124 PLANT RESPONSE 



energy. This implies that response is disproportionately 

 greater than stimulus, and that the responsive change is 

 attended by an evolution of heat and chemical by-products. 

 It would follow, however, from such rapid depreciation, that 

 fatigue must be the invariable consequence of any series of 

 responses. But it is notorious that in the responses of nerve, 

 not only is there no fatigue, but neither is there any evolution 

 of heat, nor occurrence of chemical change, that can be 

 detected. 



As the simplest case, we have hitherto considered the 

 substance acted on to be neutral in its character, that is to 

 say, not active in the sense of being able to absorb stimulus 

 and hold it latent. But if we regard the living organism as 

 a machine, three different cases are conceivable. These are : 

 first, that in which the responding substance simply con- 

 verts the energy received as stimulus into response ; secondly, 

 that in which the responding substance possesses a large 

 amount of energy, some of which is set free by the action of 

 the stimulus ; and lastly, that in which the responding sub- 

 stance is capable of absorbing and holding latent, to a greater 

 or less extent, the stimulus which it receives. 



(i) Response proportionate to stimulus. — The first of these 

 types may be illustrated by a responding circuit which con- 

 tains a magnetic motor— say a galvanometer — translating cur- 

 rent into motion. The source of stimulus may be an external 

 battery periodically closed by a tapping key. The waste of 

 energy by the production of heat may be supposed to be 

 brought down to a minimum in this circuit by using a feeble 

 current and reducing the resistance. Uniform stimuli will 

 now cause uniform responses of the responder. Response 

 will be proportionate to stimulus, and there will be no chemi- 

 cal, and no appreciable thermal, change in the responder. 

 This is a state of things which may be said to approximate 

 closely to the responsive peculiarities of the nerve. 



(2) Response disproportionately greater than stimulus : re- 

 sponding substance reduced below par. — As an example of the 

 second type, we have to imagine a responding system which 



