48 RESPONSE IN THE LIVING AND NON-LIVING 



Positive variation. The responses given by muscle 

 or nerve are, normally speaking, negative. But that of 

 retina is positive. The sign of response, however, is apt 

 to be reversed if there be any molecular modification 

 of the tissue from changes of external circumstances. 

 Thus it is often found that nerve in a stale condition 

 gives positive, instead of the normal negative variation, 

 and stale retina often gives negative, instead of the 

 usual positive. 



Curiously enough, I have on many occasions found 

 exactly parallel instances in the response of plants. 



FIG. 27. ABNORMAL POSITIVE KESPONSES IN STALE LEAF-STALK OF TURNIP 



CONVERTED INTO NORMAL NEGATIVE UNDER STRONG STIMULATION 1 



The relative intensities of stimuli in the two cases are in the ratio of 1 : 7. 



Plants when fresh, as stated, give negative responses 

 as a rule. But when somewhat faded they sometimes 

 give rise to positive response. Again, just as in the 

 modified nerve the abnormal positive response gives 

 place to the normal negative under strong and long- 

 continued stimulation, so also in the modified plant 

 the abnormal positive response passes into negative 



1 For general purposes it is immaterial whether the responses are re- 

 corded up or down. For convenience of inspection they are in general 

 recorded up. But in cases where it is necessary to discriminate the sign 

 of response, positive response will be recorded up, and negative down. 



