Natural auxins 



In the present study, the change in curvature from the start of exposure to 

 the time of subsequent recordings was regarded as a response. Figure 11 

 shows an example of the distribution of curvatures in samples of individual 

 roots under the influence of two different treatments. If a sufficiently fine 

 technique of measuring could be applied, nearly 50 per cent positive 

 curvatures would be present at any time in unstimulated roots. In general 

 practice, however, workers using 50 per cent positive curvatures as a criterion 

 for the development of a minimum response have fixed a certain angle as a 

 limit below which all curvatures are neglected. The size of this minimum 

 angle depends mainly on the technique of observation. It is thus clear that, 

 whenever such a limiting angle has been chosen, the recorded result, 



20 



-Cl 



I. 



£=0\ 



£=0-5 



__ 



1 



-1^-5 -1-6 



1-5 'i-S 



-1^-5 -1-5 

 Cupvatune 



1-5 i^-S 



7-5 10-5 

 degrees 



Figure 11. Distribution of changes in curvature from to 32 minutes in 30 unstimulated roots [E = 0) 

 and in 29 roots exposed for 0-5 minutes to \ g [E = 0-5). Curvatures measured after 32 minutes of 

 rotation at RjO-5. Note increase in variability, cf Figure 10. 



expressed as per cent positive curvatures, is not a minimum response but a 

 reaction of a certain magnitude. Possible negative curvatures, and positive 

 curvatures smaller than the fixed minimum, are not considered by this 

 method. On the other hand, the recorded result (per cent) may include a 

 number of much greater curvatures which are not evaluated quantitatively. 

 In order to detect a minimum response, therefore, the writer would strongly 

 recommend determination of the mean curvature. The presentation time 

 may then be determined by simple or logarithmic extrapolation, as shown 

 for example in Figure 8. This does not mean extrapolating back until the 

 first root has curved, but until the mean value about which the roots perform 

 their spontaneous movements is just barely positive. 



If the chosen technique is such that curvatures smaller than, for example, 

 10° are neglected, 50 per cent positive curvatures will correspond to a mean 

 curvature of roughly 10°. The exposure, E^q, which yields this mean value 

 at the time of recording (e.g. 30 minutes after the end of the exposure) is 

 longer than the presentation time. If we find that £50 is prolonged, for 

 example by the addition of auxin, this does not necessarily mean that the 

 presentation time has been prolonged. The result would be the same if 

 the presentation time were unchanged and only the velocity of bending had 

 been reduced. At the time of recording, a positive mean curvature, although 



88 



