The Nervous Mechanism of Plants

Fig. 53 . — Electric changes accompanying the passage of a wave of contraction along the Ureter (Orbeli and Brucke) : up-curve positive. [The figure is taken from Bayliss — General Physiology, p. 653.] wave disappearing before arrival at the second electrode. As far as I am aware, no satisfactory explanation lias been offered of this preliminary positive response. It finds a simple and satisfactory explanation if it be regarded as due to the positive impulse which precedes the negative.

Another method of obtaining the positive response, even in a highly conducting animal nerve, is to allow the tissue to fall into a condition of sub-tonicity, the nerve being thus rendered a semi-conductor or non-conductor. It will be shown in the next chapter that indirect stimulation gives rise to a positive impulse in both plant and animal nerve when sub-tonic, and that the restoration of full conductivity, as the result of repeated stimulatio|i, is attended by transformation of the abnormal positive to the normal negative response.

It has been shown that the conducting power is not solely dependent on the nature of the tissue, but also on the energy absorbed by it from previous stimulation which raises its tonic level. The same tissue may, under different circumstances, be a conductor or a non-conductor of excitation. How, then, does stimulation modify the properties of a tissue ? Is the modification produced only in conducting tissue, or in all tissues ? If the latter, how are these modifications manifested ?

Under favourable conditions the leaf of Mimosa responds to stimulation by a fall from a higher to a lower level of energy. Hence it is generally supposed that response to stimulus is always attended by a depletion of energy. It has, however, been shown that the response, instead of being of one kind, is of two kinds, a positive and a negative {cf. Chapter VIII.), whether the response be electric or mechanical. The positive erectile response with electrical positivity, in contrast to the negative response of fall with electrical negativity, indicates an accession of

energy. The positive and negative responses, mechanical and electrical, are but external indications of fundamental reactions associated with storage or expenditure of energy. Work is done on the plant by the energy received from the environment, and work is done the plant in the maintenance of its various life-activities. Representing the accession of energy by A, and the expenditure or depletion by D, the resultant effect is A — D, the difference between the up-hill and the down-hill work. The relative intensity of the two reactions to stimulation is, moreover, determined by the tonic condition of the tissue ; when the tonic level is above par, the D-reaction under stimulus is predominant ; but when the tonic level is below par, a condition designated as sub-tonic, the accession of energy A is the more pronounced, the tonic level being thereby raised to the normal. Turning to the protoplasmic property of conduction in a sub-tonic tissue, the resistance or block offered by the synaptic membranes is gradually removed by repeated stimulation, which is not unlike the removal of frictional resistance by the repeated working of a rusty hinge. The semi-conducting or non-conducting tissue thus becomes conducting after strong or repeated stimulation (p. 51).

The mechanical and electrical responses already described are not the only indications of protoplasmic reaction. The three fundamental manifestations of irritability— contractility, conductivity, and rh5rthmicity — are common to all living tissues. Evolution, associated with physiological division of labour, gives rise to tissue-systems specialised for the better discharge of particular functions. Nevertheless a continuity exists in all the different tissuesystems in respect of these three manifestations. Thus while the conductivity of the parenchymatous tissue is feeble, in the nervous tissue the tubular cells conduct excitation with

little decrement to a considerable distance. Again, different grades of contractility are exhibited by different tissues * it is, for example, low in ordinary pulvini, but reaches high perfection in the pulvinar tissue of Mimosa. In regard to rhythmicity, the highest development is attained in the autonomous activities of the pulsating leaflets of Desmodii/M gyrans and of growing organs. The response of protoplasm to stimulation is given mechanically, by arrest or greater activity of movement or of conduction; electrically, by variation of electromotivity.

Parallel Effects of Stimulation on Contractility, Conductivity and Rhythmicity The conclusion that a fundamental protoplasmic change is induced by stimulation is based upon observations which demonstrate that stimulation produces diametrically opposite reactions in tissues which are respectively in sub-tonic or in normal condition. The diverse manifestations of protoplasmic irritability are, as already explained, contractility, electromotivity, conductivity, and rhythmicity. In the condition of sub-tonicity, all these manifestations become depressed or arrested. The pulvinus ceases to contract, the electromotivity of the tissues becomes depressed, the conducting phloem ceases to conduct the excitatory impulse, and the pulsating activity of the leaflet of Desmodium and of the growing organ come to a stop. In optimum condition, when the tonicityis above par, the contractile and electromotive responses are at their maximum, the velocity of Gonduction attains its highest value, and the autonomic activities of Desmodium leaflet and of growing organs are at their greatest.

Induced enhancement of motile excitability. — I have shown elsewhere^ that the lost excitability of the pulvinus of Mimosa becomes restored under repeated stimulation. The mechanical and electrical responses then exhibit a staircase increase. Enhancement of conductivity— The after-effect of stimulation on the enhancement of conductivity of sub-tonic tissue has been demonstrated by the methods of both mechanical and electric response (pp. loi)- HestoTution of autonomous activity. ^Xhe arrested pulsation of Desmodium leaflet becomes revived under the action of stimulus. Stimulation also revives arrested growth.

The responsive reactions are the converse of those of the tissues when in sub-tonic condition. Motile excitability, conductivity, and rhythmic activity are all depressed by strong stimulation. Stimulation thus induces parallel modifications in conductivity, contractility, and rhythmicity. There is a particular aspect of the action of stimulus which is of fundamental importance in the life of the plant. The continuance of its normal functions depends on the maintenance by external stimulation of all the tissues in an optimum tonic condition ; for deprivation of stimulation reduces the plant to an atonic condition in which all life-activities are brought to a standstill. Beginning with the tissue at the lowest tonic level (due to prolonged deprivation of stimulation), the incidence of stimulus initiates and enhances all physiological activities to a maximum, the tonic condition of the tissue being raised at the same time to an optimum. Among the external stimuli naturally accessible to the plant, none is more potent than light. All the conditions favour the transmission of its stimulating

effect to a distance by the nervous channel, which is the phloem in the vascular tissue. The expanded lamina of the leaf, in which the vascular bundles are spread out in fine ramifications, is not merely a specialised structure for photosjmthesis, but also a catchment-basin for the stimulus of light, the excitatory effect of which is gathered into larger and larger nerve-trunks for transmission to the interior of the plant. The distribution of the vascular bundles in the interior is such that no mass of living tissue is too remote to be excited by the stimulus conducted by

these nervous channels. How reticulated they may often be, even in the trunk of a tree, is shown in the photograph of the distribution of the vascular bundles in the main stem of Papaya (fig. 54). This net-work, of which only a small portion is seen in the photograph, girdles the stem throughout its whole length, and in this particular case there were as many as twenty such layers, one within the other. Thus all parts of the plant are maintained, by means of nerve-connection, in the most intimate and rapid communication wfith each other. It can only be in yirtue of the existence of a system of nerves that the plant constitutes a single organised whole, each of whose parts is affected by every influence that falls upon any other.

The electric responses of Mimosa to indirect stimulation and those of ordinary plants exhibit the closest parallelism. The response to feeble stimulation is positive ; under strong stimulation it is transformed into negative. Fig. 54. — Distribution of vascular tissue in a single layer of stem of Papaya. The excitatory process still persists in the pulvinus when rendered immotile by excessive absorption of water, as demonstrated by its excitatory response of galvanometric negativity.

The velocity of transmission of excitation can be determined electrically by the diphasic response obtained with the Einthoven galvanometer. The velocity thus determined is similar to that obtained by the mechanical method. The nervous tissue in the petiole of Mimosa has been localised by the Electric Probe. The conducting tissue is the phloem : each vascular bundle contains two phloemstrands, one external and the other internal to the xylem.

In the semi-conducting pulvinus the impulses transmitted across it are double, positive followed by negative. Under strong stimulation the positive becomes masked by the predominant negative. No preliminary positive response to transmitted impulse had hitherto been specifically noted in animal tissues. It has been shown that, under suitable conditions, positive response can be detected in these tissues. The leaf is a catchment-basin for the stimulus of sunlight ; the transmitted excitatory effect is conducted to the interior of the plant along the conducting phloem of the vascular bundles. It is the transmitted excitation that maintains the internal activities of the plant in optimum condition.

The characteristics of the electric response to transmitted excitation have been discussed in the last two chapters. It was shown that the phloem in the vascular bundle functions as the nerve of the plant. I had long been desirous of isolating the conducting or nervous tissue of the plant in order to repeat with it the characteristic experiments hitherto carried out with the nerve of the Frog. The petiole of the Fern afforded me the necessary material. On carefully breaking the hard casing of the petiole, and pulling it apart in both directions, I was able to isolate the

vascular strands, which were long, soft, and white in colour, remarkably similar in their appearance to animal nerve (fig. 55). These threads vary in number with different species of Ferns ; it is sometimes possible to detach one of them having a length of 20 cm. or more. In the vascular strand the conducting phloem surrounds the xylem. Transverse and longitudinal sections of a vascular strand are given in figure 56. The tubular conducting cells of the

Fig. 56. Transverse and longitudinal sections of the vascular strand of a Fern. phloem are similar to those in other plants. In the following investigations the strands of the common Maiden-Hair Fern (Adiantnm) and of Nephrodium molle were found to be most suitable. The vascular strand is dissected out, special care being taken to avoid injury. It is then placed in normal saline solution for about half an hour to remove all irritation due to handling. When the external temperature is not too high, the excitability of the isolated nerve-strand remains constant for a considerable length of time. The experimental precautions to be taken are precisely the same as those observed in corresponding experiments with animal nerve; that is to say, the specimen should be placed in

a moist chamber, for in both cases the process of drying is found to induce a transient increase of excitability followed by a permanent abolition of responsiveness. In preparing an experiment, one end of the strand may be killed by the local application of hot salt-solution : the galvanometric connections are then made, one with the killed, and the other with the unkilled portion. In order to ensure that the electrical indication shall be a true responsive reaction, it is well to use a non-electrical form of stimulus One of the most perfect forms — as previously explained — ■ is the thermal. With a good specimen a single thermal shock, lasting for less than a second, will be found sufficient to induce a considerable electrical response ; a response of still greater amplitude may be obtained by the summated effects of several such stimulations.

An important characteristic of the animal nerve is its indefatigability. The responses to a long series of uniform stimuli, such as would in the case of ordinary tissues bring about marked fatigue, show little or none in that of the nerve. The same statement holds good of the plant-nerve : little fatigue is shown in a long-continued series of responses. A complete account of the effect of chemical agents on the conductivity and excitability of the plant-nerve will be found in my ' Comparative Electro-Physiology,' 1907. I give here only a few of the more important results, specially in relation to the identity of the reactions given by the plant and by the animal nerve. This will be demonstrated by records which I obtained with the nerve of the Fern and with that of the Frog. Uniform thermal stimulation was employed for both.

Effect of Previous Stimulation on the Transmission of Impulse The experiments on the electric response of the petiolelaminar preparation of ordinary plants and on the mechanical response of Mimosa, already described, show that : — (1) The normal conductivity of the plant-nerve is enhanced in consequence of previous stimulation : excitation transmitted subsequently to stimulation is above the previous intensity, and the response to the transmitted excitation is above the normal. This fact is demonstrated by the staircase-increase of the electric response (see fig. 41). It is obvious that the continuous stimulation of tetanisation confers on the nerve a higher conductivity than at the beginning, the result of which must be a great enhancement of the electrical response.

(2) It has also been shown that after long isolation from the stimuli of its natural environment the tonic condition of the plant falls below par. The conductivity of the nerve, in such a sub-tonic specimen, is greatly depressed. The electric response of the sub-tonic nerve is the abnormal positive instead of the normal negative. Long continued stimulation restores the normal conductivity, the electric response to transmitted impulse being thereby converted from the abnormal positive to the normal negative.

I will now show that the above characteristics of response in normal and sub-tonic specimens are exhibited in a very striking manner by both the isolated nerve of the Fern and that of the Frog. Experiment 63. Effect on normal nerve. — The aftereffect of tetanisation in enhancing the conducting power is exhibited in the records of figure 57. The first series of up-responses are the normal negative. The greatly increased amplitude of response to transmitted excitation after tetanisation indicates the increased conductivity of the nerve.

Experiment 64. Effect on nerve in a snh-tonic condition. ~I reproduce (fig. 58) three types of response to transmitted impulse given by plant-nerve when normal and when in a depressed condition. In the three series, the first is Fig. 57. Photographic record of effect of tetanisation, T, in inducing enhancement of normal negative response in nerve of Fern. the normal negative ; the second is the mixed or diphasic positive followed by negative ; the third is the abnormal positive of a nerve in a condition of sub-tonicity.

Fig. 58. Photographic record of three types of response, normal negative, diphasic, and abnormal positive, in nerve of Fern under different conditions. In experimenting with the isolated nerve of the Fern, I find that the diphasic response is converted to the normal negative after a short period of tetanisation. The abnormal positive response requires a longer period, of tetanisation for conversion into normal negative. This is accomplished, generally speaking, after an intermediate diphasic response in which the preliminary positive is followed by the negative.

These various effects find their natural explanation in the facts previously demonstrated that the block to the passage of impulse is gradually removed by previous stimulation. This is the reason why the conducting power of nerve in ordinary condition is raised above the normal, and why, when in the condition of sub-tonicity, the semiconducting tissue is rendered normally conducting under similar treatment, the positive response being transformed normal negative. I wish to lay special stress on these facts which wilt be found to offer the most natural and satisfactory explanation of corresponding phenomenon in the animal nerve.

Experiment 65.— The responses were taken under conditions exactly parallel to those of the experiments with plant-nerve, the mode of stimulation being thermal. The first three responses are normal negative ; the responses after tetanisation exhibit an enhancement similar to that observed in plant-nerve (fig. 59). Experiment 66. — ^The Frog’s nerve in sub-tonic condition exhibits, like the plant-nerve in a similar condition, a response of galvanometric positivity. After tetanisation the abnormal positive is found to be transformed into normal negative (fig. 60).

In his records of electrical response with Frog’s nerve under electrical stimulation, Dr. WaUer obtained responses of three different types. The first of these was the normal negative response ; the second was the diphasic ; and the third was the abnormal positive. This last he regarded as characteristic of stale nerve. The normal negative response was found by him to undergo enhancement after tetanisation, while the abnormal positive response of the stale nerve underwent a change into diphasic, or reversal to normal negative.

From the fact that carbonic acid enhances the normal negative response of nerve. Dr. Waller suggested that the enhancement of the normal negative after tetanisaiton and the tendency of the modified nerve to revert to the normal are results of a hypothetical evolution of carbonic acid in the nervous substance, due to metabolism accompanying the excitatory reactions. It must be said, however, that the production of carbonic acid in the nerve is generally regarded as extremely doubtful.

Fig. 59. Enhancement of amplitude of response, as after-effect of thermal tetanisation, in Frog's nerve. The first three responses are normal. Brief thermal tetanisation is then applied, and the responses subsequently obtained under the original stimulation are enhanced. Fid. 60. Conversion of abnormal positive (left) into normal negative response after thermal tetanisation (Frog's nerve). Since it has been shown that all these varied effects are simply explicable on the well-demonstrated fact that conductivity becomes enhanced by the gradual removal of the block under stimulation, it is quite unnecessary to assume any hypothetical evolution of carbonic acid in the excited nerve.