The Nervous Mechanism of Plants

applied to a sub-petiole of the leaf, the transmitted impulse is found to give rise to an erectile response of the leaf, shown by the up-movement of the reflected spot of light through about 15 cm. A stronger stimulus causes a brief positive followed by a very rapid and large negative response. An interesting result is obtained by successive applications of the sub-minimal stimulus. The first responses are positive, but on account of Facilitation induced by stimulation, the conductivity is gradually increased. Hence, under successive applications of sub-minimal stimulus, the response becomes gradually transformed from positive to negative through the intermediate diphasic.

The positive impulse in the petiole can be more easfly demonstrated when the specimen is in a sub-tonic condition than when it is normal. Experiment 50.— A thin stem of Mimosa was held vertical by means of a'clamp. A narrow beam of light from a small arc-lamp was made to fall upon it at a point exactly opposite to the motile leaf which was to serve as the indicator of response to positive or negative impulses transmitted across the stem. The leaf was attached to the recording lever, the successive dots in the record being at intervals of 10 seconds. Stimulation by light caused a positive or erectile movement in about 10 seconds after the application of the stimulus. When the stimulus was moderate or of short duration, the response remained positive. But with strong or prolonged stimulation the excitatory negative impiflse was conducted across the stem to the distal side, causing a very rapid fall of the leaf (fig. 33). In the present case the negative impulse reached the motile indicator in the course of about 3 minutes. The transmission is more rapid under strong stimulus. Similar effects were also obtained under unilateral electric or thermal stimulation.

In order to obtain the preliminary positive response, care has to be taken that the stimulus is of moderate intensity. In this respect, photic stimulus ohers special advantages, since its intensity can be so adjusted as not to be excessive. The experiment that has just been described is of much significance. Though the stem of Mimosa exhibits no Fig. 33. Erection of Mimosa Leaf due to Indirect Stimulation. (a) Diagram of the experiment : point of application of stimulus indicated by arrow ; (b) Erectile response (shown by downcurve) followed by rapid fall (up-curve). Successive dots at intervals of 10 seconds.

movement, and so may appear to be insensitive to stimulation, yet its perception and its reaction to stimulus are shown by the fact that, in response, it can generate and transmit two characteristic impulses : one of these is positive, and gives rise to an enhancement of turgor at the distal side ; the other, negative, induces the opposite reaction of diminution of turgor. These characteristic reactions to unilateral stimulation wiU be shown to offer the most satisfactory explanation of tropic curvature.

I have shown elsewhere ^ that increase of turgor, within limits, enhances the rate of growth, while diminution of turgor lowers the rate. I have also shown that direct stimulation induces a retardation of growth which may culminate in actual contraction. Since indirect stimulation gives rise to an enhancement of turgor, it induces an acceleration of the rate of growth. Fig. 34. Effects of Direct and Indirect Stimulation on Rate of Growth. Normal rate of growth shown in first part of record ; enhancement of the rate, due to indirect stimulation at arrow, seen in sudden erection of the curve. Direct stimulation at cross induced a sudden contraction shown by the down-curve.

Experiment 51. — The above facts are demonstrated in a record (fig. 34) obtained with my High Magnification Crescograph. Indirect stimulation enhanced the rate of growth, as shown by the resulting erect curve. Direct stimulation induced, on the other hand, an actual contraction, as seen in the reversal from' an ascending to a descending curvel When the indirect stimulation is sufficiently strong or prolonged, the response is diphasic, a preliminary acceleration followed by a depression of growth culminating in an actual contraction.

It has just been shown that while a growing organ shows retardation of growth and contraction when directly stimulated, under indirect stirnulation it exhibits acceleration of growth and expansion. Now a growing organ under unilateral stimulation, say, by light, is stimulated directly at the proximal and indirectly at the distal side : the rate of growth of the former is retarded, whilst that of the latter is accelerated. The result is a positive heliotropic curvature Under intense or long continued stimulation, the excitation reaches the distal side, neutralising the positive curvature, or reversing it to negative.

Response OF the Pulvinus of Mimosa to Unilateral Stimulation Broadly speaking, three types of tissue may be physiologically discriminated in plants. There is, first, the indifferent parenchyma in which, on account of the numerous septa, excitation remains more or less localised at the point of application of stimulus. At the other extreme, there is the highly conducting nervous tissue, in which excitation is tr^smi e to a considerable distance, the conducting nerve exhibiting no

visible change during the passage of the impulse. Between these two extremes there is an intermediate type, such as the contractile cells of the pulvinus, in which the wave of excitatory contraction passes from cell to cell at a rate slower than that of the nervous impulse. I may, convenience, distinguish this as cellular fropagatton of excitation. The phenomenon is not ^"hke the propagation of a wave of contraction from cell to cell in the muscle of

and I willnow describe its charactenstic response to indirect Stimulation applied locally on the upper surface of the pulvinus. Since the upper half is relatively inexcitable, compared with the lower half, for simplicity of explanation I confine attention to the reaction of the more excitable half of the organ. Experiment 52. — narrow beam of strong light from a small aix-lamp was thrown on the upper half of the Fig. 35* — Record of effect of continuous application of light on upper half of pulvinus of Mimosa.

Note positive erectile response (down-curve) followed by neutralisation and pronounced reversal into negative due to transverse conduction of excitation. pulvinus, and record of the response was obtained in the usual manner. The record (fig. 35) shows that after a latent period of 5 seconds, a positive erectile response was initiated, due to the expansion of the lower half of the pulvinus. Under the continued action of light, the excitatory impulse reached the more excitable lower half and caused very rapid negative response shown by the fall of the leaf, due to contraction of that half of the organ. In sensitive specimens the negative response is so abrupt

and intense, that, the writing lever is jerked off beyond the recording plate before making a dot on it. The thickness of the pnlvinns was 1*5 mm., and the distance which the excitatory impulse had to traverse to reach the lower half was therefore about 0*75 mm. The time for the transverse transmission of excitation under the action of light was found to vary in different cases from 50 to 80 seconds. The velocity of the transverse transmission of excitation in the pulvinus is therefore of the order of o-oii mm. per second, which is very much lower than the velocity in the petiole of the plant.

Other modes of stimulation gave similar results. Stimulation without mechanical disturbance can be produced by the application of a drop of dilute solution of hydrochloric acid on the upper half of the pulvinus. This produces a preliminary positive followed by a more intense negative response. The rate of transverse transmission becomes considerably enhanced under the action of a strong stimulus. By the method of mechanical response it has been shown that, in plants having motile leaves, a single stimulation gives rise to two impulses, positive and negative. The positive travels at a quicker rate and induces an enhancement of turgor and erectile response of the leaf. The negative or excitatory impulse travels at a slower rate and induces a diminution of turgor and a rapid fall of the leaf. The negative response is far more intense than the positive.

A feeble or sub-minimal stimulus gives rise only to the positive response. A minimal stimulus gives rise to a diphasic response, positive followed by negative. When the distance of transmission is reduced, the predominant negative impulse overtakes and masks the feeble positive. causes a retardation of the rate of growth. Indirect stimulation of feeble intensity causes an acceleration. The above facts explain tropic curvatures produced by unilateral stimulation ; under feeble or moderate stimulation the curvature is positive ; under strong and long continued stimulation the positive curvature passes over into negative.

Local stimulation of the upper half of the pulvinus of Mimosa gives rise to a positive response. Under strong or long continued stimulation the positive is converted into an intense negative response in consequence of the conduction of excitation to the more excitable lower half of the organ. The Laws of Direct and Indirect Stimulation are as follows : (1) The effect of all forms of Direct Stimulation IS A DIMINUTION OF TURGOR, CONTRACTION, The mechanical response of ‘sensitive’ plants to direct and indirect stimulation has been fully described in the last chapter. It was shown that under direct stimulation the irritable parenchyma of the pulvinus exhibits a diminution of turgor, a contraction, and a negative mechanical response exhibited by the fall of the leaf. Under indirect stimulation the response was either positive, diphasic, or negative, according to the intensity of stimulus, the conductivity of the intervening tissue, and the tonic condition of the plant.

These results were demonstrated in sensitive plants, in which the movement of the motile organ gives a conspicuous indication of the character of the transmitted impulse. The question now arises ; are these specific reactions manifested only by sensitive plants, or are they to be found universally in aU plants ? It used to be thought that ‘sensitive’ plants alone were excitable, their motor organs exhibiting excitatory reaction by contraction and by concomitant electric change of galvanometric negativity. Ordinary plants, on the other hand, were regarded as irresponsive, both mechanically and electrically.

I have, however, been able to show ^ that all plants, and all their organs are excitable, the state of excitation being manifested by an electric response of galvanometric negativity. To obtain an indubitable demonstration of electric response a non-electrical mode of stimulation is essential ; otherwise the leakage of the stimulating induction-current might produce a galvanometric deflection not due to true excitation. This difficulty was overcome by a device by which the plant could be subjected to mechanical stimulation, the intensity of which could be maintained uniform or increased in a graduated manner.

Experiment 53. — I obtained uniform electric responses from the petiole, stem, roots and fruits of , various plants Fig. 36. Galvanometric Record of effect of Chloroform on Electric Response of Carrot. First three responses are normal ; subsequent depression produced by chloroform. under uniform stimulation. In order to prove that the electric response of galvanometric negativity is physiological, it was ascertained that depression was induced by the action of chloroform. The first three responses (fig. 36) were normal; the application of chloroform produced rapid diminution in the amplitude of response, which completely disappeared on the death of the tissue.

Having obtained the response to direct stimulation in ordinary plants, the next point is whether or not induced excitation is transmitted to a distance ; and, if so, what are its characteristics. By means of the mechanical Positive Response in Semi-conducting or Non-conducting Tissues Experiment 54. — ^The indifferent tissue of the lamina of the leaf is practically a non-conductor. Two electric response given by sensitive plants with motile leaves, it was ascertained that two distinct impulses, a positive and a negative, were generated under indirect stimulation, and could be discriminated under suitable conditions. What has now to be determined is whether or not this evidence is corroborated by the results of the investigation of the transmitted impulse by the electrical method. Should the various modifications of the transmitted impulse, manifested in the mechanical response, find their parallel in the electrical response, then, clearly, both forms of response must have a common source.

Fig. 37. — Electric connections for response of non-conducting tissue. Stimulus applied at cross near A. Fig. 38.-- Positive Electric Response of non-conducting tissue (downcurve). contacts are made at two points A and B on the lamina {fig. 37) ; a highly sensitive reflecting galvanometer is then put in circuit, the sensitiveness of wliicli is sucli tliut a current of ampere gives deflection of i mm. at a distance of i metre. The heated tip of a glass rod is momentarily applied at a distance of 5 to 10 mm. from the contact A. The response (down-curve) is the expression of an electric change of galvanometric positivity. Recovery takes place after the passage of the impulse (fig. 38).

I will now describe experiments on the impulse transmitted along highly conducting tissue. Diphasic and Monophasic Response The excitatory impulse is propagated from point to point in an animal nerve, each excited point becoming galvanometrically negative to^any point at rest. The wave of excitation is thus accornpanied by a wave of galvanometric negativity. The result is shown diagraramatically in fig. 39, in which stimulus is applied at a point marked with a cross near the electrode A. The wave reaches A the earlier, and the deflection of the galvanometer shows that A is negative compared with the unexcited point B. The wave soon moves past A and reaches B, making that point negative in respect of A ; this causes a reversal of the former galvanometric deflection. If the conducting power of the nerve be very high, the two opposite impulses act on the galvanometer in quick succession ; and since the galvanometric inertia is considerable, the resultant deflection is practically zero.

This difficulty in obtaining a record of electric response to transmitted impulse may be overcome in three different ways : First, by the employment of an electric recorder in which the inertia of the indicator is very slight, as in the string-galvanometer of Einthoven. The diphasic response, namely the negative response at A followed by that at B, may then be obtained on a moving photographic plate. The negative response at A then appears, say, as an up-curve, and that at B, as a down-curve.

Secondly, a monophasic response at A, unopposed by any at B, may be secured by abolishing the excitability of B, as by causing local injury. This method introduces several complications: a current of injury is produced, and the response at A is then observed as a negative variation of the current of injury. The injured tissue may, however, exhibit gradual recovery, on account of which the resultant response undergoes diminution or abolition. A fresh injury has therefore to be inflicted at or near B, to abolish excitability at that point, and thus obtain the unopposed response at A.

Thirdly, these drawbacks can be overcome by employing the Method of Natural Block, in which a non-conducting tissue is interposed between the first contact A and the second contact B. The excitatory impulse is then unable to reach the second contact. The specimen, in such a case, is uninjured, and the response of A is perfectly normal. It is necessary to find suitable means of stimulation to initiate the transmitted impulse. For merely qualitative purposes, stimulation may be effected by the application of a heated rod, or by the application of a drop of acid ; but these, as already stated, are incapable of repetition with the same intensity.

In the attempt to define the fundamental electric reactions to transmitted excitation, it is desirable that the stimulus should be non-electrical and at the same tirne capable of graduation in intensity. I have succeeded in perfecting a method of stimulation by thermal shock. A single loop of thin platinum wire may be made closely to surround the experimental tissue . or a V-shaped platinum wire may be applied on the conducting tissue, a film of water being interposed between the platinum wire tissue to prevent scorching. A definite electric current sent through the platinum loop for a given length of time gives rise to a sudden thermal variation which acts as a stimulus. This method is most convenient when successive stimulations of equal intensity are required, the duration of the heating current being regulated by a metronome. The intensity of the stimulus is graduated in a predetermined manner by the adjustment of the heating current, taking care that the platinum wire is raised in temperature only through a few degrees, so as not to injure the tissue. The required temperature can be estimated, in practice, by touching the platinum with one’s finger, a heat that can be felt without inconvenience. Excitation is produced in this way either by one, or by a summated

Having indicated the most suitable method of obtaining electric response and having eliminated all complications by the employment of a non-electric mode of stimulation, the following experiments were made which show that the conducting tissue of the plant exhibits all the responsive The typical reactions are best demonstrated with an ordinary leaf, in which the parenchymatous tissue of the lamina is practically a non-conductor. The midrib is, on the other hand, a conductor of excitation : in it the conducting phloem surrounds the xylem, the bundle being concentric. The electric connections are shown in fig. 40.

contacts A and B are made on the conducting midrib to obtain the diphasic response. Experiment 55. Monophasic response»—Tht record (fig. 41) gives successive responses to uniform .thermal Pig. 41. Electric Response of Midrib to indirect stimulation. Left record shows monophasic response exhibiting staircaseincrease. Right record exhibits diphasic response. stimuli, showing a staircase-increase due to Facilitation as the after-effect of stimulation. In experiments on indirect stimulation it is generally found that the conduction is ineffective in the beginning, the response being positive. The resistance or block is gradually removed by

In (a) the method of natural block is employed to obtain monophasic response, that is, one electrode is placed on the midrib, the other on the lamina; in (b) both the Fig. 40. — Electric Connections on Leaf for (a) Monophasic and (b) Diphasic response. Stimulus applied at cross. repeated stimulation, generally in four stages. In the first stage the response is positive ; in the second it is diphasic ; in the third it is negative, exhibiting a staircase-increase ; and in the final or fourth stage, when the conductivity attains an optimum value, the responses become uniform and maximum.

Experiment 56. Preferential conductivity. — A very interesting result is obtained by the application of stimulus at the mid-point between A and B (fig. 40, b). It is found that while the centrifugal excitatory impulse of a moderate stimulus reaches B, there is no such transmission in a centripetal direction towards A, which remains unaffected. It is only under stimulation of stronger intensity that conduction takes place in both directions. Experiment 57. Diphasic response. — When the two contacts A and B are both made on the midrib (fig. 40, b), the excitation first reaches A, producing negativity at A with a resulting up-curve in response : the excitation then reaches B, and a down-curve is obtained, indicative of the later negativity at B. A record of the diphasic response is given in fig. 41. The point B should be at a sufficient distance from A, otherwise the two impulses on the galvanometer would neutralise each other.

Experiment 58. Time-relations of the Monophasic response.— The wave of excitation passes from point to point along the conducting tissue ; the excitation of any point is therefore transitory. The matter of interest is to find out the rate at which it reaches its maximum, and the rate at which it declines. It should be remembered that the excitatory process in the plant is relatively slower than in the highly conducting animal nerve. Again in the midrib of an ordinary leaf, conduction is, generally speaking, slower than in the petiole of Mimosa. The rise and fall of excitation at any point can be determined by the concomitant electric variation. The inertia of the moving indicator of an ordinary galvanometer is too great for this determination. The string-galvanometer, on the other