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, mstead of the 
usual positive. 
Curiously enough, [ have on many occasions found 
exactly parallel instances in the response of plants. 
+ —= 
ININININNG 
Fic. 27.—ABNORMAL PosiTIvE RESPONSES IN STALE LEAF-STALK OF TURNIP 
CONVERTED INTO NormMAaL NEGATIVE UNDER STRONG STIMULATION ! 
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 wp. But in cases where it is necessary to discriminate the sign 
of response, positive response will be recorded up, and negative down. 
