ive reversal experiments.

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 block method which has just been described,
there is no injury, the plant is normal throughout, and any
physiological change (which in plants will be exceedingly small during
the time of the experiment) will affect it as a whole.

[Illustration: FIG. 15.--RESPONSE IN PLANT (FROM THE STIMULATED A TO
UNSTIMULATED B) COMPLETELY IMMERSED UNDER WATER
The leaf-stalk is clamped securely in the middle with the cork C, inside
the tube T, which is filled with water, the plant being completely
immersed. Moistened threads in connection with the two
non-polarisable electrodes are led to the side tubes _t t'_. One end
of the stalk is held in ebonite forceps and vibrated. A current of
response is found to flow in the stalk from the excited A to the
unexcited B, and outside, through the liquid, from B to A. A portion
of this current, flowing through the side tubes _t t'_, produces
deflection in the galvanometer.]

#Plant response a physiological or vital response.#--I now proceed to a
demonstration of the fact that whatever be the mechanism by which they
are brought about, these plant responses are physiological in their
character. As the investigations described in the next few chapters will
show, they furnish an accurate index of physiological activity. For it
will be found that, other things being equal, whatever tends to exalt or
depress the vitality of the plant tends also to increase or diminish its
electric response. These E.M. effects are well marked, and attain
considerable value, rising sometimes, as has been said before, to as
much as .1 volt or more. They are proportional to the intensity of
stimulus.

It need hardly be added that special precautions are taken to avoid
shifting of contacts. Variation of contact, however, could not in any
case account for repeated transient responses to repeated stimuli, when
contact is made on iso-electric surfaces. Nor could it in any way
explain the reversible nature of these responses, when A and B are
stimulated alternately. These responses are obtained in the plants even
when completely immersed in water, as in the experi