sb. 
PLANTS UNDER MECHANICAL STIMULUS. 303 
It has been shown that the electric response is a faithful 
index of physiological action. It does not concern us at present 
to enter into the question as to the mechanism by which the 
response is brought about. On another occasion * I hope to 
bring forward certain facts which bear upon the subject. It 
may, however, be mentioned here, that the explanation offered 
by Kunkel of the response being due to movement of water in 
the plant is inadequate. For in that case we should expect a 
definite stimulation to be under all conditions followed by 
a definite electric response whose sign should remain invariable. 
But we find, instead, the response to be profoundly modified by 
any influence which affects the vitality of the plant. For instance, 
the response is at its maximum at an optimum temperature, 
a rise of a few degrees produces a profound depression, the 
response disappears at the maximum and minimum temperatures 
and is revived when brought back to the optimum. Anesthetics 
and poisons abolish the response. Again, we have the response 
undergoing an actual reversal when the tissue is stale. All 
these facts show that mere movement of water could not be the 
effective cause of the plant-response. 
Physiologists are now agreed that mechanical movement is 
not the only sign by which we may judge whether a tissue is 
or is not irritable. ‘We must be careful not to assume that 
irritability is restricted to motile organs. For all we know 
to the contrary, it is possessed by the protoplasm of all plant 
organs; and if in any case the action of a stimulus is not 
followed by a responsive movement, we must, before we assume 
the absence of irritability, assure ourselves that the structure 
of the organ is such that a movement is a mechanical possi- 
bility a 
We have seen that a far more universal test of the respon- 
Siveness of a tissue is its electro-motility—that is to say, its 
power of exhibiting electromotive change. This is known to be 
possessed by all living animal tissue. 
I have shown that these electric responses are given by all 
Plants and by their different organs. It has also been shown 
that in the matters of response by negative variation, of fatigue, 
* The subject of irritability and response will be found fully treated in my 
Work on “ Response in the Living and Non-Living,” to be shortly published by 
Messrs, Longmans, Green & Co. 
t Vines, Physiology of Plants, p. 372. 
