Geotropic responses in roots 



Figure 10 shows the magnitude of the standard deviation of the curvatures 

 of roots under various conditions. The squares represent roots kept in the 

 normal, vertical position. Their spontaneous movements are small, since 

 they are constantly being corrected by gravitational stimulation. The 

 upright crosses represent roots which were rotated at /?/32 without preceding 

 stimulation. Their variability is somewhat greater than that of vertical 

 roots, but still quite low. 



If roots are placed in the horizontal position, they start carrying out 

 geotropic movements, and this is attended by a sudden increase in their 

 variability (filled-in circles). Part of this variability is probably due to 

 differences in the geotropic sensitivity and reactivity of the roots. After 

 about 100 minutes, when the roots have reached an average curvature of 

 about 45°, the variability starts decreasing again, and at 5 hours the variability 

 of the previously horizontal roots is the same as in roots rotated at 7?/32. 



When roots are rotated at RjO-o, the picture is entirely different. They 

 are subjected only to small and insignificant geotropic stimuli on the klino- 

 stat, and are free to perform spontaneous movements which are only 

 slightly, if at all, modified by gravitational influences. 



The open circles represent roots which were rotated without preceding 

 stimulation. Their variability increases as an almost straight line for about 

 200 minutes and then levels off at a value (about 22°) much higher than in 

 roots rotated at i?/32. It should be borne in mind that the mean curvature 

 of these roots is very close to zero. 



When roots were first stimulated in the horizontal position for 0-5 minutes 

 and then rotated at RjO-D their variability followed a similar course (x's), 

 but did not level off at 22°. After 10 hours, the variability still seemed to be 

 increasing. The standard deviation was 46°, and the mean curvature 2-9°. 



When roots are geotropically stimulated for longer periods before rotation, 

 the spontaneous movements are superimposed on the actual response to 

 the stimulation. 



DISCUSSION AND CONCLUSIONS 



The klinostat is an interesting instrument, and the present series of results 

 do contribute to the discussion of the theory of the klinostat effect. In this 

 communication, however, only a few points which may be considered of 

 practical importance for the carrying out of geotropic experiments will be 

 considered. 



1 . Measurement of the response — As pointed out previously, the practice of 

 recording the percentage of curving roots is inconvenient for various reasons. 

 Since the technique used in the present study permitted the detection of very 

 small curvatures, few roots could be considered absolutely straight, even 

 when kept in the vertical position. This is not surprising in view of the 

 spontaneous movements which have been demonstrated. 



In individual roots, the curvature present at the start rarely exceeded ±5°. A deviation of 

 as much as ±8°, however, will have no measurable influence on the early response when a 

 group of such roots is placed in the horizontal position, since the sine of 82'' is still 0-99. On 

 the other hand, roots deviating as little as 3° from the vertical line will be stimulated by 

 5 per cent of the force of gravity and thereby induced to straighten out (sin 3^ = 0-05). 



87 



