Life Movements in Plants

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. Change of conductivity may be expected to give rise to a variation in the rate of propagation or to a variation in the magni¬ tude of the excitatory impulse that is transmitted. Thus we have several methods at our disposal for determining the induced variation of conductivity. In the first place the variation of conductivity may be measured by the in¬ duced change in the velocity of transmission of excitation. In the second place, the transmitted effect of -a sub-maximal stimulus will give rise to enhanced or diminished amplitude of mechanical response, depending on 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 the ineffectively transmitted stimulus becoming effective, or the effectively transmitted stimulus becoming ineffective.

Exclusion of the factor of Excitability.--The object of the enquiry being the pure effect of variation of conduc¬ tivity we have to assure ourselves that under the particular conditions of the experiment the complicating, factor of polar variation of excitability is eliminator!. It is to be. remembered that excitatory transmission in Mimosa takes place by means of a certain coni net ing struid of tissue which runs through the stem and th? petiole. In the experiment to be described, the constant current enters by the tip of the petiole and leaves by the stem, or vice versa. the length of the intrapolar region being 95 mm. The point of application of stimulus on the petiole is 40 mm. from the electrode at the tip of the leaf. The responding pulvinos is also at the same distance from the electrode on the stem. The point of stimulation and region of re¬ sponse are thus at the relatively great distance' of 40 mm. from either the anode or the cathode, and may therefore be regarded as situated in the indifferent region. This is found to be verified in actual experiments.

A very convincing method of demonstrating the In¬ fluence : of electric current on conductivity consists in the determination of changes induced in the velocity of trans¬ mission ' by the directive action of the current. For this purpose we have to find out the true time requir * *y the excitation to travel through a given length of tiie con¬ ducting tissue (1) In the absence the current, (2) 4 against ’ and (3) ‘with’ the direction of the current. The true time is obtained by subsiractmg the latent period of the pul vinos from the observed interval between the stimulus and re¬ sponse. How the latent period may not remain constant, but undergo change under, the action of .the ..polarising current. It has been shown that the excitability of the pnlvlnus does not undergo any change, when it is situated in the 'middle or indifferent region. The' following results

show that under parallel conditions the latent period also remains unaffected TABLE V.—SHOWING THE EFFECT OF ELECTRIC CURRENT ON THE LATENT The results of experiments with two different specimens given above 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 period observed for transmission is, as in the following cases, equal to nearly 2 seconds.

Induced changes in the Velocity of Transmission .—Having fouafi' that the average value of the latent period in sum¬ mer is OT second, we next proceed to determine the in¬ fluence of the direction of current on velocity. Experiment 41 .—As a rule, stimulus of induction shock wah applied in this and in the following experiments on the petiolfe at a distance of 15 mm. from the responding pulvinus. The recording writer was tuned e to 10 vibrations per second; the space between two succeeding dots, therefore, represents a time-interval of OT second. The middle record, N in Fig. 46, is The normal. There are 17 spaces between the application ol stimulus and the beginning of response. The total time is therefore IT seconds, and by subtracting from it the latent period of 0-1 second we obtain the true time, 1-6 seconds. The normal velocity is found by dividing the distance 15

mm. by the true interval 1*6 seconds. Thus V = 15,1*6? = 9*1 mm. per second. We shall next consider the elect of current in modifying the normal velocity. The uppermost record (1) in Fig. 46 was taken under tin* action of an ‘ up-hill,’ or 1 against ’ current of the intensity of 1*4 micro¬ amperes. It will he seen that the time interval is reduced from 1*7 seconds to 1*4 seconds ; making allowance for ihe latent .period, the velocity of transmission under 4 op-hili’ current \ l =1 o/l*3 ^ 11**> nun. per second. In the lowest record (3) we note the effect of "down-hill’ current, the time-interval between stimulus and response being prolonged to 1*95 seconds and the .velocity reduced to 8*1 mm. per second. . The . conclusion arrived at from tills mechanical mode of investigation .is thus identical with that derived from the electric method of conductivity balance referred to previously.

Thai is to say, the passage of a feeble current modi - fies conductivity for excitation in a selective manner. C 09 - ductivity is enhanced against., and diminished with, the direction of the current. Hio minimum current which induces a perceptible change of conductivity varies somewhat in different specimens. The average value of this minimal current in autumn is 1-4 microamperes. The effect of even a feebler current may be detected by employing a test stimulus which is barely effective.

TABLE VI. SHOWING EFFECTS OF UP-HILL AND DOWN-HILL CURRENTS OF Period for up-hill | Period for do wn-hill transmission. j transmission ■■ Having demonstrated the effect of direction of current on the velocity of transmission, I shall next describe other methods by which induced variations of conductivity may be exhibited. In this method we employ a minimal stimulus, the transmitted effect of whic'h under normal conditions gives rise to a feeble response. If the passage of a, current in a given direction enhances conductivity, then the

intensity of transmitted excitation will also be enhanced; tt]e minimal response will tend to become maximal. Or excitation which had hitherto been ineffectively trans¬ mitted will now become effectively transmitted. Con¬ versely, depression of conductivity will result in a diminu¬ tion or abolition of response. We may use a single break-shock of sufficient intensity as the test stimulus. It is. however, better lo employ the additive effect of a definite number of feeble m a ke - an 1 1 -break shocks.

We may again employ additive effect of a definite number of induction shocks, the alternating elements of which are exactly equal and opposite. This is secured by causing rapid reversals of the primary current by' means of a rotating commutator. The successive induction shocks of the secondary coil can thus be rendered exactly equal and opposite. Experiment 42 .—Working in this way, it is found that the transmitted excitation against the direction of current becomes effective or enhanced under & up-bill ^ current. A current, flowing with the direction of transmission, on the other hand, diminishes the intensity of transmitted excitation or blocks it altogether.

Henceforth it would be convenient to distinguish currents in the two directions ; the current in the direction of trans¬ mission will be distinguished as HtuttadrnmrntH. aim against the direction of transmission as Hetermntiuovs. AFTER-EFFECTS OF H O M O D R O M O IT § AND ' IIE T E BO I) RO MO IT S The passage of a current through a conducting tissue in a given direction causes, as we' have seen, an enhanced conductivity in an opposite direction. We nay suppose this to be brought about by a particular, molecular arrangement

induced by the current, which assisted the propagation of the excitatory disturbance in a selected direction. On the cessation of this inducing force, there may lie a rebound and a temporary reversal of previous molecular arrange¬ ment, with concomitant reversal of .the conductivity varia¬ tion. The immediate after-effect of'a current flowing in ‘ a particular direction on conductivity is likely to be, a tran¬ sient change, the sign of which would be opposite f 0 that of the direct effect. The after-effect of a heterodromous current may thus be a temporary depression, that of homodromous current, a temporary enhancement of eon'- (luctivity.

cessation of a eioon current. * homodr,.. I hl . l( . rodl , gave rise to an enhanced response. 'The depressing after¬ effect of a heterodromous current rendered the next re¬ sponse ineffective. * The following record taken during the passage of the homodromous current exhibited an abolition of response due to induced depression of conductivity. Finally, the after-effect of the homodromous current is seen to be a response larger than the normal (Fig, 47)., These experiments 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.

I shall now take up the question whether an electric current induced any selective variation of conductivity in the animal nerve, similar to that induced in the conduct¬ ing tissue of the plant. In the experiments which I am about to describe, arrangements were specially, made so that (1) the excita¬ tion had not to traverse the polar region, and (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 at the neutral region*

. In ■, the choice of experimental specimens I was fortu¬ nate enough to secure frogs of unusually large size, locally, known as “ golden frogs ” {Kara tigrina)* A preparation wasytaade "of the spine, the attached nerve,, the muscle' and. the tendon. The electrodes for constant current were applied at the extreme ends, on the spine..and op the experiment on animal nerve is similar to that employed for the corresponding investigations on the plant* I he choking coil is used to prevent the stimulating induction current from getting round the circuit of constant current. The specimen is held on an ebonite support, and every part of the apparatus insulated with the utmost care.

Iii the case of the conducting tissue of the plant a very striking proof of the influence of the direction of cur¬ rent on conductivity was afforded by the induced varia¬ tion of velocity of transmission. Equally striking is the result which I have obtained with the nerve of the frog. Experiment 44 .—The experiments described below were carried out during the cold weather. The following re¬ cords (Fig. 49), obtained by means of the pendulum myo¬ graph, exhibit the effect of the direction of current on

in the records exhibit changed rates of conduction, .The middle record is the normal, in the absence of any current. The upper record, denoted by the left-hand arrow, shows the action of a heterodromous current in shortening the period of transmission and thus enhancing the velocity ■ above the normal rate. The lower record, denoted by the right-hand arrow, exhibits the effect of a homodrom- otis current in retarding the velocity below' the norma! 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 ho mo dromons current is necessary, 1 give below a table showing the results of several experiments.

In the next method of * investigation, the induced varia¬ tion of intensity of transmitted excitation is inferred from the varying amplitude of response of the terminal muscle. Testing stimulus of sub-maximal intensity is applied at the middle *of the nerve, wherfc the constant current induces no variation of excitability. ‘ Stimulation is effected either by single break-shock or by tlm summated effects of a definite number of e^ui-alternating shocks, or by chemical stimulation

Experiment 45.—Under the action of feeble heterodrom- o'us current the transmitted excitation was always enhanc- ' ed, whatever be the form of stimulation. This is seen : illustrated in Fig. 50. Homodromous current on the other hand inhibited or blocked excitation (Fig. 51). Fioj. 50.—Ineffectively transmitted salt-tetanus becoming effective under hetero- dromons current, denoted by down-pointing arrow. * Complication due to variation of Excitabilitty of Muscle .— -"'In experiments with the plant, there was the unusual advantage in having both the point of stimulation and the responding motile organ in the middle or indifferent region Unfortunately this ideally perfect condition cannot be secured in experiments with the nerve-and-muscle prepara¬ tion of the frog. It is true that the point of stimulation in this case is chosen to lie on the nerve at the middle or indifferent region. But the, responding muscle is at one end, not very distant from the electrode applied on the tendon. It is, therefore, -necessary to find out by separate experiments any variation of excitability that might be induced in the muscle by the proximity of either the anode or * the' cathode, and make "allowance for such variation in interpreting the results obtained from investi¬ gations on variation of conductivity.. :

In the experimental arrangement employed, the hetro- dromons current is obtained by making the electrode on the spine cathode and that on the tendon anode. The depressing influence of the anode in this case may be expected to lower, to a certain extent, the normal excita¬ bility of the responding muscle. Conversely, with homo. dromous current, the tendon is made the cathode an4 under its influence the muscle might have its excitability iaised above the normal. These anticipations are fully supported by results of experiments. Sub-maximal stimulus of equi- alternating induction shock was directly applied to the muscle and records taken of (I) response under normal condi¬ tion without any current, (2) response under heterodromous current, the tendon being the anode, and {8) response underp homodromous current, the tendon being now made the cathode. It was thus found that under heterodromous current the excitability of the muscle was depressed, and under homodromous current the excitability was enhanced.

The effect of current on response to direct stimulation is thus opposite to that on response to transmitted excita¬ tion, as will be seen in the following Table. The passage of a current, therefore, induces opposing effects on the conductivity of the nerve and the excitabil¬ ity of the muscle, the resulting response being due to their differential actions. ( Under heterodromous current a more intense exclration is transmitted along the nerve, on account of induced enhancement of conductivity. But this intense excitation finds the responding muscle in a state

of depressed Excitability. In spite of this the resulting response is enhanced (Fig. 50). The enhancement of con¬ duction under heterodromous current is, in reality, much greater than is indicated in the record. Similarly, under homodromous current the depression of conduction in the nerve may be so great as to cause even an abolition of response, in spite of the enhanced excitability of the muscle (Fig. 51). The actual effects of current on conduc¬ tivity are, thus, far in excess of what are indicated in the records.

On the cessation of a current there is induced in the plant-tissue a transient conductivity change of opposite sign to that induced by the direct current {of. Expt. 43). The same I find to be the ease as regards the after-effect of current on conductivity change in animal nerve. Of this I only give a typical experiment of the direct and after- effect of homodromous current on salt-tetanus. Experiment 46. —In this experiment sufficient length of time was allowed to elapse after the application of the iait

VtG. 5i.—Direct and after-effect of homodromous current. Transmitted ex¬ citation (sait-tetanus T,) arrested under homodromous current denoted by up-point¬ ing arrow; on cessation of current represented by dotted line there is a tran¬ sient enhancement above tne normal. ©a Ike nerve, so that the muscle, in response to tie trans- mftfg^ ek^Ktatleftr exhibited air incomplete tetanus’ T. The homodromous current was next applied, with the result' of inducing a . complete block of conduction, with the- , concomitant disappearance of tetanus. The homodromous • current was gradually reduced to zero by the appropriate movement of the potentiometer slide. The after-effect of homodromous current is now seen in the transient enhance¬ ment of transmitted excitation, which lasted for nearly 40 seconds. After this the normal conductivity was restored. Repetition of the experiment gave similar results (Fig. 51).

The results that have been given are only typical of a very large number, which invariably supported the _ characteristic phenomena that have been described. It will thus be seen that with feeble or moderate current, ■ conductivity is enhanced against the direction of the current and depressed or blocked with the direction of the current. Under strong current the normal effect is liable to undergo ■ a reversal. It has thus been shown that a perfect parallelism exists in Ike conductivity variation induced in the. plant and in the animal by the directive action of the current. No explanation could be regarded as satisfactory which is not applicable / to both : cases. Now with the plant we .are able-to arrange the experimental condition in such a way that the factor of variation of excitability is completely eliminated. The various effects described about'the plant- tissue are, therefore, due entirely to. variation . of condue- tivity. The parallel phenomena observed in the case of transmission' of . excitation in the animal .nerve must, there¬ fore, be due to the-induced change of conductivity.

The action '‘of an electrical - current in inducing ' vaca¬ tion of conductivity may be enunciated under-the following laws, which are equally applicable to the conducting issue' of the plant and the nerve of the animal 1. The passage of a current induces a variation of conducti¬ 2. Under feeble intensity, heterodromous current enhances, and 3 . The after-effect of a feeble current is a transient conduc¬ action of current has been investigated by two different methods

transmitted excitations is gauged by the varying amplitudes of resulting responses. The great difficulty arising from leakage of the excit¬ ing ' induction current into the polarising circuit was successfully overcome by the interposition of a' choking The following summarises the effects of direction and intensity of an electric current, on transmission of excit- The velocity of transmission is enhanced against the direction of a leeble current, and retarded in the direction of the current.

Feeble heterodronaous current enhances conductivity ^ homodromous current, on the other hand, depresses it* Ineffectively transmitted excitation becomes effectively transmitted under heterodromous current. Effectively trans- ■ nutted excitation, on the other hand, becomes* ineffectively The after-effect of a current is a transient conductivity change, the sign of which is opposite to that induced during the passage of current. The after-effect of a hetero¬ dromous current is, thus, a transient depression, that of homodromous current, a transient enhancement of conduct¬ ivity.

The characteristic variations of conductivity induced in animal nerve by the direction and intensity of current are in every way similar to those induced in the conducting tissue of the plant. These various effects are demonstrated by the employ¬ ment of not one, but various kinds of testing stimulus, such as the excitation caused (1) by a single break- induction shock or (2) by a series of eqni-aiternating tetanising shocks or (3) by chemical stimulation.

The leaf of Mimosa pudica undergoes an almost instantaneous fall when the stimulus is applied directly on the pulvinus which is Ihe responding organ. The latent period, i.e., the interval between the application of stimulus and the resulting response is about 01 second. Indirect stimulus, i.e., application of stimulus at a distance from the pulvinus, also causes a fall of the leaf; but a longer interval will elapse between the incidence of stimulus and the response; for it will take a definite time for the excitation to be conducted through the intervening tissue. I have already shown that this conduction of excitation in plant is analogous to the transmission of nervous impulse in animal.