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CHAPTEE XXV 



THE NERVOUS MECHANISM OF PLANTS 



IT is difficult to refrain from coming to the conclusion, from 

 a consideration of the facts which have been discussed in 

 the last two chapters, that the nervous system of the 

 animal kingdom is represented in the vegetable one. That 

 plants are sensitive to variations in the conditions sur- 

 rounding them, and that the responses they make to such 

 variations are purposeful and conduce to the well-being of 

 the organism, is abundantly evident. The response to any 

 external stimulus, moreover, has been seen to be depen- 

 dent upon the plant being in a condition of tone, that is of 

 health and vigour. If its well-being 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. The lack of response is not due to a failure in 

 the motor mechanism by which the change is brought 

 about, but by an absence of power to realise the altered 

 conditions which would constitute a stimulus to an organism 

 in a condition of full health. The age of the organism 

 again has been seen to have an important influence upon 

 its power of receiving impressions and its behaviour in 

 responding to them. 



We have already called attention to the fact that the 

 responses made to stimuli of different character afford 

 clear evidence of purpose. No reply is at all haphazard, 

 but is devoted especially to some definite object which is 

 closely related to the stimulus. 



Another consideration which bears upon this question 



