Chapter 23

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 (_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.

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.

#Response recorder.#--The galvanometer used is a sensitive dead-beat D'Arsonval. The period of complete swing of the coil under experimental conditions is about 11 seconds. A current of 10^{-9} ampere produces a deflection of 1 mm. at a distance of 1 metre. For a quick and accurate method of obtaining the records, I devised the following form of response recorder. The curves are obtained directly, by tracing the excursion of the galvanometer spot of light on a revolving drum (fig. 8). The drum, on which is wrapped the paper for receiving the record, is driven by clockwork. Different speeds of revolution can be given to it by adjustment of the clock-governor, or by changing the size of the driving-wheel. The galvanometer spot is thrown down on the drum by the inclined mirror M. The galvanometer deflection takes place at right angles to the motion of the paper. A stylographic pen attached to a carrier rests on the writing surface. The carrier slides over a rod parallel to the drum. As has been said before, the galvanometer deflection takes place parallel to the drum, and as long as the plant rests unstimulated, the pen, remaining coincident with the stationary galvanometer spot on the revolving paper, describes a straight line. If, on stimulation, we trace the resulting excursion of the spot of light, by moving the carrier which holds the pen, the rising portion of the response-curve will be obtained. The galvanometer spot will then return more or less gradually to its original position, and that part of the curve which is traced during the process constitutes the recovery. The ordinate in these curves represents the E.M. variation, and the abscissa the time.

We can calibrate the value of the deflection by applying a known E.M.F. to the circuit from a compensator, and noting the deflection which results. The speed of the clock is previously adjusted so that the recording surface moves exactly through, say, one inch a minute. Of course this speed can be increased to suit the particular experiment, and in some it is as high as six inches a minute. In this simple manner very accurate records may be made. It has the additional advantage that one is able at once to see whether the specimen is suitable for the purpose of investigation. A large number of records might be taken by this means in a comparatively short time.

#Photographic recorder.#--Or the records may be made photographically. A clockwork arrangement moves a photographic plate at a known uniform rate, and a curve is traced on the plate by the moving spot of light. All the records that will be given are accurate reproductions of those obtained by one of these two methods. Photographic records are reproduced in white against a black background.

#Compensator.#--As the responses are on _variation_ of current of injury, and as the current of injury may be strong, and throw the spot of light beyond the recording surface, a potentiometer balancing arrangement may be used (fig. 9), by which the P.D. due to injury is exactly compensated; E.M. variations produced by stimulus are then taken in the usual manner. This compensating arrangement is also helpful, as has been said before, for calibrating the E.M. value of the deflection.

#Means of graduating the intensity of stimulus.#--One of the necessities in connection with quantitative measurements is to be certain that the intensity of successive stimuli is (1) constant, or (2) capable of gradual increase by known amounts. No two taps given by the hand can be made exactly alike. I have therefore devised the two following methods of stimulation, which have been found to act satisfactorily.

#The spring-tapper.#--This consists (fig. 10) of the spring proper (S), the attached rod (R) carrying at its end the tapping-head (T). A projecting rod--the lifter (L)--passes through S R. It is provided with a screw-thread, by means of which its length, projecting downwards, is regulated. This fact, as we shall see, is made to determine the height of the stroke. (C) is a cogwheel. As one of the spokes of the cogwheel is rotated past (L), the spring is lifted and released, and (T) delivers a sharp tap. The height of the lift, and therefore the intensity of the stroke, is measured by means of a graduated scale. We can increase the intensity of the stroke through a wide range (1) by increasing the projecting length of the lifter, and (2) by shortening the length of spring by a sliding catch. We may give isolated single taps or superpose a series in rapid succession according as the wheel is rotated slow or fast. The only disadvantage of the tapping method of stimulation is that in long-continued experiment the point struck is liable to be injured. The vibrational mode of stimulation to be presently described labours under no such disadvantage.

#The electric tapper.#--Instead of the simple mechanical tapper, an electromagnetic tapper may be used.

#Vibrational stimulus.#--I find that torsional vibration affords another very effective method of stimulation (fig. 11). The plant-stalk may be fixed in a vice (V), the free ends being held in tubes (C C'), provided with three clamping jaws. A rapid torsional vibration[9] may now be imparted to the stalk by means of the handle (H). The amplitude of vibration, which determines the intensity of stimulus, can be accurately measured by the graduated circle. The amplitude of vibration may be predetermined by means of the sliding stops (S S').

#Intensity of stimulus dependent on amplitude of vibration.#--I shall now describe an experiment which shows that torsional vibration is as effective as stimulation by taps, and that its stimulating intensity increases, length of stalk being constant, with amplitude of vibration. It is of course obvious that if the length of the specimen be doubled, the vibration, in order to produce the same effect, must be through twice the angle. I took a leaf-stalk of turnip and fixed it in the torsional vibrator. I then took record of responses to two successive taps, the intensity of one being nearly double that of the other. Having done this, I applied to the same stalk two successive torsional vibrations of 45° and 67° respectively. These successive responses to taps and torsional vibrations are given in fig. 12, and from them it will be seen that these two modes of stimulation may be used indifferently, with equal effect. The vibrational method has the advantage over tapping, that, while with the latter the stimulus is somewhat localised, with vibration the tissue subjected to stimulus is uniformly stimulated throughout its length.

#Effectiveness of stimulus dependent on rapidity also.# In order that successive stimuli may be equally effective another point has to be borne in mind. In all cases of stimulation of living tissue it is found that the effectiveness of a stimulus to arouse response depends on the rapidity of the onset of the disturbance. It is thus found that the stimulus of the 'break' induction shock, on a muscle for example, is more effective, by reason of its greater rapidity, than the 'make' shock. So also with the torsional vibrations of plants, I find response depending on the quickness with which the vibration is effected. I give below records of successive stimuli, given by vibrations through the same amplitude, but delivered with increasing rapidity (fig. 13).

Thus if we wish to maintain the effective intensity of stimulus constant we must meet two conditions: (1) The amplitude of vibration must be kept the same. This is done by means of the graduated circle. (2) The vibration period must be kept the same. With a little practice, this requirement is easily fulfilled.

The uniformity of stimulation which is thus attained solves the great difficulty of obtaining reliable quantitative values, by whose means alone can rigorous demonstration of the phenomena we are studying become possible.

FOOTNOTES:

[8] A preliminary account of Electric Response in Plants was given at the end of my paper on 'Electric Response of Inorganic Substances' read before the Royal Society on June 6, 1901; also at the Friday Evening Discourse, Royal Institution, May 10, 1901. A more complete account is given in my paper on 'Electric Response in Ordinary Plants under Mechanical Stimulus' read before the Linnean Society March 20, 1902.

I thank the Royal Society and the Linnean Society for permission to reproduce some of my diagrams published in their _Proceedings_.--J. C. B.

[9] By this is meant a rapid to-and-fro or complete vibration. In order that successive responses should be uniform it is essential that there should be no resultant twist, i.e. the plant at the end of vibration should be in exactly the same condition as at the beginning.