Comparative Electro-Physiology: A Physico-Physiological Study

I have also shown that the various mechanical move- ments induced by the unilateral action of light, depend (1) upon whether the stimulus remains localised on the proximal side of the organ or is conducted to the distal ; and (2) on the relative excitabilities of proximal and distal. I have shown, moreover, that all the diverse effects induced by light are demonstrably traceable to the action of these various factors in varying combination. And, finally, certain highly excitable tissues, owing to excess of energy derived from

continuous stimulation, exhibit alternations of phase, negative and positive, or vice versa, constituting multi-phasic or oscillatory response. On these considerations it is possible to summarise the principal effects caused by light, as II. | Normally — ex- citable organ subjected to | unilateral light. A 1. Moderate light, causing excitatory contraction of proxi- mal and hydro-posi- A 2. Strong light. Ex- citatory effect trans- mitted to distal, neu- tralising first.

A 3. Intense and long- continued light. Fatigue of proximal and excitatory con- traction of distal. Description . Case of Hate Action Effect observed I. | Tissuesub-tonic. | Stimulus causesincrease | Expansion or enhanced rate of growth, e.g. Pileus of Coprinus drooping in dark- ness, made re-turgid by light. Renewed growth of dark- rigored plant ex- posed to light. light, e.g. positive curvature of seed- lings of Sinapis; positive curvature of Lepidium seedlings (Oltmanns). .

Neutral effect, e.g. Sinapis and Lepfi- dium (Oltmanns) under strong and long-continued light. BI. Excitatory con- traction of proximal predominant, owing either to greater ex- citability of proximal or feeble transverse conductivity of tissue. B 2. Transmission of excitation through highly conducting tissue to more ex- citable lower or distal. Greater con- e.g. upward folding of leaflets in so-called ‘diurnal sleep’ of Robinia, Erythrina indica, and Clitorta ternatea.

e.g. downward fold- ing of leaflets in so- called ‘ diurnal sleep’ of Oxalis, Biophy- tum, and Averrhoa. III. | Tissues -which | Considerable absorp- “Initiation of multiple exhibit mul- tion of energy, im-- response in Desmo- tiple or auto- mediate or prior. dium gyrans_pre- nomous _re- _ viously at standstill ; sponse. multiple response | of swarming spores ; cf. multiple visual im- pulses in retina. Turning now to the electrical responses induced by light, our investigation is resolved into the inquiry whether these are not the electrical concomitants of those excjtatory effects which we have already been able to analyse through mechanical response. But, before proceeding to this question, I shall first briefly refer to certain ‘electrical effects of light upon green leaves’ which have been observed by Dr. Waller.' - His experiments were performed by making galvanometer connections with two halves of the same leaf, one being strongly illuminated and the other unilluminated. With leaves of different plants he obtained opposite electrical effects under the action of light. From the leaves of Iris, for example, during illumination, he obtained response of galvanometric negativity, with reversal, or positivity, on the cessation of light as its after- effect. With leaves of Zrope@olum and Mathiola, on the con- trary, he obtained positive response during illumination and subsequently negative. Beyond the suggestion that negativity may be associated with dissimilation and positivity with assimilation, Dr. Waller offers no explanation of this opposi- tion of effects observed by him. He states, however, that he regards the presence of ‘chloroplasts’ as essential to these electrical reactions under light, inasmuch as petals, he found, gave no response. Even in the case of the green leaves ‘of ordinary garden shrubs and trees,’ moreover; he found no

response. This he ascribes to their ‘low average metabolism.’ It will thus be seen that no satisfactory explanation is offered either of the mutual opposition of the electrical effects ob- served or of the way in which the presence of ‘chloroplasts’ acts as a determining factor. To turn now to the subject of my proper inquiry, it has to be determined whether those electrical responses which may be observed in vegetable tissues under the action of - light are or are not another expression of the same excita- tory reactions under light which I have already demonstrated by means of mechanical response. And it may be as well to say at the outset that it is the excitability of the tissue, and not the presence or absence of ‘chloroplasts,’ that is the critical factor in determining this electrical response. For I have obtained strong responses under light from pul- vini, stems, and other tissues which are relatively deficient in ‘chloroplasts’; and again, while the lamina of a plant rich in ‘chloroplasts’ would give but moderate response to light, the petiole of the same plant, characterised by less ‘chloro- plasts,’ would often give much stronger response. The etio- lated stem of celery, moreover, gives strong electrical response. And, finally, it is an error to suppose that petals of flowers are irresponsive to light, for I have obtained strong response from petals of Seshania coccineum and from Eucharis lily. Animal nerve, again, in which there is no chlorophyll, gives response to light.

As I have already shown the typical heliotropic effects exhibited by plants to be brought about by differential excitatory action on the proximal and distal sides of the same organ, I shall now proceed to exhibit the electrical counterparts of these experimentally. As I wish, moreover, to show that the general electrical response to the unilateral action of light is fundamentally the same as that induced by other forms of stimulus, the first experiment to be described will be one depending on the unilateral application of a non- luminous stimulus, say thermal.

thermal shocks, by means of the electro-thermic stimulator, the proximal side will undergo contraction, while the ex- pelled water, by its hydro-positive effect, will induce expan- sion on the distal side. By means of these two conspiring actions the organ will be bent towards the source of stimulus. The electrical variation on the distal side will therefore be positive ; but if the stimulus applied be suffi- ciently strong and long-continued, true excitation will be transmitted across the tissue to the distal side. This will neutralise the first mechanical movement, and the corre- sponding electrical effect will be a reversal of the previous

Fic. 233. Experimental Arrangement for Detection of Electrical Change induced at the Point transversely Distal to Point stimulated Upper point stimulated by thermal shocks from electro-thermic stimulator, the lower being the transversely distal point. positive into the excitatory negative. Similar electrical effects will also be observed if the organ be restrained from movement, or if it be so old as to have lost its power of motility. Taking now a young stem of 4ryophyllum, 1 applied a series of thermal stimuli at the proximal point (fig. 233). Periodic closure of the electrical circuit by means of a metro- nome caused rapidly succeeding thermal shocks to act on the upper or proximal side. By adjusting the heating current the stimulus was at first made moderate. It will be seen

from the photographic record (fig. 234) that this gave rise electrically to an increasing positive effect at the diametrically opposite point. This clearly shows that the latter under- went a positive turgidity variation, in consequence of the forcing-in of water expelled by excitatory contraction from the upper side. It is at this stage the indirect and not. the true excitatory effect of stimulus that is being transmitted to B. By this experiment it is also demonstrated that the positive curvature induced by the unilateral applica- tion of any stimulus is the joint effect of the direct excitatory contraction of the proximal side and the indirect or hydro-positive expansion of the distal.!

Another interesting F1G. 234. Record of Kesponse to Moderate h b b Unilateral Stimulation under the Experi- P®¢nomenon to © Cn mental Arrangement described served in this curve is that, Response of distal point by increased after the maximum effect galvanometric positivity due to hydro- positive effect. Note initiation of multi: has been reached, there ple response. . , . is a series of oscillatory multiple responses. In this result there may possibly be two factors in operation: first, after the maximum hydro-positive tension has been set up there may be a gradual percolation of the true excitatory effect, with its opposite reaction, the unstable balance thus produced manifesting itself in oscilla- tions; and, secondly, we know that increased hydrostatic

tension has the effect of initiating multiple responses, as seen 1 In records of such a response as I have just described, when exhibited by highly excitable tissues, a preliminary negative twitch, of momentary duration, may sometimes be observed. This is not due to the conduction of true excitation, but to pseudo-conduction. The sudden blow delivered by the hydrostatic wave on its arrival at the distal point is, in a highly excitable structure, sufficient of itself to induce a short-lived excitatory effect. Thus this does not represent the true transmission of excitation, but its initiation de novo by a secondary mecha- nical cause (p. 446).

in the snail’s heart, under a sufficiently high degree of internal hydrostatic pressure. As regards conduction in general, we know that a strong stimulus is transmitted to a greater distance than a weak. In the next record (fig. 235) this may be seen in an interesting manner. At first a moderate stimulus was em- ployed, and this gave rise to (2) a maximum positive varia- tion of the distal point B. The stimulus was then increased and we observe that (0) the excitatory effect, now reach- ing B, causes a reversal of the curve, owing to induced galvanometric negativity. If at the beginning we had used a stimulus of fairly strong intensity the first effect would have been a positivity of B, due to the indirect effect of stimulus ; and, secondly, the excitatory effect would have reached the point gradually, neutralising and afterwards

reversing the first. I shall FIG. 235. Record of Different Specimen now describe the correspond- under same Experimental Arrange- : ‘ ment when Stimulus is first Moderate ing effects, both mechanical and then Increased and electrical, which are in- (a) Positive response, due to hydro- positive effect ; this is converted to duced by stimulus of light. negative in (6) due to transmission of Wetake a Winiosa plant, and excitatory effect under stronger stimu- lation.

one of its primary pulvini to the action of sunlight. The effects thus induced are (1) the local contraction of the excited upper half, and the expansion of the lower half by the hydro- positive effect ; (2) the gradual percolation of true excitation to the lower half, and consequent initiation of excitatory con- traction there; and (3) the continued action of excitation and increasing contraction on the more excitable lower half. All these effects. are exhibited in the mechanical response

shown in fig. 236, where the first is seen in the up-curve, which thus indicates the joint action of contraction in the upper and expansion in the lower, giving rise to an erectile movement. After an interval of one minute the excitatory effect is seen to have reached the lower half, giving rise now to a reversed or down movement, which, on account of the greater excitability of the lower, is seen to carry the leaf downwards, much below its original position. The dotted portion of the curve shows the after effect on the cessation of the stimulating light. It is here interesting to observe that it is possible to obtain an after-effect which is posi- tive and of opposite sign to the true excitatory effect, this positivity being due to the increased internal energy consequent on the absorption Fic. 236. Mechanical Response of Of stimulus.

Pulvinus of AZmosa to Continuous Action of Light from Above applied In fig. 237 are shown the at Moment marked | electrical effects consequent - Positive heliotropic movement caused on _ stimulation by light in by excitation of upper half neu- ; tralised by transmission to distal another specimen of the side, and ultimately reversed owing ulvinus of Mimosa The to greater excitability of lower half. : , : Dotted line represents recovery on electrical contacts are made

cessation of light. Note final erec- ; : : tion of leaf above original position in this case, one with the as after-effect of absorbed stimulus. lower half of the pulvinus and the other with a distant in- different point. Stimulus of light is applied, as in the last case, on the upper half. This experimental method is free from the objection which has been urged against Dr. Waller's experiments on green leaves, that the result was complicated by the direct action of light on the electrode itself. It should be pointed out, however, that the presence of such photo-electric action is more important theoretically than practically, being of relatively small amount. A further complication which arises from the direct action of light on

one of the electrodes lies in rise of temperature. Though in all such experiments the incident light should pass to the organ through a thick stratum of water, which absorbs its heat-rays, yet the absorption of light by the tissue must necessarily occasion a slight rise of temperature. In con- nection with this should be remembered the fact I have elsewhere demonstrated, that though sudden variation of temperature acts as an excitatory agent, yet a slow and gradual rise, enhancing the internal energy, brings about only a slight positivity, opposite to the effect of true excita- tion. In this particular ex- periment, however, as the electrical contacts are not directly acted on by light, we obtain results uncom- plicated by such disturbing factors.

The first electrical effect brought about in the lower half of the pulvinus by yy. 237. Electrical Response in the icati : Lower Half of the Pulvinus of sd plication of light ii the Mimosa due to Stimulation of distal upper half is seen in Distal Upper Half by Light fig. 237 as an increasing gal- Observe the first phase of positivity, ; ee ‘ due to hydro-positive effect, con- vanometric positivity. This verted subsequently into negative is concomitant to the hydro- NF ae transmission of true ex- positive effect at the lower half, which, conspiring with the contraction of the upper, produces that up-movement of the leaf seen in the previous figure. The excitatory effect next reaches the lower half, and we there obtain increasing galvanometric negativity in consequence. This corresponds with the mechanical move- ‘ment of depression. From this experiment it is clear that light, like other forms of stimulus, induces, as its true excita- tory reaction, galvanometric negativity, the indirect or hydro- positive effect being one of galvanometric positivity. In the last case, then, we obtained a transverse trans- mission of the true excitatory effect. Similar effects are

also obtainable by longitudinal transmission. In order to do this an organ must be selected which is a fairly good conductor. I have thus been able to observe a series of responses to transmitted stimulus of light, using such speci- mens as the petiole of Bryophyllum. Light was here applied at a distance of 5 mm. from the proximal contact, and this gave rise to a series of true excitatory responses of galvano- metric negativity. Having thus established unmistakably the negative sign of the excitatory electrical variation induced and transmitted under stimulus of light, I shall next proceed: to give records of experiments in which light was applied directly. The effect ob- served in these cases is naturally much larger, as there is no enfeeblement by _ transmission. Fig. 238 shows a series of such responses, obtained at intervals of two minutes, by the application of sunlight — previously passed through a stratum of water— during five seconds only in each Hie eA? Phaeatic Be. ~case, on the petiole of a vigorous “cord of Series of Negative leaf of Bryophyllum. It will be

Sunlight of Five Seconds?’ Of galvanometric negativity there Dt eee ee Inter- is an after-effect of positivity, in Observe the positive after-effect, consequence of which the base due to increase of internal line of the series, instead of re- stimulus latent, for the increase of internal energy, we shall later see to be important, as heralding the initiation of multiple response. The exhibition of these after-effects, due to increase of latent energy, is also to be observed in the record given already of the mechanical response of Jzmosa

(fig. 236), where the leaf, after its excitatory fall, was re- erected, on cessation of stimulus, above its original height. This, as we saw, was due to a certain portion of the incident stimulus becoming latent, and thus increasing the internal energy. We shall next take up the subject of the occurrence of positive response, as sometimes induced by light. This may be the result of various different causes. There is one fact, however, in connection with the action of light which it is important to bear in mind. Thus, if we subject the lower half of the pulvinus of A/zmosa, for instance, to the action of sunlight, its responsive fall will be gradual, unlike the sudden depression caused by thermal or mechanical stimulation. This is because light, usually speaking, constitutes a stimulus of only moderate intensity. We have seen that a stimulus which falls below a certain critical level of excitatory intensity will evoke positive, instead of negative response. We have also seen that from a sub-tonic tissue the positive response is more easily obtained than from one which is highly ex- citable. Now, as the excitatory efficiency of a mechanical stimulus is very great, and as that stimulus is also incapable of finely graduated decrease, it follows that, in order to ex- hibit positive response under such stimulation, it is necessary that the tissue stimulated should be extremely depressed, or even moribund. Under such conditions I have shown (p. 83) that it is possible under feeble stimulus to obtain positive response, which, under stronger, will pass into the normal negative.

The stimulus of light, then, whose action is very moderate, discriminates more finely between tonic gradations of the tissue than can other forms of stimulus. If this tonic con- dition be very favourable, and the excitability high, the re- sponse will be by normal galvanometric negativity. If the tonic condition, however, be less favourable, the response is liable to be positive. This latter fact will be very strikingly demonstrated, in a later chapter, by experiments carried out on nerves, It will there be shown that while highly excitable

nerve gives the normal negative response, the same tissue, if its tonic condition be below par, gives a more or less per- sistent positive response. It is only when the tonic condition of the nerve has again been raised, by long-continued stimulation, that it will once more give normal response. Fatigue is another condition which is liable to give rise to the abnormal positive response. The use of the stimulus of light carries with it, also, a further limitation. A mechanical stimulus, say vibrational, throws into activity the whole mass of tissue, not only in its superficial, but also in its deeper lying strata. Now we have seen that the epidermal layer of living tissues is less excitable than those which are deeper seated. It may even, in fact, on loca! excitation, give positive response (p. 298). It is to be noticed, moreover, that light acts from outside, its excitatory influence affecting the outmost tissue first, and only by gradual percolation passing to the subjacent. Owing to these two facts, then, of the moderateness of this stimulus and the superficial character of its action, the tissue, if not highly excitable, is apt, under its application, to give positive _ response. We have seen, further, that various circumstances, such as age and season, have an important effect in varying the excitatory reaction of a tissue. We saw the effect of age exemplified in the responses given by two different specimens of roots (p. 353), in which a young root gave nega- tive and an older positive responses. Again, we shall see presently that there is a diurnal period, on account of which the state of turgor, the excitability, and the sign of response, are all alike liable to undergo periodic variations. Under the stimulus of light these varying excitabilities may be ex- pected to find varying expressions.

That sub-tonicity tends to make the response, under moderate stimulation, positive, is seen in the fact that an etiolated petiole of celery gives positive response under light Again, I have noticed that leaves of Bryophyllum, which usually give normal negative responses, sometimes exhibit positive, if the plant, during the previous night, have been subjected to unusual cold. Even in such a case, however, though the first responses are positive, successive exposures to light, by raising the tonic condition, are found to restore the response to the normal negative. The same facts receive interesting illustration in the re- sponse of growth. If the growing organ be in a normally excitatory condition, the stimulus of light, inducing negative turgidity varia- tion, causes retardation of growth. If the tissue, "|s° w0" 1s" 20" 25° 80° 35° 40°45 however, be in an ex- tremely sub-tonic con- dition, light stimulus, by increasing the internal energy, gives rise to the positive effect, that is to say, the initiation, or en-

Fic. 239. Record of Responsive Growth- variation taken under condition of balance hancement, of the rate in slightly Sub-tonic Flower-bud of Cri ; : enuha tian Gf at growth. If the erow- rane Lily under Diffuse Stimulation o ing tissue, again, be only Continuous lines represent the effect during ; 3 : application of light, the dotted line on slightly sub sek gie ‘ = withdrawal of light. The plant was shall have a preliminary originally in a sub-tonic condition, and

+4 ° application of light at x, after short positive, or enhancement, latent period, induces preliminary ac- followed by the negative celeration of growth. After this follows : the normal retardation. On withdrawal response, or retardation of light, in the dotted portion of the of the rate of growth. curve is seen the negative after-effect, eT - followed by return to the normal rate of This is seen in the fol- growth. A second and long-continued sub-tonic flower-bud of Crzzum lily, in the induced variations of the normal rate of growth under the stimulus of light (fig. 239). This record was made with the Balanced Cresco- graph, where the normal rate of growth is recorded as a horizontal line, enhancement or positive variations of the rate being represented by up-curves, and retardation, or negative variations, by down-curves. It will be noticed that

the first effect of light on this sub-tonic tissue was to induce a positive response, followed subsequently by the normal negative. Continuous stimulation is seen later to give rise to oscillatory responses. Having thus shown the continuity between the normal negative and positive responses, I give below a record of the response to light of a petiole of cauliflower (fig. 240), a specimen which usually, though not always, exhibits galvanometric positivity. Each stimulus, by exposure for five seconds, was in this case applied after an interval of two minutes. It should be mentioned here that the same tissue which gives positive response to the moderate stimulus of light will Fic. 240. Photographic Re. Show the normal negative when

of the Petiole of Cauliflower er to Light of Five Seconds’ Such as the mechanical. OE Honea ee ana The effects studied up to the present have consisted of single responses induced by light. But this stimulus also induces multiple response, as we saw in the oscillatory variations of growth in the Crinum lily in fig. 239. The same phenomenon is observed in the case of motile response. For example, I took a plant of Bzophytum, in which the leaf- lets are outspread, in the presence of diffuse light, and threw upon it direct sunlight. A series of multiple responses was now induced, under the continuous action of stimulus, a record of which is given in fig. 241. The up-curves here represent excitatory downward movements, and the down- curves their partial recoveries. Owing to the incomplete character of these recoveries, the leaflets, as the result of a series of such responses, are finally closed downwards. In the outspread diurnal position the lower half of the pulvinus of the leaflet is somewhat more turgid than the upper half, and in