Comparative Electro-Physiology: A Physico-Physiological Study

‘In the leaf, observed facts show most conclusively that the two sets of phenomena—those of the excited and those of the unexcited state—are linked together by indissoluble bands: that every change in the state of the leaf when at rest conditionates a corresponding change in the way in which it responds to stimulation, the correspondence consisting in this, that the sign, that is the direction, of the response is opposed to that of the previous state, so that, as the latter changes sign in the direction from + to |, the former changes from | to +.’ }

In making this statement, Burdon Sanderson was _ prob- ably guided by the prevalent opinion that response takes place by a negative variation of the existing current of rest. We have seen, however, that this supposition is in fact highly misleading. For, owing to such fluctuating factors as age, season, previous history, or excitation due to prepara- tion, the so-called current of rest may and frequently does undergo reversal. Thus a single excitatory effect might, as we have seen (pp. 175-177) under different circumstances, appear either as a positive or a negative variation of the existing current. The assumption of the universality of response by negative variation is thus seen to be unjustifiable.

Indeed, it would appear from the description of some of the experiments actually related by Burdon Sanderson him- self, that response did not, even in these cases, always take place by negative variation of the existing current. For instance, while in the leaf of Dzon@a in its ‘prime’ (upper surface positive) the response is negative, and while this latter becomes reversed to positive, as he tells us, in conse- quence of ‘ modification’ due to previous excitation (fig. 152), yet headmits that even in these circumstances the upper surface had first returned to positivity (zbzd. p. 447). Thus, though the responses of the leaf in its ‘prime, and of the ‘ modified ’ leaf are opposed, yet the antecedent electrical condition of the modified leaf has not in this case undergone reversal.

The suggestion, therefore, that the reversal. of response is due, in some way unexplained, to a reversal of the electrical condition of the leaf, cannot hold good. Nor does the use of the term ‘ modification’ in any way of the phenomenon. A satisfactory explanation of this reversal of response, then, still remains to be found. So much for the ‘fundamental experiment. The next experi- mental arrangement employed by Burdon Sanderson consists of a leaf which is led off by symmetrical contacts on the under surfaces of its two lobes (fig. 153). If now the right lobe was excited, by touching

FIG, 153. Experimental Con- nections with Dzonea ac- cording to the Second Experimental Method of Burdon Sanderson one of the sensitive filaments (on the upper surface) with a camel’s-hair pencil, in the neighbourhood of the leading-oft contact, it was found that the under-surface of the right lobe became first positive, and subsequently negative (fig. 154), Fic. 154. Response of Under-surface of Leaf of Dzon@za, with Electrical Connections as in Fig. 153

Mechanical excitation of upper surface of right lobe. shows relative positivity of under surface of same right its relative negativity (down curve). (Burdon Sanderson). Summarising these various observations, then, we find results which are very much at variance. First, according to the ‘fundamental experiment,’ certain leaves are seen to give rise to the positive response; other leaves, in their first positive and then negative. These latter again, after previous excitation, become so modified as to show only positive changes. And lastly, using the experimental arrangement of symmetrical contacts, a diphasic variation is obtained—positive followed by negative—on the under- surface, instead of the upper, of the lobe excited. No theory is advanced, however, by which a comprehensive explanation might be afforded of these apparently anomalous results.

But from the generalisations which I have already esta- blished, regarding the electrical signs of the hydro-positive and true excitatory effects respectively, and from the results of certain experiments on ordinary leaves which I shall presently describe, it will be found easy to arrive at a true explanation of the various observations related by Burdon Sanderson, which would otherwise have appeared inexplic- able. The fact that hydrostatic disturbance induces galvano- metric positivity, and that true excitation induces negativity, has already been clearly demonstrated under conditions from which all possible sources of complication had been elimi- nated (p. 61). The experimental arrangement adopted by Burdon Sanderson, however, laboured under the double dis- advantage, not only of a liability to confuse the hydro- positive and true excitatory effects, but also of the com- plexity arising from the differential excitability of the responding organ. It is only indeed by the closest analysis that it is possible to discriminate, in his results, between such as are due to true excitation and those arising from the hydro- positive effect.

The various electrical phenomena which are possible in an anisotropic organ in consequence of the hydro-positive and excitatory effects respectively, may be clearly exhibited, as I have already shown, by means of the mechanical response of the leaf of MW/zmosa. With regard to this, we have seen (pp. 59, 60) that direct stimulation of the pulvinus induces a negative mechanical response, or fall of the leaf, by the greater contraction of the more excitable lower half of the organ. The corresponding electrical variation would

thus consist in the greater galvanometric negativity of this more excitable lower, in relation to the less excitable upper half. If the stimulus, however, be applied at some considerable distance, so that true excitation cannot reach the responding point, then we have an erectile or positive mechanical response of the leaf. This is brought about by the relatively greater expansion of the more excitable. The corresponding electrical response will be the galvanometric positivity of this more excitable, in relation to the less excitable half of the organ. Between these two extremes lies that experi- ment in which stimulus is applied at some intermediate point, the consequence of which is that the hydro-positive wave, with its greater velocity, reaches the responding organ earlier than true excitation, thus bringing about a pre- liminary erectile or positive response, followed by the ex- citatory negative or fall of the leaf. The corresponding electrical response would therefore be diphasic, positive followed by negative.

But the occurrence of this second or negative phase is only possible when the conductivity is so great as to allow the wave of true excitation to reach the organ. We may imagine that in a very vigorous plant, with its great con- ductivity, we have found a point, at the maximum distance from which the true excitatory effect of a given stimulus is capable of transmission to the organ. With such a speci- men, in its ‘prime,’ we shall observe a diphasic effect—pre- liminary positive followed by negative. But if we took a less vigorous specimen, and applied the stimulus at the same distance from the responding point, the true excitatory wave would fail to reach the responding organ, and we should see there, only the positive effect due to hydro-positive action. Hence, two different specimens, treated in exactly the same way, may exhibit two different effects, one diphasic, and the other positive alone; this difference being due to their unequal vigour, and concomitant inequality of excitability and conductivity. This will account for the diphasic and positive responses which were exhibited by the more and

less vigorous leaves respectively of Dzonga, when stimulation was applied on the distal lobe, according to the fundamental experiment of Burdon Sanderson. We must next refer to the reason why a leaf that origin- ally gives diphasic response—positive followed by negative —undergoes such ‘ modification, in consequence of: previous excitation, as thereafter to give only positive response. We have seen that the negative element of the diphasic response is due to the arrival at the responding point of the true excitatory wave originated at the distant point of stimula- tion. Now it has been shown (p. 65), that if by any means the conductivity of an intervening region should become diminished, we may expect that the hydro-positive effect will continue to be transmitted, although the passage of true excitation is partly or wholly blocked. By-means of this selective block, I was able to unmask the hydro-positive component present in resultant response (cf. fig. 49).

I have shown elsewhere! that the conducting power of a tissue will be impaired by the fatigue consequent on previous stimulation. Thus, in the petiole of Bzophytum, | found that while the plant, when fresh, had a conductivity measured by the velocity of transmission of excitation, at a rate of 1°88 mm. per second, the same plant, when partially fatigued by four successive stimulations, had its conductivity dimi- nished, the velocity of transmisson being now only 1°54 mm. per second. The diminution in this case, then, was about 18 per cent. I shall moreover show in a later chapter that in consequence of growing fatigue the passage of true excitation may at a certain stage be arrested, the hydro- positive effect alone being then transmitted. It is thus easy to explain how it was that in Burdon Sanderson’s experi- ment, of stimulus applied on the distal lobe, the wave of true excitation became blocked, and the ‘ modified’ leaf gave positive response alone. These considerations will be found as I think, to offer a satisfactory explanation of the conflicting results arrived at by Burdon Sanderson,

I shall now, however, proceed to describe a series of ex- periments exactly parallel to the ‘fundamental experiment’ on Dionea, carried out on ordinary plants. We have seen that the inner or concave surface of the sheathing petiole of Musa is relatively more excitable than the outer or convex. Thus it corresponds with the ‘internal’ or upper surface of the leaf of Dzonga. The more excitable internal surfaces of both these, again, correspond with the more excitable lower half of the pulvinus of AZzmosa. In fig. 149 is shown an experi- mental arrangement with a specimen of JZusa which will be seen to be parallel to that of Burdon Sanderson’s funda- mental experiment on Dionea. In order to avoid any such disturbance as might conceivably arise from current-escape, if the electrical form of stimulus were used, I employed the thermal mode of stimulation. A momentary heating-current passed through a thin platinum wire gave the thermal varia- tion required, and was found to furnish a very satisfactory form of stimulus. The led-off circuit was at first placed at a distance of 16 mm. from the point of stimulation. As the stimulation was moderate, and as the conductivity of the tissue was not great, the effect induced at the respond- ing circuit was hydro-positive, the more excitable concave surface becoming positive (fig. 155 (a) ). This response is the same as the positive responses given by the ‘unmodifiable leaf’ of Dzonea (fig. 150), as well as that of a vigorous leaf which had been ‘modified’ by fatigue (fig. 152). On next taking a second pair of led-off points, at the shorter distance of 8 mm., the hydro-positive effect reached the led-off points earlier, and was followed by the true excitatory wave. This is seen as a preliminary positive response, followed by the excitatory negative (fig. 155 (2)).

This again is the same as the di-phasic response of a Lzonea leaf in its ‘prime’ (fig. 151). Inthe experimental arrangement with J/usa the led-off circuit was now brought still nearer to a distance of 4 mm. There was now little interval between the arrival at the led-off points of the hydro-positive and true excitatory effects; and since the latter is of predominant electrical expression, the former is masked by it, and we obtain here only the excitatory negative variation (fig. 155 (¢) ).

It only remains to consider the responses which Burdon Sanderson obtained with symmetrical contacts (fig. 152) on the under-surfaces of the two lobes. In the next figure (fig. 153) is reproduced his record of electrical response, obtained on mechanical stimulation of a sensitive filament situated on the upper surface of ‘the right lobe, vertically above the right-hand led-off. This response is, as will be seen, di- Fic. 155. Photographic Records of Positive, Diphasic, and Negative Responses ot Petiole of A/usa depending on the Effective Intensity of Transmitted Stimulus

(az) Here stimulus was applied at a distance and hydro-positive effect alone transmitted ; (6) Stimulus was applied nearer, and the positive effect was succeeded by the true excitatory negative; (c) Stimulus was applied very near, with the result of true excitatory negative response. phasic, its first phase being one of relative positivity of the under-surface of the excited lobe, and the second representing its subsequent relative negativity. This first phase is clearly due to the earlier transmission of the hydro-positive or indirect effect of excitation, from the stimulated point on the upper surface. It was supposed by Burdon Sanderson that the second phase of this re- sponse represented the later arrival of the same positive effect at the distal second contact, which would thus induce reversal. But it appears much more probable that this second phase of negativity is due to the arrival at the

under-surface of the wave of true excitation, initiated vertically above. This relative negativity of the under- surface may or may not be helped by the induction of positivity at the distal, due to the transmission of the hydro- positive effect. This view is supported by the fact that in a corresponding experiment on an ordinary leaf, in which the second contact was at a distance too great to allow of the effective transmission of any hydro-positive wave, the stimulation of the upper surface induced a similar diphasic response at a point diametrically opposite, on the under side. In this case the second or negative component of the response could not be due to anything but the subsequent arrival of the true excitatory wave with its concomitant negativity.

It is now clear that among the various results obtained from the study of the electrical responses of the leaf of Dzonga, there are some which do not represent true excitation at all, while in others it is only one of the two phases which is significant of this, the other being due to the hydro-positive effect. We have also seen that Burdon Sanderson at starting fell into the error of wrongly identify- ing the true excitatory electrical effect with that which was due to the hydro-positive effect, and vice versa. We have seen that there is not a single response given by the so-called excitable leaf of Dzong@a, which cannot be obtained under similar conditions from the leaves of ordinary plants also. In fact it has been by means of experiments carried out on the latter that we have been enabled to unravel all the intricacies which were offered by the recorded responses of the lamina of Dzonea. :

It has further been shown in the course of the present chapter that the leaf and stalk currents observed in Dzonga are also found in, for instance, the leaf of Ficus religiosa. These have been shown, moreover, to be due to physiological differences between an intermediate and the terminal points. The negative variation of the leaf-current, and the positive variation of the stalk-current, on the stimulation of the _ lamina, were both alike shown to be the result of the alge- braical summation of a definite excitatory current with the two opposite-directioned resting-currents. The positive variation of the leaf-current, again, on section of the petiole, has been traced to the same cause, namely the stimulatory action of mechanical section, giving rise to an excitatory current which was summated with the existing leaf-current. Finally the positive response of the concave surface of Dionega has been shown to arise, not from any specific difference between plant and animal response, but from the fact that in this particular case it was the indirect hydro- positive effect of stimulus that was transmitted, inducing an action opposite to that of true excitation.

Electrical organs in fishes—Typical instances, Zorfedo and Malepterurus— Vegetal analogues, leaf of Pterospermum and carpel of Déllenta indica or pitcher of Mefenthe—Electrical response to transmitted excitation — Response to direct excitation—Uni-directioned response to homodromous and hetero- dromous shocks—Definite-directioned response shown to be due to differential excitability— Response to equi-alternating electrical shocks—Rheotomic ob- servations-—Multiple excitations—Multiplication of terminal electromotive effect, by pile-like arrangement, in bulb of Uyzc/zs lily.

IT has been shown that by a study of the peculiarities of electrical response in plants, it is possible to obtain an insight into the obscurities of similar responses in the animal tissue. Among animal structures, there is one—the elec- _trical organ of certain fishes—the explanation of whose action offers unusual difficulties to the investigator. But I shall attempt to show in the course of the present chapter, that there are also, on the other hand, vegetable structures, the study of which will be found to elucidate the electro-motive action here involved. Taking that of the Torpedo as type, we find that the electrical organ is disposed in the form of columns, each column consisting of. numerous electrical plates, arranged in series, one over the other, like the plates in a voltaic pile. Each electrical plate consists of a rich plexus of nerve-fibres imbedded in a gelatinous mass. There are thus two surfaces, one nervous and the other non- nervous. Each disc then becomes electro-motive under the impulse from the nerve. Though the induced electro-motive force in each plate is small, yet in consequence of their serial arrangement in columns, the elements are coupled for inten- sity, and the resulting E.M.F. of discharge becomes high,

Fritsch estimates the total number of these plates in some of the Torpedos to be over 150,000. From the point of view of their development, these electrical organs in general constitute modified muscles, containing nerve-endings, The electrical fish known as Matepterurus of the Nile is an exception to this rule, inas- much as morphological evidence goes to prove that in its ~ case it is glandular, rather than muscular, elements which have been so modified.

The peculiar characteristic of the discharge of electrical organs in general, is that it takes place in a definite direction at right angles to the plates. It was Pacini who tried to establish the generalisation that the direction of the dis- charge would be found to be dependent on the morphological character of the organ. He found that as a general rule the discharge takes place in a direction from that surface of the disc which receives the nerve (henceforth to be referred to as the anterior surface) to the opposite non-nervous, or posterior, surface. Thus in the Zorfedo, where the plates are horizontal, and the anterior or nervous surface constitutes the ventral aspect of the disc, the discharge is from the ventral or anterior, to the dorsal or posterior surface. In Gymmnotus again, the plates or discs are vertical to the long axis, The anterior or nervous surface is here towards the tail-aspect, and the discharge is from tail to head. If these cases had been all, Pacini’s generalisation, as regards the direction of discharge—from the anterior nervous to the posterior non-nervous—would have been complete, and from it some attempt might have been made to offer an explana- tion of the phenomena. Unfortunately, however, this is not so, since Malepterurus presents a hitherto inexplicable ex- ception to the rule. In this fish, though the anterior or nervous surface is towards the tail-aspect as in Gymnotus, yet the discharge is in the opposite direction towards the head : that is to say, from the posterior surface to the anterior. The difficulties in the way of an explanation of the activity of these electrical organs of certain fishes are thus seen to be

very great. Is the activity something specific occurring in these fishes alone, and unrelated to other electro-motive phenomena in the animal tissues? Or is it related to the electromotive action already observed in excited muscles? In support of the latter view, it is urged that most of the electrical organs consist of modified neuro-muscular elements, Against this argument, however, as we have just seen, is the instance of Malepterurus, in which, from a morphological standpoint, the organ is to be regarded as a modified gland, and therefore not muscular in character,

There are certain peculiarities, further, about the action of these organs which call for elucidation. Among these is the question of the character of the natural current of rest, about the significance of which there have been differences of opinion. There is also the fact that the organ, under a single strong excitation, gives rise not to one, but to a series of electrical responses. We have seen that the apparently unique character of this group of organs constitutes an added difficulty in arriving ata correct theory on the subject. But it is clear that if we could succeed in discovering among vegetable organs any cases which showed similar characteristics, we should then be so much the nearer to the determination of that fundamental reaction on which the phenomenon in animal and vegetable alike depends,

In the typical case of Zorpedo, it has been seen that the conducting nerve, when entering into an electrical plate, breaks into an extensive ramification, and thus forms the nervous surface, in contradistinction to the jelly-like sub- stance in which it is imbedded, forming the opposite, and here indifferent surface of the plate. Now this arrangement is closely imitated by many ordinary leaves, in which the vascular elements break, on reaching the lamina, into a pro- fuse arborisation. |

I must here anticipate matters to say that I have discovered in the fibro-vascular bundles of plants (see ei hap. XXXII.) elements which are in every way peel eres to the nerves of animals. For an exact vegetal analogue to the electrical plate of Zorpedo, we may take certain leaves in which the ventral, or anterior, surface is formed of a prominent network of highly excitable nervous elements, while the upper consists of an indifferent and relatively inexcitable mass of tissue. An example of this may be found in the leaf of Pterospermum subertfolium (Rox.) whose lower surface is characterised by | a remarkably perfect venation, while the upper or posterior is dry and leathery. Thus the nerve passing into an elec- trical plate of Zorvpedo corresponds with the petiole attached to the leaf just described, since in the two cases alike, it is the ventral surface which contains the highly excitable nervous

elements. In the exceptional JMJalepterurus, on the other hand, - it is, as we have seen, a modified gland, and not a modified muscle, which forms the posterior surface of an individual electrical element. Morphologically speaking, the vegetal analogue is found in such organs as the carpellary leaf of Dillenta indica, or the pitcher of Nepenthe, both of which are glandular on their upper or inner surfaces. In point of structure, then, these leaf-organs are analogous to single discs or elements of the electrical organs of TZorgedo and Malepterurus respectively. But we have still, in the course of the present chapter, to inquire whether the electrical reactions are equally correspondent—that is to say, whether, on stimulation, the excitatory current in the type of vege- table organ represented by the leaf of Pterospermum is or is not, from the lower or anterior surface to the upper posterior, as in the electrical plate of Zorpedo; and, con- versely, whether in the type represented by the carpel of Dillenia or the pitcher of Wefenthe the excitatory current is from the posterior to the anterior surfaces, corresponding with the discharge in the electrical element of MWalepterurus, from the posterior glandular to the anterior non-glandular surface.

While dealing with the theory of the action of electrical organs, I shall be in a position to show that the characteristic reaction of each of these two types is governed entirely by the question of the relative excitabilities of the two surfaces. The physiological anisotropy on which the distinctive effect of the type depends is very pronounced in the representa- tive cases of the vegetal analogues which have been named. In many other cases, however, though the results under normal conditions are fairly definite, and approach one or other of the two types, yet the characteristic responses are liable to be reversed under the physiological modifications induced by age and surrounding conditions. In this way it may be said of the leaf of water-lily (Vymphea alba), of Bryophyllum calcineum, and of Coleus aromaticus that when vigorous, and in their proper season, their responses are of the first of these two types, while those of the bulb-scale of Uviedis lily, with its glandular inner surface, are of the second type.

The electrical organ of the fish may be excited indirectly by means of stimulus transmitted through the nerve; or direct stimulation may be applied, as by means of induction- shocks. Under either of these conditions the excitatory discharge is definite in its direction. In the case of Jorpedo, as already mentioned, this is always from the ventral and anterior to the dorsal or posterior surface. Turning then to the corresponding vegetable organ of the first type, I shall show that transmitted stimulus induces an effect exactly similar ; and I shall demonstrate this experimentally by means of the leaf of Vymphea alba. Suitable galvanometric connec- tions were made with the ventral anterior and with the dorsal posterior surfaces of the lamina. Thermal shocks, by means of the electro-thermic stimulator, were applied on the petiole, close to the lamina, at intervals of one minute, records being taken photographically of the resulting responses. It should be remembered here that excitation is transmitted to the lamina by the conducting nerve-like elements present in the petiole. The records (fig. 156) show that the effect of this periodically transmitted stimulation was a series of respon- sive currents, whose direction was like that of the discharge in Torpedo, from the anterior surface to the posterior.