We have in this method a great advantage over that of negative variation, for we can always verify any set of results by making corroborative 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.
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 experimental arrangement ([fig. 15]). It will be seen that in this case, where there could be no possibility of shifting of contact, or variation of surface, there is still the usual current of response.
I shall describe here a few crucial experiments only, in proof of the physiological character of electric response. The test applied by physiologists, in order to discriminate as to the physiological nature of response, consists in finding out whether the response is diminished or abolished by the action of anæsthetics, poisons, and excessively high temperature, which are known to depress or destroy vitality.
I shall therefore apply these same tests to plant responses.