Researches on Irritability of Plants/Chapter 5
CHAPTER V
THE ADDITIVE EFFECT; INFLUENCE OF LOAD, TEMPERATURE, AND INTENSITY OF STIMULUS
For obtaining the record I employed a writer which had a vibration-frequency of 20 times per second. The make and break of the primary current was effected by a metronome. In the primary circuit an electrical signal (fig. 19) was also included, which marked at the base of the figure the moments when the current was made and broken. When the current is made, an up-line is described by the writer attached to the signal. So long as the current is flowing, the writer remains in the up-position and draws a horizontal line (fig. 20). At the time of make it will be noticed that, owing to inertia, the writer was momentarily jerked somewhat above the level of this up-position. This jerked line, therefore, always marks the moment of make, and the horizontal line at the higher level the continuation of the current. When the current is broken, the writer falls suddenly to its original level. Thus a jerked up-line indicates the moment of the application of the make-shock,
Figs. 20, 21.—Records showing greater efficiency of break-shock; frequency of vibrating recorder is 20. Signal below shows by up movement 'make' and by down movement 'break.'
and the down-line the application of the break~shock. In the two accompanying figures are given records of the effects of make- and break-shocks.
Greater Effectiveness of Break-shock
In the record (fig. 20) the secondary coil was placed at the reading of ·75 unit. It will be noticed that at 'make' there was no response. But there was response at 'break,' which took place ·1 second later, the delay being due to the latent period. In the next experiment, with the same plant, the coil was pushed into the reading of 1. It will be seen (fig. 21) that excitation was here effective at 'make,' a similar delay of ·1 second being again due, as in the previous case, to the latent period. Thus we see that while the stimulus of the feebler intensity of ·75 was effective at 'break,' it took the stronger stimulus of 1 to induce response at 'make.'
Additive Effect of Stimulus
In the responsive tissue of the animal a single stimulus, by itself ineffective, is found to become effective on repetition. In order to test whether this holds good in the case of the plant also, I carried out the experiments which I
Fig. 22.—Stimulus of intensity ·5 became effective on being repeated four times.
shall now describe. With a given specimen I found that a single make-and-break shock of intensity ·75 was ineffective in inducing excitation. I then adjusted the secondary for intensity of ·5, and made a reed-interrupter interposed in the primary circuit give a series of make-and-break shocks till the leaf responded by a fall. The interrupting reed was adjusted to vibrate five times per second and the number of interruptions is recorded below in the usual manner. It will be seen in the record given in fig. 22 that the make-and-break stimulus, which singly was ineffective, here became effective on being repeated four times.
Desiring next to observe the effect of still further reducing the intensity of stimulus with the same specimen, I adjusted the secondary for an intensity of ·1. It must be remembered that this is the intensity of tetanisation, which is only one-tenth of what is perceptible to the human subject. Looking at fig. 23 it will be seen that even this very feeble stimulus became effective on being repeated 20 times.
In carrying out this experiment I had expected in a general way that a feeble stimulus, to be effective, must be repeated a greater number of times. But I was not prepared for so strictly quantitative a result as came out in these two records. If the summated effect is to prove strictly additive, then effective excitation must be equal to the individual intensity multiplied by the number of repetitions. From the record in fig. 22 the effective excitation
Fig. 23.—Stimulus of intensity ·1 became effective on being repeated twenty times.
was seen to be ·5 X 4 = 2. From the second record with the same specimen, in fig. 23, it is seen to be ·1 X 20 = 2. In other words, for effective excitation the number of additive stimuli varies inversely as the intensity of each. That this is true, within certain limits, is borne out by another set of results obtained from a different specimen, which was found to be somewhat more excitable than the former.
In order to vary the condition of the experiment I adjusted the reed-interrupter to vibrate twice in a second. There was thus an addition here of the effects of single make-and-break shocks at intervals of half a second, instead of one-fifth of a second as in the last case. In fig. 24 is seen the record of the additive effect, the intensity of stimulus being ·5. We find here that the stimulus became effective on being repeated twice.
The experiment was again repeated with the same specimen, but using the reduced stimulus-intensity of ·2. The result given in fig. 25 shows that the stimulus had to be repeated five times to become effective. We see once more in this experiment that the additive effect is strictly quantitative, and that the effective stimulation is constant under varying intensity of stimulus, being equal to the individual intensity multiplied by the number of repetition. In the
Fig. 24.—Stimulus of intensity ·5 became effective after two repetitions.
Fig. 25.—Stimulus of intensity ·2 became effective after five repetitions.
two cases here given we have a strict reaffirmation of this quantitative relation—namely, ·5 X 2 = ·2 X 5 = constant.
Influence of Load
In the response of muscle it is found that the muscle-curve is modified by the effect of the load which it has to raise during contraction. With an increasing load the height of response undergoes a progressive diminution, but the period of recovery is at the same time correspondingly shortened. In the contractile response of Mimosa a similar phenomenon is observed. In carrying out this experiment a load was placed on the arm of the horizontal lever opposite to that of the leaf-attachment, and at an equal distance from the fulcrum. The leaf, during its contractile movement, has to lift this weight. In the first experiment of the series a load of 100 milligrammes was employed. In the second this was increased to 500 mgrms., and in the last it was made 2000 mgrms. Successive records were made, for purposes of comparison, on the same part of the plate. The vertical lines under the diagram (fig. 26) are time-marks, indicating intervals of one minute. It will be seen that the height of the record, with a load of 100 mgrms. is the greatest of the three, being 41 mm. The recovery was completed in this case after the expiration
Fig 26.—Effect of load; the three records show responses under varying loads of 100, 500, and 2000 mgrms.
of 9 minutes. The height of the second, under a load of 500 mgrms. is 28 mm., but recovery is nearly complete in 6.5 minutes. And, finally, with the load of 2000 mgrms. the height is the least, being 13 mm., but the recovery is seen to be completed in 5 minutes. Thus the effect of load in the contractile response of the plant is shown to be strictly parallel with its influence on the contractile response of animal muscle.
Work performed by the Plant
The motile tissue of the plant, like that of the animal, is capable of doing work during excitatory movement. The influence of load on the height and period of response has been shown to be similar in the two cases. We may next study the effect of load on the work performed. Work is measured by the product of the weight raised and the height of the lift. In the response of muscle, if the increasing loads are represented by W₁, W₂, W₃, and the corresponding heights of response by h₁, h₂, h₃, then it is found that up to a certain limit W₁h₁ < W₂h₂ < W₃h₃; in other words, the work performed is increased under enhanced load and increasing tension.
Turning to the effect of increasing load on the response of Mimosa, we find that with a load of 100 mgrms. the height of response is 41; the value of W₁h₁ is therefore 4100; under a load of 500 mgrms., W₂h₂ = 500 X 28 = 14,000; and, lastly, with a load of 2000 mgrms., W₃h₃ = 2000 X 13 = 26,000. It is thus seen that as in the contractile response of animal, so also in that of the plant, greater amount of work is performed under increased load and higher tension.
We may now try to obtain some idea of the absolute amount of work performed and the rate of work. We shall take the case where the plant had to lift a weight of 2000 mgrms. The following data are available from fig. 26. The weight is seen to be lifted through 12 mm. in the course of ten successive dots, each representing ·1 second. The magnification of the lever was three times; the absolute lift is therefore 4 mm. The load to be lifted was 2000 mgrms.; but the weight of the leaf was 130 mgrms. and this helped the fall. The actual work performed is therefore (2000 — 130) X 4 millimetre milligrams. This was accomplished in the course of a second. Hence the absolute rate of work was 7480 mm. mgrms. per second.
Effect of Temperature
Our next inquiry is into the effect of temperature on the response of the plant. For this we have to subject the plant to different temperatures—some low, some high—and to find means of maintaining it constant at any definite temperature required. For this purpose a plant-chamber, enclosing the plant and fulfilling these conditions, had to be devised. It will be noticed that these investigations involve two opposite sets of requirements—namely, in the one case a definite lowering of the temperature of the chamber below, and in the other a definite raising of it above, the temperature of the environment.
The plant-chamber consists of a base-board with a rectangular cover. This cover is made of a light wooden framework, the sides being closed with sheets of mica. The advantage of mica is its lightness, unbreakableness, non-conductivity, and transparency. Transparency is necessary because in darkness the sensitiveness of a plant undergoes variation. The base-board consists of two halves, with a small circular opening in the middle. When these two halves of the base-board are slipped over the top of the flower-pot they form one piece, fixed together by means of suitable clasps. The base-board rests on the flower-pot and the main stem of the plant passes through the circular opening. The base-board thus forms the floor of the thermal chamber. There are grooves cut in the base-board for the reception of the wooden framework. The plant is thus enclosed except on the top. After making the necessary thread-connections of the lever with the responding leaf, the top is closed by means of two sliding-pieces of mica, with slits for the passage of the thread. There are two side-tubes, one near the top and the other near the base, for the passage in and out of a stream of cold air, when the temperature of the chamber is to be reduced. When the temperature is to be raised, an electrical heating arrangement is employed.
The requirements of cooling are, first, a weighted air-bag, provided with a stop-cock; and second, a coiled copper-pipe placed in an ice-box. By means of indiarubber tubing, connections are made, first, between the stop-cock of the air-bag and one end of the copper pipe; and, second, between the other end of the copper pipe and the upper tube of the thermal chamber. Thus by more or less opening the stop-cock of the air-bag a stream of cooled air is made to circulate through the plant-chamber, at varying rates. A steady low temperature may thus be attained by adjusting the inflow of cooled air, the degree of cooling being dependent on the rate of flow. A thermometer placed inside the chamber indicates the temperature attained.
The temperature of the room, at the time of the experiment I am about to describe, was 27° C. I desired to take three records, differing from each other by intervals of 5° C. For this purpose I reduced the temperature of the plant-chamber to 22° C. and took the first record of the series. Next, by stopping the inflow of the cooled air and opening one of the side windows, I restored the temperature of the chamber to 27° C., and after allowing a suitable interval took the second record. Lastly, by means of the electrical heating device described, the temperature of the chamber was raised to 32° C. and the third record of the series taken. It should be mentioned that in all these cases the stimulus employed was of constant intensity—namely, 2.
In fig. 27 are shown the responsive effects of an identical stimulus at these three different temperatures. At 22° C. it is seen that the height of response is small and the recovery extremely prolonged. At 27° C. we find the amplitude of response enhanced and the rate of recovery increased. At
Fig. 28.—Response taken at three different temperatures, the lowest below and the highest above, on a faster-moving plate; amplitude of response larger and steepness of curve greater, at higher temperature.
32° C. the height of response is still more enhanced and the rate:of recovery, as seen in the steepness curve, still further increased. In fig. 28 is given another set of records taken on a faster-moving plate, exhibiting the effect of temperature on the amplitude of response. It will be shown in a succeeding chapter that the latent period also is affected, being progressively decreased with rising temperature.
Influence of Stimulus-intensity on the Response
It is usually supposed that in Mimosa every effective stimulus causes the maximum response. That this is not the case comes out very clearly in careful records taken with gradually increasing stimuli. We have already seen, in fig. 12, the marked heightening of the response under an increased intensity of stimulus. In muscle, in the narrow range between minimal and maximal stimulation, there is increasing amplitude of response with increasing stimuli. But this soon attains a limit beyond which there is no further increase of responsive contraction, whatever be the stimulus-intensity employed.
In order to demonstrate a similar progressive increase in the response of Mimosa, I first determined the minimal
Fig. 29.—Increasing response under increasing intensity of stimulation.
stimulus that was barely effective in inducing a feeble response. Starting from the particular position of the secondary which gave this minimal intensity of stimulus, I very gradually increased the intensity, by moving the secondary only 5 mm. at a time nearer to the primary. At each step I took a corresponding record.
In fig. 29 a series of seven such records is shown, the successive responses being taken, as previously mentioned, under slightly increasing stimuli and at intervals of 15 minutes. It will be seen how the height of response is progressively increased. This increase is at first marked, but towards the end we note that a limit is being approached, the difference between numbers 6 and 7 of the series being very slight. After the seventh, it was found that the responses did not undergo any further increase.
The range within which the increasing effect is seen is relatively extended in the case of plants in a somewhat sub-tonic condition. But when the specimen is highly excitable the range of variation is proportionately restricted.
Summary
The break-shock is more effective in inducing excitation than the make-shock.
Stimulus, singly ineffective, becomes effective on repetition. The effective stimulation is equal to the individual intensity of stimulus multiplied by the number of repetitions.
The effect of load on the response of Mimosa is similar to that on the contractile response of muscle. With increasing load the height of response undergoes a progressive diminution With shortening of period of recovery.
Within limits, the amount of work performed by a muscle increases with the load. The same is true of work performed by the pulvinus of Mimosa.
In a given case the rate of work performed by the pulvinus of Mimosa was 7480 mm. mgrms. per second.
The effect of rising temperature on response is to enhance the amplitude and to shorten the period of recovery.
In Mimosa, increasing intensity of stimulus induces increasing amplitude of response. This, however, soon reaches a limit.