Researches on Irritability of Plants

Fic. 123.—Record showing abolition of polar excitation at high temperature: N, normal response; H, C, alternate ineffec- tive and effective excitations at high and low temperatures respectively. Testing stimulus of kathode-make was kept constant. A series of automatic records are shown in fig. 123, made by the plant. The first record of the series, N, gives the response to the descending make-current ; the amplitude of this is only moderate. The petiole was locally warmed to about 37° C., and an identical stimulus of the make of descending current was applied at a moment marked H. It will be noticed that the stimulus which was formerly effective has now become ineffective.

stimulus of descending make current applied as before at c. It will be seen that not only is the stimulus now effective, but the amplitude of response is much greater than that of normal response at 30° C. In order to test these results further, the experiment was repeated under alternate heat- ing and cooling. It will be noticed that under rising tem- perature there is an invariable failure of excitation, while under lowered temperature excitation occurred always and became maximal. Therefore one of the features which characterise the animal nerve is found also in the conduct- ing-tissue of the plant.

A very striking result of the passage of a constant current is the production of multiple responses in certain rhythmic tissues. Thus the non-ganglionated preparation of the apex of the frog’s heart is under ordinary conditions quiescent, but on the passage of a constant current it breaks forth into a rhythmic series of excitations. | In a subsequent chapter it will be shown that certain vegetable tissues exhibit multiple responses. The pulvinus of the leaflet of Biophytum is multiple-responding and behaves in many respects like the cardiac tissue. Now on maintaining a constant current of certain intensity through the petiole, the kathode being placed on a pulvinus, the particular leaflet will be found to execute a series of pulsatory movements.

Of still greater interest is the indirect effect of such stimulation. A particular leaflet of Biophytum is attached to the recording lever. A constant current is sent for a short time through the petiole, the kathode being at a distance of 10 mm. from the leaflet. The leaflet responded to the excitation caused by kathode-make ; the response here was due to the excitation transmitted through the distance of 10 mm, The first part of fig. 124 gives four

sets of responses to individual stimuli, applied at intervals of 3 minutes. After this the petiole was subjected to the action of a continuous current, represented below the record as a continuous up-line. It will be seen that under the continuous current, rhythmic excitations were initiated at the kathode, which, reaching the responding leaflet, caused multiple responses. There were five such rhythmic responses in the course of Io minutes, the period of each being 2 minutes. The

Fic. 124.—Multiple excitation in Biophytum under constant current. First four records are responses to individual stimuli of electrical current of short duration, applied at intervals of 3 minutes. The last five are multiple responses due to continuous action of current. production of such multiple excitations in a plant under constant current cannot be explained by the theory of hydro-mechanical blow. On the other hand, their simi- larity to corresponding phenomena in animal tissues is sufficiently obvious.

There is a general assumption that the sensitiveness of plant tissue is very much lower than the animal tissue. The question then arises: How sensitive is the plant as compared with the animal in the matter of excitation by a constant current ? of such excitation than the nerve-and-muscle preparation of a frog. This preparation is an exceedingly sensitive detector of induction-current. A shock too feeble to be felt by the intact human subject when passed across the body through two fingers, will, when applied to the exposed nerve of the frog, provoke vigorous movement in the attached muscle. The intact plant, ike the human subject, is not so sensitive to an induction-shock as the. bare nerve.

But the case is different when we come to the question of excitation by a constant current. In order to compare the relative excitatory effects here, in plant and animal, I carried out two different investigations, comparing the sensitiveness of the plant on the one hand with that of frog’s nerve-and-muscle preparation, and on the other, with that of intact human subject. In the first of these I placed an intact petiole of Biophytum in series with a nerve-and-muscle preparation of frog. A gradually increasing E.M.F. was applied by means of a potentiometer-slide arrangement, till one of the two specimens showed excitation at make. The intensity of the exciting current was now measured by means of micro- ammeter. The E.M.F. was then further increased till the second of the two specimens gave excitation at make, and the current again measured. From the value of these two effective currents the relative sensitiveness of the two specimens can be gauged. Instead of measuring the current, we may take, if we wish, the readings of the slide-wire for the two effective values.

These experiments were carried out in August, during the rainy season. As regards the specimen of Biophytum, it should be mentioned that the vitiated atmosphere of a town is particularly inimical to the sensitiveness of this plant. I have succeeded in raising Mimosa, Desmodium, and other sensitive plants in Calcutta, without much loss to their sensitiveness ; but I have invariably failed to do this in the case of Biophytum, It always becomes stunted

and discoloured, and ceases to exhibit its normal motility. The only alternative has been to grow the plant in the suburbs and bring it to the laboratory. Here, with one or two days’ rest after the disturbing effect of transport, it regains a fair degree of sensitiveness, but the effect of the air of the town is to bring about a daily deterioration, and in the course of a week or ten days, except in rare cases, it becomes practically insensitive. It will thus be seen that the experimental plant employed for our comparison may be taken as having had its sensitiveness reduced certainly to half.

In proceeding with the experiment, according to the method already described, it was found that the nerve-and- muscle preparation responded at the make of a current which was as feeble as ‘25 micro-ampere. The Biophytum at this time had not yet responded, but when the current intensity was increased to ‘5 micro-ampere excitation was seen to occur. From this it would appear that Biophytum had in this case fully half the susceptibility of the nerve-and- muscle preparation of frog. Bearing in mind the peculiarly unfavourable character of the circumstances to which the plant was in this case subjected, it does not seem too much to infer that the sensitiveness of the two specimens was not naturally of a different order.

It might perhaps be still more interesting to institute a comparison between the intact plant and the intact human subject. The tongue has always been considered a very sensitive detector of electrical current. According to Laserstein, the acid effect of anode is appreciated by it when the current is only 755 of a milliampere. This is equal to 6°4 micro-amperes. But I have found amongst my pupils some who could perceive by the tongue a current as feeble as 1°5 micro- ampere, whereas the Biophytum in the same circuit responded to the much smaller current of *5 micro-ampere only. This demonstrates that the plant was in this case three times as sensitive as the human tongue, A more excitable specimen

of Biophytum would doubtless prove to be about ten times as sensitive. The improbability of the theory of hydro-mechanical disturbance becomes evident when we realise that an excitatory impulse is initiated and transmitted in the plant under a stimulus that cannot even be perceived by the extremely sensitive human tongue. Nerve-excitation by constant current is enhanced by cooling and depressed by warming. This is one of the characteristics of polar excitation by constant current.

Similarly the excitability of conducting petiole of Mzmosa to polar excitation is enhanced by cold and depressed by warmth. Minimal excitation becomes maximal under cold, and ineffective under warmth. The passage of a constant current through the petiole of Biophytum gives rise to series of multiple excitations. The sensitiveness of Biophytum to an electrical current is remarkably high. Compared with the very sensitive human tongue, the sensitiveness of Biophytum is about ten times as great.

Abnormal polar reactions in Pyvotozoa—Transformation of polar reaction in leaf of Mimosa from Type II. to Type III. under strong current— Further transformation to Type IV. under stronger current—Ex- hibition of Type III. and Type IV. by leaflets of Mimosa, Biophytum, and Averrhoa cavambola—Law of polar action of strong currents. It has been shown in a preceding chapter that the charac- teristic effect of a feeble current in various plants was excitation at make of kathode, but that with a moderate current there was excitation not only at the make of kathode but also at the break of anode. These two types of effect corresponded to those recognised in the case of animal tissues under Pfluger’s Law as Stage I. and Stage II. It will be remembered that in the case of Protozoa the polar reactions have been found to be very different from these. Thus Kihne found that in Actinosphaerium excitatory con- traction took place at the make of both kathode and anode, and that excitation again took place at the break of kathode. The excitatory formula here, then, may be expressed as Km Kb Am.

It will be noticed that we have here the abnormal reactions of anode-make and kathode-break. Verworn has corroborated and extended these observations. He further finds that Amphistigma is excited by both anode and kathode at make. There was apparently no effect at break. The excitation formula in this case is therefore Km Am. From these abnormal effects it has been suggested either that the polar effects in fibrillated and unfibrillated protoplasm are more or less opposed, or that there is no law of polar action which is of universal application.

But we have seen that the unfibrillated protoplasm of the plant exhibits polar effects which are identical with those of animal tissues. It is not impossible that normal polar effects may be subject to modification from the influence of diverse factors, such for example as strength of current, physiological condition of specimen, its age, and the season of the year. When an exhaustive survey has been made, it may be found that the responses of the Protozoa were not, after all, anomalous, but have a place of their own in some transitional type of reaction given by specimens whose characteristics are definitely recognised as normal. With the object of completing such a survey, we may continue our detailed study of the polar reactions of sensitive plants under widely different conditions. And first we shall study the effects of a still further progressive increase of current, in order to see whether any new pheno- menon comes into the field of observation, taking the reactions of the pulvinus of Mzmosa first in the series.

I have employed both the bi-polar and mono-polar methods. The mode of procedure was to increase the current step by step from minimum to maximum, and note the changing types of reaction at different critical points. With vigorous specimens this can be done without any danger of the onset of fatigue. After the value of certain critical points has been determined, experiments were repeated with fresh specimens, at and about these particular critical points. I shall first describe a typical experiment with a very sensitive and vigorous specimen, and for the sake of simplicity I give in detail the effects observed at one pulvinus only, leaving it to be understood that similar effects were also induced at the other.

when the exciting current applied reached the value of 3°5 micro-amperes the characteristic effect of Type I.—namely, excitation at make of kathode Km—made its appearance. When the current was still further raised to 5°6 micro- amperes, the excitatory effect was of Type II.—namely, Km Ab, that is to say, excitation at the make of kathode and break of anode. In these two cases we have Types I. and II., with which we are already familiar. When the progressively increasing current had reached a value of 6:3 micro-amperes, however, a new and unexpected

Fic. 125.—Effect of increasing intensity of current in trans- forming response Km Ab of Type II. to response Km Am Ab of Type III. type of reaction made its appearance. With the bi-polar connections it was found that while one leaf, the kathode, underwent an excitatory fall, the other, or anode, also showed a simultaneous fallat make. After the re-erection of the leaves the current was broken, and now the anodic leaf showed excitation once more, whereas the kathodic exhibited no excitation whatever. In this new type of effect, then, we have excitation both at anode and kathode at make and

at the anode at break only. This therefore constitutes Type III., with the excitatory formula Km Am Ab. This type of excitation will be seen very clearly in the automatic record which is given in fig. 125, taken with a different specimen, where a single pulvinus was carried through a complete cycle of kathode-make, kathode-break, anode-make, and anode-break. It will be seen from the record that while under 6 volts excitation took place at kathode-make and anode-break, under a higher E.M.F. of 10 volts excitation not only took place at kathode-make and anode-break but also at anode-make. Under a stronger current Type II. has thus been transformed to Type III.

Returning to the first specimen, the current was further increased till an intensity was reached of 12°7 micro-amperes, and here was obtained another new type of reaction— namely, excitation at both kathode and anode at both make and break. The excitatory formula for this fourth type is thus Km Kb Am Ab. obtained with a given pulvinus when it was carried through the usual cycle of kathode-make, kathode-break, anode- make, and anode-break. It will be noticed that excitation took place here at each of these phasic changes.

In the case of the particular specimen whose consecutive changes have been traced, from the first type to the fourth, it will be noticed that it was subjected to a long-continued series of experiments. After this, however, I took up fresh specimens, with a view to the immediate observation in their Caiseot Pype lil: and Type IV. There could here be no possible suspicion of changes induced by previous currents too protracted. Thus in a certain fresh specimen the third-stage effect was obtained with a current of 9°I micro- amperes. When this was further increased to 12°6 micro- amperes the excitatory reaction was transformed into that of Type 1V. It will be remembered that with less sensitive specimens a higher intensity of current is necessary for the induction of any particular type of effect. Thus with a certain less sensitive Mimosa a current of I4 micro-amperes was required to induce responses of Type III. The current had in this case to be increased to 32 micro-amperes before the effects were transformed to those of Type IV.

The particular excitations that appear extraordinary in these higher types are those at anode-make and kathode- break. The anode-make effect, as has been shown, can be induced in the initial response of a fresh specimen. But to obtain that at kathode-break—since this presupposes a previous kathode-make which, if sudden, has caused excita- tion—a certain time must be allowed to elapse after make, in order that the leaf under the continuous action of the current may re-erect itself, with restoration of its sensitive- ness. Lest the long-continued action of the current should here have induced some unknown change to which the excitation at kathode-break might be attributable, I have carried out a number of test-experiments in which the kathodic increase of current was made gradually, though

excitation to occur at make. The kathode was then broken suddenly, with the result that excitation was immediately exhibited. Hence the kathode-break effect under strong current must be taken as a normal phenomenon. I have carried out more than fifty different sets of experi- ments on the characteristic effects of relatively strong currents, which have fully confirmed the results described. It was uniformly found that as the current was increased step by step, the transformation took place, as here laid down, to a higher type, and never consisted of reversion to a lower.

Taking these facts, then, as well established, the question arises whether the new, and apparently anomalous, effects cannot be explained in a simple way as a special case of these reactions with which we are already familiar ? Thus the first explanation that occurred to me was that the fall of the leaf at anode-make might be due to the arrival of excitation at the anode from the distant kathode. Against this sup- position, however, was the fact that a strong anode is known to be so depressing as to act as a block to the arrival of any excitation from outside. Overlooking this difficulty, the arrival of excitation from a very distant kathode on a different branch of the plant might be expected to take a certain interval of time. But in the cases given, the excita- tion at make of anode was, to all seeming, instantaneous. Finally, I killed a point on the distant branch by severe scorching, and made it kathode. By this means the possible excitatory effect at kathode was completely eliminated. In spite of this abolition of kathodic action, however, the excitation at anode-make continued to take place as before. Another modification of the experiment lay in previously killing the end of the petiole whose pulvinus was being experimented on. This was done, as before, by scorching. The intense excitation caused by this injury induced an excitatory action at the pulvinus, and it was only after the expiration of about an hour that the organ regained its excitability, and not even then to the fullest extent. The

kathode was placed on the injured point, and the anode at or near the pulvinus; the exciting effect was again obtained at the make of anode. The only difference in this case lay in the fact that, owing to the partial loss of sensi- bility at the pulvinus, the intensity of current for inducing one of the higher types of response had to be increased. From these experiments the new types of effects, Type III. and Type IV., appear to be normal. A further con- sideration of this question will be postponed till I have given in detail the reactions of strong currents with other species of sensitive plants.

I have already mentioned the misgiving I at first enter- tained that the somewhat unexpected appearance of excita- tion at anode-make should in reality be due to transmission from the distant -kathodic-point. I have also described the several means by which these doubts were finally set at rest, one of those being previous injury of the kathodal-point by scorching, in order to eliminate it as a possible source of excitation. I was still desirous, however, of finding some independent means of demonstrating conclusively the independent character of the excitation under a strong current at anode-make.

For this purpose I found the leaflets of Mimosa eminently suitable. The two bi-polar connections are made at about the middle points of two different sub-petioles, care being taken that the contact of each electrode is made on the midrib at the insertion-points of two opposite pulvinules. With this arrangement the character of excitation at anode- make, whether independent or transmitted effect from the distant kathode, can quite easily be discriminated, for it is impossible that any excitation should be transmitted along one sub-petiole to another without causing the fall of the intervening leaflets. In the case of independent excita- tions the fact will be displayed by the immediate fall of

leaflets at the particular points, in this case the two elec- trodes. As regards the transmitted effects of excitation, these can easily be followed by watching the serial fall of leaflets. I shall now describe an experiment in which, the con- nections being as already described, the current was increased step by step to a maximum. Beginning with a current of *75 micro-ampere, it was found that excitation took place only at the kathode-make. When the current had been increased to 2°5 micro-amperes, excitation was initiated at the kathode at make and at the anode at break. In these two cases we have the familiar effects of Type I. and Type II. With the same identical specimen, however, when the current was raised to 5 micro-amperes it was found that excitation was simultaneously initiated at the bi-polar contacts, both kathode and anode, at make. The fact that the anodic fall had nothing to do with any transmission of excitation from the kathode was evident by the slow march of the excitatory waves, sent out from the two points inde- pendently, towards each other. It may be mentioned here that this phenomenon can be made still more prominent by selecting for experiment leaflets which are a little older and not excessively sensitive. In such a case the march of the two waves can be made as slow as may be desired.