THE NERVOUS SYSTEM 55 



Three Stages of the Typical Reaction 



Analysis of the reaction of Venus' fly-trap shows 

 that there are three distinct stages in the process : 

 the adequate stimulus, supplied from without by the 

 insect touch ; the process of conduction of the stimulus 

 from the tip of the filament to the effector motor 

 mechanism of the plant, performed in this case by a 

 chain of tissue cells which in a certain sense resembles 

 the specialized nerve paths of man; and finally, the 

 chemical and motor end effect, involving all the acts 

 and organs used in the closing of the lobes and the 

 killing and digestion of the insect. 



We have seen that the presence of the insect stimulus 

 is necessary to excite the closing of the lobes, and 

 furthermore, that it must be a specific or adequate 

 stimulus. A touch, lighter or heavier than that of 

 the insect, does not stimulate. Only the stimulus 

 which throughout evolution has led to the develop- 

 ment of the responsive mechanism in the plant can 

 now activate that mechanism. Conversely, as soon 

 as the appropriate force is applied, the characteristic 

 reaction takes place, and will go on taking place as 

 long as there is sufficient energy available in the plant. 

 These observations agree with those recorded by Bose 

 in " Response in the Living and Non-Living, " and 

 " Comparative Electro-Physiology." Bose shows that 

 a wide variety of plants exhibit, under stimulation, 

 phenomena which are identical with those generally 

 supposed to be characteristic of animal tissues only; 

 namely, fatigue under often repeated stimulation ; the 

 tendency to exaltation of the response during the early 



