PLANTS UNDER MECHANICAL STIMULUS. 279 
Negative Variation in Plants. 
Taking the leaf-stalk of turnip, we kill an area on its surface, say 
B, by a burn or application of a few drops of strong potash, the 
area A being left uninjured (fig.2). A current is now observed 
Fig. 20. 
— 
. / ‘] volt 4 
r™~ 
A B 
Response by method of negative 
variation in plants. Contacts 
shown in this and other figures 
are diagrammatic. They are 
securely made by tying strips 
of cloth moistened with NaCl 
solution round A and B. These 
lead to non-polarizable elec- 
trodes, 
o- 
to flow, in the stalk, from the injured B to the uninjured A, as 
is found to be the case in the animal tissue. The Potential- 
Difference depends on the condition of the plant and the season 
in which it may have been gathered. In the experiment here 
described its value was *12 volt. 
A sharp tap was now given to the stalk, and a sudden 
diminution, or negative variation, of current occurred, the 
resting potential-difference being decreased by °026 volt. The 
transitory E.M. variation gradually disappeared with the 
recovery of the tissue from the excitation caused by the stimulus. 
A second, and stronger, tap produced a second response, causing 
& greater diminution of PD. by ‘047 volt (fig. 26), The accom- 
