Researches on Irritability of Plants

The electrical connections with the plant are diagra- matically shown, in this and other figures, by two lines. In practice the connections have to be made by means of thread moistened in dilute saline solution. In certain experiments it is necessary to avoid complications arising from electrolytic polarisation. In these it is advisable to use non-polarisable electrodes for making the connections with the plant tissue. Such non-polarisable electrodes may be of the usual U-tube type. The shorter limb of the glass U-tube is filled with kaolin paste in normal saline. A cotton thread moistened in saline protrudes from this and makes the connections with the tissue. The longer limb is filled with zinc sulphate solution, into which dips azincrod. For ordinary purposes, however, a much simpler contrivance is found effective. A narrow cork is partially hollowed out and paraffined. The well thus formed is filled with dilute saline solution. The bottom is pierced for the entry of a cotton thread into the saline. A thick silver-wire, whose surface has been covered electrolytically with a film of chloride, pierces the side of the cork and dips into the saline solution. The silver wire forms one of the two electrodes, and the cotton thread makes the necessary electrical connection with the tissue.

For the purpose of excitation we may make the two electrical connections, one at or near the pulvinus itself, and the other on the petiole at a short distance. It will be shown later that when the electrical current leaves the tissue by the pulvinus, that point becomes the seat of excitation. Thus by making the pulvinus the point of exit of current, or kathode, we may cause direct excitation. Or we may have the pulvinus included between the two electrodes, so that the electrical current passes through it (fig. 9, a). This connection we may designate the intra-electrodal. Here, in certain circumstances, the excitation throughout the tract becomes diffuse and practically instantaneous. And lastly, the two electrical connections may be made side by side, say about I cm. apart on the petiole, at a moderate distance from the pulvinus. The excitation thus caused in the petiole reaches the pulvinus, as I have already said, by conduction. This connection we may call extra-electrodal (fig. 9, 0).

I give below a series of records (fig. 10) of response to stimulation by condenser discharge. The plant was highly excitable, and excitation was caused by the discharge of ‘I microfarad condenser charged to °3 volt. Excitation may be induced by means of a single or repeated shock from an induction coil. In my own expe- rience I was at first under the impression that this mode of stimulation was not suitable for repeated quantitative experiments, as I found that the plant was liable to become insensitive owing to the fatigue or injury caused by the shock. Later, however, I was able to trace this difficulty to the

Fic. 10o.—Record of responses to stimulation by condenser discharge. employment of an intensity of shock which was in excess of a certain critical value. I had in fact been misled by the prevailing belief that the excitability of the plant was considerably lower than that of the animal. Hence I employed an intensity of current which was unnecessarily high. This induced fatigue and consequent insensitiveness. Afterwards I discovered that in so far as its sensitiveness to electrical stimulation was concerned, Mimosa was in no way inferior to the animal. Quantitative results will be given later, in justification of this statement. Avoiding, then, an intensity of stimulus which was too great, and allowing proper resting-intervals, I found that the efficiency of induction-shock as a mode of stimulation was all that could be desired. It has also the great advantage of

allowing successive stimuli to be maintained constant, or to be increased in a known manner. The induction coil consists of a primary made of a few turns of thick wire enclosing a bundle of soft iron wire—thus forming an electro-magnet—and of a secondary consisting of a larger number of turns of thin wire. The secondary coil can be made to approach or recede from the primary, by means of a slide. When a current is suddenly started in the primary coil, by pressing a key an instantaneous make-induction-current is induced in the secondary. When, by releasing the key, this current is broken, an instantaneous break-induction-current, whose direction is opposite to that of make, is induced in the secondary. Owing to the greater suddenness with which the break is effected, the intensity of the break-shock is greater and of shorter duration than that of the make-shock. The intensity of either make- or break-shock may be in- creased by bringing the secondary nearer the primary. We can obtain successive uniform shocks, of either make or break, by maintaining the distance between the two coils constant.

We may subject the tissue to shock of either make or break at will by employing an additional short-circuiting key. When this key is down, the shock from the secondary coil is practically diverted across the better conducting- path provided by the key, so that for practical purposes none passes through the plant tissue. If it is desired to cause successive excitations by make-shock only, then the short-circuit key is raised when the primary circuit is made, and pressed down when this is broken. For exciting by break-shock, the short-circuit key is pressed when the primary is made and raised when the primary is broken (fig. 11).

In the response of animal tissue it is well known that while single induction-shocks are effective in the case of quickly reacting skeletal muscles, they induce hardly any contractile effect in the more sluggish smooth muscles. Vegetal protoplasm also is commonly regarded as little capable of excitation by these shocks, behaving in this respect like the sluggish smooth muscles amongst animal tissues. Again, while the break-induction-shock of higher intensity and shorter duration is more effective in exciting the quickly reacting skeletal muscle, in the case of the sluggish smooth muscle it is the make-shock of low inten- sity and long duration that proves more efficacious. That

Fic. 11.—Arrangement for applying single make- or break-shock ; K, key in the primary circuit. The secondary circuit may be short-circuited by the second key. the inference commonly made about the reaction of vegetal protoplasm to single induction-shocks is not of universal application, is strikingly seen in the response of pulvinus of Mimosa. Here, so far at least as single induction- shocks are concerned, its reaction appears more analogous to that of skeletal than of smooth muscle; as a position of the secondary in relation to the primary can be found in which, while a single make-shock is ineffective, a single break-shock is quite efficient. In order to render the make- shock effective, the secondary has here to be pushed in nearer to the primary, thus increasing the intensity of the shock. A pair of records will be given in a later chapter

(figs. 20, 21), showing the relative ineffectiveness of the make-shock compared with the break-shock. It will be shown later that shocks individually ineffec- tive become effective by repetition. It is thus possible to excite a plant by subjecting it to a number of relatively feeble make- and break-shocks. The advantage of this mode of stimulation is that, owing to the low intensity of these shocks, the liability of the tissue to injury is very much reduced. Such alternating tetanising shocks can be produced by means of an automatic spring-interrupter, included in the primary circuits. This interrupter consists of a steel spring carrying at its free end a soft-iron armature which faces one pole of the electro-magnet of the primary. An adjustable contact-rod touches the spring and com- pletes the primary circuit. But the completion of the circuit magnetises the electro-magnet, which, pulling the armature, breaks the contact, thereby interrupting the primarycurrent. The electro-magnet is thus demagnetised, the armature is released, and the spring returns suddenly, re-establishing the circuit.. By this automatic make-and- break we obtain alternating induction-currents in the secondary.

When we wish to subject the experimental tissue to the additive effects of these shocks, of a given short duration, ‘this may be accomplished by including in the primary circuit a metronome, which in the course of a single beat closes the circuit for an approximately definite duration. If the duration of closure be, say, one-fifth of a second, and if the frequency of the spring-interrupter be 50 times per second, the number of alternating double-shocks given to the tissue would be ten.

A method of securing still greater accuracy in the duration of the tetanising shock will be described in another chapter, where also may be seen records obtained by that mode of excitation. In subsequent chapters we shall also study in detail the various characteristics of response and its time-relations. The ideal method of stimulating a plant is one in which the intensity might be maintained uniform or varied in a definite and known manner. If the stimulus exceeds a certain critical value, the tissue is injured with concomitant diminution or abolition of excitability.

One practical method of quantitative stimulation is by electro-thermic stimulus, where the plant-tissue 1s sub- jected to a sudden and definite thermal variation. The plant may also be excited by the action of a constant current. Another method of excitation is by the discharge of a condenser. And lastly, excitation may be induced in the plant by a single induction-shock or by a series. As in the skeletal muscle of animals, so in the pulvinus of Mzmosa, the break- shock is more effective than the make-shock.

Latent period of Mimosa—Apex-time—Rate of responsive movement of leaf—Effect of intensity of stimulus, fatigue, and temperature— Periodic dot-marker—Time-relations of response and recovery— Effect of season—Response of Biophytum—Response of Neptunia— Arbitrary distinction between sensitive and ordinary plants—Differ- ential response in Mimosa—Response of ordinary plants—Universal sensitiveness of plants—Standardisation of stimulus—Maximal and minimal stimuli—Extreme sensitiveness of Mimosa.

As already stated, when the pulvinus of Mimosa is sub- jected to an instantaneous stimulus, say that caused by an electric shock, a responsive movement is initiated after the lapse of a very short interval. After the completion of the fall of the leaf, the contracted pulvinus slowly recovers its original expanded condition, with consequent re-erection of the leaf. The movement of the leaf is thus a visible indication of the responsive reaction and recovery of the pulvinus under stimulus. In this entire process, we may conveniently distinguish three separate phases :—

First, there is a brief period between the incidence of stimulus and beginning of the responsive movement: the contraction has not yet manifested itself. This lost time is called the Latent Period. Secondly, after the lapse of the latent period, the leaf begins to fall, at first with increasing rapidity, which then again diminishes, tillit comes to astop. The curve described attains its maximum amplitude, corresponding to the maximum fall of the leaf. The period required, up to this

point, we shall call the Apex Time. The pulvinus remains for a short time in its contracted condition. Third and lastly, recovery of the pulvinus from the effect of stimulus begins to take place, with consequent re-erection of the leaf. This process of recovery is very much slower than the responsive fall. While the responsive fall is a matter of a few seconds only, the re-erection or recovery requires several minutes. This recovery, again, is at first rapid and at the end rela- tively slow.

Quantitative measurements of these different phases may, as we shall see, be derived from the response-curve itself. In obtaining these, there are two elements to be measured—namely, the extent and the rate of movement. The amplitude or height of the curve gives a measure of the amount of movement. Magnification or reduction of the record results, as we have seen, from two elements of adjustment—namely, the ratio between the horizontal arm of the lever and the length of the recorder, and the ratio between the distance of thread-attachment from the pulvinus and the entire length of the leaf.

In the record given in fig. 12 the length of the vibrating recorder was Io cm. and the thread-attachment with the leaf was made with the horizontal arm of the lever at a distance of 5 cm. from the fulcrum rod. The magnification of the writing-lever was therefore 2. The total length of the responding leaf was 9 cm. But the thread-attachment to the horizontal lever was made at a point on the petiole 3 cm. from the pulvinus. The responsive movement of that particular point on the petiole was therefore reduced to one-third the movement of the tip of the leaf. Thus we have a reduction to one-third brought about by the selection of the point of attachment on the petiole, and a magnification of two, due to the writing-lever. The record obtained represents in this case the actual movement of the tip of the leaf, reduced to two-thirds.

movement, we have seen that the successive dots in the curve itself give time-intervals. When it is necessary to measure short intervals, say of ‘I second, a resonant vibrator accurately tuned to ten double vibrations per second is employed. The successive dots in the curve then represent intervals of a tenth of a second each. For the correct determination of the first two phases of the responsive movement in which the time involved is short—namely, the latent period and the apex time—it is necessary to have the recording-plate moving at a rapid rate. But for determining the time-relations during the period of recovery, which is a matter of several minutes, the recording-plate has to be moved at a relatively slow rate.

I give below records (fig. 12) which show these first two elements in atypical manner. The record here, as explained before, is reduced to two-thirds. Latent Peritod.—It will be noticed that there is a short interval between the application of stimulus, represented by the vertical line, and the initiation of response. The movement is here seen to begin before an interval of ‘I second is completed. For more accurate determination of the latent period a record must be taken on a faster- moving plate. A detailed description will be found in a later chapter, where it is shown thatthe average value of the latent period may be taken as about ‘1 second. Tt will also there be observed that though in a’ given: specimen the latent period is constant, it varies slightly in. different specimens. It is also appropriately modified accord- ing to the physiological changes induced by temperature, fatigue, and the influence of the season. |

The Apex Time.—It is seen from the upper of the two curves in fig. 12 that the responsive fall practically attains its maximum near the twentieth dot. This indicates that the value of the apex time in this case is 2 seconds. As regards the rate of this responsive fall, the spacing of the successive dots, each representing an interval of ‘1 second, shows in a striking manner how the speed first accelerates and then slows down. The maximum movement is generally attained about ‘5 second after the shock. The actual rate of the maxi- mum responsive fall is here 40 mm. per.second. The rate of the responsive fall is modified by various conditions :—

(1) The speed is greater under stronger stimulus. This is well seen in fig. 12, where the lower one was taken under stimulus intensity of 1, and the upper one under stimulus intensity of 4. The gentler slope of the lower curve, and Fic. 12.—Records giving apex-time in the response of Mimosa. Lower curve is in response to stimulus 1 and upper to stimulus 4 units. more abrupt rise of the upper, clearly show the greater speed and vigour of the responsive movement under the stronger stimulus. The curves show moreover that the amount of this responsive movement is greater under stronger stimulation.

(2) In a fatigued condition the rate of the responsive fall under constant stimulus is relatively slow. Thus in a certain experiment, where the maximum rate of fall, when fresh, was 30 mm. per second, the rate was slowed down to 20 mm. per second in consequence of fatigue. In another case the rate when fresh was 50 mm. per (3) Temperature also modifies the rate of the responsive movement. Thus in a given specimen the maximum rate of fall at the relatively low temperature of 22°C. was Io mm. per second ; it became enhanced to 105 mm. per second at 25° C., and 115 mm. per second at 31° C.

The pulvinus after the attainment of the maximum fall remains more or less persistently contracted for a short time. This is shown by the horizontal portion of the curve in fig. 12. The Period of Recovery.—As this takes a relatively long time, its record has to be made on a slowly moving plate. The unit of time-measurement must therefore be relatively long. If the successive dots were to be made at the ordinary rate of 10 per second, they would become fused and continuous in the record. For this reason I have devised a contrivance by which the successive dots, in the recovery-portion of the curve, are placed at such intervals as to prevent overcrowding. A convenient interval is either 5 or I0 seconds.

The device for producing periodic dots at intervals, say of 10 seconds, consists of clockwork employed to interrupt the current actuating the vibrating recorder at particular intervals. A light six-rayed wheel is attached to the axis of the seconds-hand, and during the course of a single com- plete revolution, that is to say, in a minute, the projecting rays press the spring-key six times at intervals of Io seconds each (fig. 13). It is only during the short interval when the key is pressed that the circuit is completed and the recorder set in vibration to make its dots. Each dot made, it should be remembered, is the result of asuccession of strokes inscribed by the vibrating recorder 10 times in the second. But as the movement of the plate is slow, these successive strokes more or less superimposed make but a single large dot. Ten seconds again elapse before the next pressure of the key brings about another large dot, and in that

interval the plate has moved a certain distance. There is a second key used for short-circuiting by which the clock interrupter can be put out of action. When this is done, we obtain the usual series of dots at intervals of ‘1 second. It will be understood that if the whole response-record were to be made by the vibrator, as actuated periodically at intervals of 10 seconds, we should have a very long gap in the. Fic. 13.—Clockwork for the dot-marker; the six-rayed wheel periodically completes electric circuit.

record of the contraction-portion, since the apex is reached in 2 seconds. To obtain then a more or less continuous inscription of the entire curve, the vibrator should at first be allowed to make its normal dots ‘1 second apart. This is done by taking care to commence the experiment with the clock-interrupter short-circuited. As soon asthe apex point is reached the short-circuit is removed, and the succeeding record, during the recovery of the leaf, consists of dots

at intervals of 10 seconds. Fig. 14 gives us a record of a response reduced to two-thirds taken in this way. It will be seen that the recovery practically takes place in 16 minutes. Thus in the present case, while the pulvinus took only 3 seconds to complete the contraction, it required Fic. 14.—Response of Mimosa. Successive dots are at intervals of ;4; second in the contractile portion, and 10 seconds in the recovery portion of curve. Vertical marks below indicate intervals of 1 minute.

16 minutes to recover from it. It will be seen, further, that the rate of recovery is quicker at the beginning and slower at the end. The following is a tabular statement of the time-relations of the different phases of response and recovery :— Period of contraction Bi .. | 3 seconds ms recovery . | 16 minutes Maximum rate of contractile movement | 24 mm. per second ‘ movement of recovery | .09 ma | Average rate of contractilemovement .. 15 en i a » movement of recovery ..| .045 _ ,, Fe |

We may now briefly recapitulate the sequence of events in a typical specimen of Mimosa subjected, during the summer season, to a moderate stimulus. Response does not commence immediately ; there is a latent period of ‘r second. The responsive movement then begins and proceeds for a time with increasing speed, the maximum contraction being attained about 3 seconds after the shock. The pulvinus remains in the contracted position for a short period. After this the recovery is initiated. The rate of recovery at the beginning is relatively rapid, and very slow towards the end. The maximum rate of recovery is ‘09 mm. per second, in contrast with the maximum rate of contraction, which is 24 mm. per second. The movement of recovery is thus about three hundred times slower than the movement of contraction. The recovery is completed in about 16 minutes. ;