Response in the Living and Non-Living | Page 8

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
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 (?sculus 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.
[Illustration: 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 figure is a photographic record of another set of response-curves (fig.?7). The first three responses are for a given intensity of stimulus, and the next six in response to stimulus nearly twice as strong. It will be noticed that fatigue is exhibited in these responses. Other experiments will be described in the next chapter which show conclusively that the response was not due to any accidental circumstance but was a direct result of stimulation. But I shall first discuss the experimental arrangements and method of obtaining these graphic records.
[Illustration: FIG.?7.--RECORD OF RESPONSES IN PLANT (LEAF-STALK OF CAULIFLOWER) BY METHOD OF NEGATIVE VARIATION The first three records are for stimulus intensity 1; the next six are for intensity twice as strong; the successive responses exhibit fatigue. The vertical line to the left represents ·1?volt. The record is