METHODS OF INVESTIGATING GEOTROPISM 167 



The heliotropic curvature produced by unilateral illumination may 

 also be prevented by rotation on a klinostat, so that both the heliotropic 

 and geotropic action may be eliminated if the plant is rotated about a 

 horizontal axis at right angles to the direction of the illumination. A slight 

 phototropic action may, however, be produced if the shadow of the axis 

 of the klinostat or of the slice of bread commonly used to grow mould 

 fungi falls upon the plant for a sufficient length of time at each rotation. 

 If the axis of rotation is horizontal but parallel to the incidental rays of 

 light, the action of gravity is eliminated, but not that of light ; and the 

 same applies whether the plant is fixed so that its own axis is parallel 

 or at right angles to that of the klinostat. 



Dorsiventral organs often perform aitionastic movements under the 

 influence of changes in the diffuse external conditions. A photonastic 

 curvature may in fact be produced when a dorsiventral organ is rotated on 

 a klinostat so that it is equally illuminated on all sides. The same applies 

 to other agencies, including gravity, although under natural conditions 

 the latter never acts equally on all sides, as diffuse light may do without 

 causing any tropic curvature. 



Tropic stimulatory reactions appear to be suppressed on a klinostat 

 in the same way as when the exciting agent is equally distributed on all 

 sides, but it must be remembered that specific irritabilities and the power of 

 response to a particular excitation may be excited or modified by tropic 

 stimulation 1 . On the other hand, opposed stimuli acting on different flanks 

 in rapid succession may antagonize each other without producing any 

 responsive curvature either way. Under simultaneous stimulation of this 

 kind a dorsiventral tendril does not perform any curvature, and the same 

 effect follows when the tendril is revolved on a klinostat so that the point 

 of contact passes rapidly round and round an excitable zone. If, however, 

 the intervals between the successive stimuli are sufficiently long, a tropic 

 curvature will be produced which the stimulatory actions on the opposed 

 sides may be unable to eliminate. Finally, if both sides are equally 

 responsive, successive stimulations may be expected to produce the same 

 result as continuous diffuse excitation. 



These and other considerations show that the tropic reactions of 

 a dorsiventral organ are not always entirely eliminated on a klinostat, 

 although when the rotation is sufficiently rapid, the action of a unilateral 

 agency will usually be the same as when it is diffusely applied. Neverthe- 

 less something depends upon the point of application of the stimulus, as is 

 shown by the fact that the stimulus of gravity reawakens the growth 



1 In this way Wiesner (Die heliotropischen Erscheinungen, 1878, I, p. 55 ; 1880, II, p. 76) and 

 also H. Miiller (Flora, 1876, p. 76) were able to obtain heliotropic reactions on feebly sensitive 

 plants, which show none so long as they are exposed to geotropic induction. 



