875 
CHAPTER XXIII 
STIMULATION AND ITS RESULTS 
We may gather from what has just been said that there 
may exist for every plant, at any rate theoretically, a con- 
dition of adjustment when it is in absolute harmony with 
its environment, and when, consequently, its life is being 
regulated to the utmost advantage. We can see, however, 
that such a condition can be only momentary im any case, 
for the environment is in a constant state of change and the 
protoplasm of the organism is also exhibiting continual 
mobility. For the maintenance of health, or even of life, 
it ig essential that variations in one shall be adequately 
responded to by variations in the other, and the impossi- 
bility of securing indefinitely such a continual adjustment 
of relations is the cause of the cessation of life. 
The responses which the organism makes to such altera- 
tions in its surroundings may now be considered in greater 
detail, and we may thereby form some acquaintance with 
the causes which have led to such great diversities in form, 
structure, and habit of life as we have already seen to 
characterise large groups of plants. 
Any change in the environment which provokes some 
alteration of behaviour on the part of a plant is spoken of 
as a stimulus, and the change of behaviour is to be looked 
upon as the result of stimulation. When we come, however, 
to define more narrowly what we understand by the terms 
stimulus and stimulation, we find it is not easy to restrict 
them to such changes in the surroundings as we are able 
