Researches on Irritability of Plants

Fic. 141.—U-tube support for leaflet, and the plant chamber. ing this longer limb of the u-tube the hydrostatic pressure to which the specimen is being subjected can be varied ; different chemical solutions can also be applied internally by its means ; a stopcock allows the water to run out of the u-tube, making way for the particular solution poured in at the open end of the tube. The u-tube (fig. 141) is hinged on a rod which slides up and down inside an upright tubular support. This rod can also be rotated inside its support and clamped in any desired position. Facilities are thus secured for three

different modes of adjustment. One up and down, the second lateral, and the third, by means of the hinged support, for the inclining of the specimen. The movement of the leaflets, it must be remembered, does not always take place in a vertical plane. The object of these mechanical adjustments therefore is to enable us to place the specimen at such an angle that its to-and-fro vibration when straight shall be vertical, or have its long axis vertical when the movement is elliptical.

A light cover with mica windows can be made to enclose the specimen. By means of electric current sent through a spiral of German silver, the inside of the chamber may be heated to any desired degree. The temperature can be lowered, on the other hand, by sending through the chamber a stream of cooled air: different vapours or gases could be passed into the chamber for studying their effects on the automatic pulsation. The arm of the recording-lever is attached by means of a cocoon thread to a point about the middle of the leaflet, by means of a drop of shellac-varnish. As the pull exerted by these leaflets is very feeble, the writer has to be made extremely light. The vibration of the Resonant Recorder being about Io times per second, the record taken with it appears ascontinuous. In certain experiments it is desirable to obtain data for accurate time-measurements of different phasic movements of the leaflet. This I have been able to secure by the employment of the Oscillating Recorder, where the recording-plate, by means of an electric motor provided with an eccentric, is made to execute a to-and- fromovement. The intermittent dots thus produced may be once each second, or once in 2 seconds. As the oscillating recorder permits the employment of light grass haulm for the recorder, we may easily obtain a fair magnification pro- duced in the record. I have used both the methods— resonant and oscillating—for obtaining the records: in the former they appear continuous ; in the latter, dotted.

secured in the pulsating movement of the cut specimens, I reproduce here a continuous record lasting for four hours (fig. 142), the movements themselves being maintained uniform for more than seven hours. The run of the breadth of the plate was accomplished in one hour and twenty minutes, successive series of records being taken on the same plate from below to above. It will be seen how uniform are the successive pulsations, not only as regards Fic. 142.—Continuous record of pulsations of Desmodium leaflet for four hours; the series to be read from below to above.

the amplitude, but also the period. It is only after securing such uniformity, under normal standard conditions, that the experimenter is justified in drawing correct inferences, from variations induced in the record, on the influence of changed conditions which he has introduced. The up-and-down movement of the Desmodium leaflet is characterised by certain peculiarities. In its usual form the ‘up’ movement is executed somewhat slowly, and comes to what may be called a pause at the extreme position for a certain length of time. There is in reality no cessation of movement ; but the rate becomes extremely slow at the turning-point where ‘ up’ is reversed to ‘down’ movement and vice versa. After reaching the extreme ‘ up ’ position the ‘down’ movement is commenced, and this is accomplished in a much shorter time. After reaching the lowest position there is again a pause, when the cycle is again repeated. These normal movements and their rates are, however, subject to modification under the influence of external conditions.

These movements of Desmodium leaflet are brought about, as in the case of Mimosa, by the contraction or expansion of the pulvinus. Here, also, it is the lower half of the motile organ that is predominant in its action. A question now arises as to the significance, whether of contraction or relaxation, of the up and down movements. The question may be settled in three different ways: The contractile movement, generally speaking, is quicker ; hence the quicker down movement of the leaflet may be regarded as that due to contraction. Again, I have found that a leaflet in a state of standstill exhibits under stimu- lation an excitatory contractile movement which is down- wards. And lastly, by means of internal hydrostatic pressure, we may induce expansion of tissue. This has the effect, as we shall see in a later chapter, of shifting the pulsatory movements upwards. All these different con- siderations point to the conclusion that in Desmodium the ‘down’ position of the leaflet represents contraction and the ‘up’ position denotes expansion of the more effective lower half of the motile organ. The up-and-down move- ments of the leaflet thus correspond to the diastolic and systolic movements of the animal heart. In the records of the pulsation of Desmodium, up-curve represents down movement and vice versa.

these movements in their various phases. The movements themselves are relatively slow, much slower than the con- tractile movement of Mimosa. In that case we saw that the movement of the fall was accomplished in about 2 seconds but with Desmodium the down movement may occupy as long as 40 seconds. I was able to determine the different rates of movement in Desmodium by making the recording Fic. 143.—Record of a single pulsation of Desmodium; magnification 2°5 times. Successive dots at intervals of a second.

plate oscillate to and fro once in a second or once in two seconds. In fig. 143 is given a record of a single pulsation, mag- nified 2°5 times and taken on a fast-moving plate, the successive dots being at intervals of a second. The period of an entire pulsation was 101 seconds, of which the down movement was accomplished in 41 seconds and the up movement in 60 seconds. The spacing of the successive dots at once gives a visual representation of the changing rate. It is noticed that the leaflet attains its maximum rate during the fourteenth second of its downward journey. The maximum rate of the down-movement is ‘g mm., the

average rate being *44 mm. per second. The maximum rate of the up movement was ‘56, and the average rate -3 mm. per second. In fig. 144 is shown a record of two successive pulsations obtained with a different specimen. The amplitude is Fic. 144.—Records of two pulsations of a different specimen of Desmodium reduced to %. Successive dots are at intervals of 2 seconds. reduced to two-thirds in the diagram and the successive dots are at intervals of 2 seconds. A detailed account is given in the accompanying table of the rates of movement exhibited by the two specimens :—

specimen taken on a slower-moving plate (fig. 145). This record shows the extreme uniformity of these pulsations. How uniform the time-relations are will appear from the tabular statement below, where the intervals of time are Fic. 145.—Series of automatic pulsations of Desmodium. Successive dots at intervals of 2 seconds. measured between the attainment of highest and lowest positions :— oe Period of down | Period of up Total period movement | movement seconds seconds seconds

These figures are typical of the movement of Desmodium leaflet under normal conditions in the summer season. In winter the rate is very much slower, the period of an entire pulse being then as long as 4 minutes. Various other factors modify either the amplitude or frequency of the pulsatory movement. One factor which induces a pronounced change is that of temperature. The detailed consideration of this and of other factors will be given at some length in the following chapters.

The pulsating activity of the detached leaflet of Desmo- dium gyrans can be maintained uniform for a long time by subjecting it to internal hydrostatic pressure. The application of a shock to a leaflet in a state of standstill induces a down movement. The phase of down movement is in general quicker. Enforced expansion by increased internal hydrostatic pressure induces move- ment of the leaflet upwards. These facts indicate that the down position of the leaflet represents a ‘ systolic’ contrac- tion, and up position a ‘ diastolic’ relaxation of the motile organ.

In a typical example of the rhythmic pulsation of Desmodium leaflet the down movement is accomplished in 41 seconds. The maximum rate of down movement is ‘9 mm. per second, the average rate being ‘44 mm. per second. The period of up movement is longer, being 60 seconds ; maximum rate of up movement is °56 mm. per second, the average rate being -3 mm. per second. Effect of internal hydrostatic pressure on the pulsation of Desmodium— —Expansive erection of leaflet under increased pressure: diminution of the extent of systolic contraction—Effect of load: diminution of period—Stannius’ ligature on heart-beat—Parallel effect of ligature on pulsation of Desmodium—Arrest of pulsation by a cut and revival by electric shock.

WE have already seen that after the application of a suitable internal hydrostatic pressure the pulsation of the detached leaflet of Desmodium becomes extremely regular. Some- times a leaflet is found which has come toa state of standstill. Internal hydrostatic pressure is often found in such cases to renew the pulsatory activity. The application of internal hydrostatic pressure enables us, moreover, to understand the significance of the up-and- down movements. Such movements may be the results of contractions and expansions, as in a pulsating heart. In the case of Desmodium we can induce expansion artificially by internal hydrostatic pressure ; the shifting of the base- line up or down will then distinguish for us the result of expansion. Of the up-or-down movement again, the one corresponding to expansion will be helped by increased pressure ; the other, on the contrary, will be opposed.

increased hydrostatic pressure it is found that the base-line is shifted downwards, which in the leaflet it must be remem- bered means erection. The contractile effect is also opposed, hence we observe the extent of contraction gradually Fic. 146.—Application of increased internal hydrostatic pressure at arrow. Displacement of base-line downwards in the record indicates expansive erection of leaflet. Enforced expansion also causes decline in the extent of systolic contraction.

decreased, with the result that a line joining the apices of the successive pulsation slopes downwards (fig. 146). We will next study the effect of load. Different weights are placed on the second arm of the lever. The effect of the weight is to pull the leaflet slightly upwards. This pull will have the effect of opposing the contractile movement. I took a series of records of the pulsation of Desmodium, first without any weight, then with increasing loads of 4,, zis, and 3 grm. (fig. 147). It will be seen that the am- plitude of pulsations is in consequence progressively diminished. Without load, the amplitude is 19 mm.; under a load of z}> grm. it has become reduced to 11 mm. ;

under ;3> grm. to 8 mm., and finally, under 4 grm. to 4 mm. When the load is increased to #4; grm. the pulsation is arrested. The pulsation is also seen to become slower with increasing load. A very interesting phenomenon, often observed in a pulsating heart, is the effect of cut or ligature. Thus by the application of what is known as ‘ Stannius’ ligature’ above Fic. 147.—Effect of increasing load of 335, roo, and = grm. Amplitude of pulsation decreased, and period increased, with increasing load.

the heart, its beat may be arrested. This arrest takes place in a relaxed condition, that is to say, at diastole. The effect, however, is liable to be modified by the condition of the heart. Sometimes there is a failure of arrest. At other times, on the application of ligature there are a few vigorous pulsations followed by arrest. Effects similar to these are also induced by cut. The various explanations hitherto offered of these peculiar effects have all been pronounced untenable for one reason or another.

Seeing the remarkable parallelism which obtains be- tween the pulsation of the cardiac tissue and that of the leaflets of Desmodium, I was curious to discover whether in the case of ‘ Stannius’ ligature,’ also, there was a similar correspondence. It was a great surprise to me, on applying a cut or a ligature to the petiolule of the leaflet, about 3 mm. below the pulsating pulvinule, to find the various peculiarities recorded of the action of the heart repeated here, with striking similarity.

The most convenient way to apply what was equivalent to the ligature, was to hold the petiolule in a very small clamp and proceed to record the normal pulsation. After doing this for some time, the clamp was suddenly tightened. Fic. 148.—Effect of ligature in inducing an arrest of pulsation of Desmodium at diastole. This induced an arrest of pulsation, either at once or after one or two vigorous beats. Similar effects were also obtained by making a cut, after suitably supporting the leaflet. These results depend somewhat on the condition of the specimen. It is generally found that the nearer it is made to the pulvinule the more effective is the cut or ligature in inducing the arrest.

In fig. 148 is seen the arrest induced by ligature: it is very remarkable that here, as in the case of the pulsating heart, the arrest by ligature took place at diastole. In fig. 149 is seen the arrest induced by a cut. While the leaflet is still in the condition of arrest it is often possible to renew the pulsation by an induction-shock. These phenomena of arrest and revival are easily observable in the record. The question next arises whether any theory can be suggested as to the cause of this arrest. A possible explana- tion might lie in regarding the cut or sudden ligature as intense forms of stimulation, whose effect is somewhat persistent rather than instantaneous. We have further to suppose that this intense stimulation is conducted by the intervening petiolule. We have seen, moreover, that the leaf of Mzmosa under prolonged stimulation, after a preliminary excitatory contractile movement, assumes a

Fic. 149.—Arrest of pulsation of Desmodium by a cut applied at moment marked by first arrow. Pulsation was revived by electric shock applied at moment marked by second arrow. relaxed position indicative of over-stimulation and fatigue. After the lapse of a requisite time, however, the leaf regains its sensitiveness. In the case of the Desmodium leaflet a similar effect might conceivably be induced by the transmission of intense excitation from the stim- ulated cut or ligatured end. This question of the effect of ligature is admittedly one of great difficulty, even in the case of the cardiac tissue. The explanation here offered, as applying to the Desmodium leaflet, may be taken as suggestive and more or less tentative. Such an idea presupposes that the intervening petiolule contains some conducting-tissue by which excitation may be transmitted. In any case, the question as to the power of the petiolule to

conduct excitation applied at a distance, and thereby induce a modification in the pulsating activity of the attached leaflet, is one of very great importance. This question will be taken up in a succeeding chapter. Increased hydrostatic pressure induces an expansive erection of the leaflet of Desmodium gyrans. The enforced expansion induces a diminution in the extent of systolic contraction. Increasing load induces a diminution of amplitude and prolongation of period of pulsation. A load of #5 grm. is enough to arrest the pulsation.

A ligature applied 3 mm. below the motile organ arrests the pulsation of Desmodium leaflet at diastole. This cor- responds to the action of Stannius’ ligature on the heart. A cut also arrests the pulsation of Desmodium. After the arrest, pulsation may sometimes be revived by the action of an electrical shock. Condition of standstill brought about by depletion of energy—Renewal of pulsation by the stimulus of light—Response to stimulus of induction-shock—Multiple response under tetanisation—Determina- tion of the latent period and the apex time—Refractory period— Effect of stimulus on leaflets in sub-tonic condition—Effect of isolation on rhythmic activity—Gradual arrest of pulsation resulting from run-down of stored energy—Effect of fresh accession of energy.

THE leaflets of Desmodium exhibit in favourable circum- stances a more or less persistent rhythmic activity. But this activity may cease under conditions which are less favourable. The arrest may be due to either of two causes, of which the first is the exhaustion of that reserve of energy without which the pulsation cannot be maintained, and the second is the loss of motility of the pulvinule brought on by age or other factors. As a parallel instance to this we have the case of Biophytum, the old leaflets of which become quite insensitive, while the plant as a whole remains sensitive.

In those cases where the arrest is due to run-down of energy, the pulsation of Desmodium leaflets may often be renewed by the application of appropriate stimulus. This may be seen in the following record on the action of the stimulus of light (fig. 150). The leaflet had been reduced quiescent leaflet was then subjected to the continued action of light from a Nernst lamp. It will be noted that by the absorption of the energy of light the leaflet regained its so-called spontaneous activity.

It should be mentioned here that the effect induced by the stimulus is, to a certain extent, modified by the tonic condition of the plant. In a sub-tonic specimen, with the leaflet in a state of standstill, the action of stimulus is to renew the pulsating activity. In a different specimen, Fic. 150.—Action of light in renewing the pulsation of Des- modium leaflet at standstill. Light was applied at the point indicated by the arrow and continued afterwards.

where the rhythmic activity is feeble, the incidence of stimulus enhances the amplitude of pulsation. If the leaflet should be in a vigorous condition, excessive stimu- lation is apt to bring on fatigue, in consequence of which the pulsations become either irregular or diminished in amplitude. These various effects are found to take place not only under the action of stimulus of light but, as we shall see, under electric stimulus also. such as that of induction-shock. This can be seen in the following record (fig. 151). The leaflet had been reduced by

Fic. 151.— Response of Desmodium leaflet in a state of standstill. Single response to single stimulus. extreme sub-tonicity to a state of standstill. A single electrical stimulus of moderate intensity is seen to give rise to a single re- sponsive pulsation. Repetition of the same stimulus gave rise once more to a second response. In another instance, direct tetanising electric shock of short duration was applied to a leaflet in astate of standstill. It is seen

(fig. 152) that this application of moderately strong stimulus gave rise to renewed pulsation which persisted for a certain length of time, even on the cessation of stimulus. Having thus found that a leaflet in a state of standstill can be made to give response to instantaneous stimulation, it is possible to determine the latent period. We have seen that the latent period of the rhythmic cardiac tissue is, Fic. 152.—Multiple responses, under moderate tetanisation, in Jeaflet originally at standstill.