TECHNIQUE OF AUXIN DETERMINATIONS 49 



C. 8. Variability of the Test 



The value of the maximum angle depends (a) on the cul- 

 tural conditions, being greater for plants grown in earth 

 than in water, and (b) on unknown factors which, although 

 the plants are grown and used under constant conditions, 

 cause its value to vary from day to day. In a series of Avena 

 tests for quantitative determination of auxin it is always 

 desirable to determine the maximum angle for the day, 

 for the linear relation between concentration and curva- 

 ture only holds for smaller angles. 



Not only the maximum angle, but also the sensitiidty, 

 i.e. the mean curvature for a given concentration of auxin, 

 varies from day to day and even from hour to hour. This 

 phenomenon was first studied at Utrecht by Kogl and 

 Haagen Smit (Kogl, 1933; Kogl, Haagen Smit, and van Huls- 

 sen, 1936). They found in general that the maximum sensi- 

 tivity occurred in the early hours of the morning and the 

 minimum in the late afternoon; the variation was several 

 hundred per cent. At Copenhagen, Juel (1936) has found, 

 working under a somewhat different set of constant condi- 

 tions, that the variation from day to day is not more than 

 35 per cent. This does not necessarily conflict with the 

 Utrecht workers' results, for if the tests are carried out 

 always at the same time of day, the variation in sensitivity 

 may be relatively small. This is particularly true at the 

 time of maximum sensitivity, for there the curve is rela- 

 tively flat and the sensitivity reached is the same on differ- 

 ent days (u). 



Figure 23 shows an example of sensitivity and maximum 

 angle variations found at Pasadena. Curves A and B, deter- 

 mined on plants identically prepared (3 decapitations 

 2 hours apart), show that the sensitivity (Curve A) varies 

 parallel to the maximum angle (Curve B). Curve C shows 

 the variation in sensitivity for plants prepared in a dif- 

 ferent way (2 decapitations 40 minutes apart). It will be 

 seen that the absolute variations are about the same for 



