54 RESPONSE IN THE LIVING AND NON-LIVWe 
2. The next figure shows how little variation is pro- 
duced with low value of stimulus, but with increasing 
stimulus the response undergoes a rapid increase, 
after which it tends to approach a limit (fie. 33, @). 
3. As an extreme instance of the case just cited, 
I have often come across a curious phenomenon. 
During the gradual increase of the stimulus from a low 
value there would be apparently no response. But 
Fic. 33.—Rersponsres To INCREASING STIMULI PRODUCED BY INCREASING 
ANGLE OF VIBRATION 
(a) Record with a specimen of fresh radish. Stimuli applied at intervals of two 
minutes. The record is taken for one minute. 
(b) Record for stale radish. There is a reversed response for the feeble stimu- 
lus of 5° vibration. 
when a critical value was reached a maximum response 
would suddenly occur, and would not be exceeded when 
the stimulus was further increased. Here we have 
a parallel to what is known in animal physiology as the 
‘all or none’ principle. With the cardiac muscle, for 
example, there is a certain minimal intensity which 
is eflective in producing response, but further increase 
of stimulus produces no increase in response. 
4. From an inspection of the records of responses 
