Comparative Electro-Physiology: A Physico-Physiological Study

I showed in the first chapter of this book that one identical molecular change may be detected in different ways, according to the method of observation. Thus the same excitatory change is shown both in change of form takes place in entire independence of the other is shown, for instance, when the mechanical response of A/zmosa or Desmodium is restrained, under which condition the electro- motive response proceeds as before. Excitatory changes, similarly, may express themselves independently either by electromotive variations or by changes of electric resistance. It was, in fact, by means of the latter method, that of the variation of resistance, that I first demonstrated the responsive molecular changes which take place in inorganic matter.!

If living tissues, therefore, really respond to excitation in a manner similar to the inorganic, it should be possible to obtain from them response-records by a new method, that of Resistivity Variation alone. In order to demonstrate this inference, it will be necessary to show that such varia- and not as an after-effect. We ‘must, however, ascertain whether this method of Resistivity Variation does or does ' Bose, De la Généralité des Phénomines Moléculaires produits par ? Elec- tricité sur la matidre Inorganique et sur la Maticre Vivante. (Travaux du Congrés International de Physique. Paris, 1900.)

not give us those two opposed effects, positive and negative, which we have already seen to be exhibited by living tissues in other forms of response whether mechanical or electro- motive. Of these, again, supposing them to occur, it will also be necessary to determine whether it is the increase or decrease of resistance which corresponds to the negative and positive mechanical and electromotive responses respectively ; and finally it must be determined what are the effects of the.

various physiological modifications, induced by different agencies, on the response by resistivity variation. In this investigation many serious experimental difficulties have first to be overcome. These will be dealt with in series in the detailed description of the method to be employed. It will be well, however, to see in what important respects the conditions for the obtaining of response here are unlike those of the electromotive variation. In the latter case, it we employ an isotropic tissue, diffuse stimulation will induce similar excitatory electromotive variations in every part of the tissue. The differential electromotive variation, there- fore, on which the recording of response depends, will, under _ these circumstances, be impossible. In this case, therefore, it is necessary to injure or kill the tissue at one of the two contacts. Such artificial induction of anisotropy would not, however, be necessary under an experimental method which was not dependent on any differential action. Thus an isotropic tissue would give response by longitudinal con- traction when the stimulus was diffuse. Similarly, though an isotropic tissue fails to give an electromotive response under diffuse stimulation, yet we may expect it to exhibit response by variation of resistance. The recording of excitatory response by resistivity variation has thus one advantage over that of the electromotive variation, inas- much as the record is not affected by complications due to differential action, but is the expression of the direct effect of excitation. The question which we have next to deter- mine, then, is whether or not the excitatory variation of the living tissue is attended by any variation of its resistance,

and whether, if so, such variation is or is not of two opposite signs, according to the tonic condition of the tissue con- cerned. In subjecting this question to experimental investi- gation, it is well to employ a non-electrical form of stimula- tion, in order to avoid any possible disturbance of the galvanometer record from polarisation or current escape. The first point to be decided is the character, positive or negative, of that resistivity variation by which the true excitatory change finds expression. We have already seen that when a tissue is subjected to a gradually rising tem- perature it exhibits response, which is expressed mechani- cally as increasing expansion, and electrically as increasing positivity. When the temperature, however, has reached the definite critical point. of death, we have seen that there is a sudden excitatory effect induced, attended by a reversal of the sign ‘of response. This is expressed mechanically by a sudden contraction, and electrically by a change to galvanometric negativity. I have already ex- plained in Chapter XVI. that, in mechanical and electrical morographic curves, the abrupt point of inversion represents the death-point. I have also shown that this death-response is a true physiological response; that the temperature at which it takes place is definite in all phanerogamous plants, being at, or very near, 60° C. in normal specimens; and that it displays depression, by transposition te a lower tempera- ture, when the tissue is physiologically depressed by such influences as fatigue.'

From these facts we might expect, if a tissue sfioied response by variation of resistivity, that up to 60° C., or so, there would be a continuous one-directioned change of resistance, succeeded on reaching 60° C. by an abrupt reversal to the opposite-directioned change. In that case, it would be the second of the two, which would be indicative of true excitation. To carry out this experiment I took a radial and physiologically isotropic pistil of, Wzb¢scus, and mounted it on two non-polarisable electrodes. The specimen

was now made the fourth arm of a Wheatstone’s bridge (fig. 327), by which electrical resistance is usually determined. The plant specimens employed generally possessed high resistance, of the order of several hundreds of thousands of ohms. In the Wheatstone’s arrangement employed by me, P and Q represented the ratio-arms; R a standard. megohm, lic. 327. Diagrammatic Representation of Experimental Arrange- ment for Recording Response by Resistivity Variation

P Q, ratio arms of Wheatstone’s bridge; R, standard 1 or *5 megohm; S, specimen. or half-megohm; and s the specimen whose variations of resistance were to be determined. It is now evident that The ratio-arms, P and Q, consist of resistance-boxes, which allowed a variation of from I to 10,000 ohms. In order to obtain balance, of course, the ratio of the two had to be suitably adjusted. A highly sensitive galvanometer was used, and the electromotive force employed to obtain balance was only ‘o5 volt. This low E.M.F. was obtained by the use of a suitable potentiometer slide. It will be seen that, owing to the very low E.M.F. and the high resistance in the circuit, the current flowing through the specimen was rendered extremely feeble. This was done in order to avoid

any complication such as might result from the passage of a strong current. 7 4 In order to subject the specimen to a gradual and continuous rise of temperature, it was placed in the thermal chamber, which has already been described (fig. 131). Before the gradual raising of the temperature is initiated, an exact balance is first obtained, the galvanometer spot of light being thus adjusted to zero. This position can be maintained for an indefinite length of time, provided the specimen is subjected: to no variation of temperature. We have already seen that no, resultant electro-motive variation is induced, in consequence of stimulus, in a physiologically isotropic tissue. Any change now recorded under a gradual rise of temperature, by the movement of the galvanometer spot of light, must, therefore, be due toa resulting variation of resistance. The movement of the galvanometer spot of light is recorded in the usual manner, on a photographic plate, a down-record representing an increase of resistance, and an up-record a diminution. In order that the curve should also give indica- tions of different temperatures, light is cut off for a short time at every 2° C. of rise of temperature. Thus each of the successive gaps in the record indicates a temperature-ascent of 2° C.

Taking now the specimen of pistil of Wzdzscus, balanced as described, it was seen, on beginning gradually to raise the temperature, that the balance was upset, while the growing deflection of the galvanometer spot indicated an incréasing resistance. The method of experiment, which has been described, proved now so delicate that it was impossible to record the entire curve within the range of the photographic plate. It was, therefore, necessary to choose for record only that part of the deflection which included the interesting and significant point of inversion. The photographic record thus commences at 56° C., it being understood that there has been, before this, a larger and continuously growing deflection downwards, indicative of increasing resistance. During record the deflection continues to increase, till the

critical point is reached. And here, though the temperature still goes on ascending at the same rate as before, we see a sudden reversal in the characteristic curve of resistivity, showing that the hitherto increasing has suddenly become a decreasing resistance. This abrupt inversion represents the Fic. 328. Photographic Record of the Morographic Curve taken by Method of Resistivity Variation in Pistil of Azdéscus. Critical point of inversion at 60°8° C,

Fic. 329. Photographic Record of the Morographic Curve taken by Method of Electro-motive Variation in Petiole of Musa. Critical point of inversion at 59°6° C. Fic. 330. Photographic Record of the Morographic Curve taken by Method of Mechanical Response in Filament of 7ass¢flora. Critical excitatory effect which occurs at the point of initiation of death, and is in the present case at 60°8° C. (fig. 328). It is astonishing to find that the morographic curves obtained from different specimens, by three methods so different as the mechanical, the electro-motive, and that of

resistivity, should bear so strong a resemblance to each other, as is here seen to be the case, in the three records given side by side (figs. 328, 329, 330). The excitatory effect may thus be manifested by contraction, galvanometric negativity, or diminution of resistance. We have already seen that the electromotive is not a consequence of the mechanical re- sponse, but is exhibited independently, when physical move- ment is restrained. The response by resistivity variation likewise, is, as we shall see, an independent expression of the fundamental molecular change due to excitation.

Having thus established the fact that true excitatory response is exhibited by diminution of resistance, we‘ have next to ascertain whether this method of resistivity-variation is capable of being employed in the study of excitatory phenomena in general, with as great facility as those mechanical and electro-motive methods with which we are already familiar. In order to determine this question I employed the same Wheatstone’s bridge arrangement as before. As it was important, for reasons previously given, to use a non-electrical form of stimulus, I employed those thermal shocks which we have already found to be so reli- able. The thermal loop of platinum wire enclosed the specimen as before, without being in contact with it. A short-lived passage of heating-current, controlled by a metro- nome, would now give rise to that sudden thermal variation which we have seen to be effective in causing stimulation It should be remembered that the rise of temperature, as such, induces a responsive increase of resistance. But as, on the other hand, the sudden thermal variation acts as a stimulus, it should induce the excitatory response, by a transient diminution of resistance. In the following experi- ments, I employed the physiologically isotropic nerve of fern. The resistance of this tissue, when balanced, was found to be 400,000 ohms. It should be stated here that this specimen was in a very good tonic condition, and might be expected therefore to give normal response. It was now subjected to a series of stimuli of uniform intensity, at intervals of five

minutes. The consequent responses are seen to be uniform, and to take place by that diminution of resistance which we already know to constitute the normal mode of response (fig. 331). In order to obtain some idea of the magnitude of these resistance variations the balance was upset at the end of the response record, to the extent of 4,000 ohms. The deflection seen to the right of the figure represents the effect of this variation of resistance. The normal resistance of the tissue, including that of the non-polarisable electrodes was, as stated before, 400,000 ohms. The resistance of the electrodes themselves was 50,000 ohms. That of the tissue

FiG. 331. Response Records by Resistivity Variation, in the Nerve of Fern ; Stimuli at Intervals of Five Minutes Kesponse to stimulation is by the negative variation or diminution of resistance. The record to the right shows deflection due;to variation of resistance of 4,000 ohms. alone was thus 350,000 ohms. The variation of resistance induced by stimulus, therefore, was, in the present case, approximately I per cent. In order. next to determine whether the resistance variation was a consequence of the responsive change of form, or an independent expression of the fundamental molecular change, I clamped a nerve of fern suitably, at its two ends, to prevent any possible change of length, the two non-polarisable electrodes being led off in such a way as to include a certain length of the specimen. On carrying out the experiment in this manner I obtained response by diminution of resistance, exactly as in the last

case. It is thus seen that the response, by resistivity variation, is an independent expression of the excitatory variation. . a In taking records of the responses of animal and vege- table nerves, by the methods of mechanical and electro- motive variations, we saw that, while the normal response was negative, this was liable to become reversed to positive, under two different conditions—namely, sub-tonicity and the fatigue due to excessive stimulation. Similar reversals are observed under similar conditions, when the method of resistivity variation is employed. We saw, further, that the abnormal positive response, due to sub-tonicity, could be gradually converted into normal negative, through inter- mediate diphasic, by tetanisation—further tetanisation acting, moreover, to exalt this feeble into enhanced negative response. Parallel results are observed in the case of resistivity variation. The initial abnormal response, by increase of resistance, is found, after short tetanisation, to be converted into diphasic—an increase of resistance or positive response preceding the true excitatory or negative effect of diminution of resistance. Further tetanisation brought about the disappearance of this preliminary positive, and the enhancement of the negative, response. —

I used the same method, finally, for the observation of response and its modifications, by means of anesthetics in animal nerve. For this purpose I took a nerve of frog, and subjected it to chloroform. It will be remembered that in studying the effect of anzsthetics by the electro-motive variation method, we found it, first, to reverse the normal response to positive, and finally to’ induce an abolition, which might prove to be either temporary or permanent. The same thing is seen under the resistivity variation method. In the first series of records, given in fig. 332, we find normal responses by diminution of resistance, to a series of stimuli applied at intervals of two minutes. After the application of chloroform, the normal responses are seen to have disappeared. Stimulus now evokes

either no response or an occasional flutter, in the positive direction. We have thus seen, in the course of the present chapter, that, in addition to the mechanical and electro-motive modes of response, there is also a third mode available—namely, that by Resistivity Variation. We have also seen that the results obtained under these three methods are identical. It has been shown that the normal excitatory effect is in FIG. 332. Effect of Chloroform seen in Modification of Resistivity Variation in Frog’s Nerve

The normal responses seen to the left by diminution or negative variation of resistance were evoked by stimuli at intervals of one minute. Those to the right exhibit the effect of chloroform. The normal response is thus abolished, and we have either no response or only an occasional flutter in the positive direction. all three cases negative, consisting of mechanical contraction, galvanometric negativity, or diminution of resistance, as the case may be. In recording the morographic curve by these three methods, we find that up to the critical point of death, at or near 60° C., we obtain expansion, galvanometric posi- tivity, and increasing resistance. At that point, however, there is a sudden reversal of the curve, indicating conversion to negative, contraction, galvanometric negativity, and de- crease of resistance.

Feeble conducting power of cortical tissues—Heliotropic and geotropic eflects dependent on response of cortical tissues only—Phenomenon of correlation —FExcitability of tissue maintained in normal condition only under action of stimulus—Physiological activities of growth, ascent of sap, and motile sensibility, maintained by action of stimulus—Critical importance of energy of light—Leaf-venation a catchment-basin—Transmission of energy to remotest parts of plants—Plant thus a connected and organised entity.

IN the animal body, different kinds of tissues are possessed of different degrees of conductivity, the nerve being specialised for the rapid transmission of stimulus. And it is now seen that in the plant also we have a similar state of things, cortical tissue, for example, though excitable, having feeble conductivity, whereas the vegetal nerve possesses this power in high degree. The question next arises: What is the function subserved in the economy of the plant by a tissue so highly specialised for the rapid conduction of stimulus ? The various growth curvatures, by means of which plants place themselves under the directive action of light and gravity, are of advantage to the organism. But in bringing about these movements, the plant-nerve takes little or no part. And this is the case, even when the responsive curvature takes place at a certain distance from the point of stimulus. Here the transmission takes place slowly, through the feebly-conducting cortical tissues. For example, in Avena, curvature in consequence of such transmitted effect is observed, even when the fibro-vascular bundles have been cut across. , If, indeed, the highly conducting nervous elements ha been concerned, these curvatures in response to unilateral

stimulus could not well have taken place. This will be clear if we consider the case of a radial organ, such as the stem, unilaterally acted on by light. Here a positive heliotropic movement is induced, by which the growing organ is placed in the most favourable position as regards illumination. The peculiarity of this phenomenon lies in the responsive contraction of the side acted on by stimulus, with consequent concavity and curvature towards light. This heliotropic | movement continues until the organ has placed itself in the direction of incident radiation. When this orientation has become perfect there is no further movement, because the proximal and distal sides are now equally stimulated. Had the cortical tissue, on whose differential responsive action the curvature depended, been as highly conducting as the vegetable nerve, this particular-directioned movement would have been an impossibility, for the stimulus, instead of remaining localised on one side, would in that case have become diffused, with the result of inducing antagonistic effects on the proximal and distal sides, under which there could have been no resultant curvature. Indeed this neutral- ising action of conduction, in nullifying responsive curvature, is seen even when unilateral stimulus is excessively strong. For under these circumstances stimulus is conducted transversely, through the imperfectly conducting tissue, with the result of undoing the previous curvature. And it is obvious that had the conductivity of the tissue been higher, this neutralisation would have taken place, even under feeble stimulation.

I have also shown elsewhere that, in the responsive movements of leaves, conduction through nervous elements plays little or no part. For the blade of the leaf may be acted on by light without showing any responsive movement. Hence the lamina is not to be regarded as the perceptive organ. The organ by which, on the contrary, the responsive movements of the leaves are determined is the pulvinus or pulvinoid. This is at once perceptive and motile. When such an organ, then, is acted on directly and locally by light,

a responsive movement is\induced. These are facts which can be demonstrated by shielding the lamina and pulvinus alternately from the action of light. When the lamina alone is exposed there is no action; but when it is the pulvinus to which light is admitted, there is an immediate responsive movement. The lamina, it is true, is provided with a fine fibro-vascular network containing the nervous strands. But the stimuli received by this extensive system are ultimately conducted along the thicker channels of which it forms the terminal ramification, and serve to stimulate the plant as a whole. When such stimuli reach the petiole, then, they cannot act in that direct and unilateral manner in the motile organ which is required for the responsive movement of the leaf. The petiole, it is true, from its dorsiventral character, is unequally excitable on its two sides. But since,in the process of the transference of stimulus from the nervous to the ordinary elements there is a great loss, and since, moreover, the motile tissues in the case of most petioles are very sluggish, the diffusely transmitted stimulus induces practically no directive effect. Nor could there, in any case, have been any com- parison between the effective strength of an external stimulus acting directly and unilaterally, and a transmitted stimulus acting diffusely.

It will thus be seen that conduction through specialised nervous elements is by no means the essential factor in bringing about those numerous directive curvatures which subserve so many important functions in the life of the plant. The question, therefore, as to what is the part in the economy of the organism played by the vegetable nerve still remains to be answered. One very obscure problem in connection with Vegetable Physiology is that of Correlation. Thus various complex activities may be set up in one part of the plant, when another part, more or less distant from it, is subjected to the variation of some excitatory influence. In this way every part of the plant-organism would appear to be ez rapport with

the rest, and this intimate connection between outlying areas becomes comprehensible, when we are made aware of the easy communication afforded by the existence of specialised conducting elements. I shall now proceed to deal with. the importance ot stimulus, and its conduction to the interior of the plant, as the essential factor in sub- serving the various life-. activities of the organism. That the reception of stimu- lus is important, in main- taining the excitability of a plant, is easily seen in the case of Mzmosa, when de- prived of light, for example. Under these conditions, its motile excitability is found to disappear. And this is only restored on re-exposure to light. We have seen again, in the course of the last chapter, that the isolated vegetable nerve, deprived, as it is, of normal favourable RiGcsrin Pilostanhins Recmdcc! conditions, becomes sub-tonic

Effect of Tetanisation in En- or moribund, and then its hancing Mechanical Response of The first series of responses heightened abolished or even reversed. to second series, after intermediate Under these ‘clvcumatantes tetanisation. the normal excitability is found to be restored by the continued action of stimulus. Abnormal positive response is thus found to be converted into normal negative. Again, after an intervening period of stimulation, response of ordinary amplitude is found, as in fig. 333, to become enhanced. It is thus seen that a tissue, when cut off from the supply of stimulus, loses its normal excitability, and that a@ more or less continuous supply of

stimulus ts essential to the maintenance of the proper excitatory condition of a tissue. | | It is known that in the animal, when the conduc- tivity of a nerve is abolished by nerve degeneration, the connected muscles also rapidly waste away. Thus the maintenance of the proper excitability of various tissues is dependent on their constant reception of stimulus or energy through the mediation of the attached nerve. It is therefore highly probable that the excitability of the indifferent vegetable tissues is kept at its normal level by the reception of energy | of stimulus through the conducting nerve. |

I shall next briefly refer to a fact which I have demonstrated fully elsewhere, that all the principal physio- logical activities of the plant, such as autonomous movement, ascent of sap, and growth are fundamentally excitatory phenomena. Thus, for example, the autonomous rhythmic movements of the lateral leaflets of Desmodium gyrans come to a standstill when their store of latent energy is exhausted. And it is only by the accession of fresh stimulus from outside that these multiple responses can be renewed. We describe the state of the plant, when its internal energy is below par, by saying that it is sub-tonic, the normal tonic condition, or health of the plant, being dependent on the sum total of stimulation previously absorbed by it. Turning next to the question of the ascent of sap, I have shown that the most important factor in bringing this about is the multiple rhythmic activity of certain interior tissues of the plant. Under such circumstances as cause the tonic condi- tion of the tissues to fall below par, the rate of the ascent of sap will be lowered, or it may possibly even be brought to a standstill, owing to the depression of excitability induced. On now supplying fresh stimulus, we find the excitability to be renewed and the normal rate of ascent restored (p. 383). Another instance of this is seen in the fzleus of Coprinus which droops when kept too long in the dark, but recovers its normal turgidity on exposure to light.