LECTURE XVI. 

 IRRITABILITY. 



IT was pointed out in the first lecture (p. 7) that, amongst 

 other fundamental properties, the protoplasm of plants is 

 endowed with that of Irritability, a certain sensitiveness, that 

 is, to the influence of external agents, and we have since 

 learned (p. 301) to regard movement as one manifestation of 

 this irritability. 



In dealing now more fully with the influence of external 

 agents in inducing or preventing movement, or in modifying 

 either the rapidity or the direction of any movement which 

 the organ may be already performing, we must clearly dis- 

 tinguish their influence as merely normal conditions, upon the 

 proper combination of which the possibility of any manifesta- 

 tion of irritability depends, and their direct action upon the 

 protoplasm in inducing or arresting movement ; the former 

 we will speak of as the tonic influence of external conditions, 

 the latter as the stimulating action of external agents. 



In endeavouring to make clear the difference between 

 tonic influence and stimulating action we must, in the first 

 place, form some idea of the internal conditions of move- 

 ment. We have seen (p. 302) that a movement can only 

 take place when there is present in the protoplasm of the 

 organ a supply of readily decomposable material by the 

 decomposition of which the necessary energy is evolved. 

 Without entering at present upon a detailed discussion of the 

 mechanism of movements, we may go on to state that this 



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