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-bemg 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 mmpressions 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 
