The Nervous Mechanism of Plants

The most difficult problem is the exact determination of the time of transmission of impulse as indicated by observation of the responsive fall of the leaf. The leaf attached to a writing-lever may be made to record its responsive movement on a moving recording plate : the lever bears the writer at right angles to it. The recording plate is raised, and then allowed to descend : during its fall a brief contact is made which completes the electric circuit of the primary cod P (fig. 14), and generates a current in the secondary coil the terminals of which are applied on the petiole at A, a definite distance from the motile pulvinus B. The precise moment of stimulation is marked on the record as an arrow. The response is not immediate, but there is a definite interval between the application of stimulus and the beginning of response. This interval T is made up of the time t required for excitation to travel along the definite length of the petiole, plus the time required for starting the motile mechanism into action, which

is the latent period L of the pulvinus. For the exact determination of the true transmission-time t, the latent period L has to be subtracted from the observed interval T, V, one arm of lever attached to leaf ; w, the writer. The falling plate during descent makes electric contact of k with R' causing induction-shock by the secondary coil s. Stimulus applied at a causes response later, at h. between stimulation and response. A highly sensitive method has therefore to be devised for measurements of time-intervals shorter than a hundredth of a second.

All difficulties of recording and measuring of short intervals of time are overcome by my Kesonant Recorder (figill which the writer is not in continuous contact with the recording plate, but makes regular intermittent contacts with it so that the error due to friction is completely removed. The writer, consisting of a thin steel wire, is Fig. 15. Upper part of Resonant Recorder. (From a photograph.) Thread from clock, not shown, passes over pulley p, letting down recording-plate ; s', screw for adjusting distance of writing-point from plate ; s, screw for vertical adjustment;

T, tangent-screw for exact adjustment of plane of movement of recorder, parallel to writing-surface ; v, axis of writer supported perpendicularly at centre of circular end of magnet ; c, coercer ; m, micrometerscrew for adjustment of length of coercer ; g, smoked-glass plate. supported at the centre of one pole of an electro-magnet, which is periodically magnetised by the passage of an interrupted current. The interruption is produced by the vibrating spring C, which is designated the Coercer (fig. 15), the period of vibration of which can be accurately adjusted. Different writers are tuned exactly to vibrate 10, 100 or 200 times per second. When the Coercer is adjusted to

these vibrations, the writer is thrown into sympathetic vibration. The record then consists of a series of dots ; Fig. i6. Apparatus for determination of latent period and velocity of transmission of excitation in Mimosa. when they are, say, a 200th of a second apart, the interval between successive dots in the record is -005 second. It is not difficult to measure one-fifth of the distance between successive dots: so calculations can be carried to the thousandth part„ of a second. The intermittent contact serves two most important purposes. First, it eliminates all error arising from friction. Secondly, the distance between successive dots in the record itself enables measurement of extremely short time-intervals to be made with the highest degree of accuracy. The plant-record or phytogram is also its own chronogram.

Fig. 16 gives the general arrangement of the apparatus for experiments on the determination of the latent period and of the velocity of transmission of excitation. For direct stimulation of the pulvinus, the two electrodes are Fig. 17. Record of Latent Period of Mimosa, with 200 Vibration Recorder. attached to the petiole and the stem respectively, the shock being passed through the included motile organ. The record of an experiment for the determination of the latent period is given in fig. 17. There are 15 • 2 spaces between the incidence of stimulus and response. The writer being tuned to 200 vibrations per second, the interval between the dots is *005 second, and therefore the latent period of the specimen is -076 of a second.

The value of the latent period depends on the season and on the physiological condition of the specimen. It varies in different specimens from -05 to -12 second. The approximate average value of L may be taken as -lo second. of rest be not allowed between two successive responses the latent period becomes prolonged, indicating the effect of fatigue. Thus in a particular experiment the normal latent period of -lo second (upper record, fig. 18) was prolonged under fatigue to -14 second (lower record). Under excessive fatigue the motile excitability becomes temporarily abolished.

Experiment 31. Effect of Pig. 18. Effect of Fatigue temperature. — The latent period of a particular specimen was •14 second at 24°C. ; at 29° C. it was shortened to -lo second. At the higher temperature of 33° C., it was reduced to -07 second. Determination of Velocity of Transmission {a) Ordinary Method In order to determine the velocity of transmission induction-shock stimulus is applied to the petiole at a definite distance d from the responding pulvinus. In order to obtain the true transmission-time t, it is necessary, as previously explained, to subtract the latent period L from the observed total time T. The velocity of transmission is then found by dividing the distance by the true time. The necessary data are therefore the distance d between the stimulated point and the pulvinus, the total time T between the application of stimulus and the initiation of response, and the latent period L of the individual pulvinus.

In obtaining a record of response to indirect stimulation the two electrodes E and E' are applied on the petiole, the proximal electrode being at a distance, say, of 30 mm. from the pulvinus. After taking one or more such records, an additional record is taken of the response to direct stimulation ; this latter gives the latent period L of the particular specimen. Experiment 32. — In the experiment I am about to describe the specimen of Mimosa was very vigorous. The distance at which the stimulus was applied was 30 mm, from the responding pulvinus, and the intensity of stimulus was 3 units, which was maintained constant in successive records The frequency of the vibrating-recorder was 10 per second; hence the distance between any two

Fig. 19. Determination of Velocity of Transmission of Excitation in petiolO of Mimosa. Two lower records are in response to indirect stimulation applied at a distance of 30 mm. ; upper record of response to direct stimulation gives tlie latent period. Recorder 10 V. per second, successive dots in the record represents a time-interval of one-tenth of a second; from the record itself it is not difficult to estimate an interval of even one-fifth of that amount. The lowest of the three records (fig. 19) represents the results of the first experiment. It will be seen that the interval between the stimulation and the beginning of response is 16*2 spaces, each of the value of -i second. The total time T is therefore i • 62 seconds. After a suitable interval necessary for a complete recovery, a second record was taken, under the same conditions, on the same plate.

The minimum interval necessary for complete recovery varies from 15 to 20 minutes, depending on the condition of the specimen and the season. The total time in the second record is the same as before, namely, 1-62 seconds. The third record was taken under direct stimulation, and the latent period obtained therefrom is •12 second. The velocity of transmission obtained from the two experiments is identical, namely : In order to put the constancy of these results to a still more rigorous test, I modified the experiment in the following way, employing the Differential Method. The stimulus was first applied at a distance d from the responding pulvinus, and the total time T was found from the record. In the next experiment the distance of the point of stimulation was reduced to di, and the corresponding total time T' found in the usual manner. And lastly, a record was taken under direct stimulation for the determination of the latent period L.

These observations provide three different sets of data for the determination of the value of the velocity of transmission. By two of these is obtained the velocity in the usual manner from the distance, the total time, and the latent period. In the third, knowledge of L is not required, for T — Ti gives the time of transmission through the distance d — di. Hence three separate determinations Vi, V2, ¥„ are obtained with the same specimen. Viz. :

The rigour of the test of constancy will be gauged by the extent to which the determinations of Vi, V2, and V3 are consistent with one another. way the intensity of stimulus applied was 3 units. In the first experiment the point of application of the stimulus was at a distance of 30 mm. ; the total time was found to be 1-9 seconds. In the next experiment the distance was reduced to half, i.e, to 15 mm., and the total time was found The records from below upwards are in response to stimuli applied at distances of 30 mm., 15 mm., and directly. Recorder 10 V. per sec.

to be I second. And lastly, the latent period, under direct stimulation, was found to be -08 second (fig. 20). Thus — The three results thus obtained from independent data are seen to be extremely consistent. They bear very emphatic testimony not only to the accuracy of the method, but also to the constancy of velocity in a given specimen under unvarying external conditions. The velocity of transmission is modified by the individual vigour of the specimen, by the temperature, and by the season. In summer the velocity in thick petioles is about 30 mm. per second ; in winter it is as low as 5 mm. The velocity of nervous impulse in Frog's nerve is about

27,000 mm. per second, but in Anodon it is as low as 10 mm. per second. The velocity of nervous impulse in a thick petiole of Mimosa is thus a thousand times less than in the Frog, but about three times greater than in a lower type of animal such as Anodon. I shall presently show that the velocity in thin petioles is very much higher than in thick specimens. The velocity in the stem and in the sub-petiole is, generally speaking, considerably lower than in the petiole, being of the order of 4 mm. per second.

Experiment 34. — ^The following results present many interesting features. I first determined the velocity of transmission in the thick petiole of a particular Mimosa. The day was very bright and dry and the temperature was at an optimum degree of 33° C. (April). On account of the exceptionally favourable conditions of light and temperature, the rate of conduction in the thick petiole was found to be as high as 55 mm. per second. Lower down, the plant bore leaves of a different type, the petioles of which were extremely thin. I obtained a record of the velocity of transmission in a thin petiole, employing a recorder vibrating 20 times in a second ; the successive spacings therefore represent *05 second. The lower figure is the record of the transmitted excitation, the stimulated point on the petiole being at a distance of 20 mm. from the pulvinus. The upper record gives the latent period under direct stimulation (fig. 21). The data for the determination of the velocity are :

Total time between stimulus and response =3-5 spaces. True transmission-time through 20 mm. = 2 ‘5 — i -5 = I space = -05 second. Record of Velocity of Transraission of Excitation in Thin Petioles. The above results are by no means exceptional, for in five other determinations of the velocity in thin petioles the rate was found to be very much higher than in thick petioles. The value varied in different specimens from 400 mm. to 120 mm. per second. The average velocity in thin petioles may therefore be taken as about nine times greater than that in thick petioles. An explanation of the difference of the velocity in thick and thin petioles will be given later (see Chapter XVI.).

The lower record gives total time of transmission through 20 mm. Upper record gives the latent period. Vibrationfrequency of recorder, 20 per second. A remarkable fact noticed, in quantitative determinations is the enhancement -of velocity which results as an after-efiect of stimulation. Thus stimulus appHed for the first time gives rise to an impulse which is transmitted at a slow rate ; successive stimulations enhance the rate still further, until an optimum rate is attained which remains constant for a considerable length of time. This is the steady condition, and the effects of external agencies on conduction can only be studied after the attainment of this condition. The enhancement of conducting power by a preliminary stimulation is evidence in support of the existence of synapsoidal membranes in the conducting tissue, the resistance of which is for a time overcome by the action of a stimulus.

Specimens of Mimosa are found to be in various conditions of physiological vigour. Some are in an optimum condition, others in an unfavourable sub-tonic condition. The velocity in the former is relatively high, while in the Fig. 22. After-effect of Intense Stimulation in depressing rate of conduction in a normal specimen. latter it is low. The after-effect of intense stimulation is diametrically opposite in the two cases, as will be seen in the following experiments.

Experiment 35. After-effect of intense stimulation on normal specimens.— Rtcoxd (i) in fig. 22 gives the time of transmission through 15 mm. length of the petiole, the velocity of transmission being found to be 18-7 mm. per second. The end of the petiole beyond the point of application of the testing stimulus was then cut off, and record of velocity of transmission taken once more. It will be seen from record (2) that the excessive stimulation caused by the cut had induced a depression of the conducting power, the velocity being reduced to 10-7 mm. per second. Ex- cessive stimulation of normal specimens thus temporarily depresses the conducting power.

Experiment ^6. Aftet-effect of intense stimulation in sub-tonic specimens. — I will now describe an experiment to show that an identical agent, on account of difference in the. tonic condition of the tissue, gives rise to a diametrically opposite effect. I took a specimen in a sub-tonic condition, in which the conducting power of the tissue was so far below par that the test-stimulus applied at a distance of 15 mm. failed to be transmitted. The end of the petiole at a distance of i cm. beyond the point of application of the test-stimulus was now cut off. The after-effect of this injury was found so to enhance the conducting power that the excitation previously arrested was now effectively transmitted, the velocity being 25 mm. per second. This enhanced conducting power began slowly to decline, and after half an hour the velocity had declined to 4 ■ i mm. per second. The end of the petiole was cut off once more, and the effect of injury was again found to enhance the conducting power, the velocity of transmission being restored to 25 mm. per second.

The latent period of the pulvinus may be determined with great accuracy by means of the Resonant Recorder, by which it is possible to estimate time-intervals as short as a thousandth part of a second. The lowest value of the latent period of the pulvinus of Mimosa is -06 second, the average value of the latent period being • i second. allowing proper intervals of rest, successive values of the velocity of transmission were obtained which were constant.

Consistent results are obtained by the employment of the Difierential Method. In thick petioles the velocity in summer is about 30 mm. per second, whereas iii thin petioles the velocity may be as high as 400 mm. per second. The velocity of transmission of excitation in the petiole of Mimosa is intermediate between the velocities of the nervous impulse in higher and lower animals. The velocity in the stem and that in the sub-petiole of Mimosa are much lower than that in the petiole.

The tonic condition of a tissue has an influence on its conductivity and the induced variations of it. In a tissue in optimum condition the velocity is high, and excessive stimulation induces temporary depression of the conducting power. In a sub-tonic tissue the velocity of conduction is low, and the after-effect of intense stimulation is to initiate or enhance the rate of conduction ; thus the conducting path is canalised by the stimulus. The velocity of transmission has been shown to be constant under uniform external conditions. I will now describe the effects of external changes in inducing variations in the velocity of transmission in plants. For this purpose it is necessary to obtain two records in succession, the first under normal and the second under the changed condition. The difference reveals the effect of the changed condition on the velocity.

Experiment 37. Effect of application of glycerine.— It is known that desiccation, generally speaking, enhances the excitability of animal nerve. As glycerine, by absorption of water, causes partial desiccation, I tried its effect on conduction of excitation in the petiole of Mimosa. Enhancement of conducting power may be exhibited in two ways : either by an increase of the velocity of transmission ; or, secondly, by an enhancement of the intensity of the transmitted excitation, which would be manifested in a greater amplitude of the response of the motile indicator. In fig. 23 are given two records, one before and the other after the application of glycerine on a length of petiole through which excitation was being transmitted. The time-records demonstrate the enhanced fate of transmission after the application of glycerine. The increased intensity of transmitted excitation is also shown in the enhanced amplitude of response exhibited by the more erect curve of the upper record.

In animal nerve the velocity is increased under a rise of temperature. Conversely, under a falling temperature, the velocity is lowered, and conduction becomes blocked when the temperature is sufficiently low. I will show that precisely similar effects occur in the conduction of the excitatory impulse in Mimosa. Experiment 38. — The plant was placed in a thermal chamber, the rise of temperature of which could be regulated Fig. 23. Action of Glycerine in enhancing the speed and intensity of transmitted excitation.

Stimulus applied at the vertical line. Successive dots in record are at intervals of o-i second, i , normal response. by a suitable electric device. In fig. 24 time-records are given of the transmission of excitation at temperatures 22° C.,- 28° C. and 31° C. The experiment was carried out in winter in Calcutta, when the temperature of the room was 22° C., and the normal velocity, on account of the unfavourable season, was comparatively low. A maximal stimulus was applied at a distance of 10 mm. from the responding pulvinus. The lowest of the three records gives the transmission-time at temperature 22° C. ; the middle . record was taken at 28° C., and the uppermost one at 31° C. The records make it quite evident that

the velocity continuously increased under a rising temperature. The total time (including the latent period) at 22° C. was 2-94 seconds; at 28° C. it was 1-69 seconds ; and at 31° C. it was further shortened to i-2 seconds. In a previous experiment the variation induced in the latent period by change of temperature had been determined ; at 23° C. it was • 165 second ; at 28° C. it was Fig. 24. Effect of Temperature in enhancing Velocity of Transmission.

The three records, from below upwards, are for temperatures of 22° C., 28° C., and 31° C. respectively. ■12 second; and at 33°C..it was further shortened to •065 second. These variations are very slight as compared with the total period of transmission. After making the small corrections for the change in the latent period, the velocity of transmission at 22° C. was 3-6 mm. per second ; at 28° C. it was 6-3 mm. ; and at 31° C. it was 9 mm. per second. The results of numerous other experiments showed that the velocity was always increased by rise of temperature.

Transmission can be arrested by treating the conducting tissues in various ways ; that is, a physiological block can be induced. The method employed is illustrated by fig. 25. The electric stimulation is appHed at E and the various blocking agents are applied at B in the path of conduction. The enhancement of conducting power by warmth has been demonstrated in the experiment just described. Conversely, conductivity is depressed by lowering the temperature, and this may become so great as to induce an actual arrest of conduction. The object of the present investigation being the determination of the influence of cold on conductivity, special care was taken that the