NEKVOUS MECHANISMS 501 



organism, is abundaritly evident. Tlie response to am- external 

 stimulus, moreover, has been seen to be dependent upon the 

 plant being in a state of health and vigour. If its wellbeing 

 has been interfered with by such disturbances as deprivation of 

 light, or lack of oxygen, or exposure to too high or too low 

 a temperature, no response is given, for its irritability is in 

 abeyance or destroyed. Its own age again has been seen to 

 have an important influence upon its power of receiving im- 

 pressions and its behaviour in responding to them. 



In considering from this point of view the wa}' in which 

 plants are affected by external influences, it is noteworthy that 

 an exceedingly small stimulus is able to bring about a very con- 

 siderable effect, and that there is no direct ratio between the 

 intensity of the stimulus and the resulting movement. The 

 tendrils of Passiflora, already alluded to, are caused to move 

 by the contact with them of a small piece of thread, weighing 

 not more than ^^ of a grain, and the resulting movement is 

 of considerable extent and is prolonged for some time. The 

 sensitive hair of the leaf of Dionaea 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 vvhich 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 be exposed 

 for some time to such an illumination, and then, before the 

 curvature has begmi, be 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 gravity. There is no other explanation possible 

 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 separation of the part stimu- 

 lated and the ceUs which are the seat of the resulting action, 

 imphes that there must be in the plant a means of rapidly con- 

 ducting such external impressions from one part to another. 

 To this pomt we shall return. 



If then we admit that there is even a rudimentary nervous 



