# Researches on Irritability of Plants/Chapter 10

CHAPTER X
INFLUENCE OF INTENSITY OF STIMULUS, FATIGUE, AND TEMPERATURE ON THE LATENT PERIOD
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 procedure 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 alternating shocks passed through, excitation was simultaneous throughout the interposed tract.
This may be demonstrated in a striking manner by ​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 Mimosa, 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.
If the excitation were to prove simultaneous, however, ​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 .1 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 10 mm. and the lower when they were 80 mm. apart. It
is seen that the latent periods obtained from the two experiments are the same—namely, .08 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 employed 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.
Effect of Intensity of Stimulus
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 100, 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
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 .09 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:—
Table 1.—Effect of Intensity of Stimulus on Latent Period.
| Number. | Number. | Number. | Intensity of Stimulus. | Latent period. |
| 1 | .. | $\scriptstyle {\left\{{\begin{matrix}\ \\\ \end{matrix}}\right.}$ | 1 5 | .155 .10 |
| 2 | .. | $\scriptstyle {\left\{{\begin{matrix}\ \\\ \end{matrix}}\right.}$ | 1 2 | .14 .09 |
| 3 | .. | $\scriptstyle {\left\{{\begin{matrix}\ \\\ \end{matrix}}\right.}$ | 1 4 | .15 .11 |
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, .12 second.
It should be mentioned here that in a plant in optimum condition the latent period differs very little under strong or feeble stimulus. We have also seen, it will be remembered, that in an optimum condition of the specimen there is very little difference in the amplitude of response under strong and feeble stimulus respectively.
Effect of Fatigue
It has been shown that the successive values of the latent period become constant provided a resting-interval be
Figs. 78, 79.—Effect of fatigue.
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 prolongation 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 be stated. 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 .11 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 .1 second to .14 second. The effect of fatigue is independently seen in the record, in the relative sluggishness of the responsive 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:—
Table II.—Effect of Fatigue on the Latent Period.
| Number. | L in fresh specimen. | L′ when fatigued. |
| 1 | .11″ | .16″ |
| 2 | .10″ | .14″ |
| 3 | .10″ | .22″ |
| 4 | .11″ | .17″ |
| 5 | .8″ | .13″ |
| 6 | .11″ | .15″ |
Effect of Temperature
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 100 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
Figs. 80, 81.—Effect of temperature.
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 .14 second. 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 experiments on the effect of temperature on the latent period:—
Table III.—Effect of Temperature on the Latent Period.
| Number. | Temperature. | Latent period. |
| 1 .. $\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \ \end{matrix}}\right.}$ | 23° 28° 33° | .165″ .125″ .065″ |
| 2 .. $\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \ \end{matrix}}\right.}$ | 24° 29° 33° | .140″ .102″ .070″ |
| 3 .. $\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \ \end{matrix}}\right.}$ | 20° 25° 31° | .190″ .10″ .08″ |
Summary
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.
Fatigue prolongs the latent period.
The latent period is shortened by a rise of temperature.