ABSORPTION AND EMISSION OF ENERGY IN RESPONSE 83 



being thus negative, we must go to the other extreme of great 

 sub-tonicity if we are to be successful in demonstrating the 

 occurrence of the unmixed positive response. This considera- 

 tion leads us to expect that positive response will be evoked 

 on moderate stimulation from tissues that are either not 

 highly tonic or protoplasmically defective. I shall show in 

 Chapter XXII. that in cells of epidermis, where the proto- 

 plasmic contents have been reduced to a minimum, response 

 to moderate stimulus tends in general to be positive, Even 

 highly excitable tissues like nerve, as will be shown later ? 

 when cut off from their supply of energy, often become so 

 sub-tonic as to give positive response. I shall here show 

 how ordinary tissues exhibit this effect, when the tonic con- 

 dition is allowed to fall to such an extent as to render the 

 tissue extremely sub-tonic. For this purpose I took a cut 

 specimen of petiole of cauliflower, and kept it without water 

 for a couple of days. By this process the specimen became 

 somewhat withered. I next proceeded to take records of its 

 electrical responses under increasing stimuli. The intensity 

 of these stimuli rose from I to 10 units. It will be seen 

 from the record (fig. 54) that each stimulus up to ej evoked 

 positive response, and that it was the strong stimulus of 

 10 which gave rise to the normal response of negativity. 



This constitutes the first instance of a phenomenon which 

 I shall show later to be of very extended occurrence the 

 induction, namely, of one effect under moderate, and its 

 opposite under very feeble stimulation. It is not so easy to 

 demonstrate this fact with a highly excitable, as with a some- 

 what sub-tonic tissue, where the critical intensity of stimulus 

 for the evoking of normal response need not be impracticably 

 low. A point to be taken into account here is the after-effect 

 of sub-minimal stimulus in enhancing subsequent normal 

 excitability. Thus it is found in taking the record of 

 responses to a succession of feeble stimuli, that though they 

 are at first abnormal positive, they are afterwards converted 

 into normal negative. That it is the after-effect of the 

 previous stimulation which thus enhances previous excitability 



G 2 



