CHAPTER XXVII 



General Survey 



For the study of phenomenon of irritability, the so-called 

 ' sensitive ' plants have been selected for the purpose of 

 experimental investigation. From this it must not be 

 inferred that the fundamental reactions that have been 

 demonstrated are different in ordinary plants. By the 

 employment of the electrical mode of investigation, I have 

 shown elsewhere that not sensitive plants alone, but every 

 plant, and also every organ of every plant, is excitable. 

 Even in the matter of mechanical response, longitudinal 

 contraction under excitation may be demonstrated in the 

 case of various organs of ordinary plants ; but the extent 

 of movement in such cases is not very great. A ' sensitive 

 plant ' differs from others only in the possession of mobile 

 mechanism by which excitatory change of form is mani- 

 fested by a conspicuous movement. 



In the case of Mimosa, the responsive down move- 

 ment or the fall of the leaf is brought about by the 

 differential excitabilities of the upper and lower halves 

 of the pulvinus, the movement being magnified by the 

 long petiolar index. It is these advantageous circum- 

 stances that render the motile apparatus of Mimosa 

 a good indicator of excitation. The leaf may thus by its 

 responsive movement indicate the passage of an excitatory 

 impulse originated at a distance just as the excitation 

 transmitted through a nerve is detected by the mechanical 

 response of the attached muscle. In connection with this 

 it should be remembered that the plant indicator may 

 become inefficient under unfavourable circumstances. 

 Absorption of excess of water, for example, will annul the 



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