42 PHYTOHORMONES 



ever, under the conditions described in section C 2 (3 hours between 

 decapitation and apphcation) 1 AE = about 1.5 p.u. The Wuchsstoff- 

 Einheit (WAE) of Boysen Jensen (1933o) is defined as that amount of 

 substance which, when dissolved in 50 cc. of water and mixed with an 

 equal volume of 3 per cent agar, gives a d-value of 1 mm. 



Now that it has been shown that one of the auxins is 

 indole-3-acetic acid, which is readily available in pure 

 form, the importance of such units has largely disappeared, 

 and all activities may be expressed in comparison with the 

 effect of indole-acetic acid. However, for the better under- 

 standing of data given in the literature, the following com- 

 parisons are tabulated (Table V). It should be noted that, 

 weight for weight, auxin a is about twice as active as indole- 

 3-acetic acid, or mol for mol about 3.75 times as active 

 (Kogl, Haagen Smit, and Erxleben, 1934). 



C. 6. Positive Curvatures 



The curvature produced by auxin is a negative curvature, 

 i.e. away from the side with the agar block, the active sub- 

 stance having caused an increase of growth on the side to 

 which it is appHed. Stark (1921) and Nielsen (1924) never 

 obtained such negative curvatures by the application of 

 blocks containing plant extracts. Seubert (1925) found 

 also that the majority of substances dissolved in agar caused 

 positive curvatures. This was considered as evidence of 

 the existence of growth-retarding substances which were 

 identified by Stark with wound-substances. Seubert even 

 found that the same substance appears to be stimulating 

 or inhibiting according to its concentration; thus, high 

 concentrations of saliva or malt extract caused negative, 

 low concentrations positive curvatures. 



The absence of growth-promoting activity in plant ex- 

 tracts has been explained by Thimann (1934) ; it is due to 

 the destruction of the auxin by enzymes set free on crushing 

 the material. The production of positive curvatures, how- 

 ever, is something other than the mere absence of negative 

 curvatures, and its mechanism has been elucidated by 



