Plant Response as a Means of Physiological Investigation

Effect of temperatures maximum and minimum. — Autonomous vibrations come to a stop when the temperature is sufficiently lowered. The temperature minimum at which this occurs depends, as we should expect, on the nature of the specimen— whether Desmodiurn, Biophytum, or cardiac muscle — and also, with similar specimens, on the tonic condition. With regard to the first of these points, we have seen that the autonomous vibration of Biophytum comes to a stop below 290 C. The pulsation of Desmodiurn is said to be arrested at 2 2° C, but I find that this is a matter which is much modified by the tonic condition of the particular plant. With vigorous specimens I have seen that the vibration may persist even at so low a temperature as 170 C. The thermo-tonic minimum in Biophytum and Desmodiurn thus shows a difference, as already said, of about 120 C. ; and in the case of the frog's heart this is still lower, and is said to be about o° C.

When, again, the specimens are raised to a maximum temperature, the pulsations come to a stop. The temperature at which this takes place depends in part on the condition of the specimen. For example, with the frog's heart, this maximum is sometimes at a temperature so low as 380 C. In other cases, pulsation may be detected even at so high a temperature as 440 C. Similarly, in Desmodiurn, I have found that the maximum temperature at which arrest took place was sometimes as low as 350 C. ; but in certain specimens it did not occur till 450 C. A plant may, again, be accustomed gradually to high temperatures, and under these circumstances the maximum may be raised as much as 30 C. or 40 C. higher.

Effect of temperature on period and amplitude of response. — The most marked phenomenon of effect of temperature on automatic pulsations, whether animal or vegetable, lies, however, in the fact that the period and amplitude are both affected. When the temperature is lowered, the amplitude of pulsation is enhanced, while the frequency is diminished. Conversely, with rise of temperature, the amplitude is diminished, and frequency augmented. This is true not only of the pulsations of the rhythmic tissue of Desmodiurn, but also of those of the animal heart. This is seen in the two following records, where the first

set ofresponses in each series gives pulsations at the temperature of the room, while the second set in each gives pulsations of greater amplitude and smaller frequency, due to the lowering of temperature by several degrees (figs. 135, 136). Conversely, as already said, when the temperature is raised, the frequency is increased and the amplitude decreased. This is seen in a general way in the following photographic record (fig. 137), which I took with

Eio. 135. Photographic Records of Autonomous Pulsations in Desniodhtm, showing Increase of Amplitude and Decrease of Frequency, with Lowering of Temperature The pulsations to the left were taken at the ordinary temperature of the room, 290 C. Those to the right were taken when the temperature had been lowered to 250 C. Desmodiitniy the temperature being continuously raised, from 300 C. to 390 C. It will be seen how progressive in character is the diminution of amplitude and increase of frequency in these responses.

In another set of experiments in which I took records of the responses of Desmodium at various definite ascending temperatures, I found that at 190 C. the period of a single oscillation was 4*3 minutes. At 220 C. this was reduced to 32 minutes, or nearly to two-thirds of the period at 190 C. At 280 C. it was found to be again reduced to 21 minutes, or half. But at 400 C. it was only 1*4 minute, or one-third. Thus, while in 4-3 minutes, at 190 C. there is only a single beat, there are two beats at 2 8° C. and three beats at 400 C. in the same time. I give below (fig. 138) a record of these responses at various temperatures. The record given afterwards (fig- r39) shows how similar is the effect of temperature on the amplitude and rhythm of the pulsation of the animal heart. When the temperature of Desmodium is raised above 400 C. there appears to be an arrest of pulsation. But this need not be

Fig. 136. Effect of Lowering of Temperature in Producing Increase of Amplitude and Decrease of Frequency in Pulsation of Frog's Heart The pulsations to the left represent normal pulsations at the temperature of the room. Those to the right were taken at a temperature several degrees lower (after Brodie). Fig. 137. Photographic Record of Pulsations of Desmodium during Continuous Rise of Temperature regarded as due to heat-rigor. For in magnified records I have often noticed that here we may have very much

quicker pulsations, but of an amplitude so small as usually to pass undetected. It is only under prolonged exposure to this relatively high temperature, or after exposure to a temperature above 500 C, that true heat-rigor sets in. This maximum temperature varies in individual cases with the tonic condition of the plant. Effect of the reduction of temperature to the thermo-tonic minimum. — It has been said that, generally speaking, the amplitude of response increases with the lowering of temperature. It is evident, however, that this process must have a limit. For we know that pulsation vanishes at the thermo-tonic minimum ; before this reduction of amplitude to zero it is clear that there must be some point where the increase of amplitude due to continuous cooling must undergo reversal, or diminution. And this is what we should theoretically expect, for since it is the absorbed thermal energy that maintains the pulsation, it follows that, when this is diminished below par, the vibrational energy

Fig. 138. Record of Pulsations of Desmodium at Different Temperatures Record of Pulsations of Frog's Heart at Different Temperatures (Pembrey and Phillips) should also undergo diminution. We had an illustration of this (p, 305), when the automatically vibrating Biophytiim at 350 C. was allowed to descend to the thermo-tonic minimum. It was in that case found that there was a regular diminution of amplitude, and the pulsation afterwards disappeared below 290 C. (fig. 124).

I was next desirous of determining whether this theoretical inference could be verified in the case of Desmodium. For this purpose I rapidly cooled the plant, by means of the cooling coil, through which ice-cold brine was passed, a FlG. 140. Effect of Cooling to Thermo-tonic Minimum on Pulsation of Desmodium The first two pulses to the left were taken at the normal temperature of 29° C. ; those to the right during continuous cooling. The first response of the latter series occurred at 220 C. ; the second at 200 C. ; and the third at 170 C. Note that while in normal responses there are no sub-pulses, these are seen with increasing distinctness as the pulsation becomes continuously slower. They first make their appearance during the up movement of the first pulsation under cooling. In the two successive pulsations they are seen more and more clearly in the down as well as the up movements.

photographic record being taken of the pulsations all the time. It will be noticed that the first effect of cooling was the normal increase of amplitude and prolongation of period. This latter— which at the temperature of the room had had a value of three minutes — was now prolonged to nearly six minutes. But the most interesting fact was that, as the thermo-tonic minimum was approached, the amplitude was reduced (fig. 140). In the next experiment cooling was produced more suddenly, by application of cold water at about 40 C. to the pulvinus. We observe in this case how the quick reduction to the thermo-tonic minimum reduced the amplitude till the pulsation had come to a stop. I then allowed the leaf to return gradually to the temperature of the room, and it is very interesting to note the effect of increasing

Normal pulsations recorded to the left. Effect or application of ice-cold water is seen in the production of diminished amplitude and abolition of pulsation. Gradual return to the temperature of the room revives the pulsation in a staircase manner, the period remaining approximately constant. Note that cooling, in this and previous figure, displaced the pulsation in a downward or contracted direction. Gradual warming, conversely, is seen in this figure to produce the opposite displacement towards relaxation.

absorption of thermal energy from its surroundings. The increased energy thus absorbed is seen to give rise to increased amplitude of oscillation, in a staircase manner, which gradually approaches the original pulsation (fig. 141). In both these figures it will be noticed that cooling displaces the pulsation in a downward or contracted direction. And in the last series of fig. 141 we see that the raising of the temperature displaces it upwards or towards relaxation. These facts are of great importance, and should be borne in mind in reference to the explanation of the cause of variation of amplitude and period, which I shall bring forward.

Explanation of diminution of amplitude of pulsation with rise of temperature.— We have thus seen, as we should theoretically have expected, that with the increased absorption of energy at a higher temperature, the amplitude of vibration is also increased. How is it, then, that with the still further increase of absorption of energy, at still higher temperatures, the amplitude should undergo a diminution ? When approaching the maximum point, where heat-rigor takes place, we can understand that the excitability of the tissue would be very much decreased, with a consequent reduction of amplitude of pulsation. But at temperatures of 2 50 C. to 300 C. excitability of the tissue could not be diminished. Indeed, I shall in Chapter XXXIII. adduce considerations to show that it must, at that temperature, be highly excitable, and we should have expected that this would have produced, in addition to the increased energy, an augmentation of vibrational amplitude. But, instead of this, we obtain the curious result which has been described, of a diminution, in the cases both of cardiac muscle and of Desmodium.

It might be suggested that if increased activity, due to rise of temperature, increased the frequency of vibration, then this fact would be sufficient in itself to account for a diminution of amplitude ; for in this case, less time being allowed for each single vibration, its extent must be curtailed. But this consideration alone would not explain all the facts of the case ; for we have seen that on approaching the thermo-tonic minimum, though the frequency of vibration is reduced, and the period very much extended, yet the amplitude is also at the same time decreased (fig. 140). WTe thus see that the question of internal energy is important in this connection. An increase of internal energy may be expressed, as I shall show, in two different ways ; either, that

is to say, by increase of amplitude or by increase of frequency of pulsation. Increased internal energy, shown by: (a) Increase of amplitude, period remaining constant. — And we shall, as the simpler of the two, consider that case in which the latent energy, or tonic condition of the plant, is below par, that is to say, the case in which it is near the thermo-tonic minimum. The effect of increased absorption of energy with rising temperature would here be indicated by increasing amplitude of pulsation, the period remaining constant. Conversely the reduction of latent energy with falling temperature would be indicated, when the period is constant, by the fall of amplitude. This we find fully illustrated in records obtained with Biophytum and Desmodium. In the former, when nearing the thermo-tonic minimum, it is found that while the period remains approximately constant, that is to say, two and a half minutes, the amplitude of pulsation falls from 8 divisions at 350 C. to 5-5 divisions at 320 C. (fig. 124), In Desmodium, again, we find a converse case. Here, while the plant is rising from the thermo-tonic minimum to the normal condition, the amplitude of pulsation is seen to increase progressively, while the period of 2*4 minutes remains constant (fig. 141).

(&) Increase of frequency. — Taking next the case of a plant in the ordinary tonic condition, we find that the increase of internal activity, due to the greater absorption of energy during a rise of temperature, is exhibited by a higher frequency of vibration. The reason why, with this increase of frequency, there is a diminution of amplitude of pulsation, is now to be explained. We have seen that an increase of internal energy, as caused by rise of temperature, brings about an increase of turgor, and that this increased turgor hastens the process of recovery, and by acting antagonistically to the contractile phase of responses diminishes its amplitude. In- creased internal pressure also, generally speaking, increases the frequency of vibration. If, then, the rise of temperature increases the turgor of Desmodium, as we have found it to do

in plants in general, then the diminution of the amplitude of its vibration with higher temperature is explained ; and I shall be able to adduce independent proof that this is actually the case in Desmodium, for we have seen that the external indication of internal increase of turgor is the expansion of the organ, which produces a movement upwards, the same as that of relaxation. We found in the last series of responses in fig. 141, moreover, that when the temperature was raised gradually from the thermo-tonic minimum, the leaflet was more and more erected, or 'relaxed.' Cooling, conversely, produced diminution of turgor, and an opposite movement in the direction of fall or contraction (figs. 140 and 141).

The anomalous use of the word ' relaxation.' — We have seen that the motile organ of Destnodium, when anaesthetised, is brought to a state of standstill in a position of relaxation. Its tonic condition, by virtue of which it exhibits contractile response, has thus been abolished. We may then regard ether as having brought about a loss of tone, or as having reduced the tissue to the a-tonic condition. An apparently similar position of relaxation may, however, be attained by the active process of expansion, which is the result of an increase of internal turgor. It would thus appear that we are liable to form many wrong inferences, as to the tonic changes undergone by the organ, if we too hastily conclude that expansion is always caused by loss of tone.

Simple versus additive character of individual pulsations.—One question, regarding which opposite views have been put forward hitherto, is that of the simple or composite nature of the individual pulsations of cardiac muscle. The movement of systole may, for example, be regarded as consisting either of a single or of the additive effect of several constituent contractions. In the production of tetanus in the case of muscle and also in that of contractile vegetable tissue, we have seen several individual contractions, when following each other with sufficient rapidity, become merged in one apparently continuous contraction (figs. 49, 50). When less rapid, however, they are individually distinguishable. Similarly,

in the case of cardiac pulsation, it is possible that one apparently simple contraction may in reality consist of several, which are rendered indistinguishable by the great rapidity of their succession. Now, since the pulsatory response of Desmodium is in every way so similar to that of cardiac muscle, its analysis might be expected to throw much light on this question ; and this more especially since it possesses the added advantage that its pulsation is executed in a period about one hundred times as long as that of cardiac muscle. The constituent

elements of each pulsation, if such exist, ought thus, owing to the relative slowness of the movement, to be much more easy of detection. Now, it is often seen in watching the pulsatory movements of Desmodium that they proceed somewhat discontinuously, or by jerks. Under favourable circumstances, nevertheless, the movements of the leaflet up and down become apparently continuous. When can be brought out more easily in the case of the up movement, which is relatively slow. This is shown in a very interesting manner in the accompanying photographic record (fig. 142), where the subsidiary pulsations make themselves visible as nodules ; places where the movement is slow appear thicker, on account of the photographic irradiation-effect. During the course of the single up movement recorded in this photograph, we may count as many as twenty-five of these sub-pulses. It has been said that these subsidiary movements are more easily detected when the general movement is relatively slow ; and in connection with this5 it is extremely interesting to note the record of pulsations

Fig. 143. Photographic Record of Cyclic Groupings in Autonomous Pulsations of Desmodium, showing Sub-pulses seen in fig. 140, where successive pulsations of the same leaflet, at first apparently continuous, are made to exhibit the sub-pulses with growing distinctness, as the period becomes progressively slowed down by cooling. First we are enabled to observe the sub-pulses during the up movement, and afterwards during the down movement also. I give another photographic record also (fig. 143), in which these subsidiary pulses are seen at both the ' systole ' and ' diastole ' (see also fig. 175). Arguing from analogy, therefore, it becomes highly probable that any single pulsation of the heart also may be made up of similar discrete and constituent elements.

While dealing with this question, it must be borne in mind that in each case the contractile organ as a whole is made up of a mass of individually contractile elements, the sum of whose separate and additive actions it is which is seen as a single contraction and expansion of the whole organ. Thus, in Desmodium for example, a single pulsation of the motile organ is made up of subsidiary pulselets. The unit-pulses, again, may themselves be grouped in larger systems, either as the alternate waxings and wanings seen in periodic groupings (figs. 143 and 149), or in those periodicities of the order of an hour or so, which the plant sometimes

Fig. 144. Photographic Record of Autonomous Pulsation of Desmodium, showing Hourly Period [Speed of drum = 5 cm. in one hour.] exhibits, under the influence of periodic variations of temperature and other factors. An extremely interesting example of such an hourly periodicity is given above (fig. 144), where the mean plane of vibration is itself seen to exhibit a periodic up-and-down oscillation. There is, again, the still larger periodicity of diurnal variation of day and night. We thus see how complex may be these wave-systems, in which, superposed over large waves, are smaller waves, and on the latter still smaller wavelets.

In Desmodium, as in cardiac muscle, rise of temperature produces increased frequency with diminished amplitude of pulsation. This is true within a certain normal range of temperature. When the temperature of Desmodium is reduced to a thermo-tonic minimum — which is about 170 C. but subject to certain individual variations — the amplitude of pulsation, owing to the loss of internal energy, is decreased till there is an arrest. If now the temperature be gradually raised, the pulsations, owing to the absorption of energy, become again increased in a staircase manner, the period remaining approximately constant.

Under normal tonic conditions, the decrease of amplitude of pulsation with rising temperature — when this is not excessive — is not indicative of loss of excitability. It is due to the increase of internal energy, which hastens recovery and acts antagonistically to the responsive movement of contraction. The same explanation is probably applicable to the diminished amplitude of pulsations with rise of temperature, observable in cardiac response.

Rise of temperature, by increasing the internal energy and consequent turgor of the plant, causes expansion of the organ, thus bringing about a shifting of the pulsation towards 1 diastole.' A converse effect, or shifting towards ' systole,' is seen in Desmodium, as the result of cooling. The process of relaxation in the motile organ may be the result of entirely different causes. It may be a consequence of a loss of tone, such as results from narcotisation ; or it may be brought about by excessive turgor, caused by increased internal energy.

The apparently simple rhythmic pulsations of Desmodium can be analysed and shown to consist of subsidiary minor pulsations. The ordinary pulsations, again, may show hourly and daily periodicities. The similarities, in their fundamental characteristics, of rhythmic tissues, animal and vegetable : (i) In responses— (2) In possession of long refractory periods - (3) In incapability of tetanus — Theories regarding the causation of heart-beat — The similarities of rhythmic tissues, animal and vegetable, as seen in : ( 1 ) The effects of internal hydrostatic pressure— (2) The effects of variation of temperature—(3) The periodic groupings of response— (4) The effect of barium salt— (5) The antagonistic actions of acid and alkali — Identity of rhythmic phenomena in animal and vegetable tissues.

I HAVE, in the course of the previous chapters, shown the remarkable general similarities, extending through numerous details, between the responses in animal and vegetable tissues. These similarities, however, become still more striking when we compare the special characteristics of those plant and animal tissues which exhibit the property of rhythmicity — that is to say, those tissues which, under the action of a single strong stimulus, give rise to a multiple and rhythmic series of responses, such responses, under favourable circumstances, passing into the so-called automatic movements.