398 VEGETABLE PHYSIOLOGY 



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 move- 

 ment 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 not more 

 than V of a grain, and the resulting movement will be of 

 considerable 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 movement of a relatively bulky body. The 

 seedlings of Phalaris 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 the 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. 



If, then, we admit that there is even a rudimentary 

 nervous system in plants, we may proceed with an inquiry 

 into the degree of its differentiation, and the completeness 



