Comparative Electro-Physiology: A Physico-Physiological Study

There are, again, certain other substances, of which the normal response is negative. Thus a wire of brominated lead, for instance, when suitably prepared, is found to give an electro-motive response in which the current flows from the excited to the unexcited or away from the excited, the excited point becoming galvanometrically negative. These electro-motive responses of the inorganic have thus the same characteristics as those which have been observed in the case of animal tissues. Certain tissues, such as highly excitable muscle and nerve, give negative response—that is to say, the excited point becomes galvanometrically negative. Other tissues, again, the skin for example, give positive response. The normal responses, moreover, are sometimes found to be reversed under molecular modification, and to be re- reversed to normal response under long-continued stimu- lation.

; : Fic. 8. Fatigue in the Electric of metals, again, are subject Response of Metals to an increase or decrease which is paralleled. by the same phenomenon in the response of animal tissues under similar circumstances. That is to say, fatigue is found to depress the response of the inorganic as of the organic (fig. 8). As in the case of animal tissues, again, so also in that of metals, certain chemical substances act as excitants, enhancing the response (fig. 9), others as depressors and still a third class—such as oxalic acid—as poisons, abolishing response altogether (fig. 10).

By taking advantage of the last of these facts, we arrive at a second means of obtaining response—that is to say, the method of relative depression. If both the contact points Before * After Fic. 9. Stimulating Action of Na,CO, on Electric Response of Platinum Records to the left exhibit response before, to the right after the application of reagent. A and B be equally excited—-that is to say, subjected to diffuse stimulation—the responsive ‘currents will be opposed, and there will be no resultant galvanometric effect. This was overcome, according to the method of block, by localising excitation at one point, say, A; we might, however, neutralise altogether the counteracting excitatory effect at B by abolishing the excitability of that point, as, say, by the application of oxalic acid, A being left in its normal con- dition. If now the wire be subjected to diffuse stimulation

by vibrating it as a whole, a resultant response will occur. But by the application of oxalic acid to one contact, a resting or permanent current has been induced in the circuit. The responsive or action current originated under stimulation is now found to flow in a direction opposite to that of this resting current—that is to say, it causes a negative variation of it (fig. 11). The method of response by the so-called negative variation, which is generally employed in studying re- sponsive phenomena in animal tissues, is in reality,

: : : Fic. 10. Abolition of Response in of this method of relative siétal by Oxélic Acid depression. i Various means have been described, in the course of this chapter, for the detection and record of that excitatory change which is brought about by the upsetting of molecular equi- librium under stimulus, and the subsequent re- covery. The responsive 6 change may find expres- form of mechanical con- Fic. 11. Response by Method of Relative traction, or of the elec- Depression

trical variation of gal- _ * TePresents current of rest ; ¥ represents Or the opposite change, expressed mechanically as ex- pansion, will be evidenced by galvanometric positivity. But it must be borne in mind that neither of these expressions is consequent on the other It would be as incorrect to suppose that the electrical effect depended on the mechanical, as to assume that the mechanical was brought about by the electrical. The two are independent expressions of the same fundamental molecular change, brought about by the shock of stimulus.

Again, those various responsive phenomena and.their modi- fications which are the subject of our inquiry, are such as are induced by different external agencies. Under the influence of certain conditions, the responses of living matter undergo an abolition—the change which we associate with death. In inorganic matter also, we find a similar change of responsive- ness into irresponsiveness, to take place. The death-change in the case of living matter is thus not due to a change from the organic to the inorganic condition, but to some molecular transformation. And the nature of this very obscure trans- formation may one day be elucidated by a careful study of the changes which take place in inorganic matter, when it passes from responsivity to irresponsivity.

The word ‘ physiological’ is generally used to distinguish phenomena which are believed to be exclusively characteristic of the properties of living matter. Such phenomena, however, - are found, as our power of investigation grows, to be increas- ingly capable of analysis into physico-chemical processes. In my own use of the term ‘physiological,’ therefore, it will be understood as a convenient expression for describing the response-phenomena of plant or animal tissues, but as in no sense opposed to the word ‘ physical.’

We shall, in the following chapters, study excitatory effects in living tissues, and their variations under different conditions, using the methods of electrical response. These phenomena will be studied with special detail in the case of vegetable tissues, and it will be found that there is no responsive reaction exhibited by any one amongst the various types of animal tissues, which has not its exact corre- spondence in the vegetable. Those anomalies, further, which have been observed in the response of the animal, will be seen

to be fully elucidated by the study of similar phenomena under the simpler conditions of the plant. And finally an attempt will be made to arrive at some generalisation which will show the continuity between the simplest form of response in the inorganic and the most complex which occur in the highest type of animal tissue. Historical—Difficulties of investigation—Electrical response of pulvinus of Mimosa—Simultaneous mechanical and electrical records—Division of plants into ‘ ordinary’ and ‘sensitive’ arbitrary—Mechanical and electrical response of ‘ordinary’ plants—Direct and transmitted stimulation — All forms of stimulus induce excitatory change of galvanometric negativity.

IT has been customary to divide plants, as regards their responsiveness, into two distinct classes: ‘ordinary’ and ‘sensitive. Of these only the latter class, represented by such plants as Wzmosa and Dionea, was regarded as excitable. Hence the attention of observers desirous of investigating excitatory electro-motive phenomena in vegetable tissues was mainly attracted towards the reactions exhibited by these plants. The experiments undertaken in this field by Kunkel, Burdon Sanderson, and Munk are well known. Burdon Sanderson and Munk worked on the sensitive leaves of Dzonega and Kunkel on-those of W/zmosa.'

Electro-motive variations were observed to ‘ake place in all these plants on stimulation, but the conclusions arrived at by the investigators were not concordant. Burdon Sander- son and Munk found out that the resting-current between two selected points of Dzonza exhibited variation on applica- tion of stimulus to the leaf. They thus obtained di-phasic and sometimes even tri-phasic responses, consisting of positive and negative variations. More definite results were obtained by Burdon Sanderson, in the responsive variations of the normal leaf-current, flowing between the proximal and distal

1 For a more detailed account cf. Biedermann, ZEéectro-physiology, English translation, 1898, vol. il. pp. 1-31. negative variation. But the current in the stalk, or petiole was found to undergo the opposite change—that is to say, a positive variation. Kunkel, in working with the pulvinus of Mimosa, found that on excitation a series of opposite or oscillatory electro-motive variations was induced. He also found electro-motive differences to be induced as the result of the flexure or injury of ordinary stems He believed all these electrical phenomena to be consequent on hydrostatic disturbance or water-movement.

By means of this ‘ migration of water’ Kunkel attempted to explain all the electrical phenomena in vegetable organs. The electro-motive difference between different parts of an organ was due, according to him, to their different powers of absorption of water. The greater absorptiveness of one point, with its consequent greater movement of water, would render that point relatively positive. As against this, how-— ever, Haake pointed out that electrical differences between different points were also to be found, even in submerged plants, like Valisnaria and Nztella, in which there could not possibly be any differences of absorptiveness. This difference, therefore, he suggested, must be ascribed to some vital process, inasmuch as the P.D. is seen to undergo a change whenever the respiratory process is interfered with, as, say, by the substitution of hydrogen for oxygen.

We shall study later in greater detail the conditions on which this ‘current of rest,’ so called, actually depends (Chapter X.). But more important is the excitatory variation induced by stimulus. It has already been stated that Kunkel found oscillatory variations of the current to occur variations were difficult to reconcile with his theory of the active, single-phased displacement of water, as he did not fail to see, and he suggested that the first negative swing observed might be due to the disturbance of the diffusion- process through alterations of the protoplasm. ~

Munk attempted to explain the complicated. electrical effects which he observed in Dzon@a by assuming the existence of two different kinds of electro-motive elements, affected in opposite ways, the maximum changes being initiated in one set earlier than in the other. In this way, he thought, it might be possible to explain the occurrence of positive and negative variations, holding that the upper parenchymatous layer of the leaf, and the upper midrib, went through the negative, and the under layer and the under midrib through the positive change. |

Burdon Sanderson, in his ‘ fundamental experiment’ on the lamina of Dzone@a, had his led-off circuit connected with the upper and lower surfaces of one lobe, stimulus being applied at the other. In the experiments described in the ‘ Phil. Trans.’ of 1882, he found that the upper or inner surface of the leaf become positive on excitation. This he regarded as the true excitatory change. The upper contact now, however, after a certain interval became negative, a change which Burdon Sanderson designated as the after-effect. This after- effect he ascribed to the electrical variations caused by that movement of water which had been observed by Kunkel. But with regard to the preceding positive variation he says :

‘The excitatory disturbance which immediately follows excitation is an explosive molecular change, which by the mode of its origin, the suddenness of its incidence, and the rapidity of its propagation is distinguished from every other phenomena except the one with which I have identified it, namely, the corresponding process in the excitable tissues of animals... . The direction of the excitatory effect in the fundamental experiment is such as to indicate that in excitation excited cells become positive to unexcited, whereas in animal tissues excited parts always become negative to unexcited.’!

In a subsequent series of experiments, however, given in ‘Phil. Trans,’ for 1888 Burdon Sanderson finds the reaction of leaves ‘in their prime’ to be somewhat different. A strong negative phase was now observed on stimulation, but pre- ceded by a short-lived positive reaction. These results will be discussed in detail in a subsequent chapter, but it may be said here that from the records given by Burdon Sanderson it is difficult to know which of the various responsive phases are to be regarded as those of true excitation, and which as the results of some other cause.

It will thus be seen that the results arrived at and the theories advanced by different observers in this field are some- what at variance with eachother. This must have been due to the difficulties met with in disentangling the fundamental reaction from those various subsidiary effects which are apt to be found in combination with it. Chief among these difficulties was (1) the fact that positive and negative variations are generally measured in terms of an existing current of rest. But, as a matter of fact, these two apparently opposite responsive variations, positive and negative, are not always indicative of opposite reactions. For an identical excitatory reaction, added algebraically to the resting-current, might appear, according to circumstances, as either of the two. There was also (2) the difficulty of discriminating two opposed electrical effects, one of which was due, as I shall show, to true excitation, and the other to increase of internal energy brought about by mechanical movement of water. Under different conditions, it may be either the one or the other of these which becomes prominent. I shall hope to show that each of them is definite and dis- tinguishable from the other.

With regard to the response of plants in general, ae it may be well to state here that the first fact to be demon- strated in the course of the present work is that such responsive phenomena as may be observed in the case of sensitive plants like Dzon@a, are not unique, but occur under similar circumstances even in ordinary plants, and are characteristic of all plant organs. I shall be able to show, moreover, that an explanation of these phenomena, much

simpler than the theory of electro-motive molecules, is available. It will also be proved that the electrical response due to true excitation is quite distinct from that which is brought about by the hydrostatic disturbance, its sign being in fact opposite. This true excitatory electrical response, again, will be shown to be modified by all those conditions which affect the physiological state of the tissue. And, lastly, it will be proved that there is no breach of continuity as between the electrical responses in plant and animal, for not only is the sign of response in both cases the same, but it is also true that every type of response and modification of response, which occurs in the animal tissue, is to be found under parallel circumstances in that of the plant also.

In order to determine what is the electrical response characteristic of excitation, we first select for experiment a sensitive plant, say J/zmosa, because here, in the responsive fall of the leaf, we have a visible indication of the excitatory reaction. It is desirable at this point to say a few words re- garding the use of the terms ‘ excitatory ’ and ‘true excitation.’ We are all familiar with the fact that, when muscle is excited by stimulus, it responds in a conspicuous manner by con- traction. This is universally accepted as the phenomenon of excitation, its electrical concomitant being galvanometric negativity. Having once applied the term ‘excitatory’ to this particular aspect of molecular response in living tissues, it is of course important that we should henceforth distinguish carefully between it and its possible opposite, namely, ex- pansion, with concomitant galvanometric positivity. Under stimulation there is a contraction of, and expulsion of water from, the excited pulvinus, which brings about the depression of the leaf. It is generally. supposed that only the lower half of the pulvinus is excitable. This, however, is an error, for both upper and lower halves are excitable, and contract under stimulation. If localised stimulus be applied on the upper side, that side contracts, and, by the concavity thus induced, the leaf is erected. But, though both halves are

sensitive, yet the excitability of the lower is, generally speaking, greater, and diffuse stimulation therefore causes greater contraction of that half. Hence the resultant fall is due to the differential contraction of the two sides of the organ. The excitatory reaction of the organ, then, consists of a contraction ; expulsion of water with consequent diminu- Fic. 12. Arrangement for observing Simultaneous Mechanical and Electrical Responses Leaf stimulated by electro-thermic stimulator. Mechanical record obtained by excursion of spot of light reflected from Optic Lever, which falls on the right of drum. Electric record obtained by excursion of galvano- meter spot of light adjusted to fall on the left side of the drum.

tion of turgidity, or negative turgidity variation ; and fall of the leaf. : In order next to determine the electrical concomitant of this reaction, we make suitable electrical connections by non-polarisable electrodes, with a galvanometer. One of these is made with the pulvinus, whose excitation is to be observed, and the other with a distant indifferent point. In this way we can obtain the excitatory effect at the pulvinus, uncomplicated by that at the distal point. A spot of light, reflected from the galvanometer mirror, is thrown on the

recording drum. This spot of light, hitherto quiescent, shows, by its sudden deflection, the occurrence of the excitatory change. To show the concomitance of the mechanical and elec- trical responses, and in order to detect with certainty the exact moment of the initiation of the former, a magnifying arrangement is obtained by attaching the end of the leaf to an Optical Lever. The pull exerted by the falling leaf rotates the fulcrum-rod, carrying a light mirror. The spot of light. from this mirror moves in a vertical direction, and that — of the galvanometer horizontally or laterally. For the purpose of simultaneous record, it is necessary to have the two in one direction. The up and down movement of the spot from the Optic Mirror is therefore converted into lateral, by means of reflection from a second mirror, suitably inclined.

Stimulation may be effected in the neighbourhood of the pulvinus by means of the electro-thermic stimulator, which has the advantage of producing no mechanical dis- turbance. This consists of a V-shaped platinum wire, suddenly heated by the momentary passage of an electric current. On applying stimulus in this manner it is found ‘that the two responses—mechanical and electrical—take place at the same moment, the mechanical fall of the leaf being practically coincident with the induced electrical variation. As regards the sign of this electrical change, the excited point is found to become galvanometrically negative : that is to say, the electro-motive variation induced in the excited vegetable, is the same as that observed in animal tissues. I give below a series of these simultaneous records of mechanical and electrical response (fig. 13), obtained from Biophytum, whose lateral leaflets give motile indications. It will be seen that the responsive fall of the leaflet, and the subsequent recovery, are synchronous with the electrical variation of galvanometric negativity, and its subsequent recovery. For convenience of inspection. I shall always, unless specially stated to the contrary, represent the normal

responses of mechanical depression and galvanometric nega- tivity by up-curves. The erectile movement and galvano- metric positivity will, conversely, be represented as down. A few words may be said here as regards the syn- chronism between the two forms of response. The excitatory molecular change takes place instantaneously, and the electro-motive variation is, as far as can be judged, strictly concomitant with it. This is shown by the rheotomic method of observation, described in Chapter IV. It will there be seen that a very considerable electro-motive change has already been induced, in a period so short as ‘oI of a second after the reception by the tissue of the stimulating shock.

Fic. 13. Simultaneous Mechanical (M) and Electrical (E) Responses in Biophyium In the electrical response, then, of highly excitable tissues there is practically no latent period. But if the same elec- trical variation be recorded by the galvanometer, there will be a lag in the response, owing to the inertia of the galva- nometer needle. Similarly, in the mechanical response, though the excitatory reaction is immediate, yet the motile response is delayed, by the antagonistic actions of the upper and lower halves of the pulvinus, the sluggishness of the tissue, and the mechanical inertia of the indicating leaf. The latent period of the mechanical response of a vigorous Mimosa, owing to all these causes, I find to be about twenty-four-hundredths of a second. But this may be still

further prolonged when the tissue is in a state of depressed excitability. It will thus be seen that even when the funda- mental excitatory reaction is instantaneous, its outward expression, whether mechanical or electrical, may nevertheless appear to be subject to delay in consequence of the inertia of the particular indicator concerned. As regards these two forms of response, it should further be remembered that the mechanical and electrical responses are independent indications of the fundamental excitatory reaction, and that neither is dependent for its occurrence on the other. Thus, when the mechanical response is physically restrained, the electrical response takes place unimpeded. I shall here relate an experiment in illustration of this point.