. The chemistry and mode of action of plant growth substances; proceedings of a symposium held at Wye College, University of London, July 1955. Plant regulators; Auxin; Growth (Plants). 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
. The chemistry and mode of action of plant growth substances; proceedings of a symposium held at Wye College, University of London, July 1955. Plant regulators; Auxin; Growth (Plants). 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.
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