DETECTION AND QUANTITATIVE DETERMINATION 33 



One Avena Einheit, or AE, is the amount of growth substance present in 

 one block of agar 2 by 2 by 0.5 mm. that will cause a 10 deg. curvature 

 at 22 to 23°C. and at a relative humidity of 92 per cent (Kogl and 

 Haagen Smit, 1931, Mitt. I). 



Comparison of the Units of Different Workers. — The relation- 

 ship of the d value used by Boysen Jensen and curvature as 

 measured in degrees (Went and others) is as follows : If d is the 

 difference in length of the two sides of the curved coleoptile, 

 and (p the angle of curvature (Fig. 16), the relationship between 

 them may be stated in the following way: 



d = 



<p2ir(r + t) <p27rr ip2-Kt 



360 



360 



360 



Fig. 16. — Diagram to show relation- 

 ship of d value (used by Boysen Jensen) 

 to curvature measured in degrees 

 (used by Went and others). 



With the aid of this equation the difference in length d between 

 the convex and concave sides 

 of the coleoptile can be cal- 

 culated from the angle of cur- 

 vature. If the coleoptile is 1.5 

 mm. thick, then for d = \ mm., 

 ip is equal to an angle of 38.2 

 deg. ; this is much greater than 

 the maximum angle (20 deg.) 

 permissible for quantitative 

 work, but d values of to 0.5 

 or 0.6 mm. have a direct rela- 

 tionship with the concentration 

 of growth substance. 



Still another method for determining curvature is given by 

 Metzner (1929). 



An exact comparison between the units mentioned above is not 

 possible, because of the difTerence in the cultural and experimental 

 conditions and the size of the agar blocks employed. However, 

 to get a clearer conception of the relative values of the units, a 

 few equivalents are given (Table 1) : they are intended to be little 

 more than suggestive. 



It can be demonstrated that 1 mg. pure, crystallized auxin 

 contains 20 to 90 X 10^ AE (Kogl, Haagen Smit, and Erxleben, 

 1933, Mitt. IV) ; therefore, according to Table 1, this same amount 

 of auxin should be equivalent to 100 to 450 WAE, or 2 to 9 X 10^ 

 "units," when applied to Avena coleoptiles. 



