Comparative Electro-Physiology: A Physico-Physiological Study

We have already seen that, besides 525» Patrice. a this electrical reversal, there is another means of detection of the death-point, afforded by the con- version of the response to external stimulus from the normal negative into positive. Now it occurred to me that it would be interesting, if in the same specimen, both these tests could be applied at the same time. We could then see whether two methods so independent of each other furnished mutual corroboration or not. For this purpose I took a stem of

Amaranth, and abolished the excitability of one of the two contacts—a lateral leaf—by scalding. The electrical curve, under a continuously rising temperature, was now taken in The existing electro-motive difference the usual manner. between living and injured contacts underwent the usual Fic. 134. Record showing Inversion of Electric Curve (represented by dotted line) and Simultaneous Reversal of Electric Response in Stem of Amaranth { indicates current of injury from injured to uninjured contact, which, Normal

reaching a maximum, undergoes reversal at death-point. response up, also becomes reversed to down after death-point. increase, reaching a maximum at the death-point. Meanwhile electrical responses to uniform vibrational stimuli were taken at intervals a few degrees apart. It will be seen (fig. 134) that the electrical inversion took place at 57° C., this moderate lowering of the death-point being due, in all probability, to the slight depression caused by scalding at

the distal contact, which had not had time to pass off. The test-responses to uniform mechanical stimulation which were being taken meanwhile show a continuous diminution towards the death-point. When this, however, had been passed, the response is seen to be reversed in direction to positive. As has been said before, this positive response also disappears after a time. We thus obtain a very striking demonstration of the fact that the reversal of the electrical curve and the reversal of the sign of response are concomitant.

It may be mentioned here, in anticipation of a future chapter, that the death-point may also be obtained by the sudden inversion of the curve of electrical resistivity, and that the value obtained in this way coincides with those already given. I give below a table showing the death-points deter- mined by various methods: Specimens Method pac 1. Flower of French Marigold . | Opening or closing of flower 59° 2 Peduncle of Aliium . . | Expulsion of contained water 59° 3. Spiral tendril of Passifora . | Movement of uncurling . Z 4. Pulvinus of AZ@imosa . . | Spasmodic lateral movement | 59°-60° 5-8. Style of Datura (four speci- mens). Each gave . . | Morograph : : e} , '60? 9-12. Style of Azbzscus (four speci- mens). Each gave . : ” , ; ‘ 60°

(six specimens). Each gave » ‘ 60° Anisotropic organs . | Electric inversion It is thus seen that employing different methods and using plant-organs which are equally diverse—flowers, bulbs, petioles, and others—a death-point is determined which is very definite and practically the same for all phanero- gamous plants. Among the mechanical methods, that which depends on the thermo-mechanical curve, giving the death-point by a sudden inversion, is specially accurate. Of an equally precise character is the death-point obtained by the inversion of an electrical curve, and also that which is given by reversal of the electrical response. And it is in

the highest degree remarkable that the points of inversion — in the mechanical and electrical curves respectively, together with the point of reversal of the electrical response itself, should so exactly coincide. Repeated responses under single strong stimulus—Multiple mechanical response in Biophytum-—Multiple electrical responses in various animal and vegetable tissues— Continuity of multiple*and autonomous response—Transition from multiple response to autonomous, and vice versa—Autonomous mechanical response of Desmodium gyrans and its time-relations — Simultaneous mechanical and electrical records of automatic pulsations in DVesmodium— Double electrical pulsation, principal and subsidiary waves—-Electrical pulsation of Desmod7um \eaflet under physical restraint—Growth- ema —So-called current of rest in growing plants.

WE have seen that when a plant organ is acted on by a single stimulus of sufficient intensity, it exhibits a single excitatory effect, which may show itself in two independent ways, as mechanical and electrical response. We have also seen that part of the impinging stimulus may become latent, to find appropriate expression later. It was also shown that with increasing intensity of stimulus the amplitude of response reaches a limit. It may thus happen that a very strong stimulus, not finding adequate expression in a single response, will exhibit itself by means of repeated responses. The incident energy in such cases is held latent for a time,’ to manifest itself later in a rhythmic manner.

I have been able to demonstrate the occurrence of this multiple excitation, in response to a single strong stimulus, by several different and independent methods. The simplest and most striking of these depends on the recording of the motile effects in the leaflets of Bzophytum. In fig. 135 are seen no less than sixteen multiple pulsations resulting from a single strong thermal stimulation of the petiole bearing the leaflets. The average period of each pulsation is here about thirty seconds ; but this may vary in different cases from half of this toone minute. I have also been able to detect these multiple excitatory waves, during their transit through non-motile con- ducting tissues such as stems. ‘The imperceptible volumetric changes which occur on the arrival of excitation were here detected electrically by variations of pressure induced in an enclosing microphonic contact. In fig. 136 is seen such

Fic, 135. Multiple Mechanical Response of Biophytum, due to a Single . Strong Thermal Stimulus multiple electro-tactile response in the stem of MW/zmosa, due to a single thermal stimulus. I have also been able to record such multiple excitatory effects by means of electro-motive response. In fig. 137 is seen a photographic record of a series of such responses, given by the leaf of Bzophytum, the individual thermal stimuli being here applied at intervals of five minutes. It will here be noticed that each single stimulus gave rise to from five to eight responses, the average period of which was thirty seconds. From the corresponding mechanical

responses, it will be remembered that the average period of these had also this value. These multiple responses to a single strong stimulus, while very strikingly manifested by such plant-tissues as the pul- vinules of Azophytum, and in the animal, by the cardiac tissue, are also exhibited by almost all kinds of tissues under favourable circumstances. In fig. 138 will be seen records which show this in the case of different vegetable organs under diverse forms of stimu- ge

A . 3 Fic. 136. Multiple Electro-tactile lation. In fig. 139 1S given Response in Stem of AZ/mosa, a series of multiple responses Savi Single Strong. ’Thermal which I have obtained from frog’s stomach. I have also detected the occurrence of multiple responses in nerves of animals, which will be described in a later chapter. Fic. 137. Photographic Record of Multiple Electrical Response in Leaf of Biophytum First series of eight responses to a single thermal stimulus ; second stimulus, after interval of five minutes, evoked five responses ; third stimulus, after second interval of five minutes, gave six responses. Average period of each response, thirty seconds nearly.

Fic. 138. Multiple Electrical Responses under Different Forms of Stimulus in Different Organs (a) In Mimosa due to thermal, and (4) to chemical stimulation ; (c) in peduncle of Azophytum, due to thermal stimulus ; [N.B.— This ‘series persisted for two honrs.] (d) in hypocotyl of Zamarindus indica, due Fic. 139. Photographic Record of Multiple Electrical Re- sponse to Single Thermal Shock in Frog’s Stomach response may consequently be for instance, by adequately raising the temperature of the plant. This is seen in the following record of pulsatory responses as induced in a young leaflet of Biophytum, when the temperature was raised to 35° C. With the increase of internal energy, the turgidity of. the tissue was en- hanced, and the excessive hydro- static tension thus brought about induced autonomous. pulsations (fig. 140), as in a quiescent snail’s heart similar pulsations are in- duced by increase of the internal hydrostatic pressure. I have else- where shown that the energy which expresses itself in pulsatory

movements may be derived by the plant, either directly from immediate external sources; or from an excess of such energy already accumulated and-held latent in the tissue, aided by the incidence of external stimulus; or from the excessive accumulation of such latent energy alone. There is thus a continuity between multiple and autonomically responding plants. Azophytum, which under ordinary cir- cumstances belongs to the former of these classes, becomes converted into the latter under exceptionally favourable tonic conditions. That is to say, it responds by a single response to a single moderate stimulus, and by multiple

Fic. 140. Induction of Autonomous Response in Biophy/um at Moderately High Temperature of 35° C. Note the diminution of amplitude of response with falling temperature. The pulsations came to a stop below 29° C. responses. to a strong stimulus. Under exceptionally favour- able tonic conditions, however, it exhibits spontaneous or autonomous responses. Desmodium gyrans, on the other hand, which ordinarily exhibits autonomous response, will, under unfavourable circumstances, cease to exhibit spontaneous movements. It then exhibits a single response to a single moderate stimulus, and multiple responses to a single strong stimulus :

When the leaflet of this plant, owing to deficit of internal energy, is in a state of standstill, a renewal of the supply of stimulus will restore it to a condition of autonomous response. This is seen in the following record (fig. 141) of the response of the leaflet of Desmodium. The leaflet was in a quiescent condition, but under the action of stimulus of light, it ex- hibited multiple responses ; and these, owing to the increasing absorption of energy, showed a staircase enhancement of amplitude. On the cessation of light, the energy absorbed maintained the pulsation for some time.

It is thus the absorption of energy which is the cause of the so-called autonomous movements. The energy, as already stated, may be derived by the plant either directly from ex- ternal sources ; or from the excess already accumulated and held latent in the tissue, aided by incident external stimulus ; or from an excess of latent energy previously accumulated. Fic. 141. Initiation of Multiple Response in Lateral Leaflet of Desmodium originally at Standstill

Light applied at x and continued till the end of the sixth response, as shown by the thick line. The responses show a staircase increase with increase of absorbed energy. Pulsations persist for a short time even on the cessation of stimulus. It. would be impossible to conceive of movement without an exciting cause. Only under the action of stimulus can a living tissue give responsive indications. An ex- ternal stimulus may either give rise to an immediate responsive expression, or be partly or wholly reserved

latent form for subsequent manifestation. ‘Inner stimuli’ are simply external stimuli previously absorbed and _ held latent. A plant or animal is thus an accumulator which is constantly storing up energy from external sources, and numerous manifestations of life—often periodic in their character—are but responsive expressions of energy which | has been derived from external sources and is held latent in In Desmodium gyrans, as is well known, we have the typical example of autonomous response, its secondary leaflets executing periodic up and down or elliptical movements. The movement of the leaflet in some instances takes place by jerks, in others it is more uniform. The period of a com- plete up-and-down movement varies between two and four minutes. The length of this period is much affected by temperature, being less when this is moderately high. From the normal, or highest position, the leaflet sinks somewhat rapidly ; having reached its maximum depressed position, it rests fora while. There is next a rather slow rise to its original posi- tion. This up-and-down motion is in some cases approximately straight. In others, the pulvinule of the leaflet is slightly twisted after its descent, and the corresponding curve described becomes more circular.

In view of certain Fic. 142. Photographic Record of Autonomous peculiarities to be ob- Mechanical Pulsation in Desmodium Leaflet it is necessary here to enter into some detail regarding the time-relations of its mechanical response during the two phases of down and up movements. The great difficulty in recording the pulsatory movements of Desmodium lies in the extreme slenderness of the lateral leaflets. This is such that the friction of a light recording-lever against the recording surface is sufficient to bring these movements to a stop. This difficulty has, however, been overcome, as stated elsewhere, by means of the Optical Lever.! Fig. 142

gives a photographic record of a series of autonomous pulsations exhibited by a leaflet of Desmodium. For the accurate observation of the rate of movement of the Desmodium \eaflet during its different phases I have also been able to make records by means of a series of punctures produced by electrical sparks on a recording- surface. The sparks occur at the short gap between the end of the recording arm of a very light aluminium lever and the drum, these being connected respectively with the two electrodes of a Ruhmkorff’s coil. The electrical dis- turbance does not affect the plant, as the pulsating leaflet is separated from the other arm of the lever by a long silk thread. The primary current in the Ruhmkorff’s coil is broken at intervals of five seconds. Hence succes- sive punctures in the record

Fic. 143. Spark-record of Single | represent intervals of five | ci cia in pene of Desmodium seconds each. I give here Showing time-relations of down- and . up-movements in single pulsation (fig. 143) a record obtained of leaflet of Desmodium. Up- in this manner, of a single to be understood that the up-movement in the record represents the down-movement of the leaflet. These movements are produced by excitatory con- tractions of the lower and upper halves of the pulvinus alternately. An inspection of the record given shows that after a pause in the highest position a sudden excitatory impulse is developed in the lower half, which is gradually exhausted as the lowest position is reached. The maximum rate of movement to which this excitation gives rise is in this particular case ‘7 mm. per second. After the lowest position is attained there is a pause. The up-movement

then takes place more gradually and at a much slower rate. This movement is due to natural recovery, aided by a moderate excitatory contraction of the upper half of the pulvinus. 1 give herewith a table showing the characteristic rates of movement in the different phases of the entire pulsation. Duration of pause 40 seconds | Duration of pause. 35 seconds | Several facts are brought out in this table which are of special importance, and first we observe that the excitatory impulse which causes the down-movement is brief and quickly exhausted. -This is seen by the great distance covered

gradually slows down. This indicates a short-lived impulsive action, the subsequent movernent of the leaflet being mainly due to inertia. There is then a pause in the down-position, after which the up-movement commences. It will be noticed here that this movement is more gradual and prolonged than the characteristic movements, we may conclude that the excita- tory reaction by which the down-movement is caused is relatively more intense and more quickly exhausted than that which brings about the up-movement. We may gauge the relative intensities of the two impulses approximately, either from the maximum or the average rates of the down

impulse may therefore be taken to be roughly one and a-half times as great as that which occasions the up-movement. The total duration of the down-movement is again much less than that of the up. We have seen that a single excitation has a single con- comitant electrical pulsation. We have also seen the multiple electrical responses corresponding to multiple excitations. It remains, then, to find out whether autonomous pulsations have any electrical concomitant, and if so, of what nature. I shall here, therefore, describe experiments for the recording of the electrical pulsation of the Desmodium leaflet. For this purpose I selected specimens in which the movement of the leaflet was not spasmodic, but gradual and continuous, and where the up and down movements were pebromicnatcly ina straight line.

In order to obtain those responsive electro-motive changes which might accompany the automatic movements of the leaflets, it was necessary to make one of the electric contacts at a point on the tissue which was free from excitation, the second contact being made at a place where the excitatory reaction was at its maximum. I have shown elsewhere! that the seat of autonomous excitation in Desmodium is neither central nor peripheral, but localised at the slender pulvinated joints to which the leaflets are attached, the latter thus serving merely as indicating flags. Acting on these considerations, I made one contact with the slender pulvinule of a lateral leaflet, the other being made with the common petiole. These electrical connections were made securely by means of cotton threads moistened with normal saline solution, and attached to non-polarisable electrodes. The electro-motive variations induced in the plant now gave rise to correspond- ing deflections in the galvanometer in circuit. On taking records of these electrical responses, I was surprised to find that, corresponding with each complete mechanical vibration, there was a double electrical pulsation—a large principal followed by a smaller subsidiary wave. In a given case, where the period of the complete mechanical vibration was about 3°5 minutes, the period of the principal of these two

waves of electrical response was slightly less than I minute, and that of the subsidiary wave a little over 2°5 minutes. These double electrical pulses, corresponding to a single mechanical vibration, are at first very puzzling, and I under- took special investigations to ascertain the reason of this peculiarity. In order to obtain an insight into the relation between these mechanical and electrical responses it was necessary to take simultaneous records of the two on the same recording drum. This was accomplished by having the two recording spots of light—one from the galvanometer and one from the optic lever—thrown on the same horizontal slit, in front of the revolving drum, round which was wrapped a sensitive photographic film. The galvanometer spot of light, and consequently the electrical response record, was the lower of the two. The vertical movement of the spot of light which records the mechanical response is to be under- stood as converted into horizontal by reflection from a second mirror suitably inclined. This experimental arrange- ment is similar to that employed for simultaneous mechanical and electrical records in the case of Mimosa, as shown in fig. 12.

The record given in fig. 144 exhibits the simultaneous mechanical and electrical responses thus obtained. It will be seen that the minor electrical wave took place while the leaflet was moving up from (a) and coming to its highest position at (6). This was followed by a wave of higher amplitude but shorter period, which coincided with the movement of the leaflet again from its highest to its lowest positions. It will thus be seen that the subsidiary electrical wave of small amplitude and relatively long period coincided with the slow up-movement of the leaflet, and that the principal wave, characterised by large amplitude and short period, corresponded with the quick down-movement of the leaflet. These galvanometric deflections indicated, it must be understood, a condition of galvanometric negativity of the pulvinule at the moments of its excitatory up and down movements. The following considerations make it easy to

understand why two electrical waves correspond to one mechanical pulsation. First, we know that an excitatory change at a given point will have, as its concomitant, an electro-motive variation of galvanometric negativity. Second, the intensity of this electro- motive variation depends on the intensity of the excitatory change. And lastly, on the cessation of excitation there is an elec- trical recovery. Now we have seen from the spark-record (fig. 143) that the leaflet during one complete mechanical pul- sation is subjected to two excitatory impulses, occur- tring in the upper and lower halves of its pul- vinule alternately. It is these two excitations which give rise to the two electrical disturbances of galvanometric negativity. And the different ampli- Fic. 144. Photographic Records of tude and period in the two

etenliapos Merten] yd, Pies” canes ‘ace. fully’ abooitted ab (upper figure) represents up-movement for by the different period of leaflet; a 4 (lower figure) corre- gnd intensity of the two sponding electrical subsidiary wave ; : ; 6 a’ (upper figure) down-movement €xCitatory impulses. It of leaflet; 4 a' (lower figure) corre- yj] be remembered that sponding principal electrical wave. ; ; the excitatory impulse which produced the up-movement was the feebler and more protracted of the two. It is consequently attended by an electrical disturbance of moderate intensity and correspond- ing persistence. On the cessation of the up-movement, as we have seen, there is a pause, and during this time we find that