RESPONSEIN THELIVING AND NON-LIVING

One of the most striking effects of external disturbance on certain types of living substance is a visible change of form. Thus, a piece of muscle when pinched contracts. The external disturbance which produced this change is called the stimulus. The body which is thus capable of responding is said to be irritable or excitable. A stimulus thus produces a state of excitability which may sometimes be expressed by change of form.

Mechanical response to different kinds of stimuli.—This reaction under stimulus is seen even in the lowest organisms; in some of the amœboid rhizopods, for instance. These lumpy protoplasmic bodies, usually elongated while creeping, if mechanically jarred, contract into a spherical form. If, instead of mechanicaldisturbance, we apply salt solution, they again contract, in the same way as before. Similar effects are produced by sudden illumination, or by rise of temperature, or by electric shock. A living substance may thus be put into an excitatory state by either mechanical, chemical, thermal, electrical, or light stimulus. Not only does the point stimulated show the effect of stimulus, but that effect may sometimes be conducted even to a considerable distance. This power of conducting stimulus, though common to all living substances, is present in very different degrees. While in some forms of animal tissue irritation spreads, at a very slow rate, only to points in close neighbourhood, in other forms, as for example in nerves, conduction is very rapid and reaches far.

The visible mode of response by change of form may perhaps be best studied in a piece of muscle. When this is pinched, or an electrical shock is sent through it, it becomes shorter and broader. A responsive twitch is thus produced. The excitatory state then disappears, and the muscle is seen to relax into its normal form.

Mechanical lever recorder.—In the case of contraction of muscle, the effect is very quick, the twitch takes place in too short a time for detailed observation by ordinary means. A myographic apparatus is therefore used, by means of which the changes in the muscle are self-recorded. Thus we obtain a history of its change and recovery from the change. The muscle is connected to one end of a writing lever. When the muscle contracts, the tracing point is pulled up in onedirection, say to the right. The extent of this pull depends on the amount of contraction. A band of paper or a revolving drum-surface moves at a uniform speed at right angles to the direction of motion of the writing lever. When the muscle recovers from the stimulus, it relaxes into its original form, and the writing point traces the recovery as it moves now to the left, regaining its first position. A curve is thus described, the rising portion of which is due to contraction, and the falling portion to relaxation or recovery. The ordinate of the curve represents the intensity of response, and the abscissa the time (fig. 1).

Fig. 1.—Mechanical Lever RecorderFig. 1.—Mechanical Lever RecorderThe muscle M with the attached bone is securely held at one end, the other end being connected with the writing lever. Under the action of stimulus the contracting muscle pulls the lever and moves the tracing point to the right over the travelling recording surface P. When the muscle recovers from contraction, the tracing point returns to its original position. See on P the record of muscle curve.

Fig. 1.—Mechanical Lever Recorder

The muscle M with the attached bone is securely held at one end, the other end being connected with the writing lever. Under the action of stimulus the contracting muscle pulls the lever and moves the tracing point to the right over the travelling recording surface P. When the muscle recovers from contraction, the tracing point returns to its original position. See on P the record of muscle curve.

Characteristics of the response-curve: (1) Period, (2) Amplitude, (3) Form.—Just as a wave of sound is characterised by its (1) period, (2) amplitude, and (3) form, so may these response-curves be distinguished from each other. As regards the period, there is an enormous variation, corresponding to the functional activity of the muscle. For instance, in tortoise it may be as high as a second, whereas in the wing-muscles of many insects it is as small as 1/300 part of a second. ‘It is probable that a continuous graduated scale might, as suggested by Hermann, be drawn up in the animal kingdom, from the excessively rapid contraction ofinsects to those of tortoises and hibernating dormice.’[1]Differences in form and amplitude of curve are well illustrated by various muscles of the tortoise. The curve for the muscle of the neck, used for rapid withdrawal of the head on approach of danger, is quite different from that of the pectoral muscle of the same animal, used for its sluggish movements.

Again, progressive changes in the same muscle are well seen in the modifications of form which consecutive muscle-curves gradually undergo. In a dying muscle, for example, the amplitude of succeeding curves is continuously diminished, and the curves themselves are elongated. Numerous illustrations will be seen later, of the effect, in changing the form of the curve, of the increased excitation or depression produced by various agencies.

Thus these response records give us a means of studying the effect of stimulus, and the modification of response, under varying external conditions, advantage being taken of the mechanical contraction produced in the tissue by the stimulus. But there are other kinds of tissue where the excitation produced by stimulus is not exhibited in a visible form. In order to study these we have to use an altogether independent method, the method of electric response.

FOOTNOTES:[1]Biedermann,Electro-physiology, p. 59.

[1]Biedermann,Electro-physiology, p. 59.

Unlike muscle, a length of nerve, when mechanically or electrically excited, does not undergo any visible change. That it is thrown into an excitatory state, and that it conducts the excitatory disturbance, is shown however by the contraction produced in an attached piece of muscle, which serves as an indicator.

But the excitatory effect produced in the nerve by stimulus can also be detected by an electrical method. If an isolated piece of nerve be taken and two contacts be made on its surface by means of non-polarisable electrodes atAandB, connection being made with a galvanometer, no current will be observed, as bothAandBare in the same physico-chemical condition. The two points, that is to say, are iso-electric.

If now the nerve be excited by stimulus, similar disturbances will be evoked at bothAandB. If, further, these disturbances, reachingAandBalmost simultaneously, cause any electrical change, then,similar changes taking place at both points, and there being thus no relative difference between the two, the galvanometer will still indicate no current. This null-effect is due to the balancing action ofBas againstA. (Seefig. 2,a.)

Conditions for obtaining electric response.—If then we wish to detect the response by means of the galvanometer, one means of doing so will lie in the abolition of this balance, which may be accomplished by making one of the two points, sayB, more or less permanently irresponsive. In that case, stimulus will cause greater electrical disturbance at the more responsive point, sayA, and this will be shown by the galvanometer as a current of response. To makeBless responsive we may injure it by means of a cross-sectional cut, a burn, or the action of strong chemical reagents.

Fig. 2.—Electric Method of Detecting Nerve ResponseFig. 2.—Electric Method of Detecting Nerve Response(a) Iso-electric contacts; no current in the galvanometer.(b) The end B injured; current of injury from B to A: stimulation gives rise to an action current from A to B.(c) Non-polarisable electrode.

Fig. 2.—Electric Method of Detecting Nerve Response

(a) Iso-electric contacts; no current in the galvanometer.(b) The end B injured; current of injury from B to A: stimulation gives rise to an action current from A to B.(c) Non-polarisable electrode.

Current of injury.—We shall revert to the subject of electric response; meanwhile it is necessary to say a few words regarding the electric disturbance caused by the injury itself. Since the physico-chemical conditions of the uninjuredAand the injuredBare now no longer the same, it followsthat their electric conditions have also become different. They are no longer iso-electric. There is thus a more or less permanent or resting difference of electric potential between them. A current—the current of injury—is found to flowin the nerve, from the injured to the uninjured, and in the galvanometer, through the electrolytic contacts from the uninjured to the injured. As long as there is no further disturbance this current of injury remains approximately constant, and is therefore sometimes known as ‘the current of rest’ (fig. 2,b).

A piece of living tissue, unequally injured at the two ends, is thus seen to act like a voltaic element, comparable to a copper and zinc couple. As some confusion has arisen, on the question of whether the injured end is like the zinc or copper in such a combination, it will perhaps be well to enter upon this subject in detail.

If we take two rods, of zinc and copper respectively, in metallic contact, and further, if the pointsAandBare connected by a strip of clothsmoistened with salt solution, it will be seen that we have a complete voltaic element. A current will now flow fromBtoAin the metal (fig. 3,a) and fromAtoBthrough the electrolytes. Or instead of connectingAandBby a single strip of cloths, we may connect them by two stripss s′, leading to non-polarisable electrodesE E′. The current will then be found just the same as before, i.e. fromBtoAin the metallic part, and fromAthroughs s′toB, the wireWbeing interposed, as it were, in the electrolytic part of the circuit. If now a galvanometer be interposed atO, the current will flow fromBtoAthrough the galvanometer, i.e. from right to left. But if we interpose the galvanometer in the electrolytic part of the circuit, that is to say, atW, the same current will appear to flow in the opposite direction. Infig. 3,c, the galvanometer is so interposed, and in this case it is to be noticed that when the current in the galvanometer flows from left to right, the metal connected to the left is zinc.

Comparefig. 3,d, whereA Bis a piece of nerve of which theBend is injured. The current in the galvanometerthrough the non-polarisable electrode is from left to right. The uninjured end is therefore comparable to the zinc in a voltaic cell (is zincoid), the injured being copper-like or cuproid.[2]

Fig. 3.—Diagram showing the Correspondence between injured (B) and uninjured (A) contacts in Nerve, and Cu and Zn in a Voltaic ElementFig. 3.—Diagram showing the Correspondence between injured (B) and uninjured (A) contacts in Nerve, and Cu and Zn in a Voltaic ElementComparison of (c) and (d) will show that the injured end of B in (d) corresponds with the Cu in (c).

Fig. 3.—Diagram showing the Correspondence between injured (B) and uninjured (A) contacts in Nerve, and Cu and Zn in a Voltaic Element

Comparison of (c) and (d) will show that the injured end of B in (d) corresponds with the Cu in (c).

If the electrical condition of, say, zinc in the voltaic couple (fig. 3,c) undergo any change (and I shall show later that this can be caused by molecular disturbance), then the existing difference of potential betweenAandBwill also undergo variation. If for example the electrical condition ofAapproach that ofB, the potential difference will undergo a diminution, and the current hitherto flowing in the circuit will, as a consequence, display a diminution, ornegativevariation.

Action current.—We have seen that a current of injury—sometimes known as ‘current of rest’—flows in a nerve from the injured to the uninjured, and that the injuredBis then less excitable than the uninjuredA. If now the nerve be excited, there being a greatereffect produced atA, the existing difference of potential may thus be reduced, with a consequent diminution of the current of injury. During stimulation, therefore, a nerve exhibits a negative variation. We may express this in a different way by saying that a ‘current of action’ was produced in response to stimulus, and acted in an opposite direction to the current of injury (fig. 2,b). The action current in the nerveis from the relatively more excited to the relatively less excited.

Difficulties of present nomenclature.—We shall deal later with a method by which a responsive current of action is obtained without any antecedent current of injury. ‘Negative variation’ has then no meaning. Or, again, a current of injury may sometimes undergo a change of direction (see note,p. 12). In view of these considerations it is necessary to have at our disposal other forms of expression by which the direction of the current of response can still be designated. Keeping in touch with the old phraseology, we might then call a current ‘negative’ that flowed from the more excited to the less excited. Or, bearing in mind the fact that an uninjured contact acts as the zinc in a voltaic couple, we might call it ‘zincoid,’ and the injured contact ‘cuproid.’ Stimulation of the uninjured end, approximating it to the condition of the injured, might then be said to induce a cuproid change.The electric change produced in a normal nerve by stimulation may therefore be expressed by saying that there has been a negative variation, or that there was a current of action from the more excited to the less excited, or that stimulation has produced a cuproid change.

The electric change produced in a normal nerve by stimulation may therefore be expressed by saying that there has been a negative variation, or that there was a current of action from the more excited to the less excited, or that stimulation has produced a cuproid change.

The excitation, or molecular disturbance, produced by a stimulus has thus a concomitant electrical expression. As the excitatory state disappears with the return of the excitable tissue to its original condition, the current of action will gradually disappear.[3]The movement of the galvanometer needle during excitation of the tissue thus indicates a molecular upset by the stimulus; and the gradual creeping back of the galvanometer deflection exhibits a molecular recovery.

This transitory electrical variation constitutes the ‘response,’ and its intensity varies according to that of the stimulus.

Electric recorder.—We have thus a method of obtaining curves of response electrically. After all, it is not essentially very different from the mechanical method. In this case we use a magnetic lever (fig. 4,a), the needle of the galvanometer, which is deflected by the electromagnetic pull of the current, generated under the action of stimulus, just as the mechanical lever was deflected by the mechanical pull of the muscle contracting under stimulus.

The accompanying diagram (fig. 4,b) shows how,under the action of stimulus, the current of rest undergoes a transitory diminution, and how on the cessation of stimulus there is gradual recovery of the tissue, as exhibited in the return of the galvanometer needle to its original position.

Fig. 4.—Electric RecorderFig. 4.—Electric Recorder(a) M muscle; A uninjured, B injured ends. E E′ non-polarising electrodes connecting A and B with galvanometer G. Stimulus produces ‘negative variation’ of current of rest. Index connected with galvanometer needle records curve on travelling paper (in practice, moving galvanometer spot of light traces curve on photographic plate). Rising part of curve shows effect of stimulus; descending part, recovery.(b) O is the zero position of the galvanometer; injury produces a deflection A B; stimulus diminishes this deflection to C; C D is the recovery.

Fig. 4.—Electric Recorder

(a) M muscle; A uninjured, B injured ends. E E′ non-polarising electrodes connecting A and B with galvanometer G. Stimulus produces ‘negative variation’ of current of rest. Index connected with galvanometer needle records curve on travelling paper (in practice, moving galvanometer spot of light traces curve on photographic plate). Rising part of curve shows effect of stimulus; descending part, recovery.

(b) O is the zero position of the galvanometer; injury produces a deflection A B; stimulus diminishes this deflection to C; C D is the recovery.

Two types of response—positive and negative.—It may here be added that though stimulus in general produces a diminution of current of rest, or a negative variation (e.g. muscles and nerves), yet, in certain cases, there is an increase, or positive variation. This is seen in the response of the retina to light. Again, a tissue which normally gives a negative variation may undergo molecular changes, after which it gives a positive variation. Thus Dr. Waller finds that whereas fresh nerve always gives negative variation, stale nerve sometimes gives positive; and that retina, which when fresh gives positive, when stale, exhibits negative variation.

The following is a tabular statement of the two types of response:

I.Negative variation.—Action current from more excited to less excited—cuproid change in the excited—e.g. fresh muscle and nerve, stale retina.

II.Positive variation.—Action current from less excited to more excited—zincoid change in the excited—e.g. stale nerve, fresh retina.[4]

From this it will be seen that it is the fact of the electrical response of living substances to stimulus that is of essential importance, the signplusorminusbeing a minor consideration.

Universal applicability of the electrical mode of response.—This mode of obtaining electrical response is applicable to all living tissues, and in cases like that of muscle, where mechanical response is also available, it is found that the electrical and mechanical records are practically identical.

The two response-curves seen in the accompanying diagram (fig. 5), and taken from the same muscle by the two methods simultaneously, clearly exhibit this. Thus we see that electrical response can not only take the place of the mechanical record, but has the furtheradvantage of being applicable in cases where the latter cannot be used.

Electrical response: A measure of physiological activity.—These electrical changes are regarded as physiological, or characteristic of living tissue, for any conditions which enhance physiological activity also,pari passu, increase their intensity. Again, when the tissue is killed by poison, electrical response disappears, the tissue passing into an irresponsive condition. Anæsthetics, like chloroform, gradually diminish, and finally altogether abolish, electrical response.

Fig. 5.—Simultaneous Record of the Mechanical (M) and (E) Electrical Responses of the Muscle of Frog. (Waller.)Fig. 5.—Simultaneous Record of the Mechanical (M) and (E) Electrical Responses of the Muscle of Frog. (Waller.)

Fig. 5.—Simultaneous Record of the Mechanical (M) and (E) Electrical Responses of the Muscle of Frog. (Waller.)

From these observed facts—that living tissue gives response while a tissue that has been killed does not—it is concluded that the phenomenon of response is peculiar to living organisms.[5]The response phenomena that we have been studying are therefore considered as due to some unknown, super-physical ‘vital’ force and are thus relegated to a region beyond physical inquiry.

It may, however, be that this limitation is not justified, and surely, at least until we have explored the whole range of physical action, it cannot be asserted definitely that a particular class of phenomena is by its very nature outside that category.

Electric response in plants.—But before we proceed to the inquiry as to whether these responses are or are not due to some physical property of matter, and are to be met with even in inorganic substances, it will perhaps be advisable to see whether they are not paralleled by phenomena in the transitional world of plants. We shall thus pass from a study of response in highly complex animal tissues to those given under simpler vital conditions.

Electric response has been found by Munck, Burdon-Sanderson, and others to occur in sensitive plants. But it would be interesting to know whether these responses were confined to plants which exhibit such remarkable mechanical movements, and whether they could not also be obtained from ordinary plants where visible movements are completely absent. In this connection, Kunkel observed electrical changes in association with the injury or flexion of stems of ordinary plants.[6]My own attempt, however, was directed, not towards the obtaining of a mere qualitative response, but rather to the determination of whether throughout the whole range of response phenomena a parallelism between animal and vegetable could be detected. That is tosay, I desired to know, with regard to plants, what was the relation between intensity of stimulus and the corresponding response; what were the effects of superposition of stimuli; whether fatigue was present, and in what manner it influenced response; what were the effects of extremes of temperature on the response; and, lastly, if chemical reagents could exercise any influence in the modification of plant response, as stimulating, anæsthetic, and poisonous drugs have been found to do with nerve and muscle.

If it could be proved that the electric response served as a faithful index of the physiological activity of plants, it would then be possible successfully to attack many problems in plant physiology, the solution of which at present offers many experimental difficulties.

With animal tissues, experiments have to be carried on under many great and unavoidable difficulties. The isolated tissue, for example, is subject to unknown changes inseparable from the rapid approach of death. Plants, however, offer a great advantage in this respect, for they maintain their vitality unimpaired during a very great length of time.

In animal tissues, again, the vital conditions themselves are highly complex. Those essential factors which modify response can, therefore, be better determined under the simpler conditions which obtain in vegetable life.

In the succeeding chapters it will be shown that the response phenomena are exhibited not only by plants but by inorganic substances as well, and that theresponses are modified by various conditions in exactly the same manner as those of animal tissues. In order to show how striking are these similarities, I shall for comparison place side by side the responses of animal tissues and those I have obtained with plants and inorganic substances. For the electric response in animal tissues, I shall take the latest and most complete examples from the records made by Dr. Waller.

But before we can obtain satisfactory and conclusive results regarding plant response, many experimental difficulties will have to be surmounted. I shall now describe how this has been accomplished.[7]

FOOTNOTES:[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 excitedpositive, and towards the less excitednegative.[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, inBrain, 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.

[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.

‘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 excitedpositive, and towards the less excitednegative.

[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, inBrain, 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.

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, sayB, by the application of a few drops of strong potash, the area atAbeing left uninjured. A current is now observed to flow, in the stalk, from the injuredBto the uninjuredA, 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.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 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 beingdecreased 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.

Fig. 7.—Record of Responses in Plant (Leaf-stalk of Cauliflower) by Method of Negative VariationFig. 7.—Record of Responses in Plant (Leaf-stalk of Cauliflower) by Method of Negative VariationThe 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 to be read from right to left.

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 to be read from right to left.

Response recorder.—The galvanometer used is a sensitive dead-beat D’Arsonval. The period of complete swing of the coil under experimentalconditions is about 11 seconds. A current of 10-9ampere 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 mirrorM. 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, andas 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.

Fig. 8.—Response RecorderFig. 8.—Response Recorder

Fig. 8.—Response Recorder

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 onvariationof 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.

Fig. 9.—The CompensatorFig. 9.—The CompensatorA B is a stretched wire with added resistances R and R′. S is a storage cell. When the key K is turned to the right one scale division = ·001 volt, when turned to the left one scale division = ·01 volt. P is the plant.

Fig. 9.—The Compensator

A B is a stretched wire with added resistances R and R′. S is a storage cell. When the key K is turned to the right one scale division = ·001 volt, when turned to the left one scale division = ·01 volt. P is the plant.

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.

Fig. 10.—The Spring-tapperFig. 10.—The Spring-tapper

Fig. 10.—The Spring-tapper

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 throughS 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.

Fig. 11.—The Torsional VibratorFig. 11.—The Torsional VibratorPlant P is securely held by a vice V. The two ends are clamped by holders C C′. By means of handles H H′, torsional vibration may be imparted to either the end A or end B of the plant. The end view (b) shows how the amplitude of vibration is predetermined by means of movable stops S S′.

Fig. 11.—The Torsional Vibrator

Plant P is securely held by a vice V. The two ends are clamped by holders C C′. By means of handles H H′, torsional vibration may be imparted to either the end A or end B of the plant. The end view (b) shows how the amplitude of vibration is predetermined by means of movable stops S S′.

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 ofvibration. 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 infig. 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.

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