Geotropic responses in roots 



smaller than 10°, may actually have developed after an exposure of the 

 same duration as in the auxin-free control. 



Similar considerations can be applied to determinations of the reaction 

 time. 



2. Klinostat rotation — If the curvatures are to be recorded after rotation 

 on a klinostat, the rate of rotation should be fast enough to permit the curva- 

 tures to develop freely, and the measurements should be made so early that 

 the spontaneous movements do not create an excessive variability. If 

 klinostat rotation is too slow, the roots will follow the motion of the klinostat, 

 and the presence of maxima and minima of curvature will create difficulties 

 in measuring. 



3. Time of recording — Wlien studying the development of curvatures, 

 particularly after a stimulation of such duration that measurable angles are 

 present before the end of the exposure, the problem arises as to the correct 

 time of recording the response. It seems impossible a priori to fix any 

 particular time at which the observed curvatures may be taken as an 

 adequate measure of the response to the stimulus applied. If one wants to 

 fix a definite time at which to record the curvature, it is difficult to decide 

 whether the starting point should be the beginning, the middle, or the end of 

 the exposure. Another possibility would be to take the maximum curvature 

 as a measure of the response, regardless of when it occurs. 



A way out of this difficulty, especially when studying the influence of 

 external auxin on the geotropic responses, would be to determine the velocity 

 of curvature under the influence of a stimulation of constant intensity, 

 for example 1 g. Changes in this velocity will pi'obably be a satisfactory 

 criterion for the efiect of changes in the experimental conditions. 



SUMMARY 



1 . The acceleration of root elongation by added auxin reported in the 

 literature does not necessarily indicate a sub-optimal auxin concentration 

 in the elongation zone of normal intact roots. Such acceleration has not 

 been recorded until several hours after the addition of auxin, whereas the 

 immediate response, the one which has any bearing on normal geotropic 

 reactions, is a retardation of elongation. There is, therefore, at present no 

 necessity for changing the classical auxin theory of geotropism, which is based 

 on the assumption of a supra-optimal auxin concentration in the root. 



2. The behaviour of stimulated and unstimulated roots of Artemisia 

 absinthium (wormwood) when rotated parallel to the horizontal axis of a 

 synchronous klinostat at two velocities of rotation is described. At a velocity 

 of 1 revolution per 0-5 minutes, geotropically induced curvatures are gradually 

 enlarged during rotation. Large spontaneous movements, however, are 

 superimposed on the actual geotropic response. The variability, therefore, 

 increases enormously with the length of time of rotation. At 1 revolution in 

 32 minutes the variability is much lower, but the root tips follow the motion 

 of the klinostat, i.e. they perform spiral movements with an amplitude of 5°. 



3. Certain precautions to be taken in the recording of geotropic responses 

 are pointed out. It is recommended that the mean curvature rather than 

 the percentage of positive curvatures should be used as the measure of a 

 geotropic response. 



89 



