Researches on Irritability of Plants

If the phenomenon of transmission in the plant is one of protoplasmic change, then any factor that causes physiological variation must have a corresponding influence on its velocity. One such cause of physiological variation is found in change of temperature. If, on the other hand, the propagation had been merely of a hydrostatic blow, then a change of temperature would not have had any marked effect upon it. Thus the accurate determina- tion of the influence of temperature upon velocity of transmission becomes an important consideration in dis- criminating between the excitatory or mechanical nature of the transmitted change.

the variation of conductivity, under variation of tempera- ture. The plant was maintained at the required tempera- tures in the thermal chamber, either by the cooling device or by the electrical appliances for heating, which have been previously described. In fig. 88 time-records are given of the transmission of excitation at the three temperatures of 22° C., 28° C., and 31° C. This experiment was carried out in the Calcutta winter, when the temperature of the room was 22°C. The normal velocity of transmission in the plant was thus, owing to the season, somewhat low. Stimulus of maximal intensity 2 was applied, at a distance of Io mm. from the responding point. The lowest of the

Fic. 88.—Effect of temperature in enhancing velocity of transmission, Three records, from below upwards, are for temperatures 22° C., 28° C., and 31° C. respectively. three records gives us the period of transmission at the temperature of 22° C. The next record was taken at 28° C., and the third or topmost at 31° C. It is quite evident from these figures that the velocity is continuously increased under rising temperature. The period taken at 22° C. was 2:94 seconds; at 28° C., 1°69 second; and at 31° C., 12 second. We noted in a former experiment that the latent period undergoes a variation with changes of temperature. Thus in a given experiment, while the latent period at 23° C. was ‘165 second, at 28°C. it was ‘12 second, and at 33°C., ‘065 second. These variations are very slight as compared with the total time required for the

transmission of excitation. Hence if, in the results of the last experiment, we make the small corrections representing the variations of the latent period, the velocity of trans- mission at 22° C. will be found to be 3°6 mm. per second, at 28° C. it is increased to 6°3 mm. per second; and at 31° C. it has become g mm. per second. Thus at 31° C. it is two and a half times as great as the velocity at 22° C. The results of this and a few of many other experi- ments on the influence of temperature on velocity are given in the following table. It need only be said that the effect of rising temperature was always to induce an increase in the velocity of transmission.

Employing the method that has been described I have determined the velocity of transmission of impulse in the petiole of Biophytum, the average value of which is about 2 mm. per second. Fig. 89 shows a record obtained with a typical specimen. Stimulus was applied at a distance of 50 mm. from the responding leaflet. The record was taken by means of the Oscillating Recorder, the successive dots being at an interval of a second. It will be seen that the response took place 24°5 seconds after the appli- cation of stimulus. Making allowance for the latent period, the average value of which in Buophytum is

Fic. 89.—Record giving transmission-time in Biophytum : Successive dots at intervals of a second. ‘4 second, the velocity of transmission in this particular case is 2°I mm. per second. The velocity in the petiole of Averrhoa carambola varies from ‘5 to I mm. per second. In connection with the determination of velocity of transmission of excitation in Biophytum, I made the discovery of the curious phenomenon of preferential conductivity. It was found that the state of excitation travelled through the conducting petiole of Biophytum with greater facility in one direction than in the opposite. The experiment was carried out by employing a leaflet, situated midway in the petiole, as the motile indicator. Equal stimuli were

alternately applied at equal distances to the right and to the left of the leaflet. In one case the excitatory wave was transmitted in a centripetal direction, that is to say, towards the stem or main axis of the plant. In the other case, excitation travelled outwards towards the tip of the leaf or in a centrifugal direction. From a large number of experiments carried out in this manner it was found that the velocity is greater in the centrifugal direction.

By applying stimuli of constant intensity, and by allowing proper intervals of rest, successive values of velocity of transmission of excitation are obtained which are constant. Consistent results are also obtained by the employment of Differential Method for the determination of velocity of transmission. The automatic records afford measurement of time as short as ‘05 second. The highest velocity of transmission of excitation that has been found in the petiole of Mimosa is 30 mm. per second.

In a sub-tonic tissue the velocity of transmission of excitation is enhanced under increased intensity of stimulus. The tissue becomes a better conductor of excitation in consequence of previous stimulation, In a tissue in optimum condition the velocity of trans- mission is the same under feeble and strong stimulation. Velocity of transmission of excitation is enhanced at a higher temperature. Excitation is transmitted in both directions; but the velocity is not necessarily the same in the two cases. In Biophytum the velocity in the centrifugal direction is greater than in the centripetal.

The hydro-mechanical theory—Inconclusive character of the anesthetic experiment of Pfeffer and scalding experiment of Haberlandt— Kihne’s experiment showing transmission of excitation under intense stimulation in a rigored nerve—Error introduced by employment of excessive intensities of stimulus—Discriminative polar effect of current in excitation—Block of transmission of excitation by local application of cold—Restoration of normal conductivity by tetanising shock in tissue paralysed by cold—Electrotonic arrest of excitatory impulse—Action of various poisons in inducing block of conduction.

In the previous chapter I referred to the prevailing belief that the transmitted impulse in the plant was hydro- mechanical, unlike the excitatory impulse in the animal nerve. This view has been largely based on two well-known experiments of Pfeffer and Haberlandt. In the former of these, the effect of strong stimulus was found to travel over chloroformed parts of the petiole. Pfeffer assumed that the conductivity of this portion must have been abolished, since chloroform is known to abolish motile excitability. In the experiment of Haberlandt an intervening tissue was killed by scalding ; in spite of this, stimulus was found to be transmitted across the scalded area.

From these two experiments it was inferred that the impulse which was transmitted could not have been of a true excitatory nature. It was held that, instead of this, a strong stimulus had given rise to a variation whether of increase or diminution of hydrostatic pressure. This variation of pressure, it was assumed, had been hydro- mechanically transmitted, and on reaching the distant pulvinus had inflicted on it a blow which had proved as effective as if a mechanical stimulus had been applied locally. It is thus held that in Mimosa there is a mere transmission of stimulus but no transmission of excitation.

I shall, however, be able to show that the two experiments referred to are not as conclusive as has been supposed. But before doing this it is as well to examine the basis of the hydro-mechanical theory. According to the Dutrochet- Pfeffer theory, migration of water is the sole cause of pro- pagation of stimulus, transmission being due to movement of water in the vascular bundles. When a wound is made in the stem causing an incision of a vascular bundle, fluid exudes from the wound on account of which there is a diminution of the hydrostatic equilibrium in the bundle. There may, again, be a propagation of stimulus caused by exciting the pulvinus, when a certain quantity of fluid given out by the excited parenchyma is supposed to pass into the vascular bundle.

According to Haberlandt, flaccidity ensues in the sensi- tive parenchyma on direct stimulation of an articulation. Owing to the deformation of the cells a pressure is induced in the conducting-tissues which is propagated along them and which, reaching a new pulvinus, stimulates it as if by a blow from without. Haberlandt compares the transmission of pressure in the plant with that in an indiarubber tube filled with water.! But transmission through long, and more or less closed, capillary tubes is not the same as that through uninterrupted

1 “Tt is still harder to explain the mechanism by which a stimulus is propagated from the relaxed parenchyma of the curving pulvinus to the excitable parenchyma of an adjacent joint, after a single mechanical stimu- lus or with chemical or thermic excitation. . . And when Haberlandt compares the resulting movement of the sap “ to that within an india- rubber tube containing water at a given hydrostatic pressure in which increase of pressure at any point is propagated in the form of an undulatory wave from one end to the other,” the anatomical relations of the con- ducting-cells hardly seem to justify such a presumption. The experiments on the conductivity of Mimosa would have to be scrupulously repeated

before forming any final judgment.—Biedermann: Electro-physiology, vol. ii. p. 16 (Macmillan), indiarubber tube having a large bore. In the former case a considerable mechanical disturbance would be necessary to start the hydrostatic wave which can effectively reach-a distant point. Haberlandt supposes such a mechanical disturbance to be brought about by deformation of the mass of parenchyma in the stimulated pulvinus or by injury of the stem or petiole. But it is not at all necessary to initiate the excitatory impulse in Mimosa by stimulating the pulvinus; such an impulse may be originated in the thin petiole where there is no turgid mass of parenchyma to be deformed. Excitation may be caused, moreover, by the agency of a physiological stimulus which does not cause any injury or give rise to any mechanical disturbance.

Again, as regards the question as to whether the trans- mitted variation of pressure would always form an efficient cause of excitation, it was found by Macdougal that sudden artificial variation, whether by increase or diminution of hydrostatic pressure, brought about no responsive fall of the leaf of Mimosa.1 We now return to the detailed consideration of Pfeffer’s experiment on anesthetics and Haberlandt’s on scalding. As regards the former it has been assumed that the conduct- ing-power was arrested under chloroform. It has, however, been pointed out by Vines that though a narcotised pulvinus certainly loses its motile excitability, it does not necessarily follow that its conductivity likewise is completely abolished. In fact, instances are known to physiologists in which a tissue whose excitability has been abolished will still persist in maintaining its conducting-power. This circumstance may be demonstrated in the case of plants by taking a specimen of Biophytum and applying a strong stimulus to an old leaf the motility of whose leaflets has been abolished on account of age. Though its own leaflets do not afford any motile indication, the excitation is found conducted through the petiole of the old leaf, inducing the fall of the leaflets in a neighbouring young leaf.

' Pfeffer: Physiology of Plants, vol. iii. p. 95 (Clarendon Press). It is also extremely doubtful whether in the particular experiment with Mimosa the conducting-tissue in the in- terior could have been effectively narcotised by the external application of the anesthetic. The task would almost be as difficult as narcotising a nerve-trunk lying between muscles, by the application of chloroform on the skin out- side! In the case of the plant it is conceivable that after a very long application a small quantity of narcotic may, by absorption, get access to the internal conducting-tissue ; but narcotisation in these circumstances can only be partial. In such a case the transmitted effect of a feeble or a moderate stimulus will alone be arrested ; but the block will fail to arrest the transmitted effect of intense stimulation. These considerations will probably explain Pfeffer’s observation that, while the effect of strong injury stimulus was always transmitted across the narcotised area, a moderate mecha- nical stimulus was but occasionally transmitted.

In Haberlandt’s experiment the conducting-tissue was supposed to have been killed by scalding. If this had really been the case, then it may be supposed that under an exceptionally strong stimulus a hydrostatic disturbance had been transmitted through the dead tissue and caused stimulation of the distant leaf, as a mechanical blow de novo. But excitatory transmission in a plant is usually accomplished by a stimulus which is feeble. Strong doubt may also be entertained as to whether the tissue had really been killed. In my own experience I find it extremely difficult to be sure of killing the interior of a tissue by scalding the outside. This derives additional support from certain experiments of Kiihne on conduction of excitation in a nerve, the specimen employed being the sartorius of a frog.

“The delicate nerve which enters the middle of the sartorius by one side, divides within the muscle so that the single fibres that constitute the bifurcation branch many times dichotomously. When Kiihne threw the broad upper end of the muscle into heat rigor by dipping it into warm oil, the half which remained normal twitched on cutting the vigored portion with scissors, showing that excitable nerve-fibres could still be mechanically excited between the rigored and dead muscle-fibres, and thus carry the excitation centripetally into branches which divide above the rigored portion of the muscle.’ !

In this experiment we have an instance of transmission of excitation through heat-rigored animal tissue parallel to Haberlandt’s experiment on transmission through scalded plant-tissue. In both these cases it is evident that the scalded tissues, though under heat rigor, were not really killed ; and that the induced block or abolition of con- ductivity (caused by heat rigor, electrotonus, and so on) is after all relative. There may thus be an effective physio- logical block for normal intensities of stimulation, which would, however, fail under abnormal intensities of stimulus such as that of a burn or of acut. In Kihne’s experiment the intense excitation of scissors-cut failed to be arrested, though the conductivity of the nerve had been depressed under heat-rigor. Similar considerations will explain how the intense excitation caused by a burn or a cut may be transmitted through the narcotised or scalded areas in Mimosa.

The experiment of Kiihne shows further that the conduc- tivity may persist even after the abolition of the motile excitability. The rigored muscle is seen to have lost its motility, though the embedded nerve retained a certain amount of conductivity for excessively strong stimulus ‘of a cut. In turning our attention to Kiihne’s experiment we realise the error involved in ignoring the factor of intensity of stimulus in the matter of the effectiveness of a given block to the transmission of excitation. The necessity of discarding crude and drastic methods of excitation in researches on variation of conductivity will now have become obvious. The object of our inquiry is not to find whether a violent

mechanical disturbance is transmitted to a distance, but the determination of propagation of physiological change, under normal modes of stimulation. By employing stimulus of graduated intensity, it should be easy to determine the character of a given impulse by observing the effects of various physiological depressors in modifying the power of conduction. In order to bring the question—whether in a plant there is true transmission of excitation or mere passage of a mechanical disturbance—to a satisfactory issue, it is clear that we ought to proceed in the following way: First, we have to inquire whether it is not possible to find modes of excitation for the plant which would be purely physiological, and in which there can be no element of physical disturbance. Transmitted effect in such a case could only be due to propa- gation of excitatory protoplasmic change. We will next sub- ject the question to the final test of the physiological block which would arrest an excitatory impulse, but could have no effect on the passage of a hydro-mechanical disturbance.

I shall now describe four different lines of investigations each of which furnishes an independent proof of the excita- tory character of the transmitted impulse :— (r) On methods of excitation by the discriminative polar action of electric currents. (2) On the block of transmission of excitation by local application of cold. (4) On the action of poison in inducing block of conduc- tion. If ina muscle-and-nerve preparation of frog two electrodes are applied on the conducting-nerve, at a certain distance from the responding muscle, it is found that on sending a current through the included portion of the nerve excitation is induced, which on reaching the responding muscle brings about contraction. There is no excitatory action in the case

when the current is eStablished very gradually. The electrical current, as such, has generally speaking little or no excitatory action. It is only at the moment of its sudden initiation, or sudden cessation, that the excitatory effect is most conspicuously induced. It is found, moreover, that an excitatory effect is induced by the ‘ make’ of the current at the kathode, or the point where the current leaves the nerve ; at the ‘ break’ of the current, on the other hand, excitation is induced at the anode or the point of entry.

Precisely parallel effects I find to take place when an electrical current is sent through a portion of the conducting petiole. A detailed description of the polar effects of currents will be given in a subsequent chapter. I shall here only give what is essential to my present purpose. Two non-polarisable electrodes are applied, the proximal on the conducting petiole at a distance of Io mm. from the responding pulvinus, and the distal on the parenchyma of one of the leaflets, such tissue being non-conducting. If the current now be gradually applied by continuously increasing the E.M.F. from zero to 3 volts by means of a suitable potential slide, we shall find that there is no excitatory effect. But if the E.M.F. of 3 volts be applied suddenly, the direction of the current being such that the proximal electrode is kathode, we shall find that the kathode now becomes the seat of excitation and the leaf undergoes a responsive fall after the short and definite period required for the transmission of excitation through the intervening distance.

If the experiment be then repeated with the proximal electrode as anode, and the distal indifferent parenchyma as the kathode, we shall observe no excitatory effect. This is because the effective proximal electrode, which is anode, does not excite at ‘make.’ The excitatory effect will however be found to take place at the anode, but only at the ‘ break ’ of current. Reverting to the hydro-mechanical theory, we are con- fronted with great difficulties in accounting for the excita- tory effects in the petiole initiated locally at the electrodes.

There is no turgid mass of parenchyma here which by its deformation might cause mechanical disturbance. It may be thought that in some way exudation of sap might cause the necessary hydro-mechanical disturbance. The question now arises: How did excretion occur at the kathode at “make’? As the excitation takes place at the anode at “break ’ are we to suppose that exudation also takes place at the anode ? In the often cited instances of hydraulic transmission of stimulus of a violent blow or a cut, mechanical disturb- ance is necessarily present. But transmission of excitatory impulse is found to take place under polar excitation, in the absence of all such disturbing factors. This will be realised when in Chapter XVII it is shown that in Biophytum excitatory impulse is transmitted by the action of an electric current which is so feeble as not to be perceived by the very sensitive human tongue. This would indicate that the effect transmitted here is physiological rather than physical. In a nerve the protoplasmic change which is the basis of excitation is initiated locally at the point of kathode at “make’ and at the anode at ‘break.’ The protoplasmic change is then propagated from point to point giving rise to the excitatory impulse. In the petiole also, excitatory protoplasmic change is initiated locally at the kathode at “make ’ and at the anode at ‘ break.’ And every circum- stance indicates a point-to-point propagation of excitatory protoplasmic change in the conducting petiole.

It has been shown that the velocity of transmission is en- hanced by favourable physiological changes due to warmth. Conversely the conductivity is depressed by lowering of temperature, and this depression may become so great as to induce an actual arrest of conduction. The object of the investigation being the influence of cold on conductivity, special care has to be taken that the lowering of tempera- ture does not in any way affect either the excitability of the

point of application of stimulus or the motile sensibility of the responding pulvinus. For this reason cold is applied locally on the petiole, half-way between the point of appli- cation of stimulus of induction-shock and the pulvinus. The experimental plant was highly sensitive on account of the favourable summer season. An intensity of stimulus of -5 unit applied at a distance of 30 mm. from the pulvinus was found to be effectively transmitted. The intensity of stimulus actually employed was 2 units, which was maximal. A strip of cloth 10 mm. in breadth was wrapped round the