Response in the Living and Non-Living. Jagadis Chandra Bose

will have to be surmounted. I shall now describe how this has been accomplished.[7]

      FOOTNOTES:

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      [2] In some physiological text-books much wrong inference has been made, based on the supposition that the injured end is zinc-like.

[3] ‘The exciting cause is able to produce a particular molecular rearrangement in the nerve; this constitutes the state of excitation and is accompanied by local electrical changes as an ascertained physical concomitant.’

      ‘The excitatory state evoked by stimulus manifests itself in nerve fibres by E.M. changes, and as far as our present knowledge goes by these only. The conception of such an excitable living tissue as nerve implies that of a molecular state which is in stable equilibrium. This equilibrium can be readily upset by an external agency, the stimulus, but the term “stable” expresses the fact that a change in any direction must be succeeded by one of opposite character, this being the return of the living structure to its previous state. Thus the electrical manifestation of the excitatory state is one whose duration depends upon the time during which the external agent is able to upset and retain in a new poise the living equilibrium, and if this is extremely brief, then the recoil of the tissue causes such manifestation to be itself of very short duration.’—Text-book of Physiology, ed. by Schäfer, ii. 453.

[4] I shall here mention briefly one complication that might arise from regarding the current of injury as the current of reference, and designating the response current either positive or negative in relation to it. If this current of injury remained always invariable in direction—that is to say, from the injured to the uninjured—there would be no source of uncertainty. But it is often found, for example in the retina, that the current of injury undergoes a reversal, or is reversed from the beginning. That is to say, the direction is now from the uninjured to the injured, instead of the opposite. Confusion is thus very apt to arise. No such misunderstanding can however occur if we call the current of response towards the more excited positive, and towards the less excited negative.

[5] ‘The Electrical Sign of Life … An isolated muscle gives sign of life by contracting when stimulated … An ordinary nerve, normally connected with its terminal organs, gives sign of life by means of muscle, which by direct or reflex path is set in motion when the nerve trunk is stimulated. But such nerve separated from its natural termini, isolated from the rest of the organism, gives no sign of life when excited, either in the shape of chemical or of thermic changes, and it is only by means of an electrical change that we can ascertain whether or no it is alive … The most general and most delicate sign of life is then the electrical response.’—Waller, in Brain, pp. 3 and 4. Spring 1900.

[6] Kunkel thought the electric disturbance to be due to movement of water through the tissue. It will be shown that this explanation is inadequate.

[7] My assistant Mr. J. Bull has rendered me very efficient help in these experiments.

CHAPTER III

       ELECTRIC RESPONSE IN PLANTS—METHOD OF NEGATIVE VARIATION

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       Negative variation

       —Response recorder

       —Photographic recorder

       —Compensator

       —Means of graduating intensity of stimulus

       —Spring-tapper and torsional vibrator

       —Intensity of stimulus dependent on amplitude of vibration

       —Effectiveness of stimulus dependent on rapidity also.

      I shall first proceed to show that an electric response is evoked in plants under stimulation.[8]

      In experiments for the exhibition of electric response it is preferable to use a non-electrical form of stimulus, for there is then a certainty that the observed response is entirely due to reaction from stimulus, and not, as might be the case with electric stimulus, to mere escape of stimulating current through the tissue. For this reason, the mechanical form of stimulation is the most suitable.

      I find that all parts of the living plant give electric response to a greater or less extent. Some, however, give stronger response than others. In favourable cases, we may have an E.M. variation as high as ·1 volt. It must however be remembered that the response, being a function of physiological activity of the plant, is liable to undergo changes at different seasons of the year. Each plant has its particular season of maximum responsiveness. The leaf-stalk of horse-chestnut, for example, exhibits fairly strong response in spring and summer, but on the approach of autumn it undergoes diminution. I give here a list of specimens which will be found to exhibit fairly good response:

      Root.—Carrot (Daucus Carota), radish (Raphanus sativus).

      Stem.—Geranium (Pelargonium), vine (Vitis vinifera).

      Leaf-stalk.—Horse-chestnut (Æsculus Hippocastanum), turnip (Brassica Napus), cauliflower (Brassica oleracea), celery (Apium graveolens), Eucharis lily (Eucharis amazonica).

      Flower-stalk.—Arum lily (Richardia africana).

      Fruit.—Egg-plant (Solanum Melongena).

      Negative variation.—Taking the leaf-stalk of turnip we kill an area on its surface, say B, by the application of a few drops of strong potash, the area at A being left uninjured. A current is now observed to flow, in the stalk, from the injured B to the uninjured A, as was 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 (fig. 6, a) its value was ·13 volt.

      Fig. 6.—(a) Experiment for Exhibiting Electric Response in Plants by Method of Negative Variation. (b) Responses in Leaf-stalk of Turnip to Stimuli of Two Successive Taps, the Second being Stronger.

      A and B contacts are about 2 cm. apart, B being injured. Plant is stimulated by a tap between A and B. Stimulus acts on both A and B, but owing to injury of B, effect at A is stronger and a negative variation due to differential action occurs.

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. A second and stronger tap produced a second response, causing a greater diminution of P.D. by ·047 volt (fig. 6, b). The accompanying