Plant Response as a Means of Physiological Investigation

To obtain these multiple electrical responses in an unmistakable manner it is necessary that the excitation should reach one electrical contact and not the other, for in the latter case there would be complications arising from diphasic variation. The necessary condition may be fulfilled by applying stimulus near the proximal contact, the other contact being at a relatively great distance. The periodicity of these multiple electric responses I find to vary in different

F^ig. 114. Multiple Electrical Responses due to Single Strong Stimulus (a) In Mimosa due to thermal, and (b) to chemical stimulation ; (?) in peduncle of Biophytwn, due to thermal stimulus. [N.B.— This series persisted for two hours.] {d), in hypocotyl of Tamarindus indica, due to stimulus of cut. cases from about a half to five minutes. These multiple electrical responses, resulting from a single strong stimulus, Multiple electrotactile responses.— In the course of my experiments on the electrotactile method of detecting the excitatory wave, I discovered multiple waves, initiated by a single strong stimulus, passing through the tissue. In figllS I Slve a record of such multiple responses in the stem of Mimosa, in which it will be seen that there are four such responses, with an average period of one minute each.

Now, as these pulsations are signs of the presence of excitation, it follows from the occurrence of such multiple responses that a strong stimulus may give rise to a multiple series Fig. 115. Multiple Electrotactile Response in Stem of Mimosa, due to Single Strong Thermal Stimulus of excitations. I was therefore led to expect that this fact might be demonstrated in another and still more convincing manner, if I should succeed in finding a sensitive plant which exhibited these multiple excitatory waves byrepeated movements of the indicating motile leaflets.

Certain peculiarities of Biophytum which I had previously discovered led me to think that I might find in this plant the opportunity I sought ; for we have seen in the last chapter that in Biophytum a certain minimal intensity of stimulus induced the maximal mechanical response. With such a stimulus we obtain only one response, and if we apply a stimulus of very much more than minimal intensity it produces no greater mechanical effect. What, then, happens to the excess of stimulus ? This excess may be wasted as heat, or it may continue to exist in some latent form, and this latent stimulus may subsequently be given out rhythmically.

Multiple mechanical responses. — In view of these facts I expected a strong stimulus to give rise to those periodic waves whose existence I had been led to suspect from the observation of the recurrent electromotive and electrotactile waves in various plants. As a matter of fact it has been noticed that Biophytum, when strongly excited, exhibits two successive movements of mechanical response, the leaflets not completing their closure at once, but in the course of two twitches succeeding each other. But I expected to detect a larger number of pulsatory movements in response to a single strong stimulus.

In order to do this, however, it was necessary to prevent the complete closure of the leaflets, by which the further exhibition of mechanical response was made impossible. For this purpose I used the Optic Lever, with the light counterpoise (p. 19). In this way I succeeded in demonstrating, through mechanical response, what had already been demonstrated electrically, the fact that a single strong stimulus, of whatever form — thermal, mechanical, electrical, or photic — will induce, not one but a multiple series of

rhythmic responses in a plant. Thus on applying a thermal stimulus to the petiole of Biophytum near its insertion on the stem, an excitatory wave was found, as usual, to produce successive depressions of leaflets in a centrifugal order. But after a while the existence of a second excitatory wave became evident, by a second series of closures of leaflets. That the wave was due — not to excitation reflected in some way from the tip of the leaf, but — to a second wave starting from the original point of stimulation, was made evident by the fact that the successive fall of leaflets was again centrifugal, from near the stem to the tip of the leaf. From the records now obtained by means of the Optic Lever, attached to one of the leaflets, it was interesting to observe the very numerous rhythmic pulsations, often as many as twenty, now made visible.

As an instance of the simplest form of such multiple response, I give the following record, which was obtained under the thermal stimulation of the electrothermic stimulator. The point of application was close to the responding leaf. The average period of these pulsations was about thirty seconds (fig. 1 16) It may be stated here that the period of multiple responses of Biophytum is found to vary from about fifteen seconds to three minutes or so, depending on the condition of the plant and the intensity of the stimulus.

The question suggests itself, since multiple electromotive and electrotactile excitations are observed in Mimosa, why should not this plant also exhibit them mechanically ? The answer to this is probably found in the fact that the Biophytum leaflet is light, and easily exhibits fluctuating impulses, whereas the impulsive fall of the heavier Mimosa leaf persists, owing to greater momentum, till it is more or less complete. Again, unless an organ has at least partially recovered from fatigue, it is not susceptible of fresh excitation. The period of full recovery in the pulvinus of Mimosa is very long, being about seven minutes. We saw, in studying fatigue in

Mimosa, that when a second stimulus succeeds a first, after an interval of less than one minute, it produces no responsive effect. If, then, a second wave of excitation arrives within the refractory period of the first, we must expect that it will remain mechanically ineffective. In the multiple electrotactile responses seen in fig. 114, these successive excitations are seen to have occurred at intervals of one minute, complete recovery being accomplished in that time. In the

Fig. 116. Multiple Mechanical Response of Biophytum, due to a Single Thermal Stimulus stem, then, the period of recovery is very much shorter than in the pulvinus, and the same waves of excitation which at intervals of one minute produce response in the one case, prove ineffective in the other. Had these periodic waves of multiple excitation been three or four times as slow as they are, we might have been able to observe multiple mechanical responses of the leaf of Mimosa. In connection with this, I may state that I once observed a second mechanical response to a single strong stimulus

in the leaf of Mimosa. In this case the second response occurred four minutes after the first. We have seen that multiple electrical response is obtained when any form of strong stimulation is employed. I now tried to find out whether multiple mechanical response could be produced by chemical stimulation. One of the middle leaflets of a Biophytum leaf was attached to the recording Optic Lever, and I applied a drop of sulphuric acid 1 cm. away from the recording leaflet in the direction of the tip of the leaf. This was found to give rise to five vigorous recurrent pulsations.

Thus, though a single moderate stimulus evokes but a single response, yet under strong stimulation we obtain not only the immediate consequent response, but also a surplus of energy which remains over and is held latent in the tissue, to be given out later, after a shorter or longer interval, in the form of recurrent responses. From these experiments it is clear that a rhythmic series of effects need not have a periodic antecedent cause. As regards these pulsatory movements, it was shown on page 47 that the fall of the motile leaflet was due to a pulse of diminution, and its erection to a pulse of restoration, or increase, of turgidity. In these multiple responses, then, we have the expression of rhythmic variations of turgidity initiated by stimulus.

Among ordinary responses — that is to say, single response to single stimulus — we have observed three types, depending on the excitability of the tissue. When the excitability remains uniform, the responses are uniform. When the excitability undergoes a gradual diminution, there is a corresponding depression of response — that is to say, fatigue supervenes. And when the excitability increases by degrees, there is a correspondent enhancement of response known as the 'staircase' effect. In addition to these, I have, as explained before, also noticed some curious instances in which the excitability of the tissue appeared to undergo periodic fluctuations, in consequence of which successive responses to

uniform stimuli exhibited periodic fluctuations (p. 106). We may thus have an alternate, periodic, or cyclic fluctuation of excitability exhibited in the responses. In the first of these, the responses are alternately large and small. In the last, we may have either an ascending or descending series, which is periodically repeated. Cyclic variation in multiple response. — In multiple responses also we find all these types. And if the seat of origin of the multiple excitation be at a distance from the responding leaflet, we have in this fact an added element of variation. For we have now to consider, not simply the periodic variations of excitability of the motile organ, but also those of the excitability of the intervening tissue, causing periodic changes of conductivity. Hence, in records obtained of multiple

responses, where the seat of excitation is at a distance, we have a complex combination of variations of amplitude and of period in the pulsations. Confining our attention to the period alone, we find that multiple responses may be classified under three headings : Fig. 118. Multiple Response in Biophytum Quick rhythm becoming slow. those in which the successive periods are fairly uniform (fig. 116); those in which the pulsations are at first slow and later become quicker (fig. 117) ; and those which begin with

rapid pulsations, and afterwards slow down (fig. 118). And finally we may have any combination of these (figs. 119 and Multiple Response in Biophytuni, showing Cyclic Groupings of Amplitude and Period These cyclic changes of amplitude and period are seen in all types of rhythmic multiple responses, of which I shall show that the autonomous responses of Desmodium and the growth-responses of all plants are only special cases. These characteristics are seen not only in the rhythmic responses of vegetable, but also in those of animal, tissues. The cause of the latter is still regarded as very obscure. Hence the study of similar phenomena in plants, under simpler conditions, may be expected to throw much light on the subject.

How striking, even in their more intricate details, are the similarities between multiple responses in plant and animal, will be shown in the next three chapters. For the present I shall confine myself to the consideration of two of the most obscure instances in animal tissues, and be paralleled in the case of the Fig. 120. Multiple Re- sponse in Biophytinu, snowing Cyclic Groupings Recurrent visual impulses. — It is known that when we look for some time at a strongly illuminated object and afterwards close the eyes, we see the same image repeating itself many times in succession ; and of this phenomenon, so far

as I am aware, no satisfactory explanation has hitherto been offered. The phenomena which have been described in the case of vegetable tissues show, however, that this is nothing but an instance of multiple excitations in the retina caused by strong stimulation. In the case of the plant these recurrent excitations express themselves mechanically as twitches ; in the case of the firefly as flashes of light ; and in the retina as recurrent visual images. In these recurrent visual impulses are found all the peculiarities which I have observed in recurring excitatory impulses in Biophytum. This is shown, not only in simple cases, but also in the most complicated. I shall presently give, in illustration of this, two parallel instances of multiple excitation in Biophytum and multiple excitation in retina (see table, p. 288), in which the successive intervals undergo similar cyclic changes.

In order to measure accurately the intervals between successive visual images, I use a special stereoscope, based on my discovery of the phenomenon of binocular alternation of vision, and by this means I have found that the after-effects of light in the two eyes are not simultaneous but alternate — that is to say; when the after-image in one eye is most vivid, that in the other eye has just vanished, and vice versa. For accurate measurement of this periodicity, it is necessary to have the effect on one eye distinguished from that on the other. For this purpose I have been accustomed to use a stereoscope containing, instead of photographs, incised plates with two inclined cuts, the right eye seeing the slit inclined to the right, and the left eye that inclined to the left. When the observer looks through this stereoscope turned towards a bright sky, his two eyes are acted on by strong stimulus of light. On closing the eyes, periodic visual impulses are sent from the strongly stimulated retina just as the stimulated area of Biophytum was found to send out periodic impulses.1

1 Bose, ' Binocular Alternation of Vision,' Response in the Living and Non- Living, p. 175. Cyclic variation in the periodicity of the multiple response of Biophytum In both these cases, therefore, we begin with a comparatively long interval, which grows shorter, and then, again, is progressively lengthened. Intermittent pulsation. — We have now dealt with one of the two instances of correspondence between obscure pulsatory phenomena in animal and vegetable. The second, which remains to be considered, is that known to physicians as ' the

intermittent pulse' (fig. 121). * The term " intermittent " is employed to designate the pulse when a beat is occasionally missing from time to time, while the pulse in the intervals is perfectly regular. It is a remarkable variety of pulse, and is perhaps the least capable of explanation of any. The intermission may happen at regular and definite periods, every four, six, or up to twenty beats ; or the number of intervening pulsations may vary. The intermittent pulse may be habitual and constant, and in this case is more likely to be at definite intervals, or it may be occasional only, under the influence of some disturbing cause. . . . Occasionally nervousness and fatigue will render the intermissions more frequent.' l

In connection with this, I have observed, among the types of cyclic variation in multiple response in plants, an instance in which every third response was missing, the period of each second beat being thus approximately twice as long as that of the first It was easy to see that this was an instance of alternating fatigue, causing the particular record - ing leaflet to just miss the response every third time (p. 122). That the excitatory wave arrived in regular sequence, was seen by the fact that the neighbouring leaflets pulsated at regular intervals.

Semi-automatism. — The plant Biophytum growing in the open, under favourable conditions of heat and lightsometimes becomes so excessively sensitive that motile impulses are generated, the stimulus causing which it is often difficult to localise. A particular leaflet may have been moved by a puff of wind ; or the alighting of a small insect, or the accidental grazing of an adjacent blade of grass, may have been the original source of the impulse. But this is enough to set all the leaflets of the plant quivering in an extraordinarily lively manner. For from the excited leaflet, the impulse travels inwards, the leaflets falling in centripetal succession. The excitatory wave then reaches the stem and overflows to the other leaves. But this time the progressive closure of the leaflets proceeds in a centrifugal or outward direction. Before the first discharge, however, going through the numerous avenues, can exhaust itself, the second impulse of the multiple response may begin ; and in this way the leaflets exhibit most lively movements without any immediate

antecedent cause. In some of these cases the origin of the impulse can be traced, but there are others in which no external source of stimulus can be assigned. And here we find ourselves passing imperceptibly into the obscure region of automatism. There is much resemblance between such semi-automatic phenomena in Biophytum and the well-known instance of spontaneous movements exhibited by Desmodium gyrans, the successive mechanical responses of which exhibit all the peculiarities of multiple response as seen in Biophytum. In Desmodium as in Biophytum we have similar cyclic changes of period and of amplitude. Just as in the case of Biophytum, when we cannot determine the source of stimulus, we are tempted to regard the phenomenon as automatic, so in the case of Desmodium gyrans it is our own inability to trace out the origin of stimulus that leads us to regard the periodic movements of the plant as automatic.

Biophytum, then, under ordinary circumstances, exhibits a single response to a single stimulus ; but when the stimulus is strong, a single stimulation will produce multiple responses, and these will persist long after the cessation of the primary stimulus. Under exceptionally favourable circumstances periodic movements occur, apparently without any exciting cause. I have again found, as will be described more fully in Chapter XXIV., that Biophytum itself under favourable circumstances of light arid warmth exhibits persistent and longcontinued autonomous pulsations, which are in no way distinguishable from those of Desmodium. Even the periods of vibration are, generally speaking, similar, inasmuch as in both these plants, under different circumstances, I have recorded pulsations, the periodicities of which vary from one minute or less to four or five minutes. Thus Biophytum forms a connecting link between those plants which exhibit only ordinary response (single stimulus, single response) and those in which mechanical movements appear to be automatic. Biophytum is also particularly interesting, because in its case we find the same plant, under different

circumstances, behaving in either of these ways ; that is to say, giving ordinary response, or exhibiting automatic movements. Continuity of multipleand automatic response.— If there be thus no real breach of continuity between multiply and automatically responding plants, it should be possible, under suitable circumstances, to obtain from such a characteristically automatic plant as Desmodium all those peculiarities of responsive indication which were found in Biophytum. The latter, it will be remembered, under the condition of overexcitability, consequent on the absorption of abundance of energy of heat and light, became converted from an ordinarily responding into an automatically responding plant. Conversely we might expect Desmodium, under the opposite circumstances, to pass from the condition of giving automatic to that of giving ordinary response.

This inference I have been able to verify, for I succeeded in obtaining the requisite conditions for converting the automatically responding Desmodium into an ordinarily responding plant, in three different ways : firstly, by artificially reducing the absorbed energy of the plant, under cautious cooling, care being taken that this did not produce permanent cold-rigor ; secondly, by keeping a plant for some days in a dark room, at uniform temperature ; and thirdly, by selecting a plant for experiment in the most unfavourable season of the year — that is to say, in the winter, when it was already exhausted by flowering. It will be seen that the principle adopted in each of these cases consisted in depriving the plant of its excess of energy. The third method was the most satisfactory of all, since the leaflets were in a state of natural standstill. In every instance, the automatic movements of the leaflets came to a stop, and the motile organ was reduced, as will be shown, to the ordinarily responding condition. In choosing the leaflets for experiment, it must be remembered that their sensitiveness is liable to disappear with age. In Biophytum, for example, very old leaflets show no response. In experimenting, therefore, on leaflets of Desmodium which have been

brought to a state of standstill, care should be taken to select those which have not permanently lost their motility, but in which automatic movements have simply come to a stop for want of a sufficient reserve of latent energy. Polar excitation of Desmodium leaflet in a state of standstill. — We have seen how the leaflets of Biophytum showed kathodic excitation at make. I shall now show that a Desmodium leaflet, which has been brought to a state of standstill, either naturally or artificially, will exhibit similar effects of polar excitation ; and first, I shall take a case in which arrest was produced by cautious application of cold. I placed one electrode on the petiolule of the leaflet, and the other on the main petiole of the leaf, and applied the stimulus of condenser discharge, the petiole being made kathode. In this way I obtained from the Desmodium leaflet a series of single responses to single moderate stimuli.

I next selected specimens which, in the winter season, had come to a state of natural standstill, and tried on them the polar effects of a constant current. Electrical connections were now made at the bases of the petiolules of lateral leaflets in two neighbouring leaves. In completing the electrical circuit, one of these leaflets would be under anodic, and the other under kathodic, action. I found that a considerable electromotive force was necessary to initiate the excitatory reaction. Thus, using thirty volts, I found that the kathodic leaflet responded at make, and the anodic at break. This shows that, as regards polar effects, Desmodium in a state of standstill gives exactly the same responses as does Biophytum.