The Nervous Mechanism of Plants

Stimulation is found to give rise in Mimosa pudica to an impulse very similar to the nervous impulse in the animal. Like the nervous impulse, it causes no visible change in the tissue which conducts it ; it is conducted to a distance and, impinging on the motor organ, the pulvinus, causes the fall of the leaf. Other ‘ sensitive ’ plants present similar phenomena, among which may be mentioned Neptunia oleracea, Biophykmi sensiiivmn, .iverrhoa Carambola, and the arborescent Mimosa Spegazzinii. I have, in all my typical experiments, used Mimosa pudica, the other sensitive plants being occasionally employed for purposes of independent confirmation.

Mechanical stimulation.-— This, is effected by a superficial scratch. The Scratch-Stimulator (fig. i, a) consists of a holder, from one end of which there projects an adjustable pin-point, the usual length of projection being only a fraction of a millimetre. A minimal stimulus is produced by a single scratch. The effective intensity of the stimulu.s can, however, be increased from minimal to maximal by the additive effect of repeated scratches following rapidly one after another. The scratches, it should be remembered, are purely superficial.

Electrical stimulation.- — dhe two electrodes of an induction-coil are applied on the epidermis, electric connection being secured by means of kaolin-paste moistened with normal saline. The intensity of the stimulus can he increased from the minimal to the maximal bv moving the secondary coil nearer to the primary. A young and vertically straight specimen of Mimosa, growing in a pot, was used in the following experiments. In straight and untwisted specimens the stem bears the leaves in alternate series. Those of the odd series to the right are vertically

over each other ; so are also the leaves of the opposite even series to the left. In order to prevent shaking during the application of stimulus, the stem is securely held by a clamp without exerting undue pressure. Mechanical Stimulation.— Scratchstimulus was applied to the stem on the left, between the second and the fourth leaves, at a point situated on the vertical line that passed through the two leaves. This gave rise to impulses which were conducted simultaneously both upwards and downwards, and caused the fall of leaves 2 and 4 (fig. i, b). The distance between the point of stimulation S and the upper leaf 4 was 37 mm., the time of transmission being 10 seconds. The time of transmission downwards through 28 mm. was 20 seconds. It is to be noted that under unilateral stimulation, conduction occurred only on the stimulated side, the leaves on the opposite side remaining unaffected. When the stimulation was transferred

a, The Scratch-Stimulator ; b, effect of stimulation of moderate intensity s applied to left ; c, effect of strong stimulus s' applied to right side. from the left to the right side, the impulse was conducted only on the right side. The velocity and the possible distance of transmission depend (i) on the physiological vigour of the plant, (2) on the season, (3) on the age of the intemode, a young internode conducting more actively than an old, and (4) on the direction of propagation. It will be shown later (p. 48) that there is a preferential direction of conductivity along which the impulse travels at a quicker rate. The velocity of impulse in the stem varies from 2 to about X2 mm. per second.

Experiment 2. Electric stimulation. — Results in every way similar to those produced by mechanical stimulation were obtained with moderate electric stimulation. The impulse travelled both upwards and downwards, and the excitation of the leaves remained confined to the stimulated side. A very remarkable effect is observed when the intensity of stimulus is increased from minimal to sub-maximai. This is attained in mechanical stimulation by the additive effect of repeated scratches, in electrical stimulation by the nearer approach of the secondary to the primary coil.

I proceed to give a detailed account of a typical experiment on the effect of unilateral mechanical stimulation of moderately^ strong intensity. The impulse generated travelled both ‘above and below the stimulated point. I-'ur the present purpose it is only necessaiy to follow the track of the ascending impulses. Experiment 3.— A sub-maximal scratch-stimulus was applied unilaterally at S', to the right, and vertically below leaf I (fig. I, c). The ascending impulse lirst caused the fall of leaf I, in the course of 20 seconds. The impulse then reached leaf 3, which fell 5 seconds after leaf x. The

impulse did not, however, stop at the highest leaf on the right side, but crossed over from the right side to the left, presumably after reaching the apex. The topmost leaf on the left side, numbered 4, fell 15 seconds after the fall of 3. The impulse which had hitherto been ascending on the right side, became converted after crossing at the apex into a descending impulse on the left side, as seen by the fall of leaf 2, 20 seconds after the fall of 4. Detailed results are given in the following tabular statement.

The results obtained with the stimulus of inductionshock are in every way similar to the above. It will be shown later (p. 22) that similar effects are also produced by the stimulus of polar action of a constant current. The results are therefore independent of the mode of stimulation, provided the stimulus is moderately strong. Excitation becomes outspread under excessively strong stimulus ; I shall speak of this later. I wish to draw special attention to the results of the fundamental experiments described above, the detailed consideration of which will help to a proper understanding of the true nature of the transmission of excitation. The important facts that have been established are given below.

(1) An impulse is generated by various modes of stimulation which are known to be effective in initiating nervous impulse in the animal. (2) A superficial scratch, unattended by any exudation of sap, is effective in giving rise to an impulse. Irritation of the wood in the interior is not essential to the initiation of an impulse. effected by electric stimulus. There is, in this case, not the remotest possibility of exudation of sap nor of injury to the wood.

(4) Stimulation at any point in the vertical line between two leaves gives rise to impulses which travel simultaneously both upwards and downwards. The nervous impulse in the animal travels in both directions; the impulse in the plant does the same. (5) The conducted impulse is confined to the stimulated side. It would thus appear that there are two definite conducting strands situated on opposite sides of the stem. (6) Under stronger unilateral stimulation the ascending impulse on one side, after reaching the apex, crosses over to the opposite side and is converted into a descending impulse. This can only be explained on the supposition that there are two main conducting strands on opposite sides of the stem which meet at the apex.

The above inferences, which follow naturally from the sequence of excitatory fall of the leaves, will be "found fully confirmed by the results of anatomical investigation given in a later chapter (p. 40). That there is a continuous conducting channel between stem and leaf is shown by the successive fall of the Iea\-es after stimulation of the stem. The converse of this, namely, conduction from leaf to stem, is well shown in the following experiment. A very simple and satisfactory means of local stimulation is found in the use of a hot glowing point,

A flame is unsatisfactory for various reason.s, one of which is that the rising hot air affects other leaves in the neighbourhood. The thin and long incense stick of the Chinese, when once lighted at one end, burns slowly without any flame, and requires no further care. The burning tip i.s always a glowing point and can therefore be applied for strong stimulation at any point on the plant. When a sub-petiole of Mimosa carrying the sensitive leaflets is thus locally stimulated, the course of the impulse is easily followed : (i) by the successive closure of the leaflets upwards ; (2) by the drawing together of the sub-petioles due to excitation of the secondary pulvini ; and (3) by the fall of the leaf due to excitation of the main pulvinus. The impulse then passes into the stem, causing the fall of other leaves. The existence of a definite conducting tissue which connects the leaf with the stem will become still more clear from the anatomical account given in a subsequent chapter.

With the very definite results that have been described it will be possible to remove the prevailing misconceptions in regard to the true nature of the transmission of impulse in the plant. Different modes of stimulating the plant have been described which cause no injury, no exudation of sap, and no irritation of the enclosed wood. An impulse is nevertheless generated which is transmitted to a distance. Unilateral stimulation of moderate intensity causes an impulse which is propagated simultaneously up and down, along the same vertical line.

The course of the impulse is confined to the side stimulated, no excitation being produced on the opposite side. The conducting tissue is therefore not diffuse, but is definitely distributed in the stem. unilateral stimulus generates an impulse which, after ascending to the apex, crosses over to the opposite side. The ascending impulse on one side then becomes converted into a descending impulse on the opposite side. There must therefore be two main conducting strands which meet at or near the apex. ^

It is now possible to discuss the nature of the transmission of impulse in plants, which is commonly considered to be very different from that of the nervous impulse in the animal. Transmission in the plant has been attributed (i) to hydro-mechanical disturbance caused by stimulation, and (2) to the translocation of a chemicar stimulant by the movement of the sap. The above theory is largely based on the two wellknown experiments of Pfeffer and Haberlandt. In the former of these, the effect of strong stimulation was found to travel over a chloroformed part of the petiole of Mimosa. Pfeffer assumed that the vital conductivity of this portion, if any, must have been abolished, since chloroform is known to abolish motile excitability. In the e.xperiment of Haberlandt some of the intervening tissue was killed by scalding; in spite of this, the impulse 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 tme excitatory nature. It was held, on the contrary, that the strong stimulation had given rise to a variation, either increase or diminution, of hydrostatic pressure.’ 1 his variation of pressure, it was assumed, had been hydromechamcMly transmitted, and on reaching the distant pu vmus ad inflicted on it a blow which had proved as effective as if a mechanical stimulus had been applied : directly. It was in fact held that transmission in Mimosa

Though Pfeffer and Haberlandt advocate the theory I of hydro-mechanical transmission, they disagree in regard to the channel for the transmission of impulse. According to Pfeffer, it is the vessels of the wood which conduct it ; Haberlandt, on the other hand, insists that it is not the xylemvessels, but certain tubular cells in the phloem that are the means of transmission. I italicise the important passages in the following quotations. ' Dutrochet was the first to show that stimuli are con- ; ducted through the vascular bundles of Mimosa pudica,

I and he also came to the correct conclusion that the trans- I mission was due to pulsation of water. Pfeffer subsequently showed that the stimulus was able to travel over chloroformed parts of the stem, and Haberlandt found that dead regions of the stem and leaf retained their conductivity some time after they had been killed. We are therefore I fully justified in ascribing the transmission of stimulus to the movements and changes of pressure of water in the - vascular bundles, and when a cut is made in the stem,

a stimulus is only exercised when the knife penetrates the vascular bundles and allows the escape of a drop of water J ^ Dutrochet is often quoted in support of the contention that the wood alone is concerned in the transmission of impulse in Mimosa; I take the following account of his experiment from Biedermann's ‘ Electro-Physiology.' ' (Dutrochet) showed that the cortex was not involved I by paring away a ring of it, when the conductivity of the

\ ^ Biedermann — Electro-physiology — English translation, vol, ii. p. 12. The account given above does not support the contention that the dead wood ^ is alone concerned in conduction, since the living phloem remained with the wood. Haberlandt, as already stated, came to the conclusion that certain tubular cells in the phloem functioned in the transmission of impulse. The following quotation e.NpIains his views on the mechanical transmission of impulse :

‘ The stimulus-transmitting elements of Mimosa pudica are not situated as Dutrochet, Sachs and Pfeffer, among others, have supposed in the woody cylinder or in the hadrome portions of the primary bundles, but on the contrary occur in the leptome strands, where they takt? the form of elongated tubular cells arranged in longitudinal series. . . . The effects of incision show that stimuli are actually propagated in this system of highly turgescent tubes, and that the mode of transmission is a hydro-dynamic one. If, namely, one or more of the tubular cells are laid open by an incision in the stem or petiole, then cell-sap instantly escapes in the form of a drop of transparent liquid ; immediately afterwards the nearest pulvimis carries out the characteristic movement.'^

I wiU now briefly criticise the conclusions arrived at by Pfeffer and Haberlandt from their respective e.xperiments on the effect of narcotisation and of scalding on the conduction of impulse. As regards the former, it is e.xtremely doubtful whether the conducting tissue in the interior can be effectively narcotised by the external application of an anesthetic. The task would almost be as difficult as narcotising a nerve-trunk lying between muscles, by the application of chloroform on the .skin outside ! la the case of the plant it is conceivable that, after a very long appheation, a small quantity of the narcotic miglu by ab-

* The xylem-tissue is not completely dead, since there are sc.attertd living elements in it. Though these are not true condor tors of vxauit mn, yet the* block’ or resistance offered by them is ewercome umler intense stimulation. This abnormal diffusion is, however, a phcnonieiKUi different from the normal conduction of excitation along c'kiimte nervous channels. 2 Haberlandt — Physidogkal Mn/owj— English tranhlatiim,p.t»43. sorption get across to the internal conducting-tissue ; but narcotisation in such circumstances could only be partial, and this partial block might fail to arrest the impulse due to the intense stimulation caused by a deep wound (see also p. 37), 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. Strong doubt may, however, be entertained as to whether the tissue had really been killed throughout. In my own experience I find it extremely difficult to be sure of killing the internal tissue of an organ by scalding the outside.

Both Pfeffer and Haberlandt used a deep cut or wound to effect stimulation. The complications introduced by the employment of such a drastic method will be understood from the results of Kuhne’s experiment on conduction of excitation in a frog’s nerve. ‘ 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 Kuhne 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 rigored portion with scissors, showing that excitable nerve-fibres could still he 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.’ ^

This experiment is clearly an instance of transmission of excitation through the heat-rigored animal tissue parallel to Haberlandt’s experiment on transmission through the scalded tissue of the plant. In both these cases it is probable that the scalded tissues, though under heat-rigor, were not really killed. An induced block of conductivity is, after all, relative. There may well be an effective physiological block for normal intensities of stimulation, which would, however, fail under abnormal intensities of stimulus such as that of a burn or a cut. In Kiihne’s e.xperiment 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 most fruitful source of error in the in%’estigation of the conduction of excitation is the employment of such drastic modes of stimulation as deep wounds or burns. Though the normal transmission, under the action of moderate stimulus, is confined to the well-defined conducting tissue, the excitation becomes diffused under intense stimulation. This is demonstrated in hl.xperiment i8, described in Chapter IV. The same thing also occurs in the nervous conduction in the animal, where, under abnonnaily powerful stimulus, the excitation becomes widely diffused. The necessity for discarding crude and drastic methods of stimulation in researches on conduction will now luu'c become obvious. The object of inquiry is not to find out that a violent disturbance becomes widely diffused, but to determine the nature of the transmission of excitation under normal modes of stimulation. By employing stimulus of graduated intensity, it should be easy to determine the character of the impulse generated by observing the effects of various physiological blocks in modifying the power of conduction.

The other recently proposed theory is that conduction in Mimosa is effected by the transpiration-current carrying along the xylemvessels some stimulating substance excreted by the wood in consequence of stimulation. Ricca 1 describes the following experiment in support of this theory. He cut across a shoot of Mimosa Spegazzinii and connected the two pieces by a tube filled with water. Intense stimulation of the lower piece of the stem by a flame is, according to him, often followed by the fall of the leaves in the upper piece. Application of an extract obtained from the tissue of the internodes to the cut end of the stem was also observed to cause fall of the leaves. Snow ® obtained similar results with

In order to establish the transpiration-current theory, it is necessary to prove, (i) that stimulation is effected only by irritation of the wood ; (2) that a hypothetical stimulant is excreted as a consequence of stimulation; (3) that transmission can take place across a water-gap ; and (4) that the stimulant is transported by the transpiration-current with the same speed as the impulse which causes successive fall of the leaves. Pfeffer and Haberlandt contend that ‘ a stimulus is only exercised when the knife penetrates the vascular bundles and allows the escape of a drop of water.’ The escape of water is thus supposed to give rise to a sudden variation of pressure and to a consequent hydro-mechanical impulse. The transpiration-current theory assumes the irritation of the wood to be the antecedent of the excretion

of a stimulant which, on translocation the movement of sap, is the cause of fail of the leaves. Now the hypothetical structure on which the two theories are based inevitably falls to the ground if it can be shown that stimulation can be effected (i) without wounding the wood, and (2) without exudation of sap from the cut stem. It is remarkable that the upholders of these two theories should have been blind to the obvious fact that stimulation can be effected without wounding the plant by a superficial scratch, by rough friction, or by an electric shock of only moderate intensity. I have shown in Experiment i that the impulse generated by a superficial scratch, which could not irritate the wood, was transmitted to a distance ; there was no wound nor was there any escape of sap to cause a hydro-mechanical disturbance.