The Nervous Mechanism of Plants

Table VI. — Effect of Heterodromous and Homodromous Current OF Feeble Intensity on Velocity of Transmission. In the next method of investigation, the induced variation of intensity of transmitted excitation is inferred from Fig. 68. Ineffectively transmitted salt-tetanus becoming effective under heterodromous current, denoted by down-pointing arrow (nerve of Frog). the varying amplitude of response of the terminal muscle. Testing stimulus of sub-maximal intensity is applied at the middle of the nerve, where the constant current induces no variation of excitability. Stimulation is effected either by a single break-shock or by the summated effects of a definite number of equi-alternating shocks, or by chemical stimulus.

Experiment 73. — Under the action of feeble heterodromons current the transmitted excitation was always enhanced, whatever the form of stimulation : this is illustrated in fig. 68. Homodromous current, on the other hand, inhibited or blocked excitation (fig. 69). On the cessation of current there is induced in the planttissue a transient conductivity-change of opposite sign to that induced by the current (cf. Experiment 71). I find this to be the case also in animal nerve. I give a typical illustration of the direct- and after-effect of homodromous current on salt-tetanus.

Experiment 74. — In this experiment a sufficient length of time was allowed to elapse after the application of the salt to the nerve, so that the muscle, in response to the transmitted excitation, exhibited an incomplete tetanus T. The homodromous current was then applied, with the result of inducing a complete block of conduction and a concomitant disappearance of tetanus. The homodromous current was gradually reduced to zero by appropriate movement of the potentiometer-slide. The after-effect of the homodromous current was then shown in a transient enhancement of transmitted excitation, which lasted for nearly 40 seconds. After this the normal conductivity was restored. Repetition of the experiment gave similar results (fig. 69),

The results that have been given are typical of a very large number of experiments which invariably presented the characteristic phenomena that have been described. Thus it is established that, with feeble or moderate current, conductivity is enhanced when against the direction of the current, and depressed or blocked when with the direction of the current. Under stronger current the normal effect undergoes a reversal. It has thus been shown that a perfect parallelism exists in the conductivityvariations induced in the plant and in the animal by the directive action of a constant current. No explanation can be regarded as satisfactory which is not applicable to both cases. Now with the plant it is possible to arrange the experimental conditions in such a way that the factor of variation of excitability is completely eliminated. The various effects manifested by the plant-tissue are

Transmission of excitation (salt-tetanus x) arrested under homodromous'current, denoted by continuous line ; on cessation of current, represented by dotted line, there is a transient enhancement above the normal (nerve of Frog). therefore due entirely to variations of conductivity. The parallel phenomena observed in the transmission of excitation in the animal nerve must, therefore, also be due to induced changes of conductivity. The action of an electric current in inducing variation of conductivity may be enunciated under the following laws, which are equally applicable to the conducting tissue of the plant and to the nerve of the animal : —

Laws of Variation of Nervous Conduction under THE Action of a Constant Current (i) The passage of a current induces a variation OF conductivity, the effect depending on the direction and intensity of the current. The variation of conductivity induced by the directive action of a constant current has been investigated by two different methods : (1) The method in which the normal speed and its induced variation are automatically recorded; (2) That in which the variation in intensity of the transmitted excitation is gauged by the varying amplitude of the resulting response.

The effects of the direction and intensity of a constant electric current on the transmission of excitation through the conducting tissue of the plant, may be summarised, as follows : — The velocity of transmission is enhanced when against the direction of a feeble current, and retarded when in the direction of the current. Feeble heterodromous current enhances conductivity, homodromous current depresses it. Ineffectively transmitted excitation becomes effectively transmitted under a heterodromous current. Effectively transmitted excitation, on the other hand, becomes ineffectively transmitted under the action of a homodromous current.

The after-effect of a current is a transient conductivitychange, the sign of which is opposite to that induced during the passage of the current. The after-effect of a heterodromous current is thus a transient depression, that of homodromous current a transient enhancement, of conductivity. The characteristic variations of conductivity induced in animal nerve by the direction and intensity of a constant current are in every way similar to those induced in the conducting tissue of the plant.

The above characteristic effects undergo reversal under an intensity of current above a certain critical value. The effect of transmitted impulse is, in plants with motile leaves, manifested by the mechanical response of the pulvinus. It is therefore essential to obtain a clear idea of the anatomical and physiological characteristics of the pulvinus, which is not a simple but a highly complex organ. I confine myself here to a brief account of the pulvinus of Mimosa fudica : the subject will be treated more comprehensively in a subsequent work.

A longitudinal section of petiole and pulvinus, reproduced from a photomicrograph, is given in fig. 70, in which the following anatomical features will be noted. The two main bundles, upper and lower (as also two lateral bundles not shown in the figure), while distinct in the petiole, converge and meet in the pulvinus ; the pith becomes thereby reduced, and the cortical parenchyma increased proportionately. A very noticeable fact is that though the cyhnder of protective sclerenchyma, which extends throughout the whole length of the petiole and the pulvinus, is lignified in the petiole, the lignification of the tissue ceases in the pulvinus, as shown by the usual micro-chemical tests. The significance of the non-lignification of the sclerench3mia-cylinder in the pulvinus is obviously to allow rapid movement of the organ in response to stimulation.

examination ; for the cortical cells of the petiole pass imperceptibly into those of the pulvinus. I have, however, discovered a method, by which the line of demarcation can be brought out with extraordinary clearness.^ The phloem has been shown to be the conductor of excitation and thus to function as a nerve ; in each bundle there are two phloems, one external, the other internal. These nerve-strands of the four bundles in the petiole meet in the pulvinus and form an almost continuous ring ; it will be shown later that their nerve-ends in the pulvinus remain functionally distinct.

It is usual to regard the pulvinus as consisting of only two functional halves, an upper and a lower, the r e s p o n s i V e m o V e m eht s In reality the organ is more complex ; for it gives certain other characteristically definite 1 ‘ Physiological and Anatomical Investigations on the Conducting and Motor Tissue of Mimosa puAica* Proc. Roy. Soc., B, voL 98, Aug. 1925 . Fig. 70. Longitudinal section of petiole and pulvinus passing through upper and lower vascular bundles.

F, the vascular bundle which meets its fellows in the pulvinus ; s, protective sclerenchyma-cylinder, lignihcation of which extends only through the petiole; p (upper), pith disappearing in pulvinus ; Pt, petiole, the cells of which remain unstained ; p, contractile cells of pulvinus deeply stained. {Mimosa pudica.) responses. In addition to the down- and up-movements, the piilvinus exhibits right and left twists, that is to say, torsional clockwise and anti-clockwise responses, depending on which particular flank of the organ has been subjected to stimulation. The motor organ thus consists of four effectors for the production of four distinct types of response.

The record of torsional response is obtained as follows. In order to eliminate the effect of the weight of the leaf Stimulation of left flank of pulvinus induces an anti-clockwise, and of the right flank a clockwise, torsion. and to obtain pure torsion, the petiole is held in a hooked glass support with a smooth internal surface, which prevents any up or down movement, yet allowing freedom for torsional response. The torsion is magnified by an L- shaped piece of aluminium wire so tied to the petiole that its free arm is at right angles to the petiole. The end of the arm is connected by a silk thread to the short arm of a recording lever, thus ensuring a compound magnification of the torsional movement : a left-handed torsion produces an up-curve in the record, and a right-handed torsion produces a down-curve. The record is taken by the Oscillating Recorder, the successive dots being at definite intervals of time, which could be varied according to requirements. The same apparatus may be used for obtaining the up- and down-records, when the hooked support is removed and the short arm of the lever directly attached by a thread to the petiole (fig. 71).

The quadrants or effectors are represented diagrammatically in fig. 72. The observer standing in front of Fig. 72. Diagrammatic representation of the quadrants and their characteristic responses. Dotted arrow represents direction of incident light, and full arrow the responsive movement. Lower half of pulvinus shaded. the leaf is supposed to be looking at the stem. The two quadrants to the left and to the right are numbered (i) and (4), respectively ; the lower quadrant is numbered (2), the upper (3).

When the stimulus of light acts on the upper quadrant (3) the response is a rectilinear up-movement ; when light acts on the lower quadrant (2) from below, the response is a more energetic down-movement : stimulation of quadrant (i) or (4) induces a torsional response. Experiment 75. Effect of stimulation of left quadrant, — When a beam of light, thrown laterally on the pulvinus, strikes the quadrant (i), the response is a torsional movement which is anti-clockwise, shown as an up-curve (fig. 73). There is recovery after the cessation of exposure to light.

Effect of stimulation of right quadrant. — If the direction of stimulus be changed, so that the light strikes the right quadrant (4), the response is a clockwise torsion, shown as a down-curve (lower record, fig. 73). The intensity of light happened to be stronger in this case, and the amplitude of response was therefore relatively greater. Fig. 73. Records of torsional response due to stimulation of left and right quadrants. Experiment 77, Effect of thermal radiation. — The source of thermal radiation was a length of electricallyheated platinum wire. Stimulation of the left flank induced an anti-clockwise torsion ; of the right flank induced a clockwise torsion.

Experiment 78. Geotropic stimulation. — By means of suitable inclination, the left flank (i) of the pulvinus was exposed to the action of the vertical lines of force of gravity. The response was an anti-clock or left-handed torsion ; when the opposite flank of the organ was similarly stimulated, the response was a right-handed torsion. Inspection of fig. 72 makes it clear that in all cases the responsive torsion is such that it is the less e.xcitable upper half of the organ that is made to face the stimulus. Similar torsional response is found to be given not only by pulvinated but by all anisotropic organs.

A beam of light falling on the left flank of the pulvinus of Mimosa induces an anti-clockwise torsion. If a second beam fall on the right flank, it will induce a clockwise torsion. The two torsions being opposed, the resultant effect is determined by the effective stimulation of the two flanks. The pulvinus may thus serve as a delicate indicator by which the effective intensity of two stimulations may be compared with each other. Experiment 79- Comparison of phototropic reactions to light of different colours. — K parallel beam of light from a small arc-lamp passing through a blue glass falls on the left flank of the pulvinus ; a beam of blue light is also

thrown upon the opposite right flank, the intensity of the latter being so adjusted that the resultant torsion is zero. The blue glass on the left side is then removed, the unobstructed white'light being allowed to fall on the left flank. This causes an upset of the balance, the resultant torsion being anti-clockwise. This is due to the more effective stimulation by the unobstructed white light. A red glass is next interposed on the left side, with the result that balance is upset, this time in the opposite direction. This proves that the phototropic effect of blue light is greater than that of red light.

In this way it is possible to compare the tropic effect of one mode of stimulation with that of a totally different mode, phototropic against geotropic stimulation for example. It is enough here to draw attention to various investigations rendered possible by this very sensitive method of Torsional Balance.^ Complex Lateral and Torsional Movements under Vertical Light Very complicated movements are executed by leaflets of various plants under the action of vertical light. Thus the leaflets of Cassia alata close laterally in darkness. Under moderate diffuse illumination they open out in a lateral direction. But under strong vertical light, the pulvinules of the leaflets exhibit a torsion by which the formerly infolded surfaces of the leaflets are exposed at right angles to the light from above (fig. 74). Such complicated movements, in two directions of space, are also exhibited by other leaflets which close at night in a lateral direction.

In order to obtain an explanation of these complex movements under different intensities of light, it is necessary to discover the characteristic different excitabihties of the two halves of the pulvinule. Determination of differential excitabilities of the pulvinule. — In the leaflet of Cassia the movement of opening under the stimulus of diffuse light can only be brought about by the contraction of the outer half, which must therefore be the more excitable. This is independently demonstrated by its reaction to an electric shock. On subjecting the half-closed leaflets to diffuse electric stimulation, they open outwards in a lateral direction. The relative position of the unequally excitable halves of the pulvinule of Cassia is thus different from that of the main pulvinus of Mimosa. In the latter, the plane that divides the two halves is horizontal, the lower half being the more excitable : whereas in the pulvinule of Cassia, the plane that separates the two

Fig. 74. Leaflets of Cassia alata : open in daytime, and closed in evening. unequally excitable halves is vertical, the outer half being the more excitable. By ‘ inner half ’ is here meant that half which is inside when the leaflets are closed. Effect of strong vertical light. — When the plant is placed in a moderately lighted room, the leaflets open out laterally to the utmost. This is brought about by the contraction of the more excitable outer half of the pulvinule. If strong light be thrown from above, a new movement is superposed, namely, a torsion by which the leaflets undergo a twist so as to place their inner surface at right angles to the vertical light. In order to investigate this phenomenon in greater detail I placed the plant in a well-lighted room, the leaflets being three-quarters open under diffuse light. A very light index was attached to the leaflet for magnifying the subsequent torsional movement. A strong beam of parallel light from an arc-lamp was thrown down on the

pulvinule from above ; this fell on the junction of the more excitable outer with the less excitable inner half of the organ; the plane of separation of the two unequally excitable halves being, as previously explained, vertical The leaflet became twisted so as to expose the former infolded surface upwards, at right angles to the incident light. This is further evidence that, under lateral stimulation, a differentially excitable organ undergoes torsion by which the less excitable half is made to face the stimulus.

As a confirmatory test, strong light was made to strike the pulvinule from below, with the result that the leaflet exhibited an opposite torsion by which the infolded surface faced downwards, so as to be at right angles to the light incident from below. Under natural conditions sunlight falls from above; stimulation thus takes place at the junction of the two differentially excitable halves of the organ, the plane of separation of which is vertical The torsion induced makes the less excitable inner half turn in such a way that the infolded surfaces of the leaflets are placed perpendicular to the incident light.

The pulvinus of Mimosa has been found to consist physiologically of four different effectors characterised by definite responsive movements of the leaf. The upper and lower quadrants respond to direct stimulation by rectilinear up and down movements, respectively : the left quadrant responds by an anti-clockwise torsion ; the response of the right quadrant is a clockwise torsion. The law of torsional response is, that an anisotropic organ laterally excited by any stimulus, undergoes torsion such that its less excitable side is made to face the stimulus.

The Torsional Balance permits of simultaneous comparison of the tropic effects of two different forms of stimulation. The leaves of plants adjust themselves in various ways in relation to the incident light. The heliotropic fixed position is assumed by means of curvatures and torsions of the motile organ, which may be the pulvinus, or the petiole acting as a diffuse pulvinoid. In some cases the motile organ is both perceptive and responsive ; in others, the leaf-blade exerts a directive action, the perceptive lamina and the motor organ being separated by an intervening distance. This directive action of the lamina has been observed by Vochting in M diva verticillata, and by Haberlandt in Begonia discolor nndi in several other plants. It should be borne in mind that this characteristic reaction does not preclude the possibility of the motile organ being directly affected by the stimulus. In a nerve-and-muscle preparation, the muscle is excited not merely by indirect but also by direct stimulation. In the heliotropic adjustment of leaves in the cases just mentioned, the stimulus of light acts both directly and indirectly, the indirect stimulation being due to some impulse transmitted from the perceptive lamina. This may be generally expressed by the statement that the coarse adjustment of the leaf is brought about by direct, and the finer adjustment by indirect, stimulation.

Most leaves assume a heliotropic fixed position such that the blades are placed at right angles to the incident light with the ventral surface uppermost, the directive action being due to some as yet unknown transmitted impulse. Nor has any satisfactory explanation been forthcoming of the physiological reaction of which this movement is the expression. Suggestions have been made that the dia-hehotropic position of leaves is of obvious advantage, since it assures for the plant the maximum illumination. But such teleological considerations offer no explanation of the definite physiological reaction. It is, moreover, not true, as I show in the course of this chapter, that there is anything inherent in the irritability of the plant which constrains the surface of the leaf to place itself perpendicular to the incident light.

The dia-heliotropic phenomena have been studied not only in ‘ sensitive ’ but also in ordinary plants ; it will be shown that the responsive reactions are essentially similar in both. As a type of the former I have taken Mimosa fudica, and of the latter, Helianihus annuus. Before entering upon the experimental investigation of the subject, I will describe the dia-hehotropic phenomena, as manifested by Mimosa and Hehanthus. A photograph of the former is reproduced in fig. 75, a. The plant, grown in a pot, had been exposed to the northern sky and not to direct sunlight. It will be seen that the leaves which directly front the light have been raised, and so placed that the sub-petioles, with their leaflets, are at right angles to the strongest illumination. The lateral leaves have, on the other hand, undergone appropriate torsion, the plane of the leaflets being adjusted perpendicular to the fight : it will be noticed that the petioles to the right and the left have undergone opposite torsions. After the assumption of this position, the pot containing the plant was turned round through 180°. This brought about a new adjustment in the course of twenty minutes, the plane of all the leaflets being once more at right angles to the fight. The new adjustment necessitated a complete reversal of the former movements and torsions. Such

from the sky, not so well by direct sunlight, for reasons which will be given later. Figure 75, b shows the heliotropic adjustment of the leaves of a Sunflower, grown near a wall, the plant being exposed to light from the western sky. The adjustment is essentially similar to that seen in Mimosa. The lateral leaves, (i) and (3), have undergone appropriate torsionright-handed or left-handed — such that the leaf-blades are placed at right angles to the light with the ventral surface always facing it. The leaf number (2) has been raised, placing its lamina perpendicular to the light. A contributing factor is the bending over of the stem, due to positive heliotropic curvature, which accentuates the rise of the leaf number (2), The same bending often causes an apparent fall of the leaf marked (4). When the stem is tied to a stake, the bending over of the stem is prevented ; the leaf numbered (2) is then raised by its own heliotropic reaction ; but there is little or no fall of the opposite leaf.