Geotropic responses in roots 



present experiments, the 2-watt motor of the kUnostat used in earlier work was exchanged for 

 an 18-watt motor. The khnostat was further equipped with an automatic recording 24-mm 

 camera (Robot) and a counterweight. The camera was rotated together with the plants and 

 by means of micro-switches, the camera would make an exposure every 32 minutes or every 

 64 minutes. In addition, manual exposures could be made at any time during the rotation. 



Measurement of the geotropic responses — One of the purposes of the present 

 study was to determine the so-called presentation time under various 

 conditions. For mass-acceleration of a certain strength, for instance 1 g, 

 the presentation time is generally defined as the time during which the 

 root must be stimulated in order that a minimum response may sooner or 



Figure 2. Plant chamber. Artemisia 

 seedlings growing between agar platelets on 

 a glass slide and protected by a plastic cover. 



later become manifest. A minimum response is often defined as the develop- 

 ment of curvatures in 50 per cent of the stimulated roots. The percentage of 

 curving roots, however, is a very crude measure of the response and rather 

 inconvenient for statistical treatment. If one is particularly interested in the 

 magnitude of the reaction under various conditions, it is necessary to measure 

 the curvature itself, and not the number of reacting roots. 



Photographic recording {Figures 3, 4, and 5) was used in most of the 

 experiments. White or orange light did not show much effect, if any, on the 

 responses. Individual angles were measured to the nearest degree by means 

 of a suitably equipped horizontal microscope. 



The effect of continuous, unilateral geotropic stimulation 



Figure 3 and the upper curve in Figure 6" show the development of curvatures 

 in roots exposed continuously to unilateral geotropic stimulation in the 

 horizontal position. For about 70 minutes they seem to curve at an approxi- 

 mately constant rate of about one-half of a degree per minute. As the angle 

 of curvature increases, however, the intensity of the geotropic stimulus 

 decreases with the sine of the angle which the root tip makes with the plumb 



81 



