28 RESPONSE 1N THE LIVING AND NON-LIVING 
ture. On stimulating the stalk there was produced a 
greater excitation at A than at B, and a current of 
action was then observed to flow in the stalk from the 
more excited A to the less excited B (fig. 6). 
But we can cause this differential action to become 
evident by another means. For example, if we produce 
a block, by clamping at C between A and B (fig. 14, a), 
cs ee so that the disturbance 
2 made at A by tapping or 
vibration is prevented from 
reaching B, we shall then 
have A thrown into a rela- 

tively greater excitatory 
Current of response when 
A eines condition than B. It will 
Current of response when 
ine SOLAS 2 now be found that a cur- 
Fic. 14.—Tur Mrrnop or Brock rent of action flows in the 
(a) The plant is clamped at C, between A gtalk from A to B, that is 
and B. 
(b) Responses obtained by alternately 
stimulating the twoends. Stimulation 
of A produces upward response; of B : ne 
gives downward response. to the less excited. When 
the B end is stimulated, 
there will be a reverse current (fig. 14, 0). 
We have in this method a great advantage over 
to say, from the excited 
that of negative variation, for we can always verify any 
set of results by making corroborative reversal experi- 
ments. 
By the method of injury again, one end is made 
initially abnormal, i.e. different from the condition 
which it maintains when intact. Further, inevitable 
changes will proceed unequally at the injured and 
uninjured ends, and the conditions of the experiment 
may thus undergo unknown variations. But by the 
