TECHNIQUE OF AUXIN DETERMINATIONS 41 



Figure 19). Below the upper limit, whose position differs 

 for different active substances, the activity of a substance 

 can therefore be quantitatively expressed. The amount 

 of a substance necessary to produce a given curvature 

 under standard conditions is taken as a unit. 



04 f"^! 



Fig. 19. Relation between auxin concentration (in mg. indole-3-acetic 

 acid per liter) and curvature. Two decapitations 3 hours apart, agar blocks 

 10 mm.\ 24° C, 85% relative humidity. The scale of the abscissae of the upper 

 curve is 50 times that of the lower. Each point the mean of 30-50 plants. 



The "^rena-Einheit" (AE) of Kogl and Haagen Smit (1931) is 

 that amount of substance which when applied in a 2 cmm. block of 

 agar causes 10° curvature. The "plant unit" (p.u.) of Dolk and Thi- 

 mann (1932) is that amount of substance which when applied in a 

 10 cmm. block causes 1° curvature. These workers also define as 

 "1 unit per cc." that concentration of solution which when mixed 

 with an equal volume of 3 per cent agar and made into 10 cmm. blocks 

 gives 1° curvature. Under the conditions then used (40-60 minutes 

 between decapitation and application of block) 1 AE = 2.5 p.u. How- 



