The Nervous Mechanism of Plants

I have hitherto described the effects of a moderate stimulus, under which the irradiation of excitation takes place at the centre from one nerve-end to the next. In the animal, the resistance to the passage of the impulse is overcome under a stronger intensity or longer duration of stimulus, in consequence of which there ensues a more widespread response. I now proceed to show that effects parallel to these are obtained in the reflex of Mimosa. In order to avoid unnecessary repetition, I describe in detail two typical series of experiments on the effect (i) of increase of intensity, and (2) of duration of stimulus.

Experiment 95. The experiments in this series were carried out with the same specimen, the intensity being gradually increased from minimum to maximum, the duration of application being kept constant, namely, one second. (i) Tetanising induction-shock of -5 unit was applied to sub-petiole (i). The reflected impulse only reached sub-petiole (2). There was thus a single reflex. (ii) The intensity of the induction-current was now increased to i unit. The reflected impulse at the centre caused response not only of sub-petiole (2) but of subpetiole (3) as weU. There were thus two successive reflexes at the centre, under a moderate increase of intensity of the stimulus.

(iii) The stimulus was next increased to 3 units ; three successive reflexes now occurred at the centre, in consequence of which the sub-petioles (2), (3) and (4) responded one after another. The reflected impulse reached the subpetiole (2) 3 seconds after the fall of the leaf ; the subpetiole (3) responded 4 seconds after the response of (2) ; while the response of (4) was manifested 6 seconds after that of (3). There was always this sequence of responses in the order 2 — ^3 — 4.

Similar effects were produced under an increased intensity of stimulus, when the fourth sub-petiole, to the extreme right, was subjected to stimulation. The sequence of response of the sub-petioles was now in the reverse order, namely, 3 — 2 — i. I will now describe the effect of increasing the duration of stimulation, the intensity remaining constant. The experiment was varied by the application of the stimulus to sub-petiole (4) on the extreme right.

(i) The duration of stimulation was at first i second. The impulse after reflection at the centre only reached sub-petiole (3). The reflex was therefore single. (ii) The duration of stimulation was increased to 2 seconds; response occurred in both the sub-petioles (3) and (2) , that of (2) being the later. There were therefore two successive reflexes at the centre. (iii) The duration of stimulation was next increased to 4 seconds. This gave rise to successive responses of sub-petioles (3), (2) and (i) ; sub-petiole (2) responded 6 seconds after that of (3), and sub-petiole (i) 13 seconds after the response of (2).

It is thus apparent that increasing intensity or duration of stimulation elicits in Mimosa a corresponding outspread of response. The total delay between the application of stimulus to the first sub-petiole and the response of the second is made up of (a) the time of transmission of the afferent impulse from the periphery to the centre ; (5) ike actual ‘ lost time ’ in crossing from one nerve to another at the centre, in the first reflex ; (c) the time of efferent transmission from the centre to the periphery ; and {d) the latent period of the first sensitive leaflet which serves as the motile indicator. The actual lost time (6) at the first reflex is difficult to determine, since the values of {a), (c) and {d) are not known with suflficient exactitude.

As the length of the petiole through which the reflected impulse has to traverse is practically the same in all cases, the time-interval between the responses of sub-petioles (3) and (2) gives a fairly accurate measure of the time lost in the second reflex ; similarly the time-interval between the responses of sub-petioles (4) and (3) gives the lost time in the third reflex. The table above gives the lost time in successive reflexes in three different specimens. The lost time in the second reflex, under stimulation of moderate intensity, is in most cases 4 seconds ; it is however longer in the third reflex, varying from 6 to 13 seconds. This difference is probably due to the fact that a decrement occurs in overcoming the block at the third reflex. This explanation derives some support from the success which often attended my attempt to reduce the lost time to the normal by previous treatment. This treatment consisted in applying a preliminary stimulus to the fourth sub-petiole, the after-effect of which resulted in the partial removal of the block that existed between the third and the fourth nerve-endings at the centre. The stimulation of the first sub-petiole now gave approximately similar values of lost time in the second and third reflexes. In ordinary circumstances, the total lost time T in the second and third reflexes is 10 seconds; owing to decrement it is often as long as 21 seconds.

I may now briefly recapitulate the effect of increased intensity of stimiflus on response. Minimal stimulus applied to a sub-petiole reaches only the particular quadrant of the pulvinus with which the sub-petiole is in nervous communication; it does not proceed any further. The response induced is that characteristic of the particular quadrant reached by the stimulus. When the stimulus is increased from minimal to moderaie intensity, it does not remain localised in the one quadrant, but becomes diffused.

causing the fall of the leaf by stimulating to contraction the more excitable lower half of the pulvinus. A reflex arc IS formed by the crossing over of the excitatory impulse from one nerve-end to the next ; the reflected effLnt impulse travels along a new path and causes response of the leaflets of the next sub-petiole. A still stronger stimulus becomes more widely irradiated, aU the sub-petioles exhibiting response in serial succession. These characteristic features of the nervous reaction of Mimosa under increasing intensity of stimulus are extraordinarily similar to those of the nervous reaction in the animal, as summarised in the following quotation :

A small stimulus will discharge only along the few neurons where the resistance is lowest. Increase of the stimulus, either by increase of its strength or by summation of weak stimuli, will enable the impulse to spread alone more neurons and therefore wiU elicit a more widespread response. Only when the “ blocks” are entirely removed by the administration of strychnine, or when the stimuli are abnormally powerful and long continued, will the impulse spread to all regions of the central nervous system so that the response becomes general and inco-ordinate! instead of local and adapted to the stimulus.’ i

The question now arises, whether the characteristic action of strychnine on the reflex of the animal can also be observed in the reflex of Mimosa? In my previous works I have demonstrated the very marked similarity of the effect of drugs on plant and animal tissues. Yet it at first appeared highly improbable that a vegetable alkaloid like strychnine should exert a similar action on plant and on animal nerves. The results of some of the successful experiments on the action of strychnine on plantnerve were as astonishing as they were unexpected.

There are, however, certain inherent difficulties in working with the plant which render experimental success a matter of some uncertainty. Such are the fatigue induced by prolonged experimentation, and the absorption of the drug in excess of the proper dose. Both these factors tend to cause a prolongation of the lost time in reflex, as was ascertained in a large number of experiments. But, under favourable circumstances, the lost time was found to be reduced to zero, which cannot be accounted for by any other supposition than that of the abolition of the block under the characteristic action of strychnine. The results of some twenty successful experiments were too consistent to be ignored.

I will first describe the test by which the effect of strychnine on the reflex is to be proved. It has been shown that the total lost time T in the second and the third reflexes varied from 10 to 21 seconds. If strychnine were to cause a more or less complete removal of the block, the fact would be demonstrated by a reduction or abolition of the lost time. The greatest difficulty encountered was in the administration of the proper dose of strychnine to the plant, any excess proving to be highly toxic. In the animal, a small and definite dose can be easily injected, taking the precaution that the dose is not in excess. There is ho such facility for the plant, the only practicable method being the application of a dilute solution of strychnine hydrochloride on the pulvinus, which after absorption would reach the central nerve-endings : but the absorption of a proper quantity or dose is a question more or less of chance. Absorption of too small a dose was found to produce diminution, but not complete abolition, of the lost time ; whereas absorption of the proper quantity produced a complete removed of the Uock, the lost time being reduced to zero. Excessive absorption, on the other hand, caused an increase of lost time. A moment’s consideration will show how variable are the factors which modify the rate of absorption : it depends on the absorptive power of the

epidermis, the physiological activity and the turgor of the plant : it is also affected by the prevailing temperature and hygrometric conditions. There is a limitation in point of time for the observation of the effect of the duration of an application of strychnine. Mimosa requires a period of some 15 minutes for full recovery of its conductivity and excitability after each stimulation. Hence if the effect of 5 minutes’ application of strychnine be first observed, the next observation can only be taken at the twentieth minute, and the third at the thirty-fifth minute. If any of these periods happen to coincide with the critical period of absorption of the proper dose, only then would a complete abolition of the block be indicated by the simultaneous response of all the subpetioles. In spite of all these dif&culties, a fair number of experiments were successful, which show^ed that application of the proper dose of strychnine produces complete abolition of the block. I give below detailed account of some of the typical experiments carried out with exceptionally vigorous specimens.

Experiment 97. — The total lost time T in the second and third reflexes was found to be 12 seconds ; after application of dilute solution of strychnine, i part in 10,000, for 5 minutes, the block was partially removed, the lost time being reduced from 12 to 5 seconds. The next observation at the tw'entieth minute showed complete removal of the block, the responses of sub-petioles (2), (3) and (4) being simultaneous. Continued application produced a toxic effect ; for at the thirty-fifth minute the lost time increased from zero to 9 seconds : longer application abolished all response. The curve obtained from the data given below is given in fig. Bo, a.

In this case the period of critical absorption was 20 minutes : ^ but under conditions favourable to more rapid absorption, an application of 20 minutes would pro- Fig. 80. Curve of relation between duration of application of strychnine and the lost time. The critical period in curve (a) is about 20 minutes, in (h) it is shortened to 2 minutes. (See text.) Duration of application in minutes, and lost time in seconds. duce a toxic effect and prolong the lost time. This accounts for the result obtained a few days later (May), when the temperature was higher than usual, that the application of strychnine even for 5 minutes prolonged the lost time. I therefore took an observation with a fresh specimen a minute after the application ; the lost time was now found to be reduced from the normal 13 seconds to 2 seconds only ; an observation taken 15 minutes afterwards (duration

of application 16 minutes) showed that the lost time after reaching a minimum had become prolonged to 9 seconds. The critical period in this case was probably 2 minutes (see fig. 80, 5). I give two other examples of' experiments carried out in autumn (September) when the rate of absorption was comparatively slow. 1 therefore employed a stronger solution, one part in a thousand, and observed the effect of increased duration of the application of strychnine.

Experiment 98. — The normal lost time was found to be 15 seconds. The application of strychnine for 2 minutes caused complete abolition of the block, the responses of sub-petioles (2), (3) and (4) being simultaneous. After an additional interval of 15 minutes, retardation occurred, the lost time being prolonged to 8 seconds. Absorption of strychnine for a further period of 15 minutes abolished all response. Experiment 99. — The variation was introduced of stimulating the fourth instead of the first sub-petiole ; the sequence of response was therefore (3), (2) and (i). The normal lost time was 12 seconds. After application of strychnine for 2 minutes, the block was completely removed as shown by the simultaneous responses of subpetioles (3), (2) and (i). Prolonged application for a further period of 15 minutes caused a retardation, the lost time being now lengthened to 7 seconds.

The results given above indicate that under the proper dose of strychnine, the block becomes completely abolished, as observed in the reflex of the animal. -^The nervous mechanism of Mimosa has been shown to be far more complex than had hitherto been suspected. The question now arises : What advantage in the economy of the plant is secured by such a highly elaborated nervous mechanism ? One advantage, as already pointed out, is the co-ordinated reflex by which the leaf-surface is adjusted perpendicularly to the incident light so as to ensure the

absorption of the optimum amount of radiant energy for photosynthesis. Another important function of the nervous mechanism is the transmission of a rapid message to the motile organ for a quick reaction to avoid a threatened danger. It has been suggested that the sudden fall of the leaf serves the purpose of scaring away grazing cattle. But the cow is not sufficiently intelligent to take notice of the movement of the leaf or to be frightened by it. The reflex movements may, however, subserve the protection of the plant in a different way. The long procumbent stem of Mimosa pudica, which covers large patches of ground in the tropics, bears numerous leaves. When one of the subpetioles bearing leaflets is trampled on or bitten, the excitatory impulse is transmitted throughout the length of the plant. Confining our attention to a particular leaf, the afferent or sensory impulse causes a fall of the leaf which pre!5ses itself against the ground. The successive reflexes at the centre give rise to motor impulses which make the four sub-petioles approach each other laterally, and also cause closure of all the leaflets. Nothing could be more striking than the rapid change by which a patch of vivid green becomes transformed into thin lines of dull grey unnoticed against the dark ground. It is probable that this invisibility may serve as a means of protection.

■^en a stimulus acting on one of the sub-petioles is increased above the minimal intensity, the afferent impulse becomes reflected and transformed at the reflex-centre in the pulvinus into an efferent impulse which travels along a new path. This connotes not merely a reversal in direction, but also a considerable discharge of energy ; for there is a marked disproportion between the afferent and efferent impulses, the latter being the more intense as evidenced by its higher velocity of transmission.

in tli6 ncrvc-str^nd. of Mimoss,, of wiiicli the outer conducts the sensory and the inner the motor impulse. The impulse becomes more widespread under stronger intensity of stimulus. The resistance to the passage of impulse at the centre is completely removed by the absorption of the proper dose of strychnine. The lost time is reduced to zero, as is shown by the simultaneous response in all the sub-petioles. The irritability of an unicellular organism is exhibited by its contractility, rhythmicity and conductivity, which are different aspects of a fundamental protoplasmic reaction. These characteristics of the individual cell persist even when cell-complexes and tissue-systems are formed in multicellular organisms. In the physiological division of labour, each particular tissue becomes specialised to discharge a given function in a more efficient manner. Thus whilst all tissues are more or less contractile, high sensitiveness and power of rapid contraction have been attained by the pulvinus ' of Mimosa, which responds to stimulus of an intensity even below the threshold of human perception. I have shown elsewhere that rhythmic or pulsatory activity is exhibited very widely by different tissues, and that a continuity exists between multiple and autonomous response. The autonomous activity is conspicuously manifested by the pulsating leaflet of Desmodium gyrans ; but it requires appliances of high sensitivity to detect it in growth and in the activity which maintains the ascent of sap. There is similar continuity in the conductivity of the different tissues , the indifferent tissues are feeble conductors, while the tubular cells in the phloem-strands are most effective in conducting, even a feeble excitation, to a distance.

The intercommunication and interaction of distant organs of the plant are secured (i) by slow translocation of chemical stimulants by the movement of sap, and (2) by rapid transmission of protoplasmic excitation corresponding to the nervous impulse in the animal. Great confusion has arisen in the study of the conduction of excitation in plants from want of discrimination between the two types of action at a distance, physical and physiological. Still more serious has been the error introduced into the investigation by the application of too violent stimuli. The intense excitation thus produced remains no longer confined to the conducting tissue of the plant or the animal but becomes widely diffused (pp. 12, 41, 195).

This theory, advocated by Pfeffer and Haberlandt, is based upon the observation of the escape of a drop of water from the petiole of Mimosa pudica after a deep cut. The sudden variation of hydrostatic pressure thus produced is supposed to be hydro-mechanically transmitted to a distance so as to inflict a blow, on the excitable pulvinus, with resulting fall of the leaf. The inadequacy of this theory is evident from the fact that stimulation of the plant, and the transmission of excitation to a distance, can be effected by scratch-stimulus, by superficial friction, or by an electric shock of feeble intensity. Transmission of excitation thus takes place without any wound and resulting escape of water which might have produced a hydro-mechanical disturbance.

This is based on the supposition that stimulation is produced by some chemical irritant excreted in consequence of irritation of the wood, and that the translocation of the irritant to the distant leaf is effected by the movement of sap, the rate of which is assumed to be the same as that of the transmission of excitation. The baselessness of the theory is shown : (i) By the initiation and transmission of excitation without in any way irritating the wood (p. 14).

upwards and downwards, with and against the direction of the transpiration-current ; also by the conversion, of the ascending into a descending impulse after crossing over at the apex of the stem was applied at the extreme tip of the leaf, or of a flower-stalk ; the excitatory impulse generated travelled to a considerable distance downwards against the direction of the normal ascent of sap ; subsequent examination showed that the stimulant had not been transported, but had remained localised at the point of application (p. 18).

The theory of conduction by the transpiration-current is thus completely discredited, conduction of excitation being in no way connected with the movement of sap. The transmission of excitation is neither hydromechanical nor due to the movement of sap. The following crucial experiment proves that conduction in the plant is a propagation of protoplasmic excitation like the nervous conduction in the animal. The effect of an electric current is discriminative in its action. In the conducting nerve of the animal, a feeble current excites only on make at the kathode ; under a stronger current, excitation occurs on make at the kathode, and on break at the anode. My experiments on the polar action of an electric current prove that the reactions in the plant are identical with those in the animal. Excitation, under a feeble current, is produced only at kathodemake, and the e.xcitatory impulse thus generated is transmitted to a distance. Under a stronger current, excitation is induced both at kathode-make and at anode-break (p. 25). These results offer conclusive proof

of conduction in plants being a physiological propagation of protoplasmic excitation as in the animal. For investigating the velocity of transmission of excitation in plants, it is essential to devise a method of determination of as high a degree of accuracy as that employed for the study of the nervous impulse in the animal. This has been secured by my Resonant Recorder, in which errors arising from frictional resistance in recording have been completely removed. The dotted record permits estimation of time-intervals as short as 0- 001 second.

Latent penoi.—Th.& average value of the latent period of the pulvinus of Mimosa is o-i second. It is shortened, within limits, with rise of temperature ; it is prolonged under fatigue. Velocity of transmission. — ^The velocity in thick petioles of Mimosa, in summer, varies from 30 to 55 mm. per second. In thin petioles it is as high as 400 mm. per second {p. 64). The velocity of impulse in Mimosa is not as great as in the higher, and not so low as in the lower animals. The velocity of conduction in the sub-petiole and in the main stem of Mimosa is considerably lower than in the petiole.

The velocity is modified by the physiological condition of the plant. In winter it is very much lower than in summer. If the plant be kept in unfavourable condition, its physiological tone falls below par, when the conducting power becomes depressed or arrested. Strong stimulation is found to renew or enhance the power of conduction. The effect of intense stimulation on conduction in normal tissues is exactly the opposite, i.e. a depression or fatigue of conduction (p. 66).