368 VEGETABLE PHYSIOLOGY 



CHAPTEK XXIV 



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 

 difference 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 to 

 define more narrowly what we understand by the terms 

 stimulus and stimulation we find it is not easy to restrict 

 them however to such changes in the surroundings as we 

 are able to observe and perhaps measure by even the most 

 delicate instruments at our disposal. 



