Researches on Irritability of Plants

generally speaking, longer than that of the ordinary skeletal tissue. In the plant, again, while the average value of the latent period of ordinarily responding Mimosa is ‘1 second that of the multiple responding Biophytum is *4 second. The latent period of motile leaflet of Desmodium is of the same order. It varies from ‘4 second to ‘5 second. In order to study in detail the time-relations, I took ona fast-moving plate the response of Desmodium leaflet, origin- ally in a state of standstill, to the action of induction-shock. The frequency of oscillation of the recording-plate was once in a second, hence successive dots represent intervals of

FIG. 153.—Response of Desmodium leaflet originally at standstill. Successive dots at intervals of 1 second. I second. The entire response, consisting of contractile down-movement and subsequent recovery, was accom- plished in the course of 2 minutes and 45 seconds; the leaflet attained its maximum down or contractile position 45 seconds after the application of stimulus. The top of the response-curve is seen to be somewhat flattened, indi- cating a persistent contraction from which recovery takes place slowly. The period of relaxation is much longer, being 120 seconds ; the record in fig. 153 shows recovery which is not quite complete.

A striking characteristic of the rhythmic cardiac tissue of the animal is its long refractory period. It is found that by applying successive stimuli there is a minimum resting- interval, the diminution of which brings about such a loss of excitability as to abolish response. This minimum interval is known as the Refractory Period, because short of this period the tissue takes no account of stimulus or is refractory to it. In the cardiac muscle the refractory period lasts during the entire period of the contraction.

In the response of the leaflet of Desmodium also we observe in this respect a very striking similarity, in its Fic. 154.—Record demonstrating refractory period in Desmodium. In the lower record the second stimulus, applied after 45 seconds, is seen to be ineffective, having fallen within the refractory period. In the upper record the second stimulus, applied after an interval of go seconds, is seen to be effective. possession of a long refractory period. This will be seen in fig. 154, where the leaflet in a state of standstill was sub- jected to two successive stimuli of induction-shocks. In the lower record of the two, after the responsive movement due to the first stimulus, a second was applied after an interval of 45 seconds. The leaflet, however, took no _ account of it, there being induced no second response. The refractory period in this case is seen to be longer than 45 seconds. After a suitable interval the same leaflet was subjected to two successive stimuli, and: at an interval of go seconds. It is seen from the upper record that the second stimulus was effective, having fallen on the leaflet beyond the refractory limit.

I stated in a previous chapter that, strictly speaking, there is. no such thing as spontaneous movement ; that the energy which expresses itself in the pulsating activity of the plant is in reality derived from external sources. It is thus the stimulus supplied by the environment which is held latent by the plant-tissue to find expression later in rhythmic pulsations. For the experimental demonstration of this theory we may carry out the following investigations :

(x) The effect.of stimulus on the pulsating activity of a specimen in a moderately sub-tonic condition. (2) The effect of stimulus on a leaflet in a state of stand- still. The renewal of pulsation by absorption of energy has already been shown in the records given in figs. 150, I5I, 152, and 153. (3) Observation of the effect of gradual depletion of energy on the pulsation of the leaflets. (4) Effect of fresh stimulation on specimens in which the stored energy has been allowed to run down.

We take a specimen in which the pulsating activity is moderate or feeble, and subject it to stimulation. If the rhythmic activity is the result or after-effect of stimulus previously absorbed, then a more vigorous pulsation may be expected from the greater accession of energy. This inference is found fully verified in the record given in fig. 155. The specimen had been kept in the dark, and its pulsating activity was only moderate as seen in the first part of the record. The leaflet was then subjected to the stimulus of light from a Nernst electric lamp for half an hour. The record taken after this interval shows a marked enhancement in the amplitude of pulsation.

Having observed the effect of accession of energy, it is still more interesting to note the effect of the converse process of gradual depletion of energy. I had hitherto been relying on chance specimens for exhibition of the condition of standstill. As the previous history of the particular specimen was not known from moment to moment, no definite information was available as to the cause of the stoppage of pulsation, except the very natural inference that it must have been due to the run-down of the stored energy. In order to verify this inference, I now proceeded deliberately to isolate an active specimen from all accession of energy from outside, and observe the effect of gradual depletion of energy that had been stored up. The cut specimen, mounted in the usual

Fic. 155.—After-effect of stimulation on pulsation of Desmodium leaflet in sub-tonic condition. manner, was kept in a dark room, and a continuous record of its pulsations was taken all the time. In taking a series of such records under isolation, it was found that the persistence of the rhythmic activity depended on the vigour of the specimen—that is to say, on the storage of energy in the tissue. Thus a vigorous specimen exhibited persistent activity for more than twenty hours ; its pulsations showed great uniformity for the first twelve hours, after which the amplitude began to decline and the rhythmic beat came to a stop at the twenty-first hour.

In another specimen, less vigorous, the pulsations of the isolated specimen came to an end at the ninth hour. A continuous record from the third to the sixth hour is seen in fig. 142, given on page 294; the series of records are to be read from below to above each series, lasting for one hour and twenty minutes. It is seen that the amplitude is Fic. 156.—Effect of depletion of energy on the pulsation of isolated specimen of Desmodium gyrans. practically constant ; the period is also constant in the first two series, there being twenty-seven pulsations in each in the course of eighty minutes. The period is, however,

F1G. 157.—Gradual stoppage of pulsation in isolated leaflet of Desmodium. slightly lengthened in the third series, there being twenty-six pulsations in the given time. The record was continued (fig. 156) during the seventh, eighth, and part of the ninth hour. In these two series, the effect of the depletion of stored energy is well seen in the regular diminution of the amplitude and the prolongation of the period; there are now, in the lower or earlier series, twenty-three pulsations in the course of eighty minutes, instead of twenty-seven, which was found at the beginning. The period becomes still more prolonged in the next series, where there are only twenty pulsations. The pulsating activity of this specimen was found completely arrested at the end of the ninth hour.

In another specimen less vigorous, the arrest took place at the seventh hour. I give a record taken during the seventh hour (fig. 157) which shows the process of arrest in a very interesting manner. The arrest of pulsation seen here is not due to death or loss of sensibility, but merely to the run down of stored energy. I shall presently show that the activity of the leaflets can in these cases be renewed by the incidence of an external stimulus. Before describing the experiments, it is well to anticipate the modifying results brought on by the varying loss of stored energy. The extent of depletion will, it is obvious, depend on the length of time during which the specimen had been kept isolated from the external supply. At the moment of arrest of pulsation the specimen wil] still have a certain reserve, but not enough to cause an overflow. At this stage a stimulus of even short duration is likely to give rise to multiple responses. If we allow a longer time to elapse after the stoppage of pulsation, then the storage-level will be much lowered. A more intense stimulus or one of longer duration will now be effective ; the response is more likely to be single rather than multiple. And lastly, if we allow a very long time to elapse after the cessation of pulsation, the vitality will be found to have so far declined as to bring about the condition of death. To recapitulate: the effect of fresh incidence of stimulus on a leaflet brought to a state of standstill by isolation will be modified according to the period which had been allowed

to elapse after the cessation of pulsation. Stimulus applied within a short time of stoppage will give rise to multiple responses ; after a long period, an identical stimulus will be found far less effective. After a still longer interval, which is critical, even a strong stimulus will fail to evoke any response. These theoretical considerations will be seen verified in the following series of records. In fig. 158, stimulus of light of two seconds’ duration was applied after half an hour of the stoppage of the pulsation of the leaflet. The response is

Fic. 158.—Effect of stimulus in renewing pulsation of Desmodium brought to standstill through depletion of energy. Note two series of multiple responses due to stimulus of light of two seconds’ duration. seen to be multiple even under a stimulus of this short duration. The stimulus was applied once more, with similar results. In the next series of records, stimulus was applied on a specimen which had been in a state of standstill for five hours. The store of energy here had undergone a consider- able decline. Stimulus of light of two seconds’ duration was found ineffective ; it was only after an application lasting for half a minute that a response took place (fig. 159). The response was single, instead of multiple as in the last case. Stimulus was applied a second time, but of still

longer duration of one minute. The response is now found to consist of a large, followed by two small, pulsations. I next took another series of records, with a specimen which had been in a state of standstill for seven hours. Fic. 159.—Response of Desmodium leaflet at standstill to stimulus of light of °5 and 1 minute’s duration. Stimulus of light was successively applied for one, two, three and four minutes. The general result is somewhat similar to those that have been seen in the previous record. The noticeable differences are, first the diminished amplitude

Fic. 160.—Response of a depressed specimen of Desmodium to stimulus of light applied successively for one, two, three, and four minutes. of response, and the exhibition of decline in the successive responses. The specimen was nearing the critical condition of death, and after a while there was a cessation of all response. In other specimens the critical period was found to be exceeded when the state of standstill had been allowed to persist for nine or ten hours. In these, stimulus failed completely to evoke any response.

The leaflet of Desmodium comes to a state of standstill by depletion of its store of energy. The pulsation may be revived by the action of various stimuli, such as that of light or of induction-shock. Desmodium leaflet in a state of standstill gives a single response to a single stimulus of induction-shock of moderate intensity. In a typical case the latent period is ‘4 second, the apex time 45 seconds, and the period of relaxation 120 seconds. The response-curve exhibits a flattened top.

The response of Desmodium, like that of the cardiac tissue, is characterised by a long refractory period. The rhythmic activity of the leaflet of Desmodium comes to an end when its store of energy is depleted. A leaflet isolated from external sources of stimulation is thus gradually brought to a state of standstill. In this condition, response occurs under fresh stimulation. If the depletion of energy had not been excessive, then a moderate stimulus gives rise to multiple responses. But under greater depletion even a very strong stimulus induces only a single response.

Effect of electric shock on Desmodium leaflet—Incapability of tetanus —Extra pulsation induced by electric shock—Relative effectiveness of electric stimulus at diastolic phase—Effect of transmitted excita- tion on normal pulsation of heart and on pulsation of Desmodium leaflet—Effects of acceleration and inhibition. In the previous chapter we studied the action of electric shock on the leaflet of Desmodium in a state of standstill. We will now inquire into the effect of induction-shock on the pulsating leaflet.

Fic. 161.—Application of strong tetanising shock at arrow brings about diminished amplitude of pulsation. In the case of cardiac tissue it is known that the excita- bility is least at the commencement of systole. A moderate stimulus applied at this period induces hardly any effect. During the occurrence of diastole, however, excitability is relatively great. Hence, if stimulus be applied at diastole, it is followed by an extra contraction. Turning to Desmodium, I find that similar characteristics obtain in this case also. The application of stimulus at the beginning of the contractile movement, which corresponds to systole, has little or no effect. But during the movement of

Fics. 162, 163.—Extra pulsation induced by induction-shock applied at diastolic phase. relaxation, which corresponds to diastole, the application of an induction-shock induces the arrest of relaxation and gives rise to an extra or interpolated pulsation. I give here two records, out of many, obtained from two different specimens, in both of which a momentary electrical shock was applied about the middle of the phase of diastolic relaxation. The arrest of relaxation, and reversal to the opposite phase of contraction, giving rise to an extra pulse, are here clearly seen in both cases (figs. 162, 163).

On exciting the nerve that goes to the heart, two opposite effects have been observed. In some cases there is induced a diminished amplitude of pulsation: in other cases an augmentation. It has been supposed that this nerve contains two different kinds of fibres, the accelerators and the inhibitors. In the case of lower animals, such as the tortoise, these have been isolated by Gaskell. But the accelerator fibres have not been distinguished in the mam- malian vagus. It has been found that, generally speaking, in a vigorously beating heart the effect of vagus-stimulation is to induce depression, whereas in a sluggish heart the same stimulation is apt to induce an augmentation.

Fic. 164.—Inhibitory effect of transmitted excitation on the pulsa- tion of vigorous leaflet of Desmodium. The line below indicates duration of transmitted excitation; note the gradual removal of inhibitory effect on cessation of stimulation. Desmodium, the question arises whether the organ is in communication with any conducting channel by which distant excitation might be transmitted to it. The fact of such transmission, if it occurred, would be tested by its modifying influence upon the normal pulsation. In order, then, to subject this question to the test of experiment, I made suitable electrical connections—one contact being on the petiolule, 5 mm. below the contractile pulvinule, and the other still lower down on the petiole. Normal responses were first taken, after which indirect stimulation was applied — by means of tetanising electrical shocks of moderate intensity. The first specimen employed was very vigorous, as will be observed from the amplitude of its pulsations (fig.164). It

will be noticed that stimulation, thus applied at a distance, was transmitted and induced an inhibitory effect in diminish- ing the amplitude of normal pulsation. On the cessation of excitation the pulsations are seen gradually to regain their normal amplitude. This inhibitory effect is what takes place more frequently, and may be regarded as typical. Somewhat exceptional is the converse effect of augmentation, seen in the next record (fig. 165). The particular specimen was less vigorous ; this fact is seen in the smaller amplitude of its normal pulsations, the magnification being the same in the two cases. After these normal pulses had been

Fic. 165.—Augmented pulsation induced by transmitted excitation in less vigorous specimen of Desmodium. recorded, indirect stimulation of moderate intensity was applied, as in the previous case. It will be noticed that, in consequence of this, there occurred a marked enhancement of the amplitude of pulsation. Even on considerably raising the intensity of the stimulation this enhancement of pulsation still persisted. A remarkable parallelism has thus been shown to exist between the responsive characteristics of rhythmic animal and vegetal tissues. This is seen in their incapability of tetanus, in their prolonged refractory period, in the extra pulsation induced by electric shock at the diastolic phase, and in the transmitted excitation causing in different circumstances effects either of inhibition or acceleration. Other similarities, equally remarkable, will be found in the

effect of temperature and of drugs on the pulsating activity of animal and vegetal tissues. The rhythmic tissue of Desmodium, like the rhythmic cardiac tissue, is incapable of tetanus. Pulsating leaflet of Desmodium, like the pulsating heart, is more susceptible to excitation at diastole than at systole. An extra pulsation is induced by an electric shock applied during the diastolic phase. Transmitted excitation affects the normal pulsations of rhythmic tissues—animal or vegetal—in a similar manner. In certain circumstances the effect is one of inhibition ; in other circumstances the effect is one of acceleration.

Effect of lowering of temperature on rhythmic pulsation of cardiac tissue— Similar effect on the pulsation of Desmodium—Increase of systolic limit during cooling—Minimum temperature for arrest of pulsation— Arrest by cooling and subsequent revival by warming—Increase of diastolic limit during warming—Fffect of rise of temperature on the pul- sation of frog’s heart—Similar effect on the pulsation of Desmodium— Effect of rise above and return to normal temperature—Diminution of systolic contraction during rise of temperature—Increase of systolic contraction during fall of temperature—Permanent arrest due to heat-rigor.

In the course of our study of automatic pulsation of Des- modium we shall find striking similarities between the rhythmic activities of the plant and animal tissues. In the present chapter we will study in detail the effect of tem- perature in modifying the amplitude and period of the spontaneous movements. I have already described the thermal chamber by means of which the temperature of the plant can be regulated. Temperature, as we have seen, may be raised to any degree by the adjusting of the heating current which passes through a coil of German silver. Lowering of temperature, on the other hand; is effected by allowing a stream of cooled air to pass through the chamber containing the specimen.

In the rhythmic pulsation of frog’s heart the marked effect of variation of temperature is to change the period and modify the amplitude of pulsation. Lowering of tempera- ture has the effect of lengthening the period and enhancing the amplitude. This is seen in the following record (fig. 166), where the normal pulsations are modified in consequence of lowering the temperature through a few degrees. The following experiment, carried out on the leaflet of Desmodium, shows that the effect of lowering of temperature on it is precisely the same. The temperature of the room at the time of the experiment was 30° C. A record of three normal pulsations was taken at this normal temperature.

Fic. 166.—Effect of lowering of temperature in increasing amplitude, and decrease of frequency of pulsation of frog’s heart. Series to the left represent normal pulsa- tions at the temperature of room; series to the right were recorded at a temperature several degrees lower. (Brodie.) The specimen was then gradually cooled by sending through the chamber a stream of cold air, the record being taken all the time. The successive dots in the diagram are at intervals of 2 seconds. Hence the period of complete pulsation can be accurately determined.