Researches on Irritability of Plants/Chapter 9
CHAPTER IX
DETERMINATION OF THE LATENT PERIOD
When the motile pulvinus of Mimosa is subjected to an exciting shock, a short time elapses between the incidence of this shock and the initiation of the responsive movement. This short interval is known as the Latent Period. In a responding muscle, similarly, contraction does not occur instantaneously on the application of stimulus. The latent period in this case is determined from the record of the muscle-twitch. When after the application of stimulus the muscle has not yet begun to contract, the record appears as a straight line. Then on the commencement of contraction, the recording-lever is jerked up and the curve likewise bends upwards. The length of the straight portion of the record, between a mark that represents the incidence of the shock and the flexure at the initiation of response, gives us the duration of the latent period. We have, however, to determine the time-value of this length. This is done by means of a sinuous curve drawn below the record by a tuning-fork, vibrating 100 or 200 times in a second. Experimenting in this manner, the latent period for frog's muscle has been determined at about .01 second.
There are still several difficulties to be encountered in making this determination with any great exactness. As the muscle-record and the time-record are separate, certain error is likely to be introduced in inferring the time-value of any point on the muscle-curve (fig. 64). This error becomes relatively serious when the total time to be measured is very small. There is, again, the difficulty of exactly determining the point of flexure which represents the beginning of mechanical response. More troublesome still is the error due to the inertia of the recording-lever. On account of this and the mechanical inertia of the responding muscle itself, the latent period thus obtained appears somewhat in excess of the true value.
In the apparatus which I employed, these difficulties have been reduced to a minimum. In the first place, the curve of response or phytogram is at the same time a chronogram. The error which might arise from an inference based on a neighbouring time-record is thus eliminated. I will later explain also the means that make it possible to determine the point of flexure, representing the beginning of the responsive movement, with relative accuracy. And lastly, the error due to the inertia of the recording part of the apparatus is reduced to a minimum by making the writing-lever excessively light. In the muscle recorders the weight of the recording-lever is about 3.5 grams. The lever which I employ weighs only .04 gram. The recording part of my apparatus is thus nearly a hundred times lighter than that used for muscle records.
The accuracy of the time-record when made by the response recorder itself may be gauged from records giving simultaneous tracings of the exciting standard tuning-fork (100 D.V.) and the resonant vibration in the recorder induced by it. This latter had been previously tuned to give exactly 100 double vibrations in a second. A light aluminium stylus attached to the tuning-fork traced a sinuous line on a falling plate of smoked glass. The top of the vibrating recorder was so adjusted as to make successive dots during its vibration, simultaneously with the tuning-fork tracings. It will be seen from the record (fig. 65) that, corresponding to the crest of each tuning-fork wave and slightly to its right, we have a dot. The record given represents a period of fourteen one-hundredths of a second, there being fourteen crests made by the tuning-fork
Fig. 65.—Simultaneous record of vibrating-recorder and 100 D.V. tuning-fork exciter.
time-marker, and exactly coincident with these are the fourteen dots made by the vibrating recorder. The interval between any two dots, therefore, is an accurate measurement of one-hundredth part of a second. If the plate be moving at a uniform rate, the interval between these dots will be uniform. But the accuracy of the time-measurements in the curve is independent of the rate of movement of the plate, fur we calculate not by the distance but by the number of the dots. In the present figure the record was made on a plate which had been released and during its fall was acquiring increasing speed. The tuning-fork waves are thus gradually broadening out, and in exact correspondence to this the intervals between the dots are lengthening. When the phonograph motor which lets down the plate is just released, there is a short interval during which both that and the dependent plate are acquiring increasing velocity. After this the velocity becomes uniform. If this uniformity should be required throughout the record, the tracing of response may be taken during this later period only.
The mode of procedure, therefore, is first to make the recording-writer vibrate at its own definite frequency of, say, 100 times per second. The recording-plate is then released and later, when its motion has become uniform, we pass through the pulvinus an electrical stimulus of an instantaneous break-shock. There should be a mark made on the recording-plate corresponding exactly to the moment of stimulation. The horizontal record, consisting of a series of dotted points representing one-hundredth of a second, is suddenly deflected upwards on the initiation of the responsive fall of the leaf. The number of dots intervening between the mark of stimulation and this point gives us the value of the latent period for the specimen.
Stimulation cannot be effected by hand at any exact predetermined point on the record. This must be done automatically by the moving plate itself. We cannot again give an instantaneous break-shock without previously completing the primary of the Ruhmkorff coil, which causes a disturbing make-shock.
In order to avoid this the secondary electrodes, during make, should be short-circuited by means of a thick conducting-wire; the secondary shock is thus practically diverted from the plant through the path of least resistance, which is the conducting wire. All these requirements are provided for in practice by the special mechanical devices of the apparatus.
Device for Automatic Stimulation
The essential parts of the automatic arrangement by which a break-shock is given, at a predetermined point on the recording-plate, are shown in fig. 66.
The recording-plate is allowed to drop by pressing the handle K. The winding disc is attached to the revolving axis of a phonograph motor. The disc is wound in a right-handed direction, which at the same time winds the spring
of the phonograph motor. The circumference of the disc is the same as the length of the recording-plate. One complete turn pulls the recording-plate up to its highest position. A projecting catch below the disc is caught by a pin attached to the spring-handle K, when a complete turn has been made: the recording-plate is thus held arrested at its highest position. When desired, a pressure on the handle K releases the disc, the axis of the motor begins to unwind, and the plate is allowed to fall. The motor is fully wound at the beginning, and the partial unwinding during one revolution is exactly compensated before the next observation by the winding necessary to pull up the plate. Owing to the constancy of this winding, the rate of fall in successive experiments is kept the same. The pressure of the handle K, which releases the plate, also causes 'make' of the current in the primary coil. This circuit of the primary coil is completed in addition through a contact-breaking device.
This consists of a long strip of ebonite, fixed along one edge of the recording-plate carrier. On the lower end of the ebonite a conducting-strip of platinum is sunk in and provided with a binding-screw. In front of this slides a rod with contact-point tipped with platinum. This can be adjusted up or down by means of a fine micrometer-screw, A. When the recording plate is released, carrying with it the conducting-strip, the primary circuit is broken as soon as the line of junction between platinum and ebonite is reached. This sudden interruption of the primary current gives rise in the secondary coil to an instantaneous break-shock, which passes through the plant. In order that shocks in successive experiments shall always be given at the same definite predetermined position in the fall of the plate, the following device is adopted: The recording-plate, as we have seen in a previous chapter, slides up and down a vertical support of triangular section. A movable peg fixed in the support holds it temporarily at a certain selected point chosen as that at which, during the descent of the plate, the shock is to be given automatically to the plant. For the purpose of adjustment a galvanometer is interposed in the primary circuit. So long as the point of contact-rod is in touch with the conducting-strip, so long there will be a deflection in the galvanometer. By means of its screw-adjustment, the rod is gradually raised till the line of junction between platinum and ebonite is exactly reached; the deflection in the galvanometer will now cease suddenly. In this way the point of interruption or 'break' is determined with precision. By pulling the thread in connection with one arm of the recording-lever, we then trace a slightly curved line on the smoked plate. This indicates the exact position in succeeding records of the moment of application of stimulus. This mark of stimulation is shown in the printed records as a vertical line. After making this mark on the plate, the peg is removed. It is easy to see that in successive experiments stimulation will occur at that definite moment which corresponds to this marked line of stimulation.
K′ represents a key-device by which the make-shock is prevented from exciting the plant. One end of a lever carries a bent metal-rod of U-shape, which is partly immersed in cups of mercury by means of a spring. During the depressed position of this key, the secondary coil is short-circuited. When the handle K is slightly pressed, there is a 'make' of the primary current. But the make-shock is short-circuited as K′ is still in the depressed position. Further pressure of the handle K lifts K′ up, removing the short-circuit of the secondary coil. When the break-shock is given by the contact-breaker of the falling plate, there is no short-circuit to divert the shock, which now passes through the plant and excites it.
The sequence of these events, then, is as follows:—
By turning the disc D the recording-plate is lifted and held arrested in the up-position. The pressure of the handle K releases the plate-carrier, which then begins to fall. At the same time, the primary circuit is completed and a make-shock is induced in the secondary. But this make-shock is diverted by the short-circuit key K′, which is still in the depressed position. Further pressure of the handle K removes the short-circuit by lifting the ends of K′. All this takes place during the continuance of one pressure of the handle. During the descent of the plate, stimulation due to instantaneous break-shock takes place at the definite moment corresponding to the stimulation mark.
Electric connections are appropriately made with the plant by means of threads moistened in dilute saline solution. One electrode of the secondary coil is thus connected with the stem of the specimen: the moistened thread in connection with the other electrode is lightly wound round the pulvinus. It is sometimes preferable, for reasons previously explained, to make this contact with
Fig. 67—Two successive records, exhibiting identity of latent period. Recorder 100 D.V. per second.
glycerin. The connections are so made that the current of the break-shock enters by the stem and leaves by the pulvinus, the latter being thus the kathode. We shall understand later the reason of this, in as much as the kathode is the point of excitation.
I now describe the record of an experiment (fig. 67) carried out in summer for the purpose of determining the latent period in a specimen of Mimosa. Stimulus was applied at the point marked by the vertical line, and the upper of the two records was the first taken. The vibrating-recorder employed had been tuned to exactly 100 vibrations per second; successive dots therefore represent intervals of .01 second. It will be seen that the responsive movement begins to occur between the tenth and eleventh dots, and very near the latter. There are thus 10.9 spaces, each of the value of .01 second, and the latent period is therefore .109 second. In order to test to what extent successive experiments might give concordant results, I took a second record with the same specimen, which appears in fig. 67 as the lower of the two, having given the plant an interval of rest of 20 minutes after the taking of the first record. It will be seen that the second record is essentially a replica of the first, thus demonstrating that with proper precautions
Fig. 68—Record of highly excitable specimen, taken with 100 D.V. recorder on a slowly moving plate.
successive experiments on the value of the latent period will give results which are of extraordinary constancy.
By making the travel of the recording-plate very rapid, the successive dots become more widely spaced and the minute time-intervals involved are made more conspicuous. But this has the disadvantage of rendering the flexure of the curve representing the responsive movement less abrupt, making the exact point of initiation of response somewhat more difficult to discriminate. Going, on the other hand, to the opposite extreme of making the travel of the recording-plate slow, the flexure of the curve becomes more abrupt, enabling us the better to detect the point of initiation of the responsive movement. The time-dots, however, are now closer together. This can be seen in another record (fig. 68) obtained with a vigorous specimen. Here the number of spaces before the initiation of response is eight, the latent period being therefore .08 second. The closeness of the time-dots is not any great difficulty here, as with the help of a magnifying glass it is quite easy to make the necessary observation.
For the determination of the latent period in plants this accuracy of an order higher than hundredths of a second is more than ample. But such a limit is easily exceeded. As an example of this, I give a record (fig. 69) made with a different recorder, whose frequency was an octave higher than the last—namely, 200 double vibrations
Fig. 69.—Record of L of Mimosa with a 200 D.V. recorder.
each second. The successive dots are therefore in this case part of a second apart. It has been said already that by slowing the travel of the recording plate the abruptness of the flexure of the curve would be increased, the spaces between the dots being at the same time shortened. But we may obtain wider spacings without losing this sharpness of flexure, by making a magnified photographic reproduction of the curve, as shown in the next figure (fig. 70), which is a reproduction of the first part of the record in fig. 69 enlarged about three times by photographic means. In this way it is not difficult to measure, say, one-fifth of the distance between two successive dots, themselves representing an interval of 1200 part of a second. In other words, the calculation can be carried into thousandths of a second. In the present case there are 15.2 spaces between stimulus and initiation of response. The latent period of the specimen is therefore .076 of a second.
I have been able, moreover, to construct a vibrating-recorder whose frequency is 500 times per second, a fact which enables an easy determination of time-intervals of less than a thousandth of a second to be made. These recorders, owing to their excessive lightness, possess the additional advantage of having a very small moment of inertia. It is obvious, therefore, that the employment of such recorders not only bears favourable comparison with those at present used in animal physiology, but would also
Fig. 70.—The previous record magnified.
have the advantage of reducing the error due to inertia to the lowest possible minimum, and of making the record itself its own chronogram.
It has been said that owing to the extreme lightness of the vibrating-recorder, the slight error usually due to instrumental inertia is here negligible. To what extent this is true may be judged by taking records from the same leaf with two separate recorders of different sizes and comparing the results. If the factors of inertia were prominent, then two such determinations of an identical latent period would give results varying somewhat from each other. I therefore took two different records from the same specimen, using the same stimulus but varying the mode of record—that is to say, the vibrator used in one case had been tuned to 50 vibrations per second, the length of the recorder being 12 cm. The speed of the recording-plate was in this case relatively slow. The result is shown in fig. 71. The other record in fig. 72 was taken immediately afterwards from the same specimen with a vibrator tuned to 100 double vibrations per second, the recording-plate
moving at a faster rate. It will be seen from fig. 71 that the time-interval in the first case is represented by 8.5 spaces, each representing .02 second, therefore proving the latent period L to be .17 second. This, it should be mentioned, was an autumn specimen, in which the latent period is somewhat longer than in summer. In the second record, fig. 72, under its different speed and with the vibrator giving 100 vibrations per second—we find the intervening spaces to be 17. This gives the latent period as again .17 second. The identity of these values shows that the inertia of the recorder has but little effect on the results obtained.
The latent period in any given specimen of the pulvinus of Mimosa is, as we have seen, under uniform conditions extremely constant. It differs, however, in different specimens and from season to season. A thin specimen has in general a shorter latent period than one which is stouter. Perhaps this fact is illustrated, with a certain exaggeration, in the case of Neptunia, the leaf and pulvinus of which are comparatively thick. In any case, we have already seen
Fig. 73.—Record of latent period of Neptunia with 10 D.V. recorder.
that in its responsive movements, relatively to Mimosa pudica, it is very sluggish. In order to determine the latent period I employed a slow vibrator, that is to say, one which vibrates with a frequency of 10 per second. It will be seen by reference to fig. 73 that the responsive movement began after the sixth dot, the latent period being thus .6 second, or six times the value of the average latent period in Mimosa.
With Mimosa pudica I have carried out more than a hundred different determinations, and give below a tabular statement of seventy of those values which occurred most frequently amongst these. Specimens giving a latent period shorter than .08 second or longer than .12 second in summer may be regarded as rather exceptional.
Tabular Statement of Values of Latent Period L in Numbers of Different Specimens of Mimosa.
| Number of specimens | Value of L | ||
| 10 | .. | .. | .08″ |
| 9 | .. | .. | .09″ |
| 26 | .. | .. | .10″ |
| 10 | .. | .. | .11″ |
| 15 | .. | .. | .12″ |
The shortest latent period that I have come across is .06 second obtained in summer and when the temperature was specially high. The longest in summer was .14 second. Most specimens have a latent period not appreciably differing from .1 second. This may be regarded as approximately the average value for summer. In winter and with sluggish specimens the latent period may be prolonged to a value of something like twice as much, that is to say, .2 second, more or less.
Summary
Latent period of Mimosa may be determined with great accuracy by means of Resonant Recorder. This enables the measurement of time-interval shorter than .005 second.
Error due to inertia is reduced to a minimum on account of extreme lightness of plant-recorder, which is nearly a hundred times lighter than muscle-recorder.
Successive values of latent period with the same specimen are found to be constant. The results are not modified by employment of different recorders.
The shortest value of latent period given by a vigorous Mimosa leaf in summer is .06 second, the average value being .1 second.
The latent period of leaf of Neptunia oleracea is .6 second.