. TECHNIQUE OF AUXIN DETERMINATIONS 33 



the side to which the agar block has been apphed. Instead of 

 measuring the angle (which is directly proportional to the dif- 

 ference in growth), the radius of curvature, r, thickness of the 



Fig. 13. Measurement of Avena curvatures. The vertical lines on the base 

 of the transparent celluloid protractor are brought parallel to the uncurved 

 base of the plant on photograph, B, while the lines on the moving arm, A, are 

 brought parallel to the extreme curved tip. The angle (23J^°) is read directly. 

 (From Went, Rec. trav. hot. need. 25: 1-116, 1928.) 



plant, t, and length of the curved zone, I, are measured (see 

 Figure 14). Then the difference, d, between the growth on the 



two sides is expressed hy d = — Assuming a thickness of 1.5 



mm., a rf- value of 0.1 mm. corresponds to 4° curvature, 0.5 

 mm. to IQ'', and 1.0 mm. to 38.5°. This procedure involves 

 the making of three measurements, of which, however, t is 

 taken as constant. Satisfactory measurement of the other 

 two, r and I, depends upon the assumption that the curved 



