Researches on Irritability of Plants

I5°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

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 promi- nent, 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 I00 double vibrations per second, the recording-plate

Fics. 71, 72.—Two successive records taken with the same leaf; upper with 50 D.V. recorder and slow-moving plate; lower with 100 D.V. recorder and faster-moving plate. Latent period in either case is *I7 second, proving that value of Lis unaffected by any peculiarity of the recorder. 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 ‘o2 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

‘I7 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 speci- mens 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

Fic. 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 Io 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 ‘o8 second or longer than ‘12 second in summer may be regarded as rather exceptional. 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 ‘I second. This may be regarded as approxi- mately 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.

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 speci- men 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 ‘I second. The latent period of leaf of Neptunia oleracea is *6 second. Diffuse stimulation under alternating shock—Effect of intensity of stimulus on Latent Peviod—Influence of optimum condition—Effect of fatigue—Effect of temperature. I WILL now describe different experiments carried out for the purpose of observing the effect of varying external conditions—such as the intensity of stimulus, fatigue, and temperature—on the latent period. The mode of pro- cedure adopted was first to take a record giving the latent period under standard conditions, and then to make further records under conditions similar in all respects to the first, except in regard to the one special factor whose influence was to be determined.

As some of the experiments in question necessitated a long period of observation, lasting sometimes over an hour, it became necessary to eliminate one source of possible uncertainty—namely, the effect of electrolytic contact on the pulvinus. It has been shown that there is no variation of excitability induced in the pulvinus where the contact is made with glycerin. It would, however, be preferable to effect direct stimulation without placing either of the electrodes on the pulvinus. In connection with this it was found that if one of the two electrodes were placed on the petiole slightly to the right of the pulvinus, and the other on the stem immediately below it, and a few rapidly alter- nating shocks passed through, excitation was simultaneous throughout the interposed tract.

experimenting with sub-petioles of Mimosa bearing numerous pairs of sensitive leaflets. The two electrical connections are made on the middle points of two neighbouring sub- petioles. If a single induction-shock be passed, then it will be found that the point by which the current leaves the petiole—the kathode—becomes the seat of excitation, which is transmitted serially to the neighbouring leaflets. The characteristics of this phenomenon will be dealt with in detail in a subsequent chapter. If instead of a single shock a few alternating-shocks of moderately strong intensity be next passed in rapid succession through the sub-petiole, it will be found that the excitation has become diffuse, the leaflets in the intrapolar tract exhibiting excitation simultaneosly. _ This fact of simultaneous excitation in an interposed tract may be demonstrated by the following experiment giving quantitative results: Two successive records are taken of the response of the pulvinus of Muzmosa, the exciting electrode being first placed with the interposed pulvinus 10-mm. apart and again 80 mm. apart. The distance of the pulvinus, in the first case, would be 5 mm. from either electrode, and in the second case, 40 mm. The average velocity of transmission of excitation, as will be seen in the next chapter, may be taken as approximately 16 mm. per second in summer. If the excitation in the interposed tract is not simultaneous, but locally initiated at the points of application of the electrodes, we may expect to find that the periods intervening between the beginning of stimulation and the initiation of response will differ greatly from each other in the two cases. In the first case, where either electrode is distant from the pulvinus by 5 mm., the delay in the response may be expected to be of the order of *3 second.

In the second case, where either electrode is distant from the pulvinus by 40 mm., we may expect the delay caused by transmission to be about 2°5 seconds. we should find no difference of time as between the two cases ; and this stimulation would be equivalent to direct stimulation and be expected to give us a latent period of the order of ‘I second. I give below (fig. 74) the record of this experiment, which shows that under alternating shocks the excitation is simultaneous, and that the value of the latent period is then independent of the points of application of the electrodes, provided the pulvinus be included in the tract. The upper of the two records was taken when the electrodes were Io mm. and the lower when they were 80 mm. apart. It

Fic. 74.—Simultaneous excitation in interposed tract under alternating shock. In upper record two electrodes were I cm. apart, in lower record, 8 cm. apart. is seen that the latent periods obtained from the two experi- ments are the same—namely, ‘o8 second—which is of the order of other determinations already obtained. Had the stimulation not been simultaneous this value would have been increased to at least 3 second or to 2°5 seconds in the respective instances. The identity of results in the two cases shows, moreover, that we are measuring the effect of a constant factor, which is the latent period.

For the purpose of applying alternating shocks I em- ployed as usual a Ruhmkorff coil. In this the spring vibrator in the primary was so adjusted as to cause 100 interruptions per second. This would give in the secondary circuit 200 alternating shocks per second, of which 100 would be due to make and the alternating 100 to break. In order to subject the plant to brief alternating shocks applied at definite moments, and having definite duration, the sliding interrupter and its connections already described were appropriately modified. The vertical sliding contact- plate now consists of a conducting platinum sheet, except for a narrow non-conducting interruption made of a piece of inlaid ebonite. The sliding-plate and the contact rod now short-circuit the secondary, except during the brief interval when the narrow strip of ebonite removes the short-circuit. It is during this definite interval that the plant is subjected to the alternating shock. The breadth of the strip is so chosen that the duration of the shock is about ‘05 second. During this time the plant will receive 10 alternating shocks, 5 of make and 5 of break. If desired, a simple device may be introduced by which the duration of the shock can be modified. This modification consists in inlaying a right- angled triangle instead of a linear piece of ebonite on the metallic plate. The base of the right-angled triangle is kept horizontal. It follows that, by adjusting the rod from right to left, the interval of the removal of short- circuit can undergo continuous reduction of duration.

The mark which indicates the beginning of stimulation is made in the usual manner, and in successive experiments the stimulation is initiated at this precise moment. The duration of stimulus is also constant. In the following experiments, it is to be remembered, we are to determine the effect of changing one factor whilst maintaining others constant. Thus we have in each case to take one record under standard and one or more under modified conditions.

The series of experiments to be described below have in each case been repeated at least twelve times, with results that were invariably concordant. I content myself, however, with giving two records in each case, obtained from different specimens. In order to test still further the reliability of these results I was careful, with each pair of figures given for comparison, to employ two different recorders, the vibration-frequency of the first being 100, and of the second or companion-set 50 per second.

In fig. 75, with vibrating recorder of Ioo, are given two records testing the effect of intensity of stimulus on the latent period. The lower of the two was obtained with the minimum stimulus of 1; and the latent period is seen to be "155 second. In the upper of the two records we Fics. 75, 76.—Effect of intensity of stimulus on latent period: the upper record in each is due to stronger stimulus. have the result of the maximal stimulus of 5. The latent period is now found to be reduced to ‘1 second.

In fig. 76, with vibrating recorder of 50 and taking a different specimen, we find a precisely similar result. The lower of the two records, with the minimal stimulus of 1, shows a latent period of ‘14 second. The upper, with stimulus 2, which in this individual case was maximal, shows a latent period of ‘og second. It is interesting to note alike in figs. 75 and 76 the great vigour of the responsive movement under higher intensity of stimulus, as seen in the abruptness of the rise of the curve and the wider spacing of the successive dots.

The following table shows the effect of intensity of stimulus on the latent period :— Number. Intensity of Stimulus./Latent period. 5 Io ; f I 14 | | I ons 3) 4 fe Fic. 77.—Constancy of latent period when stimulus is above maximal. Lower record under stimulus 2; upper record under stimulus 5. Recorder 100 D.V. It is thus seen that with the increase of the intensity of stimulus there is a corresponding reduction of the latent period. But it would appear from further experiments that a limit is soon reached, when the stimulus begins to be maximal. A further increase of the intensity of stimulation above this point will have little or no effect in reducing the latent period. This is shown in fig. 77, which gives a pair of records taken with a vibrator having

a frequency of 100 double vibrations per second. The lower of these two records represents the effect of a stimulus of 2, which was here maximal. The upper was taken with the increased intensity of stimulus of 5. In the two cases the latent period was practically the same—namely, ‘I2 second. It should be mentioned here that ina plant in optimum condition the latent period differs very little under strong or feeble stimulus. We have also seen, it will be remem- bered, that in an optimum condition of the specimen there is very little difference in the amplitude of response under strong and feeble stimulus respectively.

It has been shown that the successive values of the latent period become constant provided a resting-interval be allowed for complete protoplasmic recovery. The period required for full recovery I find to be about 20 to 25 minutes in summer, more or less. If this resting-interval be shortened, the effect of fatigue is seen in the prolonga- tion of the latent period ; if this shortening be carried too far, then the motile excitability is temporarily abolished. I give below a pair of records which exhibit the prolongation of latent period on account of fatigue.

The mode of procedure is first to obtain the normal record with a fresh specimen under a maximal stimulus of intensity 3. This is the intensity which is always used unless the contrary bestated. In order to exhibit the effect of fatigue, the second record is taken after a period of rest of only 15 minutes. The upper record (vibration-frequency 100) gives the normal value of L to be ‘1r second; the lower record shows that the latent period has been prolonged to ‘16 second on account of fatigue (fig. 78).

The experiment was repeated with a different specimen and with a vibrating recorder giving 50 vibrations per second. The record (fig. 79) shows again that the latent period is prolonged under fatigue, from the normal ‘I second to 14 second. The effect of fatigue is independently seen in the record, in the relative sluggishness of the respon- sive movement. The slope in the response of the fresh specimen is almost vertical, with successive dots very widely spaced. In the response of the fatigued specimen a great contrast is observed in both these respects.

I give below a tabular statement showing results of different experiments on the effect of fatigue :— Number. L in fresh specimen.|L’ when fatigued. | to} uN 4 Sit Ly The effect of temperature on the latent period is shown in the next two sets of records (figs. 80, 81). In fig. 80 we have three sets of records, taken with a too D.V. recorder at the three different temperatures of 23° C., 28°C., and 33° C. respectively, under a uniform stimulus-intensity of 2. These temperatures were maintained by means of the thermal chamber, heated electrically. From the lowest record at a temperature of 23° C. the latent period is seen to be ‘165 second. At 28° C. in the middle record, it is found

to be reduced to ‘125 second. And at 33° C. it becomes still further reduced to ‘065 second. In fig. 81 these results are corroborated by records taken with a different specimen, under stimulus-intensity of 2, the vibration-frequency of recorder being 50 D.V. The three records are for temperatures of 24° C., 29° C., and 33° C. respectively. The shortening of the latent period with rising temperature is also shown here in a very striking manner. The lowest of the records, taken at 24° C., gives us a latent period of ‘14second. The next, at 29° C., shows a reduction to ‘102 second. And the last and highest, at 33° C., gives us a latent period of only ‘07 second.

The increase of vigour in the responsive movement under rising temperature is also very clearly apparent in the record. It will thus be seen that the latent period decreases with rising temperature. The following table gives the results of several experi- ments on the effect of temperature on the latent period :— Alternating electric shocks of moderate intensity induce simultaneous excitation throughout the interposed tract. Latent period is in general shorter under stronger intensity of stimulus. There is no further variation above a maximal.

In the optimum condition of the plant the latent period is the same for feeble and strong stimulus. Detection of transmitted excitation by means of electromotive variation— Specific tissue for conduction of excitation—Hydro-mechanical theory of transmission of stimulus—Propagation of excitatory protoplasmic change—Physiological test—Automatic record of transmission-period—Conditions for obtaining constant velocity— Determination of velocity of transmission in Mzmosa—Differential method of determining velocity—Constancy of results—Tabular statement of different determinations of velocity—Effect of intensity of stimulus on velocity of transmission—Effects on sub-tonic tissue and on tissue in optimum condition—After-effect of stimulus in enhancing conductivity—Effect of optimum condition—Disturbing action of leakage of exciting current—Effect of fatigue—Effect of temperature—Velocity of transmission in Biophytum and Averrhoa— Direction of preferential conduction.

WE have hitherto dealt with the reaction of tissues which exhibit the excitatory condition by motile response, as in pulvinus in the case of the plant and muscle in the case of animal. In the animal, again, we meet with certain conducting-tissues in which the excitatory protoplasmic change is transmitted to adistance, and, should one of these nerves happen to lead to a contractile muscle, the trans- mission of the excitatory change is conspicuously exhibited by the contraction of the terminal organ.

We now come to the question whether there is a trans- mission of a true excitatory change in the plant, and if so whether there is in it any specific conducting-tissue, corre- sponding to the nerve of the animal, for the conveyance of excitation ? Since the transmission of excitation depends on the propagation of a protoplasmic change, it follows that a conducting-tissue must be characterised by a more or less protoplasmic continuity. Should the plant possess

any tissue analogous to the nerve, then it is in the fibro- vascular bundle that we must look for it. In a nerve-and-muscle preparation the transmitted excitation is detected by the contraction of the terminal muscle. Even in the absence of any terminal contractile organ, we can detect the passage of excitation by an elec- trical method. It is known that the excitation of a living tissue is attended by a concomitant electrical change of galvanometric negativity. If we make suitable galvano- metric connections with two points on a nerve, and we stimulate the nerve at a distant point, we shall find that the arrival of excitation from the distant stimulated point is at a proper moment signalised in the galvanometer by a deflection of a definite sign.