Natural auxins 



of agar, probably because the resistance is lower there than in other direc- 

 tions. The geotropic movements induced during rotation at /?/32, on the 

 other hand, most likely take place in all directions, so that the root tip 

 follows a spiral. Deviations as small as ±2-b° would not be hindered much 

 by the agar. The upper curve in Figure 9 shows the course of the curvatures of 

 roots which have been stimulated in the horizontal position for 32 minutes. 

 The average curvature at the end of the exposure was about 13°. The curva- 

 tures increase during the first 16 minutes of rotation at i?/32 and reach a 

 maximum of about 21°. During the second halfofthe revolution the curvature 



"^ fs - 



Figure 10. Variability of 

 curvatures, a measure of the 

 extent of the spontaneous move- 

 ments. Ordinate: standard 

 deviation of changes in curvature 

 font zero time to the time of 

 each of the subsequent readings. 

 Abscissa : time after first read- 

 ing. Most points based on 25- 

 35 degrees of freedom {exception: 

 6 upper xV; /= 17 or 8). 



Time 



is reduced by about 5°. During subsequent revolutions the amplitude is 

 approximately constant, but both the maxima and the minima become 

 lower and lower, so that after about 10 hours the root tip is pendling between 

 3 and 8 degrees. This pendling, probably a spiral movement, goes on for at 

 least 12 more hours. 



Spontaneous movements. Variability 



When unstimulated roots are rotated at /?/32, they remain approxi- 

 mately straight, apart from the spiral pendling of their tips. At i?/0-5 on the 

 other hand, the roots will carry out spontaneous movements, resulting in 

 large irregular curvatures. A measure of these spontaneous movements may 

 be obtained by taking the standard deviation of the changes in curvature 

 which take place after the initial measurement. 



86 



