Researches on Irritability of Plants

Fic. 24.—Stimulus of*intensity ‘5 Fic. 25.—Stimulus of intensity ‘2 became effective after two became effective after five repetitions. repetitions, two cases here given we have a strict reaffirmation of this quantitative relation—namely, *5 x 2 =°2 X 5 = constant. 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 correspond- ingly 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. Inthe 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 Fic. 26.—Effect of load ; the three records show responses under varying loads of 100, 500, and 2000 mgrms.

of 9g 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. 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 increas- ing loads are represented by W,, W,, W;, and the correspond- ing heights of response by h,, h,, h,, then it is found that up to a certain limit W,h, < W.h, < W.,h,; 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 W,h, is therefore 4100 ; under a load of 500 mgrms., Wh, = 500 X 28 = 14,000 ; and, lastly, with a load of 2000 mgrms., W,h, = 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 ‘I second. The mag- nification of the lever was three times ; the absolute lift is therefore 4mm. 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.

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 onthe top. After making the neces- sary 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. In order to raise the temperature of the plant-chamber an electrical device is employed. Inside the rectangular frame there is a coil of wire of German silver, the ends of the wire being led outside to two binding- screws. An electrical current from an outside battery is led through this wire, a variable resistance being also interposed in the circuit. The heat generated inside the chamber can be increased or de- creased by changing the intensity of the current; this is accom- plished by varying the adjustable resistance. In this manner it is quite easy to raise the temperature inside the plant-chamber to any degree that is desired, and to maintain it constant as long as

of response seen to be _— at the time of the experiment I am ing 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 Fic. 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 tempera- ture 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. 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

Fic. 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 ap- proached, 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. The break-shock is more effective in inducing excita- tion than the make-shock. Stimulus, singly ineffective, becomes effective on repeti- tion. 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 progres- sive 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 pul- vinus 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. Necessity of uniform stimulation—The Periodic Starter—The Auto- matic Exciter—Electrolytic contact-maker—The complete Response- recorder—The factor of tonicity—Uniform responses—Fatigue under shortened period of rest—Growing fatigue—Alternating fatigue— Staircase response—Explanation of erection of leaf under continuous stimulation—Fatigue-relaxation in plant and animal—Response under single stimulus and under tetanisation.

Some of the effects brought about by varying external conditions on the excitability of the plant have now been noted. Certain other variations may, however, be induced in the excitability, in consequence of the after-effect of the stimulus itself, even when the external conditions are main- tained constant. We may trace these induced internal changes in the modification of the response-records. It is clear that we can only be assured of the occurrence of such internal changes from the observed variation of response-record if we have been able in the first place to keep the plant under unvarying external conditions. This, taking certain special precautions as regards light, tempera- ture, and so forth, presents no difficulty. But, in the second place, we have to be specially careful that the testing- stimulus itself shall be absolutely constant in successive experiments. The problem then resolves itself into the successful devising of some arrangement by which records may be taken automatically at definite pre-determined intervals of time. The stimulus of unvarying intensity must also be made to act automatically upon the specimen.

in the record will be due to changes of excitability induced as an after-effect of the stimulus itself. Maintaining the stimulus-intensity absolutely constant is not so easy to secure with a single make- or break-shock, since the intensity of such a shock is liable to variation, according to the degree of suddenness with which it is effected ; but the total additive value of a group of such shocks may be expected to be fairly constant. For this reason, therefore, tetanising shocks caused by a vibrating interrupter would be preferable, provided the duration of these shocks, depending on the duration of closure of current in the primary circuit, be maintained in successive experiments rigorously equal. Such constancy cannot be arrived at if the closure of the circuit be caried out by hand, or even by metronome. Some special mechanical device must therefore be adopted for this purpose.

It is further necessary, in order to maintain constancy of conditions, that identical periods of recovery should be allowed in successive records. For this the stimulus must be applied at accurate and pre-determined intervals of time. The ideal condition, then, for the final elimination of all uncertainties due to the personal factor, is that the plant attached to the recording apparatus should be auto- matically excited by a stimulus absolutely constant, make its own responsive records, go through its own period of recovery, and embark on the same cycle over again without assistance at any point on the part of the observer.

These demands have been fully met in the devices and adjustments now to be described, consisting as they do of two chief elements—namely, the Periodic Starter and the Automatic Exciter. By the former the time-interval between successive stimulations is regulated ; by the latter the stimulation itself, of a definite duration, is applied. In the case of Mimosa recovery from excitation is practically completed in a period of about Io to 20 minutes, according to the season and the condition of the plant. In practice, therefore, we require arrangements by which successive stimulations can be automatically effected at these intervals. As we require a slowly moving plate for the purpose of these records, the plate-carrier is let down by a thread which is wound round a wheel attached to the minute-hand axis of the driving-clock. To the same axis is also screwed one or other of the three separate discs, bearing equidistant projecting rods, either 3, 4, or 6 in

number. During one complete revolution, which takes an hour, these rods will press and release a spring at intervals of 20, 15, or I0 minutes, as the case may be (fig. 30). If the primary circuit of the induction coil provided with a spring-interrupter be closed for a definite period of time, say ‘I second, then the number of interruptions, with con- sequent induction-shocks, will also be definite. What is wanted is some contrivance for release, through which the Periodic Starter can close the main circuit for a definite length of time, say ‘I second. It might at first sight appear that this could be secured by an electrical contact made by the revolving radial-rods already referred to, but the

period of such contact would be impracticably long— more than 15 seconds. So continuous a tetanisation would undoubtedly fatigue or even injure the tissue. The contact made by a seconds- hand, again, though sufficiently brief, would have the serious defect that its movements were jerky and would therefore make the duration of contact unequal. I succeeded in overcoming these difficulties by using a released revolving disc, which could be made to complete an electrical circuit for any definite short period that was required. For this I employed a phonograph motor, an axis of which, carrying a disc, could be adjusted to revolve once in a second.

This disc is usually held stationary by a lever-clutch, and can be started only by the pressure on it of the re- volving rod of the Periodic Starter. It is re-arrested after one complete revolution and is not again released till the next rod comes into position, after an interval of Io, 15, or 20 minutes, as the case may be. There is also attached to the disc a sector whose arc is one-tenth the circum- ference of the circle. This sector, during the revolution, will press against a closing-key, the period of closure being then ‘Isecond. By increasing or diminishing this arc the time of closure, and with it the duration of the tetanising shock, can be correspondingly changed. It is necessary that the sector should, at the moment of the release, be at the greatest possible distance from the closing-key. By the time it reaches this key it will have acquired a constant and definite velocity. Thus the periods of closure, and consequent duration of the exciting-shock, will be identical in successive experiments.

There is another possible source of variation which must be guarded against. The electrodes of the secondary coil are connected with the plant by means of moistened threads. These threads, in long-continued experiments, may become more or less dried up, the electrical resistance being thus increased in an unknown manner. The intensity of the exciting current may, under these conditions, undergo a change. This difficulty has been overcome by a con- trivance for keeping the thread uniformly moist. This will be understood from the diagram (fig. 31) of the electrolytic contact-maker: Twosmallcellsaremadeofcork; the upper cell is filled with very dilute saline solution, a little of which also lies in the bottom of the lower cell. A bent piece of silver-wire coated with a deposit of chloride, and fixed to the horizontal metallic-rod, pricks through two cells and dips into. the solutions above and below. The moistened thread coming through a hole near the bottom of the upper chamber makes one loop round that portion of the plant speci- men where electrical connec- tion is desired, turns back into the lower cell (which it enters through the open aper- ture), and dips into the saline solution. It will be seen that apart from capillary action, owing to the upper cham- ber being at a higher level, Fic. 31.—Electrolytic contact. the thread will be kept con- stantly moist by the slow

streaming down of the solution. The current from the coil, again, will have two entries by means of the doubled thread, the resistance being thus halved. The second electrode of the coil is connected with the other contact-point on the plant in a similar manner. The resistance offered by the plant tissue is relatively high, being of the order of a million ohms. The resistance of the electrolytic contacts, on the other hand, need be no higher than a few thousand ohms. By thus making the resistance of the moist contact relatively small, the total resistance of the circuit remains practically the same, especially since we guard against any variation that might

Fic. 32.—Photograph of Duplex Resonant Recorder with plant and accessories, be induced in the moist thread by drying. The horizontal rod holding the cork of the contact-maker is soldered to a tube which can move up and down a vertical rod. These adjustments enable the point of electrolytic contact to be brought to a level with the point of connection on the specimen. The rod is insulated on ebonite. All the practical difficulties having thus been eliminated, I shall now proceed to show the various records obtained under this mode of periodic stimulation of uniform intensity. In fig. 32 is given a photograph of the apparatus with its accessories. The recorder is of duplex type, for taking two sets of records at the same time.

Before entering upon the detailed consideration of the results of these experiments, I may say that at the beginning of this investigation my attention was roused by the apparently capricious variations in the responses obtained under conditions which were rigidly uniform. A long-continued investigation through the different seasons of the year has given me the clue to what at first appeared to be so anomalous. The outward response, it is obvious, is dependent on two factors—the intensity of the impinging stimulus and the capacity for reply possessed by the plant itself. It is easy to see that the second of these factors must be dependent on the vigour of the plant, or in other words, on its tonic condition, which in its turn is modified by the environmental condition. Thus in unfavour- able circumstances the plant may fall into an atonic or sluggish condition. The absorption of energy from with- out, by whatever form of stimulation, will improve the tonic condition of the plant, with consequent enhancement of excitability.

Taking a plant in a subtonic condition, then, we may expect that any application of stimulus will increase its excitability, a fact which will find expression in a growing amplitude of response. This enhancement of excitability will reach a limit at which the plant will be in an optimum condition. After reaching this climax there may be a reversal, with decline of excitability, a state of things which we associate with fatigue. It must be remembered that in Nature, according to the conditions of its environment, a plant may be found in any of the three states. One specimen may be found in the pre-optimum or subtonic condition ; another may be near the optimum condition, and this we shall designate as the normal ; a third may be found in the post-optimum condition predisposed to fatigue. The first and third of these conditions may be distinguished from each other by means of testing blows or stimuli. If the plant be in the former condition, these will evoke responses of increasing amplitude ; in the latter, they will show a decline.