The Nervous Mechanism of Plants

The important generalisation which I have endeavoured to establish is the identity of the physiological mechanism in the plant- and in the animal-nerve. This finds complete demonstration in the similarity of reaction not only of normal nerves, but also of modified nerves. When any point in the conducting tissue is acted on by an external stimulus, it receives the stimulation and is thrown into a state of excitation. This excitation is then conducted along the length of the tissue, and may be made

outwardly manifest at some distant point by means of a ^ suitable indication such as a fig. 61. Diagrammatic representaare caUed into play : namely, tion of the Conductivity Balance. first, the sensitiveness at the s, thermal stimulator ; c and c', the point of reception of the conducting arms of the balance ; . . - - . , - - . , Differential excitatory electrical Receptivity ; secondly, the effects at e and e' recorded by ^ r • • j: the excitation, Conductivity ; and thirdly, the manifestation of an excitatory effect at the distant responding region, which I term Responsivity.

The measurement of variation of these under change of external conditions is attended by numerous difficulties, which have ultimately been overcome by my device of the Conductivity Balance (fig. 6i). When the stimulus, given at S, is of sufficient intensity, the excitatory wave travels along both arms of the balance, through the conducting regions C and C', and induces excitatory electromotive effects at the two responsive points E and E'. The excitatory electrical effects at E and E' are opposed ; and when they are equal they balance each other ; and then the resulting galvanometer-deflection is reduced to zero.

Determination of Variation of Conductivity DUE to Chemical Agents Detailed account of investigations on receptivity and responsivity will be found in my work already referred to. I will here describe some of the results of the determination of variation of conductivity induced by the action of chemical agents. The balanced record is taken at the beginning ; the given chemical agent is then applied on a length of about i cm. at C on the right conducting arm (see fig. 61). If the effect of the agent is an increased conductivity of C, then the excitation transmitted to the right hand side E will be greater, and the upsetting of the balance will cause a resulting response upwards. Conversel}T a down-response will indicate that the effect of the agent has been to depress the conductivity. Again, it is possible to compare the relative effects on conductivity-variation induced by two different agents which are applied simultaneously, one on the arm C, and the other on C.

Experiment 67. — -As a typical example I describe the effect of dilute ( '5 per cent.) solution of Na2C03 on the conductivity of the nerve of the Fern. Inspection of figure 62 shows that a great and immediate enhancement of conductivity was produced, causing resultant up-responses which were particularly marked during the first four minutes. This increased conductivity then underwent continuous decrease and reversal into growing depression, as shown by the increasing down-responses. This record deserves special attention, inasmuch as it affords an insight

into a phenomenon which could not otherwise have been suspected. Greater conductivity is usually associated with increased velocity of transmission. It would, however, appear that the term conductivity reaUy covers two different phenomena which may not always be concomitant. That is to say, an increase of conductivity may mean either a greater speed of transmission of excitation, or a greater intensity of the excitation transmitted. The first four responses (fig. 62) show the induced enhancement of con-

Fig. 62. Photographic record of effect of dilute (-5 per cent.) solution of NagCOg on conductivity (Fern-nerve). ductivity by the fact that they are up-curves only. The fifth response, however, shows a marked preliminary twitch in the downward direction, followed by an up-response of some amplitude. This shows that the excitatory effect reached the right end E later than the left end E', though the intensity still remained greater. The continued action of the reagent subsequently reduced the intensity also, so that this diphasic response ultimately became converted into a purely monophasic down-response.

The method thus provides an unique means of discriminating between the two distinct elements in conduction, namely, speed and intensity. Experiment 68. — A special investigation was next undertaken of the effect of a constant electrical current in modifying normal conduction of excitation. The results obtained show that, under a feeble current, conduction is enhanced in an electric up-hill direction, that is to say, from the region of low to one of high electric potential ; and conversely, that it is retarded or abolished in a downhill direction, i.e, from a region of high to one of low potential. Further discussion of this totally unexpected result is given in the next chapter.

The vascular strands of the Fern can be readily isolated. They include the phloem which is the tissue conducting excitatory impulse, and which, Tike that of Mimosa, consists of long tubular cells. The isolated strand can be used experimentally, and may be described as a nerve. A transmitted impulse gives rise, in the isolated nerve of the Fern, to a response of gal vanometric negativity similar to that given by animal nerve. Tetanisation enhances the conducting power in both plant and animal nerve.

When the nerve is in the condition of sub-tonicity, the response to a transmitted impulse is positive. This abnormal positive is transformed, after tetanisation, into the normal negative. The characteristic responses of plant-nerve and animalnerve are in every way similar, in both the normal and the modified conditions, Tetanisation has the same effect on both. These common characteristics prove that the physiological mechanism of excitatory conduction is the same in the plant and in the animal.

The Conductivity Balance enables accurate determination to be made of the effect of chemical agents on the conductivity of the nerve. A constant electric current of feeble intensity enhances conduction in an electric up-hill direction; conversely, conduction is depressed or abolished in an electric down-hill direction. A PROBLEM of great interest, which has attracted my attention for several years, is the question whether excitation travels better in a conducting tissue with or against the direction of an electric current passing through it. The definite results obtained with my Conductivity Balance on conduction in the isolated nerve of the Fern were so unexpected that I undertook to reinvestigate the subject by a different method, which has given satisfactory results. The investigation was carried out not only with the conducting tissue of Mimosa, but also with the nerve of the Frog.^

The velocity of transmission in Mimosa is found from the mechanical response of the leaf registered by my Resonant Recorder, the writer being tuned to inscribe ten dots per second. After taking the record for normal transmission, two other records are obtained, with an electric current flowing along the petiole either with or against the direction of transmission of excitation. The experimental arrangement is shown diagrammatically in fig. 63. After attaching the petiole to the recording lever, indirect stimulation is applied, generally speaking, at a distance of 15 mm. from the pulvinus. The stimulus of electric shock is given by means of a sliding induction-coil : the intensity of the shock is adjusted by

^ ' The Influence of Homodromous and Heterodromous Current on Transmission of Excitation in Plant and Animal/ Proc. Roy. Soc,, B. Vol. 88, 1914. gradually varying the distance between the secondary and the primary coils, till a minimally effective stimulus is Fig. 63. Complete apparatus for investigation of the variation of conductivity in Mimosa. A, storage-cell ; s, potentiometer-slide, which, by alternate movement to right or left, continuously increases or decreases the E.M.F. of the constant current ; k, switch-key for putting constant current ' on ’ and ‘ off ' without variation of resistance ; E, e', electrodes of induction-coil for stimulation ; c, choking coil ; g, micro-ammeter,

found. The constant current is conveyed to the plant by non-polarisable electrodes placed one on the stena and the other on the tip of a sub-petiole, at a distance from each other of about 95 mm. The point of stimulation and the responding pulvinus are both situated at a considerable distance from the anode or the kathode, in the indifferent region in which there is no polar variation of excitability. By means of a PohFs commutator or reverser, the constant current can be maintained either * with ' or ' against ' the direction of transmission of excitation. The transmission in the former case is ' down-hill,' and in the latter case ' up-hill.' Electrical connections are so arranged that when the commutator is tilted to the right, the transmission is down-hill, when tilted to the left, up-hill.

The electrical resistance offered by the 95 mm. length of stem and petiole was found to be from two to three million ohms. The intensity of the constant current flowing through the plant can be read by unplugging the key which short-circuits the micro-ammeter G. The choking coil C prevents the alternating induction-current from flowing into the polarising circuit, and causing direct stimulation of the pulvinus. Before describing the experimental results, it is as well to enter briefly into the question of the external indication by which the conducting power may be gauged. A change of conductivity may be expected to give rise to a variation in the rate of propagation or to a variation in the magnitude of the excitatory impulse that is transmitted. There are thus several methods available for determining the induced variation of conductivity. In the first place, the variation of conductivity may be measured by the induced change in the velocity of transmission of excitation. In the second place, the transmitted impulse will give rise to an enhanced or a diminished amplitude of mechanical response determined by the increase or decrease of conductivity brought about by the directive action of the current. And, finally, the enhancement or depression of conductivity may be demonstrated by an ineffectively transmitted stimulus becoming effective, or an effectively transmitted stimulus becoming ineffective.

A very convincing method of demonstrating the influence of a constant electric current on conductivity consists in the determination of the changes induced in the velocity of transmission by the directive action of the current. It is necessary to ascertain the true time required for the excitation to travel through a given length of the conducting tissue (i) in the absence of the current, (2) ‘ against,’ and (3) ‘ with,’ the direction of the current. The true time is obtained by subtracting the latent period of the pulvinus from the observed interval between stimulation and response. Now the latent period may not remain constant, but may undergo change under the action of the polarising current. It has been stated that the excitability of the pulvinus does not undergo any change when it is situated in the middle or indifferent region. The following results prove that, under parallel conditions, the latent period also remains unaffected.

Table IV. — The Effect of Constant Electric Current ON THE Latent Period (Mimosa). These results (Table IV.) show that a current applied under the given conditions has practically no effect on the latent period, the slight variation being of the order of one-hundredth part of a second. This is quite negligible when the total observed transmission-time is, as in the following cases, equal to nearly 2 seconds. Having found that the average value of the latent period in summer is O'l sec., the determination of the influence of the direction of the current on velocity can be undertaken.

Experiment 69.— In this and in the following experiments the stimulus of induction-shock was, as a rule. Fig. 64. Record showing enhancement of transmission ' uphill'or against the constant current (uppermost curve) and retardation of velocity ‘ down-hill ' or with the current (lowest curve). N, normal record in the absence of current. indicates uphill' and down-hill ' transmission. applied on the petiole at a distance of 15 mm. from the pulvinus. The recording writer was tuned to 10 vibrations per second ; the space between two succeeding dots, therefore, represents a time-interval of 0 • i second. The middle record, N in fig. 64, is the normal. There are seventeen spaces between the application of stimulus and the beginning of response. The total time is therefore 1-7 seconds, and by subtracting from it the latent period of o-i second, the true time, i • 6 seconds, is obtained. The normal velocity is found by dividing the distance 15 mm. by the true time 1-6 seconds ; thus V=i5-i-i-6=9-4 mm. per second.

With regard to the effect of current in modifying the normal velocity, the uppermost record (i), fig. 64, was taken under the action of an ‘ up-hill,’ or ‘ against,’ current of the intensity of i'4 micro-amperes. It will be seen that the time was reduced from 1-7 seconds to i - 4 seconds ; making allowance for the latent period, the velocity of transmission under ‘ up-hill ’ current Vj = 15 4- 1 -3 = 11-5 mm. per second. The lowest record (3) gives the effect of a ‘ down-hill ’ current ; the time between stimulation and response was prolonged to 1-95 seconds and the velocity reduced to 8-i mm. per second. The conclusion arrived^ at from the results of this mechanical mode of investigation is identical with that based upon the results obtained by the electric method of the Conductivity Balance referred to previously : it is as follows : — The passage of a feeble constant current modifies conductivity for excitation in a selective manner ; it enhances conductivity when the direction of the constant current is opposite to that in which the excitation is travelling; it diminishes it when the direction of the constant current is the same as that in which the excitation is travelling. The results obtained with fouj different specimens are given in Table V.

The minimum current which induces a perceptible change of conductivity varies somewhat in different specimens. The average value of the minimal current in autumn is 1-4 microamperes. The effect of even a feeble current may be detected by emplopng a test-stimulus which is barely effective. Table V. — Showing Efiiects of Up-hill and Down-hill Current OF Feeble Intensity on Time of Transmission through 15 mm. Having demonstrated the effect of the direction of current on the velocity of transmission, I go on to describe other methods by which induced variation of conductivity may be detected.

In this method a minimal stimulus is employed, such that the transmitted impulse gives rise, under normal conditions, to a feeble response. If the passage of a constant current in a given direction enhances conductivity, then the intensity of the transmitted excitation will also be enhanced ; the minimal response will tend to become maximal ; excitation which had hitherto been ineffectively transmitted will now become effectively transmitted. Conversely, depression of conductivity will result in a diminution or abolition of response. A single breakshock of sufficient intensity may serve as the test-stimulus ; or the additive effect of a definite number of inductionshocks, the alternating elements of which are exactly equal and opposite, which is secured by causing rapid reversal of the primary current by means of a rotating commutator.

Experiment 70. — Working in this way, it was found that an impulse transmitted against the direction of the constant current became effective or was enhanced. A current, flowing in the same direction as that of the transmitted impulse, on the other hand, diminished the intensity of transmitted excitation or blocked it altogether. It will be convenient to designate the currents in the two directions ; the current in the direction of transmission will be termed Homodromous, and that against the direction oi ixQXismissiori Heterodr omous.

The passage of a current through a conducting tissue in a given direction causes, as has been shown, enhanced conductivity in an opposite direction. It may be supposed that this is brought about by a particular molecular arrangement induced by the current, which assists the propagation of the excitatory disturbance in a selected direction. On the cessation of this inducing force, there will be a rebound and a temporary reversal of the previous molecular arrangement, with concomitant reversal of the conductivity-variation. The immediate after-effect of a current flowing in a particular direction on conductivity is likely to be a transient change, the sign of which would be opposite to that of the direct effect. The after-effect of a heterodromous current may thus be a temporary depression, that of a homodromous current a temporary enhancement of conductivity.

Experiment 71. — This anticipation is fully justified in the following experiment : — The first two responses Fig. 65. Direct and after-effect of heterodromous and homodromous currents. First two records, N, n, normal ; J, , enhanced transmission under heterodromous current ; 4 > arrest of conduction as an aftereffect of heterodromous current. Next record shows arrest under homodromous current. Last record f shows enhancement of conduction greater than normal, as an after-effect of homodromous current. {Dotted arrow indicates the aftereffect on cessation of a given current,* f homodromous and 4 heterodromous current.) (Mimosa.)

were normal, after which the passage of a heterodromous current gave rise to an enhanced response. The depressing after-effect of the heterodromous current rendered the next stimulation ineffective. The next record, taken during the passage of the homodromous current, exhibits an abolition of response due to an induced depression of conductivity ; and the last record, the after-effect of the homodromous current as a response larger than the normal (fig. 65). These results show that the after-effect of cessation of a current in a given direction is a transient conductivity-variation of which the sign is opposite to that induced by the continuation of the current.

Influence of Direction of Constant Current on THE Conductivity of Animal Nerve I now take up the question whether an electric current induces any selective variation of conductivity in the animal nerve, similar to that observed in the conducting tissue of the plant. In the experiments which I am about to describe, special arrangements were made such that (i) the excitation had not to traverse the polar region, and that (2) the point of stimulation was at a relatively great distance from either pole. The fulfilment of the latter condition ensured the point of stimulation being placed in the neutral region.

In the choice of experimental specimens I was fortunate enough to secure Frogs of an unusually large size, locally known as ‘ Golden Frogs ’ {Rana iigrina). A preparation was made of the spine, the attached nerve, the muscle and the tendon. The electrodes for the constant current were applied at the extreme ends, on the spine and on the tendon (fig. 66). The following are the measurements, in a typical case, of the different parts of the preparation. Length of spine between the electrode and the nerve = 40 mm. ; length of nerve = 90 mm. ; length of muscle = 50 mm. ;

length of tendon =30 mm. Stimulus was applied in all cases on the nerve, at a point midway between the two Fig. 66. Experimental arrangement for study of the effect on conductivity of Frog’s nerve of the directive action of an electric current. N, nerve ; s, point of application of stimulus in the middle or indifferent region. electrodes, and at a minimum distance of loo mm. from either electrode. The point of stimulation is, therefore, situated in the indifferent region.

Induced Variation of Velocity of Transmission IN Animal Nerve Striking evidence of the influence of the direction of a constant current on conductivity in plants was afforded by the induced variation of the velocity of transmission. Equally striking is the result which I have obtained with the nerve of the Frog. Experiment 72.— The experiments described below were carried out during the cold weather. The following records (fig. 67), obtained by means of the pendulum-

Fig. 67. Efiect of constant electric current in inducing variation in velocity of transmission in animal nerve (Frog). N, normal record ; upper record shows enhancement, and lower record retardation of velocity of transmission under heterodromous and homodromous currents respectively. myograph, exhibit the effect of the direction of the constant current on the time of transmission through a given length of nerve. The latent period of muscle being constant, the variations in the records exhibit the actual changed rates of conduction. The middle record is the normal, in the absence of any current. The upper record, denoted by the lefthand arrow, shows the action of a heterodromous current in shortening the transmission-time and thus enhancing the velocity above the normal rate. The lower record, denoted by the right-hand arrow, exhibits the effect of a homodromous current in retarding the velocity below the normal rate. I find that a very feeble heterodromous current is enough to induce a considerable increase of velocity, which soon reaches a limit. For inducing retardation of velocity, a relatively strong homodromous current is necessary. I give on p. 146 a table showing the results of several experiments.