THE MECHANISM OF THE ACTION 



135 



ing the lock. The importance of Thimann's discovery, then, 

 lies in the fact that we are now able to study separately 

 each property of the key, and are warned against ascribing 

 the activity of a substance to those details in its chemical 

 structure which only affect its secondary properties. 



Is it possible now to test for one of these properties, espe- 

 cially for the primary one of growth promotion, without 



300* 



200" 



100' 



■ 



10 



10 



10 mol 



Fig. 45. Relation between auxin concentration and curvature in the pea 

 test. A, indole-3-acetic acid; B, indole-3-propionic acid; C, indole-3-butyric 

 acid; D, naphthalene-acetic acid; E, auxin a and h (obtained by extraction of 

 corn meal). 



being confused by the interference of others? In other 

 words, can we test for the wards of the key independently 

 of the grooves? To some extent at least, we can, by using 

 Went's pea test. We have seen that substances inactive, 

 or of only small activity, on Avena may be highly active 

 in causing pea stem curvatures. Such substances may be 

 regarded as possessing primary growlh-promoting activity. 

 The relation between molar concentration and acti\'ity 

 on pea stems has been compared for a number of substances 

 which show widely varying activity in the Avena test (u).- 

 The result is that auxin a, indole-3-acetiCj indole-3-propiojnLic,- 



