Response in the Living and Non-Living

‘001 volt, when turned to the. left one scale riati ns yrod c d division=‘01 volt. P is the plant. va O l uce 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 measure- . ments is to be certain that the imtensity 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 (8), the attached rod (rk) carrying at its end the tapping-head (1). A projecting rod—the lifter (L)—passes through sr. It is provided with a screwthread, 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 (r) 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’ 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

Plant P is securely held by a vice V. The two ends are clamped by holders CC’. 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 (0) shows how the amplitude of vibration is predetermined by means of movable stops S S’. the graduated circle. The amplitude of vibration may be predetermined by means of the sliding stops (ss’). 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

2 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. : 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

The end B is injured. A tap was given between A and B and this gave the response-curve a. A stronger tap gave the response b. By means of the handle H, a torsional vibration of 45° was now imparted, this gave theresponsec. Vibration through 67° gave d. 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.

Fic. 13.—Ineuvence Stimuli, given by vibrations through oF SUDDENNESS ON the same amplitude, but delivered The curves a, b, c,d, are é 3 z - responses to vibra- Thus if we wish to maintain the 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 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.

Method of block—Advantages of block method—Plant response a physiological phenomenon—Abolition of response by anesthetics and poisons —Apbolition of response when plant is killed by hot water. I SHALL now proceed to describe another and independent method which I devised for obtaining plant response. It has the advantage of offering us a complementary means of verifying the results found by the method of negative variation. As it is also, in itself, for reasons which will be shown later, a more perfect mode of inquiry, it enables us to investigate problems which would otherwise have been difficult to attempt. When electrolytic contacts are made on the uninjured surfaces of the stalk at A and B, the two points, being practically similar in every way, are iso-electric, and little or no current will flow in the galvanometer. If now the whole stalk be uniformly stimulated, and if both ends A and B be equally excited at the same moment, it is clear that there will still be no responsive current, owing to balancing action at the two ends. This difficulty as regards the obtaining of response was overcome in the method of negative variation, where the excitability of one end was depressed by chemical reagents or injury, or abolished by excessive tempera-

ture. On stimulating the stalk there was produced a greater excitation at A than at B, and a current of action was then observed to flow in the stalk from the more excited A to the less excited B (fig. 6). But we can cause this differential action to become evident by another means. For example, if we produce a block, by clamping at C between A and B (fig. 14, a), (a) The plant is clamped at C, between A gtalk from A to B, that is and B. (b) Responses obtained by alternately stimulating the twoends. Stimulation

of A produces upward response; of B : ne gives downward response. to the less excited. When the B end is stimulated, there will be a reverse current (fig. 14, 0). We have in this method a great advantage over that of negative variation, for we can always verify any set of results by making corroborative reversal experiments. By the method of injury again, one end is made initially abnormal, i.e. different from the condition which it maintains when intact. Further, inevitable changes will proceed unequally at the injured and uninjured ends, and the conditions of the experiment may thus undergo unknown variations. But by the

small during the time of the experiment) will affect it as a whole. Plant response a physiological or vital response.—I now proceed to a demonstration of the fact that whatever be the mechanism by which they are brought about, these plant responses are physiological in their character. As the investigations described in the next few chapters will show, they furnish an accurate index of physiological activity. For it will be found that, other things being equal, whatever tends to exalt or depress the vitality of the plant tends also to increase or diminish These E.M.

considerable value, rising sometimes, as has been said before, to as much as ‘1 volt or more. They are proportional to the intensity of stimulus. It need hardly be added that special precautions are taken to securely in the middle with the cork C, inside the tube T, which is filled with water, the plant being completely immersed. Moistened threads in connection with the two non-polarisable electrodes are led to the side tubes tt’. One end of the stalk is held in ebonite forceps and vibrated. A currentof response is found to flow in the stalk from the excited A to the unexcited B, and outside, through the liquid, from B to A. A portion of this current, flowing through the side tubes ¢ t’, produces deflection in the galvanometer.

in any way explain the reversible nature of these responses, when A and B are stimulated alternately. These responses are obtained in the plants even when completely immersed in water, as in the experimental arrangement (fig. 15). It will be seen that in this case, where there could be no possibility of shifting of contact, or variation of surface, there is still the usual current of response. I shall describe here a few crucial experiments only, in proof of the physiological character of electric response. The test applied by physiologists, in order to discriminate as to the physiological nature of response, consists in finding out whether the response is diminished or abolished by the action of anzesthetics, poisons, and excessively high temperature, which are known to depress or destroy vitality.

I shall therefore apply these same tests to plant responses. Effect of anesthetics and poisons.—Ordinary anesthetics, like chloroform, and poisons, like mercuric chloride, are known to produce a profound depression or abolish all signs of response in the living tissue. For the purpose of experiment, I took two groups of stalks, with leaves attached, exactly similar to each other in every respect. In order that the leaf-stalks might absorb chloroform I dipped their cut ends in chloroform-water, a certain amount of which they absorbed, the process being helped by the transpiration from the leaves. The second group of stalks was placed simply in water, in order to serve for control experiment. The narcotic action of chloroform, finally

culminating in death, soon became visually evident. The leaves began to droop, a peculiar death-discolouration began to spread from the mid rib along the venation of the leaves. Another peculiarity was also observed. The aphides feeding on the leaves died even before the appearance of the discoloured patches, whereas on the leaves of the stalks placed in water these little creatures maintained their accustomed activity, nor did any discolouration occur. In order to study the effect of poison, another set was placed in water containing a small quantity of mercuric chloride. The leaves here underwent the same change of appearance, and the aphides met with the same untimely fate, as in the case of those subjected to the action of chloroform. There was hardly any visible change in the appearance of the stalks themselves, which were to all outer seeming as living as ever, indications of death being apparent only on the leaf surfaces. I give below the results of several sets of experiments, from which it would appear that whereas there was strong normal response in the group of stalks kept in water, there was practically a total abolition of all response in those ansesthetised or poisoned.

Experiments on the effect of anzsthetics and poisons. A batch of ten leaf-stalks of plane-tree was placed with the cut ends in water, and leaves in air; an equal number was immersed in chloroform-water; a third batch was placed in 5 per cent. solution of mercuric chloride. Similarly a batch of three horse-chestnut leaf-stalks was put in water, another batch in chloroform-water, and a third batch in mercuric chloride solution. abolished. Effect of high temperature.—It is well known that plants are killed when subjected to high temperatures.

vibration as the stimulus, obtained strong responses at both ends A and B. I then immersed the same stalk for a short time in hot water at about 65° C., and again stimulated it as before. But at neither A nor B could any response now be evoked. As all the external conditions were the same in the first and second parts of this experiment, the only difference being that in one the stalk was alive and in the other killed, we have here further and conclusive proof of the physiological character of electric response in plants.

The same facts may be demonstrated in a still more striking manner by first obtaining two similar but opposite responses in a fresh stalk, at A and B, and then killing one half, say B, by immersing only that half of the stalk in hot water. The stalk is replaced in the apparatus, and it is now found that whereas the A half gives strong response, the end B gives none. In the experiments on negative variation, it was tacitly assumed that the variation is due to a differential action, stimulus producing a greater excitation at the uninjured than at the injured end. The block method enables us to test the correctness of this assumption. The B end of the stalk is injured or killed by a few drops of strong potash, the other end being uninjured. There is a clamp between A and B. The end A is stimulated and a strong response is obtained. The end B is now stimulated, and there is little or no response. The block is now removed and the plant stimulated throughout its length. Though the stimulus now acts on both ends, yet, owing to the irresponsive condition of B, there is a resultant response, which from its direction is found

to be due to the responsive action of A. This would not have been the case if the end B had been uninjured. We have thus experimentally verified the assumption that in the same tissue an uninjured portion will be thrown into a greater excitatory state than an ines by the action of the same stimulus. Effect of single stimulus— Superposition of stimuli — Additive effect— Staircase effect—Fatigue—No fatigue when sufficient interval between stimuli—A pparent fatigue when stimulation frequency is increased— Fatigue under continuous stimulation.

Effect of single stimulus.—In a muscle a single stimulus gives rise to a single twitch which may be recorded either mechanically or electrically. If there is no fatigue, the successive responses to uniform stimuli are exactly similar. Muscle when strongly stimulated often exhibits fatigue, and successive responses therefore become feebler and feebler. In nerves, however, there is practically no fatigue and successive records are alike. Similarly, in plants, we shall find some exhibiting marked fatigue and others very little.

Superposition of stimuli.—lf instead of a single stimulus a succession of stimuli be superposed, it happens that a second shock is received before recovery from the first has taken place. Individual effects will then become more or less fused. When the frequency is sufficiently increased, the intermittent effects are fused, and we find an almost unbroken curve. When for example the muscle attains its maximum contraction (corresponding to the frequency and strength of stimuli) it

is thrown into a state of complete tetanus, in which it appears to be held rigid. If the rapidity be not sufficient for this, we have the jagged curve of incomplete tetanus. If there is not much fatigue, the upper part of the tetanic curve is approximately horizontal, but in cases where fatigue sets in quickly, the fact is shown by the rapid decline of the curve. With regard to all these points we find strict parallels in plant response. In cases where there is

@ (6) no fatigue, the successive responses are identical (fig. Fissdly? Shree wees of effects, analogous to the Rapipiy SuccEEpine Srmvxr tetanus of muscle (fig. 17). And lastly, the influence of fatigue in plants is to produce a modification of response-curve exactly similar to that of muscle (see below). One effect of superposition of stimuli may be mentioned here. Additive effect.—It is found in animal responses that there is a minimum intensity of stimulus, below which no response can be evoked. But even a sub-minimal stimulus will, though singly

same stimulus when rapidly super- __ « : te posed thirty times, produced the tissues sometimes exhibit large effect (0). (Leaf-stalk of turnip.) what is known as the ‘ staircase effect,’ that is to say, the heights of successive responses are gradually increased, though the stimuli are maintained constant. This is exhibited typically by cardiac muscle, though it is not unknown even in nerve. The cause is obscure, but it seems to depend on the condition of the tissue. It appears as if the molecular slugeishness of tissue were in these cases only gradually removed under stimulation, and the increased effects were due to increased mole-

explanation, I have sometimes observed the same staircase effect in Fyg-,10-> Stumesse plants (fig. 19). Fatigue.—It is assumed that in living substances like muscle, fatigue is caused by the break down or dissimilation of tissue by stimulus. And till this waste is repaired by the process of building-up or assimilation, the functional activity of the tissue will remain below par. There may also be an accumulation of the products of dissimilation—‘ the fatigue stuffs ’—and these latter may act as poisons or chemical depressants.

In an animal it is supposed that the nutritive blood supply performs the two-fold task of bringing material for assimilation and removing the fatigue products, thus causing the disappearance of fatigue. This explanation, however, is shown to be insufficient by the fact that an excised bloodless muscle recovers from fatigue after a short period of rest. It is obvious that here the fatigue has been removed by means other than that of renewed assimilation and removal of fatigue products by the circulating blood. It may therefore be instructive to study certain phases of fatigue exhibited under simpler conditions in vegetable tissue, where the constructive processes are in abeyance, and there is no active circulation for the removal of fatigue products.

It has been said before that the E.M. variation caused by stimulus is the concomitant of a disturbance of the molecules of the responsive tissues from their normal equilibrium, and that the curve of recovery exhibits the restoration of the tissue to equilibrium. No fatigue when sufficient interval between successive stimuli— We may thus gather from a study of the response-curve some indication of the molecular distortion experienced by the excited tissue. Let us first take the case of an experiment whose. record is given

in fig. 20, a. It will be seen from that curve that one minute after the application of stimulus there is a complete recovery of the tissue; the molecular condition is exactly the same at the end of recovery as in the beginning of stimulation. The second and succeeding response-curves therefore are exactly similar to the first, provided a sufficient interval has been allowed in each case for complete recovery. There is, in such a case, no diminution in intensity of response, that is to say, no fatigue.