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CHAPTER XXIV 
THE NERVOUS MECHANISM OF PLANTS 
Ir 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 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 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 
