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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 in any case, 
for the environment is in a constant state of change and 
the protoplasm of the organism is also exhibiting continual 
motility. For the maintenance of health or even of life 
it is 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 
alterations in its surroundings may now be considered 
in greater detail, and we may thereby form some acquaint- 
ance with the causes which have led to such great diver- 
sities 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, how- 
ever, 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 to observe and perhaps measure by even the most 
delicate instruments at our disposal. 
