Researches on Irritability of Plants

Polar effects of currents on pulsation of Desmodium gyvans—Reduction of systolic contraction by anodic action—Diminution of diastolic expansion by kathodic action—Arrest at systole by make of kathode and diastolic expansion by break of kathode—Arrest at diastole by make of anode, and systolic contraction by break of anode—Effects of ascending and descending currents of feeble and strong intensity in nerve-and-muscle preparation—Parallel effects in petiole-and-pulvinus,

WE have seen that under currents of moderate intensity the polar effects of anode and kathode are more or less antithetic. The kathode excites at make, whereas the anode excites at break. If the kathode at make induces contraction, then it would appear probable that at the make of anode there may be produced an expansion. At break, again, supposing the same antithesis to be maintained, the contractile effect at the anode will have a contrasted effect of expansion at the kathode.

Biedermann has shown that in a beating heart the point of application of the anode, during make, remains expanded as a dark red blistered swelling, even when the rest of the heart, during contraction, is becoming pallid. This shows that during the continuation of the anode an expansive reaction is induced in the tissue. An effect similar to this is induced at the kathodic point at break. I have been able to demonstrate the contrasted effects of anode and kathode, in a still more striking manner, in the case of rhythmic pulsation of the leaflet of Desmodium gyrans.

In a future chapter it will be shown how remarkable are the similarities of the rhythmic tissue of the plant and animal. In the case of the Desmodium leaflets the pulsations are seen to take place in a very regular manner, the period of one complete pulsation being about 3 minutes. Of the up and down movements of the leaflet, the down movement takes place more quickly, corresponding to the systolic con- traction of the rhythmic cardiac tissue. The systolic move- ment in Desmodium takes place in the course of about I minute and Io seconds, the slower diastolic expansion being accomplished in the course of about I minute and 50 seconds. In the record, the quicker systolic movement is represented by the up-curve. The extent of contraction is thus repre- sented by the upper limit of the curve of response ; the lower limit indicates the extent of diastolic expansion. If the leaflet is executing its greatest possible amplitude of pulsa- tion, then an external agent will be unable to increase it any further; but the extent of contraction or expansion can be individually reduced under agencies which have an opposing tendency. This if the continuation of anode tends to induce expansion, then during a cycle of pulsating activity it would oppose the contraction at the systolic phase. The effect would be a diminution of contraction ; in the record this would appear as progressive diminution of heights of responses.

If the application of kathode, on the other hand, induces contraction, this would oppose the diastolic expansion ; the amplitude of pulsation will be progressively diminished, with continuously diminishing relaxations, and the base-line would be shifted upwards. In order to demonstrate these contrasted polar reactions the experiment was carried out by making the pulvinule of the leaflet alternately anode or kathode. One electric connection is made with the pulvinule by means of a thin thread, special care being taken that this inno way interfered with the free pulsation of the leaflet ; the second connection is made with an indifferent point lower down in the petiole.

I shall first describe the expansive effect of anode-make and the contractile effect of kathode-make ; these particular effects are easy to demonstrate, the current applied being of moderate intensity. In fig. 116 is seen the anodic effect ; the first two pulsations are normal, after which the anode was applied. This is seen to result in a continuous lessening of the height of successive contractions. The application of kathode, on the other hand, is found to induce a precisely opposite effect (fig. 117); here the diastolic expansion is opposed, which results in a con- tinuously diminishing relaxation. In fig. 116 a line joining

contraction in pulsation of posing expansion. Note the Desmodium gyvans. Note the gradual diminution of diastolic gradual diminution of systolic expansion. the apices of successive contractions is seen to descend, while in fig. 117, under the action of kathode, the line joining the extreme points of the diastolic excursion is seen to ascend. The same fact is seen again in fig. 118, where a single specimen is subjected first to anode and then to kathode. The contrasted effects of anode and kathode in this case are very obvious.

By employing a suitable intensity of current it is some- times possible to exhibit the contrasted effects of the kathode and anode by the actual arrest of pulsation. By the make of kathode the arrest takes place towards systole, and by the make of anode towards diastole. In such cases it is possible to demonstrate further the remarkable effects of the break of kathode and break of anode. The difficulty in this experiment lies in the fact that the application of an intensity of current, greater than is exactly sufficient to induce the arrest, is apt to bring about fatigue, with the abolition of pulsation. I have several times succeeded in

Fic. 118.—Alternate effects of anode and kathode in dimin- ishing systolic contraction and diastolic expansion. inducing an arrest which was not followed by a permanent abolition of excitability. In these cases it is possible to exhibit the very interesting effects of break of kathode and of anode. In fig. 11g is seen the arrest at systole induced Fic. 119.—Arrest at systole by Fic. 120.—Arrest at diastole the make of kathode and by the make of anode and diastolic expansion as im- systolic contraction as im- mediate effect of break of mediate effect of break kathode. of anode.

by the make of kathode. The current was then broken, and as an immediate result an expansive or diastolic phase of pulsation was obtained, followed by several other pulses of somewhat diminished amplitude. The effect of break of anode, on the other hand, is exactly opposite. In fig. 120 the pulsation was arrested towards diastole by the make of anode. The break of anode was immediately attended by an abrupt termination of the standstill. A pulse of systolic contraction followed, and was succeeded by the renewal of ordinary pulsation. It is thus seen that while the make of kathode and break of anode induce contraction, the make of anode and break of kathode induce the reverse effect of expansion.

The contrasted effects of anode and kathode, and of make and break, are also exhibited by induced variations of excitability. These are well seen in characteristic differences of effects induced by a constant current in a muscle-and-nerve preparation, these being modified by the strength of current. Thus employing a feeble current it is found that both ascending and descending currents in the nerve induce excitation of the terminal muscle at make but not at break. With strong currents, on the other hand, excitation only takes place at the make of the descending current and break of the ascending current. We shall presently see how these different effects are explicable on the assumption of the contrasted effects of anode and kathode, and of make and break.

The obscurity of the subject lies in the fact that there is no visible indication of an excitatory change in the nerve. The case would have been different had the conducting- tissue been provided with indicators which could signal the passage of excitation. Such a conducting-tissue is provided by the petiole of Bzophytum, where the successive closures of the lateral leaflets indicate the transmission of excitation through the central conducting-strand. We shall see how the effects visually manifested in Biophytum elucidate the characteristic effects observed in the nerve-and-muscle preparation.

Fig. 121 shows arrangements of parallel experiments in nerve of frog and conducting petiole of Biophytum. The polarising electrodes are applied to the right, the terminal motile organ, muscle or leaflet, being tothe left. The series to the left represent the effect of descending, those to the right the effect of ascending, current. In the case of the descending current the terminal indicator is to the left of kK, and the conducting tissue between K and A. In the case of the ascending current the terminal indicator is to the left of A. Transmitted excitation can be detected by the

Fic. 121.—Effects of Descending and Ascending currents at make and break on nerve-muscle and petiole-pulvinus. Excited leaflets shaded dark. Vertical series to left represent effects of descending currents ; series to right effects of ascending currents. With feeble current, terminal leaflets excited by both descending and ascending currents at make and not at break. With strong currents, excitation of terminal leaflets takes place only at make of descending and break of ascending currents. Two upper pairs of leaves exhibit effects of feeble current at make and break. The two lower pairs of leaves show effects of strong current.

contraction of muscle or leaflet, to the left of K in the case of descending, and to the left of A in the case of ascending current. But the characteristic effect at the electrodal points, K and A themselves, or the overflow of the effect between the two points, cannot be detected in the case of frog’s nerve. It can, however, be detected in the case of conducting-tissue of Biophytwm on account of the presence of the lateral motile-leaflets. I will now describe experiments carried out on Biophy- tum with feeble and strong currents, and show how they throw light on parallel experiments with nerve-and-muscle preparation of animal. In Biophytum the excited leaflets are indicated by dark shading, which will also clearly exhibit the extent of excitatory overflow. The point of initiation of excitation, and direction of transmission, are indicated by arrows.

In the nerve-and-muscle preparation it is known that (1) excitation takes place at the make of descending current ; (2) excitation also occurs at the make of ascending current ; (3) no excitation occurs at the break of descending, (4) or ascending, currents. Accounts of parallel experiments with Biophytum will now be described :— (1) Make of descending current—The kathode is proximal to the extra-polar or terminal leaflets to the left. At make, excitation is seen to be initiated only at K, the two waves proceeding in opposite directions. Five pairs of leaflets thus fall in the extra-polar and three pairs in the intra-polar regions. This excitatory overflow in the intra- polar region cannot be observed in the animal nerve on account of absence of a visible indicator.

(2) Make of ascending current.—In fixing our attention to the terminal or extra-polar leaflets to the left of A, we find occurrence of excitation corresponding to the excitation of muscle at the make of the ascending current. That the excitation was really initiated at the distal kathode, and traversed without hindrance through the feeble anode, will clearly be seen from the serial closure of the leaflets initiated at K. Eight pairs of leaflets underwent excitatory fall to the left, and one pair to the right of K.

There is no excitation, since feeble anode does not excite at break. We now take up the question of the effect of strong currents :-— (1) Make of descending current.—In the nerve-and-muscle preparation there is excitation of muscle at the make. Here the kathode is proximal, and we obtain the normal excitatory effect of kathode-make. In the corresponding experiment with Biophytum we find excitation transmitted to the terminal or extra-polar region to the left, five pairs of leaflets undergoing closure. Only two pairs of leaflets closed in the intra-polar region, further progress of excitation being arrested by the depressing action of the anode.

(2) Make of ascending current.—Unlike the action of feeble current, there is no excitatory effect in the nerve- and-muscle preparation at the make of strong ascending current. This is explained on the supposition that the excitation at the distal kathode cannot traverse the region of the strong anode with its depressed excitability. The proof of this assumption is strikingly afforded by the corresponding experiment with Biophytuwm. We observe the initiation of excitation at the kathode, but the pro- gress of excitation is arrested near the region of anode. Hence, in spite of the occurrence of excitation, there could be no transmitted effect in the terminal extra-polar region.

(3) Break of descending current.—In a nerve-and-muscle preparation there is no excitation at break of a descending current. Here, though the distal anode excites at break, the intervening kathodic region is assumed to undergo depression at break. Hence a block occurs to the trans- mission of excitation. with Biophytum. We here observe excitation initiated at the anode at break, two pairs of leaflets undergoing closure, and the further progress of the excitatory wave is arrested before reaching the depressed region of kathode- break.

(4) Break of ascending current.—Excitation occurs in a nerve-and-muscle preparation at the break of strong ascend- ing current. Here the anode is proximal, and excitation induced at break reaches the terminal organ without hindrance. In the corresponding experiment with Biophytum we observe five pairs of leaflets undergoing closure in the terminal extra-polar region, the excitation being initiated at the anode at break. Excitation also traversed the intra-polar region, two pairs of leaflets undergoing closure. Further progress in this direction was, however, arrested by the depressing action of kathode-break.

The contrasted effects of anode and kathode are exhibited by appropriate modification in the pulsating activity of Desmodium gyrans. The anodic effect of expansion is seen in the reduction of normal limit of systolic contraction. The kathodic effect of contraction is observed in the reduction of normal limit of diastolic expansion. The immediate effect of break is the reverse of that at make or continuation of current. The diastolic arrest by anode is followed at its break by systolic contraction.

The systolic arrest by kathode is succeeded at its break by diastolic expansion. In nerve-and-muscle preparation the effects of ascending and descending currents are found modified by the intensity of the current. Effects in every way parallel are observed in experimenting with petiole-pulvinus of Biophytum. These characteristic modifications are easily traceable in Biophytum to the contrasted effects of anode and kathode, and of make and break. Excitability is enhanced by the make of kathode and break of anode. It is depressed by the make of anode and break of kathode.

Excitability of conducting tissue to induction-shock diminished by cooling— Nerve-excitation by constant current enhanced by cooling—Excitation of conducting-tissue of Mimosa by constant current enhanced by cooling and depressed by warming—Ineffective stimulus becoming effective under cooling and vice vevsa—Multiple response induced in Biophytum by the passage of constant current—Comparison of sensitiveness of plant and animal—Minimum current for excitation of human tongue—Relatively higher sensitiveness of Biophytum.

that due to temperature—appeared at first very puzzling. The temperature of Calcutta in summer is high. More- subjected in summer was often as high as 35° C. In these circumstances it was found that the indirect stimulation of the pulvinus of Mimosa by the action of constant current often became ineffective. It was quite easy in spring to excite the leaf of Mimosa by the transmitted excitation due to the make of kathode when the kathodic point was at a distance of several centimetres from the pulvinus. But in summer there was hardly any transmitted excitation even when the exciting kathode was at a comparatively short distance from the pulvinus. This ineffectiveness might be due to the impairment of conductivity or excitability. It could not be due to the

loss of conductivity, for we have seen in a previous chapter that the conductivity is enhanced by a rise of temperature. The pulvinus, again, was found extremely sensitive. Yet, in spite of the high conductivity and motile excitability, the transmitted effect of excitation was often found ineffective at a high temperature. Thus the only remaining factor to which the change might be attributed was the excitability of the conducting tissue itself. Taking the parallel case of the animal nerve, it is known that the excitability of a nerve is diminished by local cooling, the stimulus being that due to break induction- shock. But Gotch and Macdonald have made the very interesting observation that the effect is reversed in the case of stimulation by constant current ; here the nerve excita- bility is enhanced by lowering of temperature. By employ- ing a descending current—with the kathode proximal to the contractile muscle—they found that the make-excitation which was ineffective when the nerve was locally warmed to 30° C. became effective when locally cooled to 5° C.

It occurred to me that the failure of indirect stimulation by the closure of constant current in Mimosa might be due to the depression of excitability of the conducting petiole, in consequence of the high temperature. Should this prove to be the case, then this specific reaction would afford a very striking demonstration of the characteristic similarities in the conducting-tissues of the animal and plant. The effect of cold in modifying the excitability of the conducting animal nerve is, as said before, dependent on the mode of stimulation. We shall now see whether this holds good in the case of the plants also. First, in order to determine the effect of cold on the exciting efficiency of the break induction-shock, the two electrodes from the secondary coil were placed on the petiole, one 20 mm. from the pulvinus and the other further away. It has been shown that with a single induction-shock it is the kathode which causes excitation. The electrodes of the secondary coil are so connected with the petiole as to render the proximal contact

the kathode, and therefore the point of excitation. Records are then taken under the same stimulus alternately, (1) with the excited point as warm as the general temperature of the room, which was 30° C., and (2) with the tempera- ture lowered to about 5° C. by the application of cooled water. It is essential that the cooling should be effected gradually, for sudden variation of temperature of itself causes excitation. The series of records in fig. 122 shows the result. It is seen that while the excitation is effective at H, H, H, when

Fic. 122.—Effect of cold on excitability to induction-shock : n, CG, alternate effective and ineffective excitation at moderately warm and low temperatures respectively. Testing stimulus, a single break induction-shock was maintained constant. the stimulated point is warm, it becomes ineffective at C, C, when the excited point is cooled. This proves that in the conducting-tissue of the plant lowering of temperature depresses the excitatory efficiency of a break induction-shock.

For studying the effect of cold on the exciting efficiency of the constant current, I next made suitable electrical connection with two points on the petiole, the proximal kathode, kK, being at a distance of Io mm. from the pulvinus. Excitation was produced by the make of the descending current, An E.M,F, was applied which caused minimal response ; this was found to be 2 volts, the temperature of the room being 30° C. The point K was now alternately raised and lowered in temperature. This was effected by means of a stream of hot or cold water applied at the point. It is essential that the warming or cooling should be effected gradually, for, as stated above, sudden variation of temperature of itself causes excitation.