TECHNIQUE OF AUXIN DETERMINATIONS 55 



terminal bud has reached less than 5 mm. in length. The top is cut off 

 at 5 mm. below the terminal bud and the stem is split centrally length- 

 wise with a sharp safety razor blade, for a distance of about 3 cm. 

 The split section is then cut off a few mm. below the split, and washed 

 for an hour in water. Serial dilutions of the substance to be tested are 

 prepared, volumes of about 20 cc. being convenient, and poured into 

 Petri dishes. These solutions should not be more acid than pH 4, since 

 then acid curvatures, in the opposite (outward) direction, may inter- 

 fere with the auxin curvatures. A number of such sections (5-8) are 

 transferred to these solutions, in which they are left for 6-24 hours in 

 darkness. At the end of 6 hours the curvature is completed and re- 

 mains stationary, the appearance being as in Figure 25. The angles of 

 curvature are then measured with a suit- 

 able protractor either directly on the 

 plants or on a shadowgraph. This angle 

 is that between the tangents to the ex- 

 treme curved tip and at the point A 

 (Figure 26) where the direction of curva- 

 ture changes. The curvatures obtained 

 are, within about a 10° range, independ- 

 ent of temperature. The plants may be 

 prepared in diffuse white light without 

 affecting the curvatures. A curious fea- 

 ture of these curvatures is that they are 

 inhibited by heavy metals in concentra- 

 tions which do not appear to be other- 

 wise toxic. Thus M/4000 solutions of 

 Cu salts, M/1000 of Xi, or M/lOO of IVIn or Zn inhibit curvature prac- 

 tically completely, although the stems remain apparently healthy (u). 



When the curvature is plotted against the logarithm of 



the concentration, a straight line is obtained within certain 



concentration limits (see Figure 45). The slope of this Hne 



will be, in general, different for different compounds and 



from it activities can be conveniently compared by the 



relationships : 



C_ 



Co 



where a is the mean observed curvature, 



C the concentration, in moles per liter, 

 Co the concentration at which the curvature is zero, 



Fig. 26. Method of meas- 

 uring curvature of split stem 

 of Pisum. 



a = K log 



and 



f 



