Comparative Electro-Physiology: A Physico-Physiological Study

rhythmic excitation. When the tonic condition of the plant falls below par, growth is arrested, however large the amount of formative material present. But if stimulus be applied, while the plant is in this state of growth-standstill, there is a renewal of responsive growth. the response of growth is seen in the development of certain seedlings. These, when grown in the dark, become diseased | and perish. The growth and the development of organs cease, even though the cotyledons still contain considerable quantities of unused formative substance. They become moribund, as the expression of their loss of tonic condition, which, as we have seen, depends upon the supply of stimulus from outside. And in the present case the critical element is the stimulus of light.

citability of a tissue, is shown in the following photographic records of the mechanical response of vegetal nerve. The first three responses show the extent of normal response to a given electrical stimulus applied in the dark. The specimen was then subjected to the light of an electrical arc lamp, and the next series show the consequent enhancement of response, under the same stimulus as before. Light was now cut off, and after an interval response once more taken. This is seen to have been of the same amplitude as at the beginning (fig. 334). Thus light is seen to enhance excitability.

In the cases which we have just been considering we have, for the sake of simplicity, confined our attention to a single factor—namely the photo-tonic—among the many which finally determine the general tonic condition of the plant. And we have found that when the organism is deprived of this source of stimulus, its motile activity, its suctional activity, and its response of growth, all disappear. When the plant, however, is restored to the direct action of light, all these various activities reappear. Now it is clear that this illumination cannot. directly penetrate to many of those interior tissues, whose activity is nevertheless essential to the

maintenance of life. How, then, is the external stimulus conveyed to these? It is evident that this can only be accomplished through the agency of the nervous elements. This fact, of the transmission of the excitatory effect of an external stimulus from one part of the plant to another at a distance, there to maintain the tonic condition, is again still more clearly seen in the well-known experiment on the sensitiveness of Wzmosa when partially kept in the dark. If one branch of this plant be covered by a dark box, while the rest of it is exposed to light, it is found that the leaves of the

FIG. 334. Photographic Record showing Enhancement of Excitability under Action of Light in Nerve of Fern First series, normal mechanical responses to electrical stimuli, in dark ; second series, the same, taken under light; third series, taken after withdrawal of light. first undergo no loss of motile sensitiveness. It is thus evident that the photo-tonic stimulus has been transmitted from the illuminated to the unilluminated portions of the plant, through conducting channels, in order to maintain the normal excitatory condition. |

Next comes @ie very interesting experiment of Sachs, in which a long shoot of Cucurbita was made to grow inside a dark box, the rest of the plant being exposed to light. The covered part of the plant, under these circumstances, showed normal growth of stem and leaves. Normal.flowers and a large fruit were also produced in the same confinement. The tendrils inside the box, moreover, were found to be fully as sensitive as those outside. The transmission of stimulus by the plant, in such a way as effectively to maintain such complex life-activities as motility and growth, even in the absence of direct stimulation, is. thus fully demonstrated. And we may gather an idea from this fact of the funda- mental importance, to the life of the plant, of those nervous elements by which this is rendered possible.

One of the most important functions of the venation ot the leaf, not hitherto suspected, is now made clear to us: Among external stimuli, none perhaps is so essential, or so universally and easily available to green plants, as energy otf light. And we now see that the fine ramification of fibro- vascular elements over as wide an area as possible in the leaf, provides a virtual catchment- basin for the reception of stimu- lus. The expanded lamina is Re Sages STistginurine ore Ties thus not merely a specialised

Bevan heel ot hapaye photo-synthesis, but also a sen- There are at least 20: such layers eb ‘ of stimulus, the effect of which is gathered into larger and larger nerve-trunks, in the course of its transmission downwards into the body of the plant. And even in the interior of the plant the distribution o1 these is such that no mass of tissue is too remote to be ex- cited by the stimulus conducted through the nervous ele- ments buried in them. How reticulated they may be, even in the trunk, is seen in the accompanying photograph of the distribution of fibro-vascular elements in the main stem of Papaya (fig. 335). This network, of which only a small portion is seen in the photograph, girdles the stem through- out its whole length, and in this particular case there were as Many as twenty such layers, one within the other.

It is thus seen how all parts of the plant are, by means of nerve-conduction, maintained in the most intimate communi- cation with each other. It is, then, in virtue of the existence of such nerves, that the plant constitutes a single organised whole, each of whose parts is affected by every influence that falls upon any other. | Extra-polar effects of electrotonic currents on vegetal nerve—Electrotonic variation of excitability—Bernstein’s polarisation decrement —Hermann’s polarisation increment—Investigation into the law of electrotonic variation of conductivity —Investigation on variation of excitability—Conductivity en- hanced when excitation travels from places of lower to higher electric potential, and depressed in opposite direction—When feeble, anode enhances and kathode depresses excitability—All electrotonic phenomena reducible to combined action of these factors—Explanation of apparent anomalies.

WHEN an electrical current is Jed through a portion of a nerve— entering, say, at A, and leaving by K—it is found that electro-motive changes are induced by it in the extra-polar Fig. 336 shows kat-electronus, E near K being galvanometrically negative. Fig. 337 shows an-electronus, E near A being now galvanometri- cally positive. regions. On the kathodic side, the electric potential near K is found to be lowered in reference to a point further away. On the anodic side similarly, the electric potential of a point near A is found to be raised. These changes induced in the electric potential are indicated by the galvanometric nega- tivity of the point near the kathode, and positivity of that near the anode (figs. 336, 337). In the tissue itself the current is assumed to flow in a direction contrary to that in

the external galvanometric circuit, as indicated by the dotted arrow.' In the medullated animal nerve, the electrotonic currents increase with the intensity of the polarising current. In the vegetal nerve, I have obtained exactly similar results. Taking afresh and vigorous specimen, the polarising elec- trodes were placed at a distance of 2°5 cm. from each other, the pair of extra-polar electrodes, where the electrotonic effects are observed, being separated from these by 2 cm. and divided from each other by 2 cm. also. The value of the acting polarising E.M.F. could be varied by the use of a

Fic, 338. Extra-polar Electrotonic Effects under an Acting E,M.F. which rises from °6 to 1°4 Volts A, an-electrotonic deflections seen to left ; K, kat-electrotonic to right. potentiometer arrangement. In order to induce in the extra- polar electrodes, an-electrotonic and kat-electrotonic effects alternately, the current in the polarising circuit can be sent in one direction or the other by means of a reversing-key. The record of the galvanometer deflection, in the extra-polar circuit, gives a measure of the electrotonic effect induced. In fig. 338 is seen such a record of. effects both an-electro- tonic and kat-electrotonic, taken while the acting E.M.F. was increased from ‘6 to 1°4 volt, by steps of ‘2 volt at a time.

' Certain considerations, which need only be referred to here, cast some doubt on the validity of this assumption. But as it is so widely current in physiological literature, I shall confine myself, in dealing with the subject of the electrotonic current and its variations, to those indications in the external circuit which are afforded by the galvanometer. I give here a table which shows the galvanometric deflection corresponding to each particular E.M.F.

In this particular experiment, it will be seen that the an-electrotonic and kat-electrotonic effects are practically equal. But, to be more accurate, the an-electrotonic are slightly lower with low E.M.F., and slightly higher with high, than the corresponding kat-electrotonic deflections. A constant electrical current is thus seen to induce electro- motive variations, outside its poles, in the vegetal nerve. We next turn to the question of the variation of excitability induced in a tissue, by the passage of a con- stant current. On this subject the most important con- tributions have been made by Bernstein and Hermann. Bernstein, experimenting on the sciatic nerve of frog, found that excitation induced a polarisation decrement. This experiment is illustrated in the following diagram (figs. 339,

Figs. 339, 340. Diagrams illustrating Bernstein’s Electrotonic Decrement Fig. 339 shows decrement of kat-electrotonic, and fig. 340 of an-electro- tonic currents, under stimulation at s. In this and following figures the inside thin arrow indicates direction of polarising current, the outside thick arrow the direction of responsive current. 340). In fig. 339 the kathodal effect is seen induced in the extra-polar circuit. When the nerve is now excited

by tetanising electric shocks, a diminution of the extra-polar current is induced. When the anodal effect is induced in the extra-polar circuit, by reversal of polarising current, the an-electrotonic current, opposed in direction to the former kat-electrotonic, also undergoes diminution on excitation of the nerve (fig. 340). It has been suggested that this diminution of electrotonic current was due to a supposed diminution, during excitation, of the susceptibility of the nerve to polarisation.

But this explanation is negatived by an experiment of Hermann, showing the occurrence of polarisation increment during excitation. In figs. 341 and 342 we havea polarising _ and exciting circuit in series, excitation being caused by the Figs. 341, 342. Diagrams representing Hermann’s Polarisation- increment under Tetanising Shocks Inside thin arrow indicates the direction of polarising current ; the outside thick arrow, the direction of excitatory current.

secondary coil of an inductiorium. With such an arrange- ment, the polarising current, whether from left to right or from right to left, is found, during excitation, to undergo an augmentation. Hermann refers these facts to alterations of intensity in the negative wave of excitation, during its passage through the nerve, when the latter is polarised. ‘ It is, indeed, more pronounced at any point of the nerve, the more strongly positive and weakly negative the polarisation of the latter, ze. it increases when it is becoming algebraically more positive, and diminishes when it advances upon more negative points’ (Hermann’s Law of the ‘ Polarisation Increment’ of excitation).!

It would thus appear that the observations hitherto made, as to the effects of electrotonus on excitatory response, are of a somewhat discordant character. I shall, however, be able to show that their complexity is due to the combination of the different effects of the polarisation current on conductivity and on excitability. These separate effects may, according to circumstances, either conspire or act antagonistically. Hence the great variety of results, which appears at first sight incapable of a consistent explanation.

In order, then, to discover the laws by which an electric. current induces a variation of conductivity and excitability, we must first determine the pure effect of the current on conductivity, apart from any excitatory variation; and, secondly, its effect on excitability, uncomplicated by any variation of conductivity. | | To take conductivity first: the ideally perfect arrange- ment would be to have the polarising electrodes, in relation to the region whose conductivity-variation is to be tested, at a distance so great that they could exert no predominant an- or kat-electrotonic influence upon it. In a led-off circuit, moreover, a differential action, unless proper,pre- cautions are taken, is exerted on two electrodes placed side by side. It is, therefore, desirable to remove one of these outside the sphere of action. Such, then, being the con- ditions to be observed, in order to eliminate the effect of the poles themselves, and thus determine the influence of the direction of current on conductivity alone, I took a petiole of fern 20cm. in length, and connected its ends through a reversing-key with a Daniell cell (E.M.F.= 1 volt). The responding galvanometer-circuit had one electrode about the middle of the petiole, near the insertion of a certain lateral leaflet, the other electrode being connected with the lamina of the same leaflet, whose midrib, however, was cut across to prevent transmission of the excitatory effect (figs. 343, 344). It is thus seen that the led-off electrodes are at a relatively great distance from the polarising electrodes, and further, owing to one electrode being placed out of the way, on the lateral leaflet, and the other symmetrically between anode and .kathode, it is clear that anodal or kathodal action is

reduced to wz. The excitation from the stimulator is transmitted across the intervening conducting region, either along the slope of a falling electrical potential, that is to say, from-a galvanometrically positive to a galvanometrically negative point, or against that direction, namely, in an electrically uphill manner, from the galvanometrically negative to the galvanometrically positive. Now, if the direction of an electrical current have an effect on the conduction of excitation, this fact will be detected by the modification induced in the normal response during the passage of the current. :

The results of the present experiment will be found to determine this question. Excitation was induced by means Fic. 343. : Fic. 344. Figs. 343, 344. Experiment with Petiole of Fern demonstrating Variation of Conductivity by Polarising Current of the thermal stimulator, and the normal responses taken, shown in fig. 345 a, as ‘up. The polarising current was now sent from left to right; hence excitation will now be transmitted through the intervening conducting region in an electrically downhill manner, or in the direction of the falling potential—that is to say, from the region of the anode to that of the kathode. It will be seen presently that conduction is retarded or abolished when excitation is made to travel elec- trically downhill from the anode to the kathode.' If this be so, we shall expect to detect the fact by the diminution of the amplitude of the normal response, or even by its actual reversal. For we have seen that when the excitatory re- action of galvanometric negativity is sufficiently retarded, its opposite, the positive effect, often makes its appearance alone.

1 These remarks apply to a feeble or moderate rate of fall of potential. In fig. 345 4, this is seen to have actually occurred. There will be other cases where, the depression of conductivity induced being not too great, it will be possible to watch the gradually lessening amplitude of response until it ends in actual reversal. We have next to determine the effect on conductivity, of the passage of excitation in an uphill direction—that is to say, from the kathodic to the anodic region. For this purpose the polarising current was reversed

Fic. 345. Photographic Records of Responses taken in last Experiment, when Excitation was transmitted with and against the Polarising Current a, Normal response of petiole of fern to transmitted excitation; 4, Reversal of response when excitation was travelling electrically downhill or with the current; c, Normal response once more ; @, Enhanced response due to increase of conductivity when excitation travels electri- cally uphill, or against the polarising current. . Upward arrow 4 indicates that polarising current is in same direction as normal response. Downward arrow { shows polarising current in opposite direction. The same in the two following figures.

by means of a reversing key, the left-hand end of the petiole being now made the kathode. Before doing this, however, I stopped the current, and took a second set of records of normal responses. It will be seen that by reason of the cessation of the previously acting left-to-right current, leaving an after-effect, these were slightly enhanced above the first normal responses (fig. 345 ¢c). On now reversing the current, conductivity was found to be enhanced, as seen in the greater amplitude of response (fig. 345 @).

I next undertook an investigation into the effect on conductivity, of variations of intensity in a moderate polaris- ing current. This is shown in fig. 346, in which excitation travels electrically downhill—that is to say, from the anodic to the kathodic region. In a we have the normal response before the passage of the current. In 4 we have the re- sponses reduced by the diminution of conductivity con- sequent on the application of ‘1 volt for polarisation. On the application of *5 volt in c, there was a tendency towards

Fic. 346. Photographic Record of Modification of Conduction during Passage of Excitation from Anodic to Kathodic Region, under Increasing Intensity of Polarising E.M.F. a, Normal response ; 4, Diminished response where terminal E.M.F. was "I volt; c, Response still further diminished and rendered diphasic under *5 volt ; @, Response reversed under I volt. reversal, the response being now diphasic, positive followed by negative. Finally, on the application of 1 volt in -d, we see the response reversed to positive, the conduction of the true excitatory effect being here altogether abolished.

In a second set of experiments, carried out on a fresh specimen, I investigated the effect of an increasing intensity of the polarising current, when the excitation was made to travel, electrically uphill, from the kathodic to the anodic region. It will be seen, from figure 347, that the application of a polarising E.M.F. of 1 volt increased the conductivity, as seen in the heightened responses shown in 4, as compared with the normal responses in a. The application of higher polarising E.M.F. of °5, 1, and 1°5 volts respectively, now - induced appropriate increments of conductivity, as seen in ¢, d, and e. I was unable to use an E.M.F. of higher than I'5 volts because the galvanometer spot of light became unsteady. It is to be borne in mind that the specimens in these experiments were 20 cm. long, and the maximum potential gradient employed was only ‘07 volt per cm. In experimenting on electrotonic effects, it must be remembered that the E.M.F. employed is, generally speaking, feeble.

Fic. 347. Photographic Record showing. Enhanced Conduction from Kathodic to Anodic Region a, Normal responses; 4, c, d, ¢, Responses gradually enhancing under increasing polarising current. From the results described, then, we arrive at the follow- ing law of the effect of a moderate or feeble constant electric current on conductivity. A moderate polarising E.M.F. induces variations of conduc- tivity. The conductivity is increased in the direction from the kathodic to the anodic region, and depressed in the opposite,

Having thus demonstrated the pure effect of a constant electric current on conductivity, we have next to study the unmixed effect of polarisation on excitability. We. have seen that when two equally excitable points in the same circuit are simultaneously excited by an identical stimulus, there is no resultant response, since the two excitatory effects balance each other. The galvanometric effect is then zero. But if one of the two have its excitability en- hanced in any way, this balance will be disturbed, and a resultant current will flow through the circuit, the more ex- citable contact becoming galvanometrically negative. I now took a long piece of isolated vegetal nerve and connected it with the galvanometer at E and E’,, a secondary coil, giving equi-alternating electric shocks, being also in the circuit (fig. 348). The two longitudinal con- tacts E’ and E being more or less equally excitable, there was at first no. resultant re- sponse to stimulation. E’ was now made the anode, an E.M.F. of +1 volt being used for the purpose, and a perma- nent current was found to flow in the galvano- meter in the direction of E’GE shown by the thin inner arrow, E’ being galvanometrically