THE NERVOUS MECHANISM OP PLANTS 407 



devoted especially to some definite object which is closely 

 related to the stimulus. 



Another consideration which bears upon this question 

 is that an extremely small stimulus is able to bring about a 

 very considerable effect, and that there is no direct or simple 

 ratio between the intensity of the stimulus and the extent 

 of the response, whether this takes the form of movement 

 or chemical change. The tendrils of Passiflora, already 

 alluded to, can be caused to move by the contact with 

 them of a small piece of thread, weighing hot more than ^V 

 of a grain, and the resulting movement will be of consider- 

 able extent and prolonged for some time. The sensitive 

 hair of the leaf of Dioncea needs only a touch to cause a 

 rapid movement of the whole leaf-blade ; the pricking of 

 the staminal filament of Berberis causes a considerable 

 bending of a relatively bulky body. The seedlings of 

 PJialaris bend with some speed towards a light which is 

 not sufficient to cause a visible shadow at the distance at 

 which they are placed from it. 



It can hardly be imagined that such slight disturbances 

 can act mechanically upon the parts that move. This 

 point is illustrated by an observation made by Wiesner, 

 that if a part which responds only to the stimulus of lateral 

 light is exposed for some time to such an illumination, and 

 then, before the heliotropic curvature has begun, is removed 

 into darkness, it will slowly bend towards the side which 

 has been stimulated. The same observation has been 

 made by other observers in the case of the stimulus of 

 gravitation. There is no explanation possible other than 

 that the stimulus brings about changes in the protoplasm 

 of the cells of the moving part, which slowly modify their 

 relation to the water of their contents, so that a great 

 alteration of their turgidity results. Moreover, the separa- 

 tion of the part stimulated and the cells which are the 

 seat of the resulting action, implies that there must be in 

 the plant a means of more or less rapidly conducting such 

 external impressions from one part to another. 



