The experimental arrangement is diagrammatically shown in Fig. 45. After attaching the petiole to the recording lever, indirect stimulus is applied, generally speaking, at a distance of 15 mm. from the responding pulvinus. Stimulus of electric shock is applied in the usual manner, by means of a sliding induction coil. The intensity of the induction shock is adjusted by gradually changing the distance between the secondary and the primary, till a minimally effective stimulus is found. In the study of the effect of direction of constant current on conductivity, non-polarisable electrodes make suitable electric connections, one with the stem and the other with 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 thus situated at a considerable distance from the anode or the cathode, in the indifferent region in which there is no polar variation of ex­cit­abil­ity. By means of a Pohl’s 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 will 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. 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. 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 induced 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 conductivity, we have to assure ourselves that under the particular conditions of the experiment the complicating factor of polar variation of ex­cit­abil­ity is eliminated. It is to be remembered that excitatory trans­mission in Mimosa takes place by means of a certain conduct­ing strand of tissue which runs through the stem and the petiole. In the experiment to be described, the constant current enters by the tip of the petiole and leaves by the stem, or vice versâ, 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 pulvinus is also at the same distance from the electrode on the stem. The point of stimulation and region of response 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.

EFFECTS OF DIRECTION OF CURRENT ON VELOCITY OF TRANSMISSION.

A very convincing method of demonstrating the influence of electric current on conductivity consists in the determination of changes induced in the velocity of trans­mission by the directive action of the current. For this purpose we have to find out the true time required by the excitation to travel through a given length of the conduct­ing tissue (1) 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 the stimulus and response. Now 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 pulvinus 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 PERIOD.

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 found that the average value of the latent period in summer is 0.1 second, we next proceed to determine the influence of the direction of current on velocity.

Experiment 41.—As a rule, stimulus of induction shock was applied in this and in the following experiments on the petiole at a distance of 15 mm. from the responding pulvinus. The recording writer was tuned to 10 vibrations per second; the space between two succeeding dots, therefore, represents a time-interval of 0.1 second. The middle record, N in Fig. 46, is the normal. There are 17 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 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 = 1 5/1.6 = 9.4 mm. per second. We shall next consider the effect of current in modifying the normal velocity. The uppermost record (1) in Fig. 46 was taken under the action of an ‘up-hill,’ or ‘against’ current of the intensity of 1.4 microampères. It will be seen that the time interval is reduced from 1.7 seconds to 1.4 seconds; making allowance for the latent period, the velocity of trans­mission under ‘up-hill’ current V₁ = 1 5/1.3 = 11.5 mm. 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 this mechanical mode of in­ves­ti­ga­tion is thus identical with that derived from the electric method of conductivity balance referred to previously.

That is to say, the passage of a feeble current modifies conductivity for excitation in a selective manner. Conductivity is enhanced against, and diminished with, the direction of the current.

The 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 microampères. 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 FEEBLE INTENSITY ON PERIOD OF TRANSMISSION THROUGH 15 MM.

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.

DETERMINATION OF VARIATION OF CONDUCTIVITY BY METHOD OF MINIMAL STIMULUS AND RESPONSE.

In this method we employ a minimal stimulus, the transmitted effect of which 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; the minimal response will tend to become maximal. Or 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. We may use a single break-shock of sufficient intensity as the test stimulus. It is, however, better to employ the additive effect of a definite number of feeble make-and-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-hill’ 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 trans­mission will be distinguished as Homodromous, and against the direction of trans­mission as Heterodromous.

AFTER-EFFECTS OF HOMODROMOUS AND HETERODROMOUS CURRENTS.

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 may 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 be a rebound and a temporary reversal of 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 43.—This inference will be found fully justified in the following experiment:—The first two responses are normal, after which the hetero­dromous current gave rise to an enhanced response. The depressing after-effect of a hetero­dromous current rendered the next response 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.

PART II—INFLUENCE OF DIRECTION OF ELECTRIC CURRENT ON CONDUCTION OF EXCITATION IN ANIMAL NERVE.

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 conduct­ing tissue of the plant.

THE METHOD OF EXPERIMENT.

In the experiments which I am about to describe, arrangements were specially made so that (1) the excitation 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 fortunate enough to secure frogs of unusually large size, locally known as “golden frogs” (Rana tigrina). A preparation was made 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 on the tendon (Fig. 48). 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 is applied in all cases on the nerve, midway between the two electrodes this point being at a minimum distance of 100 mm. from either electrode. The point of stimulation is, therefore, situated at an indifferent region.

Great precautions have to be taken to guard against the leakage of current. The general arrangement for the experiment on animal nerve is similar to that employed for the corresponding in­ves­ti­ga­tions on the plant. The 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.

VARIATION OF VELOCITY OF TRANSMISSION.

In the case of the conduct­ing tissue of the plant a very striking proof of the influence of the direction of current on conductivity was afforded by the induced variation 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 records (Fig. 49), obtained by means of the pendulum myograph, exhibit the effect of the direction of current on the period of trans­mission through a given length of nerve. The latent period of muscle being constant, the variations 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 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 below a table showing the results of several experiments.

TABLE V—EFFECT OF HETERODROMOUS AND HOMODROMOUS CURRENT OF FEEBLE INTENSITY ON VELOCITY OF TRANSMISSION.

VARIATION OF INTENSITY OF TRANSMITTED EXCITATION UNDER HETERODROMOUS AND HOMODROMOUS CURRENTS.

In the next method of in­ves­ti­ga­tion, the induced variation 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, where the constant current induces no variation of ex­cit­abil­ity. Stimulation is effected either by single break-shock or by the summated effects of a definite number of equi-alternating shocks, or by chemical stimulation

Experiment 45.—Under the action of feeble heterodromous current the transmitted excitation was always enhanced, 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).

In the experimental arrangement employed, the heterodromous 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 ex­cit­abil­ity of the responding muscle. Conversely, with homodromous current, the tendon is made the cathode and under its influence the muscle might have its ex­cit­abil­ity raised 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 (1) response under normal condition without any current, (2) response under heterodromous current, the tendon being the anode, and (3) response under 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 excitation, as will be seen in the following Table.

TABLE VIII.—INFLUENCE OF DIRECTION OF CURRENT ON DIRECT AND TRANSMITTED EFFECTS OF STIMULATION.

The passage of a current, therefore, induces opposing effects on the conductivity of the nerve and the ex­cit­abil­ity of the muscle, the resulting response being due to their differential actions. Under hetero­dromous current a more intense excitation 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 conduction 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 conductivity are, thus, far in excess of what are indicated in the records.

AFTER-EFFECTS OF HETERODROMOUS AND HOMODROMOUS CURRENTS.

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 (cf. Expt. 43). The same I find to be the case 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 salt on the nerve, so that the muscle, in response to the transmitted excitation, exhibited an 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 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. 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 the 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 conductivity. The parallel phenomena observed in the case of trans­mission of excitation in the animal nerve must, therefore, be due to the induced change of conductivity.

The action of an electrical 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 the nerve of the animal:—

LAWS OF VARIATION OF NERVOUS CONDUCTION UNDER THE ACTION OF ELECTRIC CURRENTS.

1. The passage of a current induces a variation of conductivity, the effect depending on the direction and intensity of current.

2. Under feeble intensity, hetero­dromous current enhances, and homo­dromous current depresses, the conduction of excitation.

3. The after-effect of a feeble current is a transient conductivity variation, the sign of which is opposite that induced during the continuation of current.

SUMMARY.

The variation of conductivity induced by the directive action of 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 the intensity of transmitted excitations is gauged by the varying amplitudes of resulting responses.

The great difficulty arising from leakage of the exciting induction current into the polarising circuit was successfully overcome by the interposition of a choking coil.

The following summarises the effects of direction and intensity of an electric current, on trans­mission of excitation through the conduct­ing tissue of the plant.

The velocity of trans­mission is enhanced against the direction of a feeble current, and retarded in the direction of the current.

Feeble hetero­dromous current enhances conductivity, homo­dromous current, on the other hand, depresses it.

Ineffectively transmitted excitation becomes effectively transmitted under hetero­dromous current. Effectively transmitted excitation, on the other hand, becomes ineffectively transmitted under the action of homo­dromous current.

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 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 current are in every way similar to those induced in the conduct­ing tissue of the plant.

These various effects are demonstrated by the employment 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 equi-alternating tetanising shocks or (3) by chemical stimulation.

VIII.—EFFECT OF INDIRECT STIMULUS ON PULVINATED ORGANS

By

Sir J. C. Bose,

Assisted by

Guruprasanna Das, L.M.S.

The leaf of Mimosa pudica undergoes an almost instantaneous fall when the stimulus is applied directly on the pulvinus which is the responding organ. The latent period, i.e., the interval between the application of stimulus and the resulting response is about 0.1 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.

The power of conduction varies widely in different plants. In the petiole of Mimosa pudica the velocity may be as high as 30 mm. per second. In the stem the velocity is considerably less, i.e., about 6 mm. per second in the longitudinal direction; but conduction across the stem is a very much slower process. In the petiole of Averrhoa the longitudinal velocity is of the order of 1 mm. per second.

DUAL CHARACTER OF THE TRANSMITTED IMPULSE.

The record of the transmitted effect of stimulus is found to exhibit a remarkable preliminary variation. This was detected by my delicate recorders, which gave magnifications from fifty to hundred times. I shall give a detailed account of a typical experiment carried out with Averrhoa carambola, which will bring out clearly the char­ac­ter­is­tic effects of Indirect Stimulus.

Experiment 47.—Stimulus of electric shock applied at a point on the long petiole of Averrhoa causes successive fall of pairs of leaflets. In the experiment to be described one of the leaflets of the plant was attached to the recorder. Stimulus was applied at a distance of 50 mm. The successive dots in the record are at intervals of a second. It will be noticed that two distinct impulses—a positive and a negative—were generated by the action of Indirect Stimulus. The positive impulse reached the responding organ after 1.5 second and caused an erectile movement. The velocity of the positive impulse in the present case is 33 mm. per second. The normal excitatory negative impulse reached the motile organ 44 seconds after the application of stimulus, and caused a very rapid fall of the leaflet, the fall being far more pronounced than the positive movement of erection (Fig. 52). In this and in all subsequent records, the positive and negative responses offer a great contrast. The movement in response to positive reaction is slow, whereas that due to negative reaction is very abrupt, almost ‘explosive,’ the successive dots being now very wide apart. As regards the velocity of impulse the relation is reversed, the positive being the quicker of the two. In the present case, the velocity of the excitatory negative impulse is 1.1 mm. per second, as against 33 mm. of the positive impulse.

The negative impulse is due to the comparatively slow propagation of the excitatory protoplasmic change, which brings about a diminution of turgor in the pulvinus and fall of the responding leaflet. The erectile movement of the leaflet by the positive impulse must be due to an increase of turgor, brought on evidently, by the forcing in of water. This presupposes a forcing out of water somewhere else, probably at the point of application of stimulus. It may be supposed that an active contraction occurred in plant cells under direct stimulus, in consequence of which water was forced out giving rise to a hydraulic wave. On this supposition the positive impulse is to be regarded as hydro-mechanical. I have, however, not yet been able to devise a direct experimental test to settle the question.

EFFECT OF DISTANCE OF APPLICATION OF STIMULUS.

In the last experiment the stimulus was applied at the moderate distance of 50 mm. Let us now consider the respective effects, first, of an increase, and second, of a decrease of the intervening distance. In a tissue whose conduct­ing power is not great, the excitatory impulse is weakened, even to extinction in trans­mission through a long distance. Thus the negative impulse may fail to reach the responding organ, when the stimulus is feeble or the intervening distance long or semi-conduct­ing. Hence, under the above conditions, stimulus applied at a distance will give rise only to a positive response.

A reduction of the intervening distance will give rise to a different result. As the negative response is the more intense of the two, the feeble positive will be masked by the superposed negative. The separate exhibition of the two responses is only possible by a sufficient lag of the negative impulse behind the positive. This lag increases with increase of length of trans­mission and decreases with the diminution of the length. Hence the application of stimulus near the responding organ will give rise only to a negative response, in spite of the presence of the positive, which becomes masked by the predominant negative.[P]

These inferences have been fully borne out by results of experiments carried out with various specimens of plants under the action of diverse forms of stimuli. In all cases, application of stimulus at a distance causes a pure positive response; moderate reduction of the distance induces a diphasic response—a positive followed by a negative; further diminution of distance gives rise to a resultant negative response, the positive being masked by the predominant negative.

From what has been said it will be understood that the exhibition of positive response is favoured by the conditions, that the transmitting tissue should be semi-conduct­ing, and the stimulus feeble. It is thus easier to exhibit the positive effect with the feebly conduct­ing petiole of Averrhoa than with the better conduct­ing petiole of Mimosa. It is, however, possible to obtain positive response in the Mimosa by application of indirect stimulus to the stem in which conduction is less rapid than in the petioles.

TABLE IX.—PERIODS OF TRANSMISSION OF POSITIVE AND NEGATIVE IMPULSES IN THE PETIOLE OF AVERRHOA AND STEM OF MIMOSA.

EFFECTS OF DIRECT AND INDIRECT STIMULUS.

From the results given in course of the Paper we are able to formulate the following laws about the effects of Direct and Indirect Stimulus on pulvinated organs:—

1. Effect of all forms of Direct stimulus is a diminution of turgor, a contraction and a negative mechanical response.

2. Effect of Indirect stimulus is an increase of turgor, an expansion and a positive mechanical response.

3. Prolonged application of indirect stimulus of moderate intensity gives rise to a diphasic, positive mechanical response followed by the negative.

4. If the intervening tissue be highly conduct­ing, the transmitted positive effect becomes masked by the predominant negative.

The laws of Effects of Direct and Indirect stimulus hold good not merely in the case of sensitive plants, but universally for all plants. This aspect of the subject will be treated in fuller detail in later Papers of this series.

IX.—MODIFYING INFLUENCE OF TONIC CONDITION ON RESPONSE

By

Sir J. C. Bose

Assisted by

Guruprasanna Das.

In experiments with different pulvinated organs, great difference is noticed as regards their ex­cit­abil­ity. If electric shock of increasing intensity from a secondary coil be passed through the pulvini of Mimosa, Neptunia, and Erythrina arranged in series, it would be found that Mimosa would be the first to respond; a nearer approach of the secondary coil to the primary would be necessary for Neptunia to show sign of excitation. Erythrina would require a far greater intensity of electric shock to induce excitatory movement. Organs of different plants may thus be arranged, according to their ex­cit­abil­ity, in a vertical series, the one at the top being the most excitable. The specific ex­cit­abil­ity of a given organ is different in different species.

In addition to this char­ac­ter­is­tic difference, an identical organ may, on account of favourable or unfavourable conditions, exhibit wide variation in ex­cit­abil­ity. Thus under favourable conditions of light, warmth and other factors, the ex­cit­abil­ity of an organ is greatly enhanced. In the absence of these favourable tonic conditions the ex­cit­abil­ity is depressed or even abolished. I shall, for convenience, distinguish the different tonic conditions of the plant as normal, hyper-tonic and sub-tonic. In the first case, stimulus of moderate intensity will induce excitation; in the second, the ex­cit­abil­ity being exceptionally high, very feeble stimulus will be found to precipitate excitatory reaction. But a tissue in a sub-tonic condition will require a very strong stimulus to bring about excitation. The ex­cit­abil­ity of an organ is thus determined by two factors: the specific ex­cit­abil­ity, and the tonic condition of the tissue.

THEORY OF ASSIMILATION AND DISSIMILATION.

A muscle contracts under stimulus; this is assumed to be due to some explosive chemical change which leaves the tissue in a condition less capable of functioning, or in a condition below par. Herring designates this as a process of dissimilation. The ex­cit­abil­ity of the muscle is restored after suitable periods of rest, by the opposite metabolic change of assimilation. “Assimilation and Dissimilation must be conceived as two closely interwoven processes, which constitute the metabolism (unknown to us in its intrinsic nature) of the living substance. Excitability diminishes in proportion with the duration of D-stimulus, or, as it is usually expressed, the substance fatigues itself. It is perfectly intelligible that a progressive fatigue and decrement of the magnitude of contraction must ensue. The only point that is difficult to elucidate is the initial staircase increment of the twitches, more especially in excised, bloodless muscle, which seems in direct contradiction with the previous theory.”[Q]

With reference to Herring’s theory given above, Bayliss in his “Principles of General Physiology” (1915), page 377 says, “In the phenomenon of metabolism, two processes must be distinguished, the building up of a complex system or substance of high potential energy, ‘anabolism,’ and the breaking down of such a system, ‘catabolism,’ giving off energy in other forms. The tendency of much recent work, however, is to throw doubt on the universality of this opposition of anabolism and catabolism as explanatory of physio­logic­al activity in general.”

The results obtained with the response of plants to stimulus may perhaps throw some light on the obscurities that surround the subject. They show that the two processes may be present simultaneously, and that the ‘down’ change induced by stimulus may, in certain instances, be more than compensated by the ‘up’ change.[R] I shall, for convenience, designate the physico-chemical modi­fi­ca­tion, associated with the excitatory negative mechanical and electrical response of plants, as the “D” change; this is attended by run down of energy. The positive mechanical and electrical response must therefore connote opposite physico-chemical change, with increase of potential energy. This I shall designate as the “A” change, which by increasing the latent energy, enhances the functional activity of the tissue. That stimulus may give rise simultaneously to both A, and D, effects, finds strong support in the dual reactions exhibited in plant-response. Under indirect stimulus, the two responses are seen separately, the more intense negative following the feeble positive. When by the reduction of the intervening distance, stimulus is made direct, the resultant response, as previously stated, is negative; and this is due not to the total absence of the positive but to its being masked by the predominant negative. Let us next consider the question of unmasking this positive element in the resultant negative response.

UNMASKING OF THE POSITIVE EFFECT.

Under favourable conditions of the environment, the ex­cit­abil­ity of the organs is at its maximum. A given stimulus will bring about an intense excitation, and the ‘down’ D-change will therefore be very much greater than the A-change. Let us now consider the case at the opposite extreme where, owing to unfavourable condition, the ex­cit­abil­ity is at its lowest. Under stimulus the excitatory D-change will now be relatively feeble compared to the A-change, by which the potential energy of the system becomes increased. In such a case successive stimuli will increase the functional activity of the tissue, and bring about staircase response. Biedermann mentions the staircase response of excised bloodless muscle as offering difficulty of explanation. It is obvious that the physio­logic­al condition of the excised muscle must have fallen below par. The staircase response in such a tissue is thus explained from considerations that have just been adduced.

The results obtained with Mimosa not only corroborate them, but add incontestable proof of the simultaneous existence of both A and D changes. The physio­logic­al condition of a plant, Mimosa for example, is greatly modified by the favourable or unfavourable condition of the environment. In a hyper-tonic condition its ex­cit­abil­ity becomes very great; in this condition the plant responds to its maximum even under very feeble stimulus. Here the D-change is relatively great, and successive responses are apt to show sign of fatigue.