a common receptor in 

 the auxin responses. Other substances, the structure of which is 

 often very closely related to some auxin, cause growth effects of a dif- 

 ferent sign and also counteract the effect of externally applied auxin. 

 These substances may be assumed to compete with the auxins at the 

 receptor site, but they may also interfere with auxin uptake and 

 transport, and with auxin metabolism. Nor should a direct effect 

 at the receptor sites of a sign opposite to that of the auxins be ex- 

 cluded at the present stage of knowledge (5). Such substances have 

 generally been called antiauxins, and for the present it seems best to 

 retain this usage. The explanation of the antiauxin action is still 

 largely hypothetical, and an antiauxin in the strict sense (19) could 

 easily be specified by the prefix competitive. 



In order to illustrate the grow'th effects of auxins and antiauxins 

 as delimited in the present study, some results with the standard 

 auxin lAA and with the antiauxin 2-NMSeA are show^n in Figure 1. 

 The most powerful antiauxin known to the present author, 2-NMSeA, 

 strongly stimulates the growth of wheat seedling roots; it also stimu- 

 lates the growth of flax roots but gives only growth depression when 

 tested on oat coleopiile sections. The inhibition of flax root growth 

 caused by the auxin 2,4-D is effectively restored by 2-NMSeA. In a 

 series of 5 independent experiments, the growth in 10 "M 2,4-D was 

 23.4, and in lO^M 2,4-D + 3 X 10 "^ 2-NMSeA 67.0 per cent of the 

 control growth. Also the growth of lAA-inhibited flax roots is restored, 

 but to a somewhat less extent. On the other hand, the inhibition of 

 (oleoptile growth caused by 10-^ to lO-^'Af 2-NMSeA is completely 

 iinnihilated in the presence of 10-*'A/ lAA. 



The situation is complicated, however, by the existence of sub- 

 stances intermediate between the typical auxins and the typical anti- 

 auxins (1,4,6,8). These substances normally give an inhibition of 

 oat coleoptile cylinder growth at low concentrations, which inhibition 



