OF VITAL MOTION. 53 



causes. In the Mimosa sensitiva, for example, where 

 the life of the plant is quickened into its highest in- 

 tensity, the vital motion is distinctly referrible to 

 those associated influences which proceed from the 

 sun. The leaves open and expand in the morning, 

 and become raised on their footstalks; and in the 

 evening they again close and droop: or, in other 

 words, the irritable tissues pass into a state equivalent 

 to expansion under the operation of light and warmth, 

 and return to the contracted condition when these in- 

 fluences are withdrawn. They are in fact acted upon 

 in the same manner as the rudimentary structures with 

 which we were concerned in the first chapter. 



The sudden contraction when the plant is touched 

 would seem to point to an external agent, and to meet 

 its explanation without the assumption of any mystical 

 and inherent principle. The first question in this 

 matter concerns the nature of the agent, and then we 

 have to inquire whether there is an abstraction or a 

 communication of influence when the contact is made. 

 The agent, whatever it is, must be physical in its 

 character, for it may be connected with an inorganic 

 instrument, and therefore we have to choose which of 

 the several correlated expressions of physical force is 

 the one concerned. It is not heat, or light, or chemical 

 affinity, because there may be no evidence of the pre- 

 sence of these in the instrument ; it is not the mere 

 communication of ordinary motion, for the results are 



