Comparative Electro-Physiology: A Physico-Physiological Study

electrical recovery takes place; but before this is complete, and while there is still a certain residual galvanometric negativity, there occurs that more intense and short-lived excitatory reaction which finds mechanical expression in the downward-movement. The same reaction finds electrical expression in a brief and intense response of. galvanometric negativity ; on the expenditure of this excitatory impulse there is again an electrical recovery, which becomes prac- tically complete. 2

In the instance given in the spark-record, it was found that the relative intensity of the down to the up impulse was approximately as I'5 is to 1. And it is interesting to see that in the photographic record of the electrical responses (fig. 144), the ratio of the amplitudes of the corresponding electrical waves is also the same. In other instances, the relative intensity of the principal wave is still higher. I give below two tables showing the absolute. values of the electro- motive variations in two different cases.

It might: be thought that these two electrical waves had been induced by the mechanical movement of the leaflet as such. We have seen, however, that the electrical response is a concomitant of the excitatory condition, whether such excitation be followed by any mechanical response or not. This we saw in the absence of mechanical movement in the case of ordinary plants. The same was found also in the case of sensitive plants when responsive mechanical move- ments were prevented from taking place by physical restraint (p. 20). The mechanical and electrical responses are thus independent modes of expression of a single funda- mental excitatory process. In order to demonstrate this in the case of the autonomous pulsation of Desmodium, | first obtained simultaneous mechanical and electrical responses of

Fic. 145. Photographic Record of Simultaneous Mechanical and Electrical Pulsation in Leaflet of Desmodium, before and after Physical Restraint of Leaflet. The first part of this record shows both mechanical and electrical pulsa- tion. In the second part, leaflet was physically restrained, as seen in the mechanical record, becoming horizontal. Electrical pulsation now seen to persist with even greater vigour than before. the leaflet (ig. 145). In the next part of the same record the mechanical movement of the leaflet was restrained, as seen in the upper mechanical record, which here becomes a straight line. But the lower record, which gives the electrical re- sponse, still shows the double electrical pulsation unimpeded. Indeed, so far from the mechanical response having been the cause of the electrical, we find that on its arrest, at least in this particular case, the latter becomes very

much enhanced. In fact, it appears as if the fundamental excitatory reaction, being now deprived of one of its two modes of expression, exhibited the other with the greater energy. . We thus see that not only does the electrical response sive us a means of detecting the action of external stimulus on a tissue, but that the same mode of indication enables us further to demon- strate the existence of those internal ex- citations which may find mechanical ex- pression in the so- called ‘autonomous’ movements,

One such autono- mous pulsation pre- ‘sent in all plants is that of growth, and by means of the highly magnified re- cord given by the Crescograph. I have eee ue to: conaest Fic. 146. Crescographic Record of Multiple of the additive effects Growth-responses in Peduncle of Crocus of multiple minute The ordinate represents the extent of responsive elongations in mm. ; the abscissa, time in seconds. pulsatory movements. Wesee this in fig. 146, which gives a series of records of multiple growth responses obtained with the peduncle of Crocus at different times of the day. In the present case, the average period of each pulsation is twenty seconds. In the case of relatively slow pulsations like these, if one electrical connection be made with the growing-point, where such movements are in pro- gress, and the other with an old leaf in which they have

ceased, the galvanometer-spot of light is thrown into a state of oscillation, indicative of the local excitatory reactions at | the growing-point. These. multiple pulsations of growth consist of alternating positive and negative turgidity-variations. In dealing with the concomitant electrical response, however, we have seen (p. 64) that the induced galvanometric negativity, owing to its greater intensity, always overpowers the galvanometric positivity, if the two occur in rapid succession. In growth- pulsation, the constituent pulses are often extremely rapid. Hence a growing-point may be expected to exhibit, galvano- metrically, a resultant negativity. This consideration may explain the observation of Johannes Miiller-Hetlingen, other- wise unexplained, that the growing-points of both shoot and root, in the seedling of Pzsum sativum, are negative, as compared with the indifferent cotyledons.

Observations of Burdon Sanderson on leaf-response in Dionea—Leaf-and- stalk currents—Their opposite variations under stimulus—Similar leaf-and- stalk currents shown to exist in ordinary leaf of F2cus religiosa—Opposite- directioned currents in Czt¢rus decumana—True explanation of these resting- currents and their variations—Electrical effect of section of petiole on Dzonca and Ficus religtosa—Fundamental experiment of Burdon Sanderson on lamina of Dzonea—Subsequent results—Experimental arrangement with symmetrical contacts—Parallel experiments on sheathing leaf of A/usa— Explanation of various results.

IT was pointed out in Chapter II. that progress in the in- vestigation of the subject of excitatory phenomena in plants had been long delayed, in consequence of the prevalent idea that only motile plant-organs were ‘excitable.’ The atten- tion of investigators was thus mainly confined within the narrow range of the so-called ‘sensitive’ plants, such as Dionea. It was also shown, in the same place, that the results already arrived at by observers in this field had not been altogether concordant, and presented many anomalies,

As it has now been demonstrated, however, in the course of previous chapters, that ordinary plants are fully sensitive, it will be well to proceed to show that the various effects observed in the ‘sensitive’ Dzong@a may be still better studied in ordinary leaves. It will be possible, moreover, by follow- ing this line of inquiry, to determine those general laws, of which the peculiarities observed in Dzonga are only instances ; and thus we shall be the better able to offer an explanation of such cases as now appear anomalous. Se

Before doing this, I shall briefly recapitulate the principal effects observed by Burdon Sanderson in the leaf of Dzonea. These observations relate firstly to the existing current of rest in the petiole and midrib, and the variations of this resting-current, whether under excitation of the lamina, or by section of the petiole, or again, by the action of electro- tonus; and secondly, to the induction of variations of a transverse current between the upper and lower surfaces, of the lamina. As regards the current in the petiole and its prolongation the midrib, which I shall distinguish as ‘the longitudinal petiolar current, Burdon Sanderson found this

(a) Leaf- and stalk-currents in Dzonga. Natural current, N, flows out- wards < A-, s being stalk-current, and L leaf-current. Stimulation of lamina at x gives rise to responsive current, R, from right to left, inducing negative variation of leaf-current and positive variation of stalk-current. When stimulus, however, is applied on left at x, responsive current, R, is from left to right, inducing ¢ffects exactly opposite of former, viz. negative variation of stalk-current and positive variation of leaf-current; (4) Leaf- and stalk-currents in Ficus rel- giosa similar to those of Déowea. Natural current flows outwards <A->. Stimulation of lamina at x gives rise to responsive current, R, inducing negative variation of leaf- and positive variation of stalk- currents; (c) Leaf- and stalk-currents of C7ztrus decumana, opposite to those of Ficus and Dionea, > A<-. Stimulation at x induces positive variation of leaf and negative variation of stalk-currents.

to flow in the midrib, from the end proximal to the stalk to the distal end. This he designated as the ‘normal leaf- current.’ He further found that if electrical connections were made, so that one contact was near the lamina, and the other away from it, the stalk-current was opposite in direction to the leaf-current (fig. 147 (a) ). On stimulation of the lamina, these resting leaf-and-stalk currents were found to undergo responsive variations. But these changes were exactly opposite to each other. That is to say, the leaf-current underwent a negative, and the stalk-

current a positive, variation. No explanation has as yet been offered, regarding either the existence of these opposite- directioned currents of rest, or the. apparently anomalous result, that an identical stimulus would induce, in one case a-negative, and in the other a positive, variation of them. seen (p. 176), that if an intermediate point be physio- logically less excitable than either of the two terminal points, then a resting current will flow from the less to the more excitable. This is the particular current-distribution in the leaf of Dzonga. It is not a unique phenomenon, for I have noticed other such instances in ordinary leaves. The point of junction of the petiole with the lamina of Ficus religiosa, for example, is galvanometrically the most negative point in that petiole-and-midrib. _The currents here also, then, as in the case of Dzon@a, flow outwards from the point of junction—the leaf-current towards the tip of the leaf, and the stalk-current in the opposite direction (fig. 147 (0) ). _ We also saw, however, in the same place, that there may be instances in which an intermediate point is more excitable than either of the two terminal. When this is so, the currents of rest will be reversed in direction, and flow inwards. This I find to be the case in the leaf of Cztrus decumana (fig. 147 (c) ).

Next with regard to the excitatory variation of these resting-currents in leaf and stalk, we must remember that the effect of stimulation is to give rise to a true excitatory current, flowing away from the excited. If then there be already a resting-current, the responsive current will be added to this algebraically. When the lamina to the right is excited, the responsive current flows from right to left. This would naturally, in the case of Dzonea, induce a negative variation of the leaf-current, and a positive variation of the stalk-current (fig. 147 (a) ). The same thing is seen on stimulating the lamina of Ficus religiosa, where also the excitatory current, being of opposite sign to the leaf-current, and of the same sign as the stalk-current, induces a negative

variation of the former, and positive variation of the latter (fig. 147 (6) ). The same stimulus thus induces effects which are apparently opposite. Or an interesting variation of the phenomenon may be obtained, on repeating the experiment with the leaf of Cztvus. Here, on stimulating the lamina, we observe a positive variation of the leaf-current, and a negative variation of the stalk-current (fig. 146 (c)). This is because the currents of reference or resting-currents are the _ opposite of those in Dzonea and Ficus religiosa.

Another series of variations exactly the reverse of these, and therefore at first sight anomalous, is caused by simply changing the point of application of stimulus, from the right end on the lamina, to the left end on the stalk. The direc- tion of the excitatory current is thus reversed, being now from left to right (fig. 147 (a) ). By algebraical summation, there now occurs a negative variation of the stalk-current, and a positive variation of the leaf-current, in Dzon@a and Ficus religtosa, while the very opposite takes place in Cztrus.

I shall here draw attention once more to those errors to which an investigator becomes liable when he infers that positive and negative variations must necessarily be the expression of assimilatory and dissimilatory processes. For we have just seen that the same responsive current, by alge- braical summation with two opposite-directioned resting- currents, may appear to be both positive and negative, at one and the same time. Again, with a single resting-current, it is possible to obtain either a positive or a negative varia- tion, according as the same stimulus is applied to the right or the left. It is now abundantly clear that the one uni- versal effect of stimulus is to give rise to a responsive current which flows from the more to the less excited portions of the tissue. If there be already an existing current, the responsive current is added to this algebraically, and induces, according to circumstances, either a positive or a negative variation. Much confusion, and many erroneous inferences would be avoided, if instead of looking at these variable indications attention were centred on the one constant criterion, namely

that the excitatory current always flows from the more to the less excited portions of the tissue. Another effect observed by Burdon Sanderson was, that on cutting the petiole across, the existing normal leaf-current was increased, the amount of this increase being determined by the length of the petiole cut off, in such a way that the shorter the petiole left, the stronger the leaf-current became. In Nature (vol. x. p. 128), he suggested an explanation of this phenomenon. In the leaf of Dzonga, as already said, there is a resting-current in the stalk, opposed in direction to that in the leaf. Thus ‘the electrical conditions on opposite sides of the joint between stalk and leaf are antagonistic to each other ; consequently, so long as the leaf and stalk are united each. prevents or diminishes the manifestation of electro- motive force by the other.’ He thus inferred that the pro- gressive removal of the antagonistic element, by section of the stalk, would serve to enhance the intensity of the leaf-current.

Taking the ordinary leaf of Fzcus religiosa, | have myself been able to obtain results precisely similar to those described in Dzonea, by making successive sections of the petiole, at shorter and shorter distances from the point of junction. The leaf-current at each section underwent an increment. The parallelism of the two sets of effects will be seen from the following table. =e eh Sy a ae ' Length of stalk Galvanometric deflection | Length of stalk | Galvanometric deflection

Burdon Sanderson’s suggested explanation that the suc- cessive augmentations of the leaf-current were due to suc- cessive removals of the antagonistic element, by section, is quite untenable. He failed to see that the effect was, on the contrary, due to the increasing excitatory action of the sections themselves. Similar results may be obtained, even without the bodily removal of the supposed antagonistic element, if, instead, we apply an increasing intensity of stimulus, as say, by contact of a hot wire at points nearer and nearer to that of junction. In the case of the trans- verse section, the cut acts as a stimulus, and the respon- sive current flows from the left to the right... Algebraical summation of this with the existing leaf-current, which is also from left to right, causes an increase, or positive variation of it, ina manner exactly the converse of the negative varia- tion induced in the leaf, when the stimulus was applied on the lamina. - As the section is made nearer and nearer to the point of junction, the degree of stimulation, and the con- sequent positive variation of the resting-current, must become greater and greater.

And lastly, in the case of the longitudinal lesfieasPauk Burdon Sanderson found that if a current from a battery were directed through a leaf-stalk, at the same time that the two ends of the midrib were led off to the galvanometer, the difference previously existing between the ends of the midrib would be increased, if the current led through the leaf-stalk were in the same direction with the leaf-current, and diminished, if it were in the opposite direction. A similar effect, as seen in the conducting tissues of ordinary plants, will be studied in detail, when we take up the question of the extra-polar effects’ induced by electrotonic currents (Chap. XXXIX.). |

We have already seen that, by means of induced varia- tion of the longitudinal stalk-current, under the stimulation caused by section of the petiole, it is easy to obtain an un- mistakable indication of the nature of the true excitatory electrical change. Burdon Sanderson, however, laboured under the disadvantage, as already said, of having failed to recognise that a section acts as a stimulus. His _ investi- gation, therefore, on the character of the excitatory variation,

was chiefly carried out by means of experiments on electrical variations induced in the lamina. These depended (1) on variations in the cross-difference of existing potential between the upper and lower surfaces, according to his ‘fundamental experiment, and (2) on electrical variations in the led-offs of symmetrical surfaces of contact on the under-side of opposite lobes. The results which he obtained, however, by these methods, appear to the reader to have been very conflicting, and in fact the experimental methods described by him would seem to have been open to many sources of complica- tion of which he himself was unaware.

Fic. 148. Burdon Sanderson’s Funda- Fic. 149. Parallel Experiment in mental Experiment on Dzonea Leaf Sheathing Petiole of Musa Electrical stimulus applied on distal Thermal stimulus applied on distal lobe, 7, induces responsive effect side induces responsive effect on on led-off circuit fw. Upper or led-off circuit. Upper or ‘internal’ internal surface, f, more excitable surface more excitable than lower. I shall deal first with Burdon Sanderson’s ‘fundamental experiment, of which the excitatory electrodes are seen on the left lobe, and the led-off on the right in fig, 148, In fig. 149 is given a diagram of a parallel experiment carried out by myself on the petiole of Musa. ‘According to Burdon- Sanderson, as the result of excitation, a + current is induced in the right lobe of Dzonea (fig. 150). This means, of course, that the upper or more excitable surface of the right lobe has become positive to the lower. This current, how- ever, he termed ‘excitatory,’ regarding it as the analogue of the ‘action-current’ known to animal physiology. After this first phase, when a certain interval had elapsed, he

observed a second phase to set in, in which the upper surface became relatively negative to the lower. This negative change, which he called the ‘after-effect,’ he described as taking place at that moment at which the mechanical effect of excitation also made itself evident. This negative phase—called by him the ‘after-effect’— Burdon Sanderson regarded as connected with those electrical changes which had been observed by Kunkel to be induced by movement of water in the tissues. The first effect on the

Fics. 150,151,152. Recordsjof Electrical Responses of Different Leaves of Dionea according to Fundamental Experiment of Burdon Sanderson Fig. 150. Positive response of certain leaves of Dzonea. Time-marks 20 per second (Burdon Sanderson). Fig. 151. Diphasic response of leaf of Droxea ‘in its prime.’ Positive followed by negative. Time-marks Io per second (Burdon Sanderson). Fig. 152. Positive response of same leaf when ‘ modified’ by previous stimulation. Time-marks Io per second (Burdon Sanderson). The above récords were obtained with capillary electrometer.

contrary, which immediately preceded this, and was charac- terised by relative positivity of the upper surface, he regarded, as already mentioned, as the true excitatory or action-effect. The following is from his summary : ‘The first phase of the variation--the effect which immediately follows excitation, and has an opposite sign to the after-effect, and a much higher electro-motive force—does not admit of a similar explanation: for it cannot be imagined that a change which spreads over the whole lamina in less than one-twentieth of a second can be dependent on migration of water. The excita- tory 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 phenomenon except the one with which I have identified it—namely, the corresponding process in the excitable tissues of animals. Of the nature of this preliminary disturbance (to which alone the term ex- citatory variation ought to be applied, it alone being the analogue of the ‘action-current’ of animal physio- logy) we know nothing. ... The direction of the ex- citatory 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. The ap- parent discrepancy will probably find its explanation in the difference of the structural relations of the electro- motive surfaces.’ }

_ From this quotation it will be seen that Burdon Sanderson had fallen into the basic error of mistaking what I have demonstrated to be the hydro-positive, for the true excitatory effect, and wece versa. In a subsequent Paper again (Phz/. Trans. vol. 179, 1889) Burdon Sanderson published certain results, which differed from those referred to above. He had previously found that usually speaking the upper surface of each lobe was negative to the lower. Later, however, he came to the conclusion that in the leaf of Dzonga in its ‘prime, the upper surface was positive to the under. On repeating his ‘fundamental experiment’ moreover, with these vigorous leaves, he found that instead of the pronounced positive response which he had previously observed, he now obtained a short-lived positive effect succeeded by a strong negative (fig. 151). He was unable to offer any definite explanation of this difference between the two sets of results, but suggested that it might arise, in some way, from changes of the resting-current.