#Fatigue under continuous stimulation.#--The effect of fatigue is exhibited in 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 (fig. 24).

[Ill.u.s.tration: FIG. 24.--RAPID FATIGUE UNDER CONTINUOUS STIMULATION IN (_a_) MUSCLE; (_b_) IN LEAF-STALK OF CELERY]

The effect of rest in producing molecular recovery, and hence in the removal of fatigue, is well ill.u.s.trated 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 subsequently, owing to partial removal of residual strain, is found to exhibit greater response.

[Ill.u.s.tration: FIG. 25.--EFFECT 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.]

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.

FOOTNOTES:

[10] Biedermann, _Electro-physiology_, p. 86.

[11] Biedermann, _loc. cit._

CHAPTER VI

PLANT RESPONSE--ON DIPHASIC VARIATION

Diphasic variation--Positive after-effect and positive response--Radial E.M. variation.

When 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 point 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, and 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, _b_). 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 point 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.

[Ill.u.s.tration: FIG. 26.--DIPHASIC VARIATION]

(2) Stimulus was next applied near B. The resulting response was now downward (fig. 26, _b_).

(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 earlier 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 over-shooting 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 circ.u.mstances. 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, instead of the usual positive.

[Ill.u.s.tration: FIG. 27.--ABNORMAL POSITIVE RESPONSES IN STALE LEAF-STALK OF TURNIP CONVERTED INTO NORMAL NEGATIVE UNDER STRONG STIMULATION[12]

The relative intensities of stimuli in the two cases are in the ratio of 1:7.]

Curiously enough, I have on many occasions found exactly parallel instances in the response of plants. 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 long-continued stimulation, so also in the modified plant the abnormal positive response pa.s.ses into negative (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).

[Ill.u.s.tration: FIG. 28. ABNORMAL POSITIVE Pa.s.sING INTO NORMAL NEGATIVE IN A STALE SPECIMEN OF LEAF-STALK OF CAULIFLOWER 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.)]

The plant thus gives a reversed response under abnormal conditions of staleness. I have sometimes 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). Hence it is seen that when a wave of disturbance is propagated along the plant, there is a concomitant wave of radial E.M. variation. The swaying of a tree by the wind would thus appear to give rise to a radial E.M.F.

[Ill.u.s.tration: FIG. 29.--RADIAL E.M. VARIATION]

FOOTNOTES:

[12] For general purposes it is immaterial whether the responses are recorded up or down. For convenience of inspection they are in general recorded _up_. But in cases where it is necessary to discriminate the sign of response, positive response will be recorded up, and negative down.

CHAPTER VII

PLANT RESPONSE--ON THE RELATION BETWEEN STIMULUS AND RESPONSE

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 E.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 great. When the stimulus is carried beyond moderate limits, the response, owing to fatigue or other causes, may sometimes undergo an actual diminution.

[Ill.u.s.tration: FIG. 30.--CURVES SHOWING THE RELATION BETWEEN THE INTENSITY OF STIMULUS AND RESPONSE Abscissae indicate increasing intensity of stimulus. Ordinates indicate magnitude of response. (Waller.)]

I have obtained very interesting results, with reference to the relation between stimulus and response, when 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 (_A_) the stimulus was applied by means of the spring-tapper, and in the second (_B_) by torsional vibration.

[Ill.u.s.tration: FIG. 31 Taps of increasing strength 1:2:3:4 producing increased response in leaf stalk of turnip.]

(_A_) The first stimulus was given by a fall of the lever through _h_, the second through 2 _h_, and so on. The response-curves clearly show increasing effect with increased stimulus (fig. 31).