406 VEGETABLE PHYSIOLOGY 



CHAPTER XXIV 



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 surrounding 

 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 dependent 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 sensitiveness 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 con- 

 ditions 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 suggest 

 definite purpose. No reply is at all haphazard, but is 



