Response in the Living and Non-Living

val between each pair of (a) ) ©) F : . 2 Fic. 20.—Recorp sHowinc DIMINUTION oF stimult, in which the RESPONSE WHEN SuFFIcIeNT TIME Is NoT ° > . ALLOWED FoR Fut Recovery height of twitch does not iets. sist ee In (a) stimuli were applied at intervals of one diminish even after prominute; in (0) the intervals were reduced to half a minute; this caused a diminution of tracted excitation and response. In (c) the original rhythm is re- v 4 / 49 L stored, and the response is found to be enno fatigue appears.’ | Bees haha sv ya Apparent fatigue when stimulation frequency increased.—If the rhythm of stimulation frequency be now changed, and made quicker, certain remarkable modifications will appear in the response-curves. In fig. 20, the first part shows the responses at one minute interval, by which time the individual recovery was complete.

a minute, instead of one, while the stimuli were maintained at the same intensity as before. It will be noticed (fig. 20, 6) that these responses appear much feebler than the first set, in spite of the equality of stimulus. An inspection of the figure may perhaps throw some light on the subject. It will be seen that when greater frequency of stimulation was introduced, the tissue had not yet had time to effect complete recovery from previous strain. The molecular swing towards equilibrium had not yet abated, when the new stimulus, with its opposing impulse, was received. There is thus a diminution of height in the resultant response. The original rhythm of one minute was now restored, and the succeeding curves (fig. 20, c) at once show increased Fic. 21.—Faticue Iv response. An analogous instance may

Vibration of 30° at inter. be cited in the case of muscle re- Ore pene ae Bare sponse, where ‘the height of twitch diminishes more rapidly in proportion as the excitation interval is shorter.’ ! From what has just been said it would appear that one of the causes of diminution of response, or fatigue, is the residual strain. This is clearly seen in fig. 21, in a record which I obtained with celerystalk. It will be noticed there that, owing to the imperfect molecular recovery during the time allowed, the succeeding heights of the responses have undergone a continuous diminution. Fig. 22 gives a

photographic record of fatigue in the leaf-stalk of cauliflower. It is evident that residual strain, other things being equal, will be greater if the stimuli have been excessive. This is well seen in fig. 23, where the set of first three curves Ais for stimulus intensity of 45° vibration, and the second set B, with an . augmented response, for stimulus intensity of 90° vibration. On reverting 1 7 os 6 ro Fic. 22. — Faticur in € to stimulus intensity of 45°, the Ee Ee

responses are seen to have under- _ CX™7EOWEE Stimulus: 30° vibration gone a great diminution as compared ®t, intervals of one F c minute. with the first set a. Here is seen marked fatigue, the result of overstrain from excessive stimulation. If this fatigue be really due to residual strain effect, Fie. 23.—Errect oF OvEeRsTRAIN IN Propucine FaticuE Successive stimuli applied at intervals of one minute. The intensity of stimulus in C is the same as that of A, but response is feebler owing to previous over-stimulation. Fatigue is to a great extent removed after fifteen minutes’ rest, and the responses in D are stronger than those in C. The vertical line between arrows represents ‘05 volt. (Turnip leaf-stalk.) responses to regain their former height after a period of rest. In order to verify this, therefore, I renewed the

will at once be seen from record D how far the fatigue had been removed. One peculiarity that will be noticed in these curves is that, owing to the presence of comparatively little residual strain, the first response of each set is relatively large. The succeeding responses are approximately equal where the residual strains are similar. The first response of A shows this because it had had long previous rest. The first of B shows it because we are

there passing for the first time to increased stimulation. The first of c does not show it, because there Fic. 24.—Rarip FaricuE uNDER Constimulation.—The effect of TINUOUS STIMULATION IN(@) MuscLeE; fatioue is exhibited in (bo) iy Lear-sTaLK OF CELERY 5 marked degree when a tissue is subjected to continuous stimulation. In cases where there is marked fatigue, as for instance in certain muscles, the top of the tetanic curve undergoes rapid decline. A similar effect is obtained also with plants (fie. 24).

The eflect of rest in producing molecular recovery, and hence in the removal of fatigue, is well illustrated in the following set of photographic records (fig. 25). The first shows the curve obtained with a fresh plant. The effect is seen to be very large. Two minutes were allowed for recovery, and then stimulation was repeated during another two minutes. The response in this case is seen to be decidedly smaller. A third case is somewhat similar to the second. A period of rest of five minutes was now allowed, and the curve obtained

Fic. 25.—E¥Frect oF Continuous VIBRATION (THROUGH 50°) IN CaRRoT In the first three records, two minutes’ stimulation is followed by two minutes’ recovery. The last record was taken after the specimen had a rest of five minutes. The response, owing to removal of fatigue by rest, is stronger. subsequently, owing to partial removal of residual strain, is found to exhibit greater response. The results thus arrived at, under the .simple conditions of vegetable life, free as they are from all possible complications and uncertainties, may perhaps throw some light on the obscure phenomena of fatigue in animal tissues.

Diphasic variation—Positive after-effect and positive response— Radial E.M. variation. WuHEN a plant is stimulated at any point, a molecular disturbance—the excitatory wave—is propagated outwards from the point of its initiation. Diphasic variation.—This wave of molecular disturbance is attended by a wave of electrical disturbance. (Usually speaking, the electrical relation between disturbed and less disturbed is that of copper to zinc.) It takes some time for a disturbance to travel from one point to another, and its intensity may undergo a diminution as it recedes further from its pomt of origin. Suppose a disturbance originated at C; if two points are taken near each other, as A and B, the disturbance will reach them almost at the same time, and with the same intensity. The electric disturbance will be the same in both. The effect produced at A and B will balance each other and there will be no resultant current.

By killing or otherwise reducing the sensibility of B as is done in the method of injury, there is no response at B, aud we obtain the unbalanced response, due to disturbance at A; the same effect is obtained by putting a clamp between A and B, so that the disturbance may not reach B. But we may get response even without injury or block. If we have the contacts at A and B, and if we give a tap nearer A than B (fig. 26, a), then we have (1) the disturbance reaching A earlier than B. (2) The disturbance reaching A is much stronger than at B. The disturbance at B may be so comparatively feeble as to be negligible.

It will thus be seen that we might obtain responses even without injury or block, in cases where the disturbance is enfeebled in reaching a distant point. In such a case on giving a tap near A a responsive current would be produced in one direction, and in the opposite direction when the tap is given near B (fig. 26, 6). Theoretically, then, we might find a neutral point between A and B, so that, on originating the disturbance there, the waves of disturbance would reach A and B at the same instant and with the same intensity. If, further, the rate of recovery be the same for both points, then the electric disturbances produced at A and B will continue to balance each other, and the galvanometer will show no current. On taking a cylindrical root of radish I have sometimes succeeded in finding a neutral point, which, being disturbed, did not give rise to any resultant current. But disturbing a point to the right or to the left gave rise to opposite currents.

It is, however, difficult to obtain an absolutely cylindrical specimen, as it always tapers in one direction. The conductivity towards the tip of the root is not exactly the same as that in the ascending direction. It is therefore difficult to fix an absolutely neutral point, but a pot may be found which approaches this very nearly, and on stimulating the stalk near this, a very interesting diphasic variation has been observed. In a specimen of cauliflower-stalk, (1) stimulus was applied very much nearer A than B (the feeble disturbance reaching B was negligible). The resulting response was upward and the recovery took place in about sixty seconds.

(2) Stimulus was next applied near B. The resulting response was now downward (fig. 26, 0). (3) The stimulus was now applied near the approximately neutral point N. In this case, owing to a slight difference in the rates of propagation in the two directions, a very interesting diphasic variation was produced (fig. 26, c). From the record it will be seen that the disturbance arrived earher at A than at B. This produced an upward response. But during the

subsidence of the disturbance at A, the wave reached B. The effect of this was to produce a current in the opposite direction. This apparently hastened the recovery of A (from 60 seconds to 12 seconds). The excitation of A now disappeared, and the second phase of response, that due to excitation of B, was fully displayed. | Positive after-effect.—If we regard the response due to excitation of A as negative, the later effect on B would appear as a subsequent positive variation.

In the response of nerve, for example, where contacts are made at two surfaces, injured and uninjured, there is sometimes observed, first a negative variation, and then a positive after-effect. This may sometimes at least be due to the proximal uninjured contact first giving the usual negative variation, and the more distant contact of injury giving rise, later, to the opposite, that is to* say, apparently positive, response. There is always a chance of an after-effect due to this cause, unless (1) the injured end be completely killed and rendered quite irresponsive, or (2) there be an effective block between A and B, so that the disturbance due to stimulus can only act on one, and not on the other.

I have found cases where, even when there was a perfect block, a positive after-effect occurred. It would thus appear that if molecular distortion from stimulus give rise to a negative variation, then during the process of molecular recovery there may be overshooting of the equilibrium position, which may be exhibited as a positive variation. Positive variation—The responses given by muscle or nerve are, normally speaking, negative. But that of retina is positive. The sign of response, however, is apt to be reversed if there be any molecular modification of the tissue from changes of external circumstances. Thus it is often found that nerve in a stale condition gives positive, instead of the normal negative variation, and stale retina often gives negative, mstead of the usual positive.

Curiously enough, [ have on many occasions found exactly parallel instances in the response of plants. Fic. 27.—ABNORMAL PosiTIvE RESPONSES IN STALE LEAF-STALK OF TURNIP CONVERTED INTO NormMAaL NEGATIVE UNDER STRONG STIMULATION ! The relative intensities of stimuli in the two cases are in the ratio of 1 : 7. Plants when fresh, as stated, give negative responses as a rule. But when somewhat faded they sometimes give rise to positive response. Again, just as in the modified nerve the abnormal positive response gives place to the normal negative under strong and longcontinued stimulation, so also in the modified plant the abnormal positive response passes into negative

1 For general purposes it is immaterial whether the responses are recorded up or down. For convenience of inspection they are in general recorded wp. But in cases where it is necessary to discriminate the sign of response, positive response will be recorded up, and negative down. (fig. 27) under strong stimulation. I was able in some cases to trace this process of gradual reversal, by continuously increasing the intensity of stimulus. It was then found that as the stimulus was increased, the positive at a certain point underwent a reversal into the normal negative response (fig. 28).

The plant thus gives a reversed response under abnormal conditions of staleness. 1 have sometimes Fic. 28.—Apnnormat PosiTIvE PAssinc IntTO Normal NEGATIVE IN A STALE Stimulus was gradually increased from 1 to 10, by means of spring-tapper. When the stimulus intensity was 10, the response became reversed into normal negative. (Parts of 8 and 9 are out of the plate.) found similar reversal of response when the plant is subjected to the abnormal conditions of excessively high or low temperature.

Radial E.M. variation. We have seen that a current of response flows in the plant from the relatively more to the relatively less excited. A theoretically important experiment is the following: A thick stem of plant stalk was taken and a hole bored so as to make one contact with the interior of the tissue, the other being on the surface. After a while the current of injury was found to disappear. On exciting the stem by taps or torsional vibration, a responsive current was observed which flowed inwards from the more disturbed outer surface to the shielded core inside (fig. 29). Henee it is seen that when a wave of disturbance is propagated

Fic. 29.—Rapi1at E.M. Variation along the plant, there is a concomitant wave of radial H.M. variation. The swaying of a tree by the wind Increased response with increasing stimulus—Apparent diminution of response with excessively strong stimulus. As already said, in the living tissue, molecular disturbance induced by stimulus is accompanied by an electric disturbance, which gradually disappears with the return of the disturbed molecules to their position of equilibrium. The greater the molecular distortion produced by the stimulus, the greater is the electric variation produced. The electric response is thus an outward expression of a molecular disturbance produced by an external agency, the stimulus.

Curve of relation between stimulus and response.— In the curve showing the relation between stimulus and response in nerve and muscle, it is found that the molecular effect as exhibited either by contraction or H.M. variation in muscle, or simply by E.M. variation in nerve, is at first slight. In the second part, there is a rapidly increasing effect with increased stimulus. Finally, a tendency shows itself to approach a limit of response. Thus we find the curve at first slightly convex, then straight and ascending, and lastly, concave to the abscissa (fig. 30).

In muscle the limit of response is reached much sooner than in nerve. As will be seen, the range of variation of stimulus in these curves is not very Fia. 30.—CurvES SHOWING THE RELATION BETWEEN THE INTENSITY OF Absciss indicate increasing intensity of stimulus. Ordinates indicate magnitude of response. ( Waller.) jwhen experimenting with plants. These results are suggestive of various types of response met with in animal tissues. 1. In order to obtain the simplest type of effects, not complicated by secondary phenomena, one has to choose specimens which exhibit little fatigue. Having procured these, I undertook two series

of experiments. In the first (4) the stimulus was applied by means of the spring-tapper, and in the second (5) by (A) The first stimulus was given by a fall of the lever through /, the second through 2 h, and so on. The response-curves clearly show increasing eftect with increased stimulus (fig. 31). Fig. 32.—iNcREASED RESPONSE WITH INCREASING VIBRATIONAL STIMULI Stimuli applied at intervals of three minutes. Vertical line='1 volt. 2. The next figure shows how little variation is produced with low value of stimulus, but with increasing stimulus the response undergoes a rapid increase, after which it tends to approach a limit (fie. 33, @).

3. As an extreme instance of the case just cited, I have often come across a curious phenomenon. During the gradual increase of the stimulus from a low value there would be apparently no response. But Fic. 33.—Rersponsres To INCREASING STIMULI PRODUCED BY INCREASING (a) Record with a specimen of fresh radish. Stimuli applied at intervals of two minutes. The record is taken for one minute. (b) Record for stale radish. There is a reversed response for the feeble stimulus of 5° vibration.

when a critical value was reached a maximum response would suddenly occur, and would not be exceeded when the stimulus was further increased. Here we have a parallel to what is known in animal physiology as the ‘all or none’ principle. With the cardiac muscle, for example, there is a certain minimal intensity which is eflective in producing response, but further increase of stimulus produces no increase in response. which are given, it will be seen that the slope of a curve which shows the relation of stimulus to response will at first be slight, the curve will then ascend rapidly, and at high values of stimulus tend to become horizontal. The curve as a whole becomes, first shghtly convex to the abscissa, then straight and ascending, and lastly concave. A far more pronounced convexity in the first part is shown in some cases, especially when the specimen is stale. This is due to the fact that under these circumstances response is apt to begin with an actual reversal of sign, the plant under feebler than a certain critical intensity of stimulus giving positive, instead of the normal negative, response (fig. 33, 6).

Diminution of response with excessively strong stimulus.—It is found that in animal tissues there is sometimes an actual diminution of response with excessive increase of stimulus. Thus Waller finds, in working with retina, that as the intensity of light stimulus is gradually increased, the response at first increases, and then sometimes undergoes a diminution. This phenomenon is unfortunately complicated by fatigue, itself regarded as obscure. It is therefore difficult to say whether the diminution of response is due to fatigue or to some reversing action of an excessively strong stimulus.

From fig. 33, 6, above, it is seen that there was an actual reversal of response in the lower portion of the curve. It is therefore not improbable that there may be more than one point of reversal. In physical phenomena we are, however, acquainted with numerous instances of reversals. For example, a common effect of magnetisation is to produce an elongation of an iron rod. But Bidwell finds that as the magnetising force is pushed to an extreme, at a certain point elongation ceases and is succeeded, with further increase of magnetising force, by an actual contraction. Again a photographic plate,.when exposed continuously to light, gives at first a negative image. Still longer exposure produces a positive. Then again we have anegative. There is thus produced a series of recurrent reversals. In photographic prints of flashes of lightning, two kinds of images are observed, one, the positive—when the lightning discharge is moderately intense—and the other, negative, the so-called ‘ dark lightning "—due to the reversal action of an intensely strong discharge.

In studying the changes of conductivity produced in metallic particles by the stimulus of Hertzian radiation, I have often noticed that whereas feeble radiation produces one effect, strong radiation produces the opposite. Again, under the continuous action of electric radiation, I have frequently found recurrent reversals.’ Diminution of response under strong stimulus traced to fatigue.—But there are instances in plant response where the diminution effect can be definitely traced to fatigue. The records of these cases are extremely suggestive as to the manner in which the diminvtion is brought about. The accompanying figures (fig. 34) give records of responses to increasing stimulus. They were made with specimens of cauliflower-stalks, one of which (a) showed little fatigue, while in the other (4)