612 SENSORY SYSTEM 



less pronounced shortening of presentation-time and reaction-time, a result 

 which proves that stimulation was actually intensified under the condi- 

 tions of experiment. F. Darwin made use of a large tuning-fork which 

 was set in vibration by electrical means. By this method also a very 

 pronounced acceleration of geotropic curvature was found to take place. 



4. The following very elegant experiment, which strongly supports 

 the statolith theory, has been devised by Buder. 313 The experimental 

 material (consisting of radicles of Lepidium sativum) is placed hori- 

 zontally for about 12-15 minutes, until all the falling starch- 

 grains have taken up their position upon the physically lower walls 

 of the statocysts. The experimental plants are now turned through 

 180, and left in the new position for precisely the same length 

 of time as that spent in the first position. The stimuli perceived 

 in the two diametrically opposed positions neutralise one another ; 

 hence, at the end of the second period, the statoliths are all located 

 on one side of the statocysts, but no tendency to curve geotro- 

 pically has as yet been induced. During the third stage of the 

 experiment, the radicles are subjected to intermittent stimulation, 

 being placed alternately in the two diametrically opposed horizontal 

 positions for periods of 8-10 seconds, or longer, the periods being 

 in all cases made too short to allow of any rearrangement of the 

 statoliths. The total duration of this stage amounts to about 20 

 or 30 minutes. During the fourth and last stage, the experimental 

 plants are placed on the klinostat, whereupon geotropic curvature 

 is found to result after 40-90 minutes; this curvature is always 

 such that the side which faced downwards when the statoliths 

 were also resting on the physically lower walls, during the period 

 of intermittent stimulation, becomes convex. Concordant results have 

 been obtained by Buder in analogous experiments performed upon 

 inflorescence-axes of Capsella Bursa pastoris ; in this case the inter- 

 mittent stimulation was effected by means of rapid rotation (one revolu- 

 tion in 7-8 seconds) on the klinostat. These results seem entirely 

 inexplicable except on the basis of the statolith theory. 



5. It has been shown by F. Darwin and Miss Pertz, 314 and again, 

 more recently, by Buder, that migration of the statoliths can be 

 induced by the action of centrifugal forces of smaller intensity than 

 the normal acceleration of gravity (g) ; in such cases the rearrange- 

 ment naturally takes a considerable time. The amount of retardation 

 agrees very closely with the prolongation of the presentation-time 

 observed by Bach, when working with mass-accelerations of smaller 

 value than g. This coincidence also seems to support the statolith 

 theory, as has already been pointed out by Buder. 315 



