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CHAPTEE 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 

 mobility. 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 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 



