Life Movements in Plants

I also..determined the relative values of the minimal inten¬ sity of stimulus, effective in causing transmission of excita¬ tion in the three cases. Adopting as before the intensity of electric stimulus which causes bare^ perception in a human being as the unit, I find that the effective stimulus for a fully grown young petiole is 0*3 unit, while the very yonng required 2*5 units, ^ and the very old 5 units. Hence* it may be said that the conducting power of a very young is an eighth, and of the very old one-sixteenth of the conductivity of the fully grown young specimen. "

It will thus be seen that the conducting power of a very young petiole is feebler than in a fully grown speci¬ men. The conducting tissue, it is true, is present, but the power of conduction has not become fully developed. This .ypower is, as we shall see later, conferred’by the stimulus of the environment. In a very old specimen the diminution ’of conducting power is due to the general physiological I have already shown that transmission in the plant is a process fundamentally similar to that taking place in

the animal nerve; it has also been shown that the effects of various physical and chemical agents are the same in the conducting tissues of plant and of animal. known that desiccation, generally speaking, enhances the excitability of the animal nerve. As glycerine, by absorp¬ tion of water, causes partial desiccation, I tried its effect on conduction of excitation in the petiole of Mimosa. Enhancement of conducting power may be exhibited in two ways: first, by an increase of velocity of transmis¬ sion ; and, secondly, by an enhancement of the intensity of the transmitted excitation, which would give rise to a greater amplitude of response of the motile indicator. In Fig. 41 are given two records, N, before, and the other after

Fig. 41— Action of glycerine in enhancing the speed and intensity of transmitted excitation. Stimulus applied at the vertical line. Successive dots in record are at intervals of 0*1 sec. the application of glycerine on a length of petiole through which excitation was being transmitted. The time-records demonstrate conclusively the enhanced rate of transmission after the application of glycerine. The increased intensity of transmitted excitation is also sden in the enhanced

amplitude of response seen in the more erect curve in the upper record. Different specimens of Mimosa are found to exhibit differences in physiological vigour. Some are in an opti¬ mum condition, others in an unfavourable or sub-tonic condition. I shall now describe certain characteristic differences of conductivity exhibited by tissues in different conditions. Effect of intensity of stimulus on velocity of transmis¬ sion.—In a specimen at optimum condition, the velocity remains constant under varying intensities of stimulus. Thus the velocity of transmission in a specimen was deter¬ mined under a stimulus intensity of 0‘5 unit; the next determination was made with a'stimulus of four times the previous intensity, i.e., 2 units. In both these cases the velocity remained constant. But when the specimen is in a sub-tonic condition, the velocity is found to increase with the intensity of the stimulus. Thus the velocity of conduction of a specimen of Mimosa in a sub-tonic condition was found to be 5-9 mm. per second under a stimulus of 0-5 unit; with the intensity raised to 2‘5 units, the velocity was enhanced to 8'3 mm. per second.

After-effect of stimulus.—In experimenting with a parti¬ cular specimen of Mimosa I found that on account of its sub-tonic condition, the conducting power of the petiole Was practically absent. Previous stimulation was, however, found to confer the power of conduction as an after¬ effect. It is thus seen that stimulus canalises a path for conduction. The effect of excessive stimulus in a., specimen in an optimum condition is to induce a temporary depression of conductivity ; the effect of strong stimulus on a suh-tonic

specimen is precisely the opposite, namely, an enhancement of conductivity. I give below accounts of two typical experiments carried out with petiole-pulvimis preparation of Mimosa . Excessive stimulation in these cases was caused by injury. A cut stem with entire leaf was taken, and stimulus applied' at - a distance of 15 mm. from the. pulvimis. From the normal record (1) in Fig. 42 the velocity of- transmission was Fig. 42.—Effect of injury, depressing rate of conduction in normal specimen ;

found to be 187 mm. per sec. The end of the petiole beyond the point of application of the testing stimulus was now cut off, and record of velocity of transmission taken once more. It will be seen from record (2) that the excessive stimulus caused by injury had induced a depression in the conducting power, the velocity being v reduced to 107 mm. per sec. Excessive stimulation of normal specimens is thus seen to depress temporarily the conducting power.

Actio%i of Injury on Sub-tonic Specimens : Experiment 37 .—I -$ill now describe a very interesting experiment which shows how ah identical agent may, on account of difference in the tonic condition of the tissue, give rise metrically opposite effects. In demonstrating this, I-took a specimen in a sub-tonic condition, in which the conducting power of the tissue was so far below par, that the test- stimulus applied at a distance of 15 mm. failed to be transmitted (Fig. 43). The end of the petiole at a distance

Fig. 43 .—' Effect of injury in enhancing we conducting ^ ■» norma! specimen!; (1) Ineffective transmission becoming effective at (2) after section; (3). decline after half an hoar, and (4) increased conductivity after a fresh cut. ^ of 1 cm. beyond the point of application of test-stimulus was now cut off. The after-effect of this injury was found to enhance the conducting power so that the stimulus previously arrested was. now effectively transmitted, the velocity being 25 mm. per sec. This enhanced conducting power began slowly to decline, and after half an hour the velocity had declined to 4T mm. per see. The end of the petiole was cut once more, and the effect of injury was again found to enhance the conducting power, the velocity of . transmission being restored to 25 mm, per sem

There are two different types of propagation Of excit¬ ation: by convection, and by conduction. In the former the excited yfett undergoes deformation and causes mechanical stimulation of the next; example of this type-is seen in the stamen of Berberis . The conduction of excitation con¬ sists, on the other hand, of propagation of excitatory protoplasmic change. The tranmission in the petiole of Mimosa is a phenomenon of conduction. This conduction takes place along vascular elements. The conductivity is very much greater 1 in the longitudinal than in the transverse direction.

Rise of temperature enhances, and fall of temperature lowers, the rate of condoction. Excitation is transmitted in both directions ; the centrifugal velocity is greater than the centripetal. Dessication of conducting tissue by glycerine enhances the conducting power. Local application of cold depresses or arrests the conduction. Application of poison permanent¬ ly abolishes the power of conduction. Conductivity is modified by the effect of season, being higher in summer than in winter.

The power of conduction is also modified by age. In young specimens the conducting power is low, the conduc¬ tivity^ is at its maximum in fully grown organs ; but a decline of conductivity sets in with age. The tonic condition of a tissue has an influence on conductivity. In an optimum condition, the velocity is the same for feeble or strong stimulus. Excessive stimulation induces a temporary depression of the conducting power. The effect's are different in a sub-tonic tissue : velocity of transmission increases with intensity of stimulus ; after¬ effect of stimulus is to initiate or enhance the conducting power. The conducting path is canalised by stimulus.

j have in my previous works * described investigations on the conduction of excitation in Mimosa pudica. It was there shown that the various' characteristics of the propa¬ gation of excitation in the conducting tissue of the plant are in every way similar to those in the animal nerve. Hence it appeared probable that any newly found pheno¬ menon in the one case was likely to lead to discovery of a similar phenomenon in the other. As the transmission of excitation is a phenomenon of propagation of molecular disturbance in the conducting vehicle, it appeared that the excitatory impulse could be controlled by inducing in the conducting tissue two opposite ‘molecular dispositions’, using that term in the widest sense. The possibility of accomplishing this by the directive action of an electric current had attracted my attention for many years.

I have previously carried out an electric method of in¬ vestigation, dealing with the influence of electric current on conductivity. The method of Conductivity Balance which I devised for this purpose! was found very suitable. Isolated conducting tissues of certain plants were found to exhibit , bose-- Comparative Electro-Physiology - (1907). Longmans, Green and Co. t Ibid, p.478. transmitted effect of excitatory electric change of galvano- metric negativity, which at the favourable season of the year was of sufficient intensity to be recorded by a sensitive galvanometer. A long strand of the conducting tissue was taken and two electric connections were made with a galvanometer, 2 r few centimetres from the free ends. Thermal stimulus was applied at the middle, when two excitatory waves with their concomitant electric changes were transmitted outwards. By suitably moving the point of application of stimulus nearer or further away from one of the two electric contacts, an exact balance was obtained. This was the case when the resultant galvanometer deflec¬ tion was reduced to zero. If now an electrical current be sent along the length of the conducting tissue, the two excitatory waves sent outwards from the central stimulated point will encounter the electric current in different ways ; one of the excitatory waves will travel with, and the other against the direction of the current. If the power of transmitting excitation is modified by the direction of an■ electric current then the magnitudes of transmitted excitations will be differ¬ ent in the two cases, with the result of the upsetting of the donductivity Balance. From the results of experiments carried out by this method on the effect of feeble current on conductivity, the conclusion was arrived at that excitation is better conducted against the direction of the current than with it. In other words, the influence of an electric current.is to confer a preferential or selective direc¬ tion of conductivity for excitation, the tissue becoming a better conductor in an electric up-hill direction compared with a down-hill.

4. he results were so unexpected that I have for long been desirous of teliing the validity of this conclusion by inde¬ pendent method of inquiry. I shall presently give full account of the perfected method, and the various difficulties which had to bo overcome to render it practical. Before doing his I shall describe a simple method which I have devised for demonstrating the principal results. The petiole of Averrhoa bilimbi has a large number of paired leaflets, which, on excitation, undergo downward closure.* Feeble stimulus is applied at a point in the petiole, and the transmission of excitation is visibly mani¬ fested by the serial fall of the leaflets. The distance to which the excitation reaches is a measure of normal power of conduction. Any variation of conductivity, by the passage of an electric current in one direction or the othe* is detected by the enhancement or diminution of the dis¬ tance through which excitation is transmitted. I shall describe the special precautions to be taken in carrying oai this investigation.

Electric stimulus of induction shock of definite intensity and duration is supplied at the middle of the petiole at EE (Fig 44; The leaflets to the left of E, are not necessary for^ the purpose of this experiment and therefore removed. The intensity of the induction shock may be varied in the usual manner by removing the secondary coil nearer or farther from the primary. The duration of the shock is always maintained constant. On application of electric stimulus excitation is transmitted along the petiole, the dis¬ tance of transmission depending on the intensity of stimulus. With feeble stimulus two pairs of leaflets may undergo an excitatory fail ; with stronger stimulus the transmission is extended to the end of the petiole, and all the leaflets exhibit movements of closure. We shall now study the modifying influence of a constant current on conduction of excitation. C is an electric cell, R the reversing key by which the electric current could be sent from right to left or in the opposite direction. When the current is sent from right to the left, the excitatory impulse initiated at EE' travels against the direction of the current in an ‘up-hill’ direction. When the current is reversed it flows in the petiole from left to right and the transmitted impulse travels with the current or in a 4 down-hill 9 direction.

Two complications are introduced on the completion of the electric circuit ■■ of the constant current: the first, is the distributing effect of leakage of the induction current used for excitation, and second,.the polar variation of excitation induced by the constant current. Leakage of induction current .—Before completing the constant current circuit, fjie alternating induction current goes only through the path EE'. On completion of the constant current circuit, the alternating induction current not only passes through the shorter path EE' but also by the circuitous path of the constant current circuit. The escaping induction current would thus excite all the leaflets directly and not by its transmitted section. This difficulty is fully overcome by the interposition of a

Liking coil which will be described below. A sampler, thigh leas perfect, device may he employed toredoce an ScaUy elimiBate the leakage. Th.s ~->f " r wire placed onteide EE'. The leakage ot mdac Polar action of current on excitability.—It is we ^own t jr electric current induces a local * £ a distance from th& ^°^J &nced conductivity conferred it will be shown that the en overpo wers any Tmnsmiuim of txcM to f — , Uol may he .hall gi™ here a» aocon ^ ■ b lpMilneB oI Ave „ hM

Jefi A necessary precaution is to increase the current gradu¬ ally by means of k suitable potentiometer slide, to its full value. The reason for this will be given later* The in¬ tensity of the constant current employed was J*4 micro-' amperes. Now oh exciting the' petiole by the previous stimulus, the conducting power was found to be greatly enhanced. The. excitatory impulse now reached, the end of the petiole, and caused six pairs of leaflets to fall. -

Transmission of excitation '‘Down-hill’ : Experiment 89 .— In continuation of the previous experiment, the constant electric current was reversed, its directions being now from left to right.. Transmission of excitation was now in a down-hill direction. On applying the induction shock stimu¬ lus of the same intensity as before, the conducting power of the petiole, was found to be abolished, none of the leaflets exhibiting any. sign of excitation. "This modification of the conducting .power persists during the passage of the constant current. On cessation of the current the original ecfeducti-ng'-power is- found to be restored. B will thus be seen that the power of conduction is capable of modifica¬ tion, and. that the passage of an electric current of moderate intensity induces enhanced power of conduction in an ‘up-hill* and diminished conductivity in a ‘down-hill* direction.

■ Tn my 4 -Researches on Irritability of Plants 9 - I have shown -how intimately connected are the various physio¬ logical reactions in the plant and in .the animal, and I .response in plant and- animal will lead to further dis¬ coveries in physiology in general. This surmise has been fully. justified, as will be seen in the following experi¬ ments carried . out on the nerve-and-muscie preparation of - a ,fro£.- It is best to carry out the experiments with vigor¬ ous specimens ; this ensures success, even in long continued experiments, which can then be repeated with unfailing certainty for hours. It is also an advantage to use a large frog for its relatively great length of the nerve.

Directive action of current on conduction of excitation in a nerve-and-muscle preparation: Experiment 40 .—A preparation was made with a length of the spine and two nerves leading to the muscles. The specimen is supported in a suitable manner, and electric connections made with the toes, one for the entrance and the other for exit of the constant current. The current thus entered, say,, by /the le F t toe ascended the muscle and went up ihe_ nerve, on the left side, and descended through the other nerve on the right side along the muscle and thence to the right 'toe. Before the passage of the constant electric current the spinal nerve was stimulated by an induction shock of definite intensity. The nervous impulse was conducted by the two nerves, one to the left and the other to the right, and caused a feeble twitch of the respective muscles. A feeble current of 1-5 micro-ampere was sent along the nerve-and-muscle circuit, ascending by the left and de¬ scending by the right side. It will be seen that excita¬ tion initiated at the spine is propagated ‘against’ the electric current on the left side, and ‘with’ the current on the right side. On repetition of previous electric

stimulus the effect of directive action of current was at once manifested by the left limb being thrown into a state of strong tetanic contraction, whereas the right limb remained quiescent. By changing the direction of the constant current the induced enhancement of con¬ ductivity of the nerve was quickly transferred from the left to the right side, the depression or arrest of condacr tion being simultaneously transferred to the' left side. Turning the reversing key one way or the other brought

abgut supra or non-conducting state of the nerve, and this condition was maintained. throughout the duration of the current. I shall next describe a more perfect method for obtain¬ ing quantitative results both with plant and animal. In order v to demonstrate the universality of the phenomenon, I next used Mimosa pudica instead of Averrhaa, for experiments on plants. For determination of normal velocity of transmission of excitation and the induced variation of that velocity, I employed the automatic method of recording the velocity of transmission of excitation in Mimosa, where the excit¬ atory fall of the motile leaf gave a signal for the arrival of the excitation initiated at a distant point. In this method the responding leaf is attached to a light lever the writer being placed at right angles to it. The record is taken on a smoked glass plate, which during its descent, makes an instantaneous electric contact, in consequence of which a stimulating shock is applied at a given point of the petiole. A mark in the recording plate indicates the mopaent of application of stimulus. After a definite interval the excitation is conducted to the responding pul- vinus, when the excitatory fall of the leaf pulls the writer suddenly to the left. From the curve traced in this manner the time-interval between the application of stimulus and the initiation of response can be found, and the normal rate of % transmission of excitation through a given length of the conducting tissue deduced. The experiment is then repeated with an electric current flowing along the petiole with or against the direction of transmission of excitation. The records .thus obtained enable us to determine the influence of the direction of the current on the rate of transmission. I. shall presently describe the various diffi¬ culties which have to be overcome before the method ‘fust indicated can be rendered practical.

The scope of investigation will be best describe!, accord¬ ing to the following plan*:— Effect of feeble current on velocity of transmission of excitation ‘ up-hill ’ or ‘ down-hill.’ Determination of variation of conductivity by the method of minimal stimulus and response. Variation of velocity of transmission under the action of current. I may here say a few words of the manner in which the period of transmission can be found from the record given by my Resonant Recorder, fully described in my previous paper. The writer attached to the recording lever of this instrument is maintained by electromagnetic means in a state of to-and-fro vibration. The record thus con¬ sists of a series of dots made by the tapping writer, which is tuned to vibrate at a definite rate, say, 10 times per second. In a particular ease whose record is given in CJurve 1 (Pig. 46), indirect stimulus of electric stoc.k was applied at a distance of 15 mm. from the responding

* For fuller account >ee Bose-‘The influence of Homoctromous and Hetero- drcmous Electric Current on Transmission of Excitation in Plant and Animal. Proe. R. S. B.. Voi. 88, 1915. pulvinus. There are 15 intervening dots between the moment of application of stimulus and the beginning of response; the time-interval is therefore 1*5 seconds. The latent period of the motile pulvinus is obtained from a record of direct stimulation; the average value of this in summer is OT second. Hence the true period of trans¬ mission is 1-4 seconds for a distance of 15 mm. The velocity determined in this particular case is therefore 10’7 mm. per second.

Precaution has to be taken against another source of disturbance, namely, the excitation caused by the sudden commencement or the cessation of the constant current. I have shown elsewhere* that the sudden initiation or cessa¬ tion of the current induces an excitatory reaction in the plant-tissue similar to that in. the animal tissue. This difficulty is removed by the introduction of a sliding potentiometer, which allows the applied electromotive force to be gradually increased from zero to the maximum or decreased from the maximum to zero.

The experimental arrangement is diagrammatically shown in Fig. 45. After attaching the petiole to the recording leW, indirect stimulus is applied, generally speaking, at a distance of 15 mm. from the responding pulvinus. Stimu¬ lus 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, on2 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

]«k}Iar variation of excitability. By means of a PotiPs com¬ mutator or reverser, the constant current can be maintained either u 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 connexions are so arranged that when tfie commutator “is tilted to the right, the transmission is down-hill, when tilted to the left, up-hill. The electrical resistance offered by the 95mm, 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 0 prevents the alternating induction current from flowing into the polarising circuit and causing direct stimulation of the fpulvinus.