Researches on Irritability of Plants/Chapter 4
CHAPTER IV
TIME-RELATIONS OF THE RESPONSIVE MOVEMENT AND STANDARDISATION OF STIMULUS
As already stated, when the pulvinus of Mimosa is subjected 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, till it comes to a stop. 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 relatively 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 10 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.
As regards the time-measurements of the responsive 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 ·1 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 a typical manner. The record here, as explained before, is reduced to two-thirds.
Latent Period.—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 ·1 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 that the average value of the latent period may be taken as about ·1 second. It 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 according 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 maximum 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
Fig. 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 second, which under pronounced fatigue was reduced to 8 mm.
(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 10 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 10 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 complete revolution, that is to say, in a minute, the projecting rays press the spring-key six times at intervals of 10 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 a succession 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
Fig. 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 as the 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
Fig. 14.—Response of Mimosa. Successive dots are at intervals of 110 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.... „recovery.... Maximum rate of contractile movement „„movement of recovery Average rate of contractile movement.. „„movement of recovery.. |
3 seconds 16 minutes 24 mm. per second ·09„„ 15 „„ ·045„„ | ||||
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 ·1 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.
A stronger stimulus, generally speaking, requires a longer period for recovery. The influence of season is also a factor to be taken into consideration. Under the physiological depression induced by winter, the responsive process is appropriately modified. The excitability of the tissue becomes depressed. An intensity of stimulus which in summer was effective, becomes in winter ineffective. To evoke response much stronger stimulus has to be employed. The latent period is prolonged and the amplitude of response reduced. And lastly, in winter there is, generally speaking, a great prolongation of the period of recovery. In summer, with vigorous specimens, recovery may be practically complete in as short a time as 8 minutes. But owing to sluggishness induced in winter, on the other hand, the recovery may be prolonged to 25 minutes or more. In a severe winter response may even be abolished altogether.
I have hitherto dealt in some detail with the responsive movement of Mimosa. In contrast with this may be cited other examples in which the excitatory reaction may be either more rapid or extremely sluggish.
Response of Biophytum.—As an instance of relatively quick reaction I give (fig. 15) a record of response of leaflet of Biophytum. The maximum fall was here attained in the course of a second after the shock. The recovery was completed in the course of only 3 minutes.
Response of Neptunia.—In marked contrast with the quick reaction of Biophytum, I may cite the very slow action
Fig. 15.—Record of response of leaflet of Biophytum. Vertical marks below record indicate intervals of ·5 minute.
of the primary leaf of Neptunia oleracea. In fig. 16 is given a record of its response under an exciting induction-shock of moderate intensity. The clock-interrupter was so adjusted that the successive dots should be at intervals of half a
Fig. 16.—Response of leaf of Neptunia. Successive dots are at intervals of ·5 minute in the contractile portion, and 1 minute in the recovery portion of curve.
minute during contraction, and at intervals of a minute during recovery. It will be seen that the maximum fall was attained 3 minutes after stimulation, and the recovery was not completed even after 40 minutes, which was the duration of this particular record. The recovery was completed after a further period of 20 minutes, that is to say, the total period of recovery was an hour. The apex time or period of contraction is shortened by the application of a stronger stimulus, but the period of recovery then becomes very much prolonged. The following tabular statement will display the range of variation in the speed of reaction of these sensitive plants:—
| Table showing Apex-times and Periods of Recovery in Different Plants | |||||||||
| Specimen | Apex time | Period of recovery | |||||||
| Biophytum sensitivum.. | 1 second | 3 minutes | |||||||
| Mimosa pudica.... | 3 seconds | 16 minutes | |||||||
| Neptunia oleracea.... | 180 seconds | 60 minutes | |||||||
Arbitrary Distinction of Sensitive and Ordinary Plants
The arbitrary distinction that is generally drawn between the so-called sensitive and ordinary plants may briefly be referred to here. In the case of Mimosa, it is generally supposed that the lower half of the pulvinus is alone sensitive; this however is not an accurate statement. By local application of stimulus it can be shown that the upper half also undergoes a feeble contraction, causing an 'up' movement of the leaf.
The localised stimulus may be applied by means of the electro-thermic stimulator. Application of stimulus of moderate intensity on the upper half of the pulvinus will be found to give rise to an erectile response. Another practical method of local application of stimulus is by means of sunlight. A narrow beam may thus be thrown on the upper half of the pulvinus; this will be found to give rise to erectile or 'up' response. Two such responses are shown in fig. 17, where the stimulus of sunlight was applied for 2 minutes followed by a period of recovery for 13 minutes.
If the stimulus applied on the upper half be strong or long continued, then the excitatory effect is transmitted across the pulvinus to the more excitable lower half. In these circumstances the 'up' is converted to 'down' response, on account of the greater contraction of the lower half of the pulvinus. Thus under any form of diffuse stimulation the resultant response in Mimosa is brought about by the differential excitabilities of the upper and the lower halves of the pulvinus. We should also bear in mind that the slight differential contraction-effect in Mimosa leaf is very much magnified by the long petiolar
Fig. 17.—'Up' response (represented by down curve) due to local stimulation of upper half of pulvinus of Mimosa.
index. There are, again, numerous pulvinar organs whose responsive movements have passed unnoticed. In Desmodium gyrans there are two conspicuous pulvini; the primary pulvinus is at the junction of the petiole with the stem; there is a secondary pulvinus at the junction of the petiole with the terminal leaflet. The primary pulvinus appears at first sight to be insensitive. But on attaching the primary petiole of Desmodium with the writing-lever, I obtained the series of responses under a very feeble electric shock, as seen in fig. 18. In this particular case the recovery is practically complete in 15 minutes. Other pulvini also exhibit differential contraction under diffuse stimulation. Thus the terminal leaflet of the bean plant (Vicia Fava) exhibits responsive down movement, though here recovery is very protracted. But we have seen that the recovery of Neptunia is also a very slow process.
The responsive movement of Mimosa is due, as has been noted, to the unequal excitabilities of the upper and lower halves of the pulvinus. The excitability of the tissue is again modified by the state of turgor. In Mimosa there is induced a periodic variation in the relative turgescence of the two halves of the pulvinus. On account of this the differential excitability, on which the motile response of
Fig. 18.—Series of responses of leaf of Desmodium gyrans, under electrical stimulation.
Mimosa depends, undergoes great variation. The sensitiveness of this plant is in consequence often found to disappear completely at certain hours of the day. I shall, moreover, show in Chapter VII that the leaf of Mimosa becomes insensitive when its pulvinus absorbs an excess of water. Thus the mechanical movement of the sensitive plants on which depended the assumption that 'ordinary' plants were insensitive, rests on a basis which is very unreliable.
Responsive movements may, on the other hand, be demonstrated in ordinary plants by the employment of a suitable contrivance. In a radial organ diffuse stimulation induces equal contractions on all sides, which balance each other. Hence lateral movement, dependent on differential contraction, cannot take place. But if we take a hollow tubular organ of some ordinary plant, say the peduncle of daffodil, it is clear that the protected inner side of the tube must be the more excitable. When this is cut in the form of a spiral strip and excited by means of an electric shock, We observe a responsive movement to take place by curling, due to the greater contraction of the inside of the strip. This mechanical response is at its maximum at that season which is optimum for the plant. When the plant is killed its response disappears.
It will be seen that the division of plants into sensitive and insensitive is without any justification. Moreover, by adopting the electric mode of investigation, I have shown that every plant and every organ of the plant is sensitive and responds to stimulus by a definite electric variation.
We have hitherto referred but vaguely to the question of the intensity of the induction-shock employed as stimulus to induce response. We have observed that on making and breaking a current in the primary coil, instantaneous currents are induced in the secondary. The intensity of the induction-current employed for giving a shock depends in the first place on the intensity of the primary current; secondly, on the suddenness with which the primary current is made or broken; and lastly, on the relative distance separating the secondary from the primary coil. The intensity of the current can be maintained uniform if we always employ the same battery, say a 4-volt accumulator or storage-cell. As the break of a current is accomplished more quickly than make, the break-shock, as we have seen, is more intense than the make-shock. The plant may, therefore, be excited by a single make-shock, or by a single break-shock or a double make-and-break shock, or by a sequence of make-and-break shocks, of definite duration, according to the particular requirements of the experiment.
The intensity of the shock moreover may, as already shown, be increased by sliding the secondary nearer and nearer to the primary coil. At a great distance the intensity of the shock is very feeble, whereas in the nearest position it is most intense. If a scale be placed to mark the relative position of the secondary to the primary, we may be assured of obtaining an identical intensity of shock whenever we place the secondary at the same point on the scale; or we can obtain an increasing intensity of stimulus by progressive movement along the scale towards the primary. There is, however, no simple relation between the distance and the intensity—that is to say, equal decrement of distance does not mean equal increment of intensity. All that we are sure of, is that the sliding in of the secondary coil secures an increasing intensity of stimulation. In order to be certain of obtaining quantitative values of intensity, the scale has to be specially calibrated.
Standardisation of Stimulus
In subjecting the plant to the secondary shock, if we begin with feeble intensity of stimulus, by placing the secondary at a great distance, and gradually increase the intensity by sliding the secondary nearer and nearer, we shall obtain that scale-reading at which the stimulus begins to be effective. This particular intensity, the feeblest that is effective, we designate the minimal stimulus. As We now proceed to increase the stimulation by pushing the secondary nearer to the primary, we find the amplitude of the response is progressively enhanced, and ultimately we reach an intensity beyond which there is no further increment of amplitude. This intensity we designate the maximal stimulus. When the plant is in an exceedingly vigorous condition, the minimal intensity is low and the range between maximal and minimal is narrow. But if the plant be in a less favourable tonic condition, then the minimal stimulus is relatively high and the range between minimal and maximal is wider.
As different induction-coils have different constants, the value of the intensity of stimulus as obtained from the scale-reading of a particular coil gives us no idea of the absolute intensity. It appeared desirable, nevertheless, in making quantitative experiments, to adopt some unit of stimulus in terms of which other intensities might be expressed. It would be well, moreover, to select this unit in some way not quite arbitrary, so that it might carry a significance more or less universal. The unit intensity of exciting shock which I have adopted for these reasons is that which barely induces in ourselves a perceptible sensation. The observer dips two fingers, one of each hand, into two troughs of saline solution, which are in series with the experimental Mimosa and the secondary coil. The plant tissue is interposed so as to ensure an identical current to pass through the experimental individual and the plant. The resistance offered by the plant tissue is very great; in the case of Mimosa under the usual mode of connection, it is about half a million ohms. At the beginning the secondary is placed at a great distance from the primary. The vibrating interrupter of the primary is next started and the secondary gradually pushed in, till at a certain scale-reading the observer, who is kept in ignorance of the position of the secondary, just begins to perceive the shock. This process is repeated several times in the case of the individual observer, and the mean of various consecutive readings, which ought not to differ from each other to any extent, is taken as the unit for that particular individual. The same observation is repeated with some ten different individuals, and the mean of these ten readings is finally adopted as that reading of the unit intensity which is to serve as the standard.
Though this reading cannot be regarded as absolute and invariable, yet, in the particular circumstances of the case, it is fairly definite and on the whole satisfactory. It gives us a general idea, moreover, of that intensity which will be effective in stimulating the plant, in terms of the minimal stimulus capable of evoking sensation in man. Having thus obtained the scale-reading corresponding to this unit, we calibrate other positions of the scale in terms of this unit. In this manner the scale is marked so as to indicate intensities of ·1, ·5, 1, 2, 3, 4, 5, and so on. The calibration is carried out by means of a ballistic galvanometer. In subsequent chapters we shall employ these practical units, which will thus have a definite significance.
Having shared the prevailing belief that the sensitiveness of the plant was very feeble compared with that of the animal, I was considerably surprised to find that the intensity of induction-shock which is barely sufficient to induce sensation in man is quite enough to cause excitatory fall in a Mimosa of moderate sensitiveness. Indeed, I found that in the case of a highly excitable specimen an intensity only one-tenth of this was sufficient to excite it. In other words, under this particular test Mimosa may prove ten times as sensitive as a human subject! Later on I shall give details of measurements which will show that, as far as electric mode of stimulation is concerned, the plant is in no way inferior to the animal in sensitiveness.
Summary
The extent of responsive fall in Mimosa increases with increasing intensity of stimulus. The rate of movement is also greater under stronger stimulus.
The rate of responsive movement becomes slower under fatigue. In a given case the normal maximum rate of movement of 50 mm. per second was reduced to 8 mm. under fatigue.
Temperature enhances the rate of movement. A rate of 10 mm. per second at a temperature of 22° C. was found enhanced to 105 mm. per second when the temperature was raised to 28° C.
In a typical case of Mimosa, in summer, the latent period was found to be one-tenth of a second. The maximum contraction was attained in 3 seconds and the recovery completed in 15 minutes. The rate of recovery was relatively rapid at the beginning and very slow towards the end. The maximum rate of recovery was ·09 mm. per second in contrast with the maximum rate of contraction of 24 mm. per second. The movement of recovery was about three hundred times slower than the movement of excitatory contraction.
A stronger stimulus, generally speaking, requires a longer period for recovery.
Under the physiological depression induced by winter the responsive reactions are modified. The latent period is prolonged and amplitude of response reduced. The period of recovery may also become protracted.
Different plants exhibit different characteristics of response. Biophytum sensitivum may be taken as a type of quickly reacting plant, while Neptunia oleracea is very sluggish in its reactions. In Biophytum the apex time is reached in a second and the recovery accomplished in 3 minutes. In Neptunia the apex time is reached in 180 seconds, and recovery completed in 60 minutes.
Mechanical response of Mimosa is due to differential contraction of the upper and lower halves of pulvinus.
Erectile responses of Mimosa may be obtained by local stimulation of the upper half of pulvinus.
Distinction of plants into sensitive and ordinary is arbitrary. Under suitable conditions, ordinary plants, so-called, may be made to exhibit motile response.
By means of electric response it may be shown that every plant, and every organ of the plant, is sensitive and responds to stimulation by a definite electric change.
The sensitiveness of Mimosa to electrical stimulus is high and may even exceed that of a human subject.