[Ill.u.s.tration: FIG. 32.--INCREASED RESPONSE WITH INCREASING VIBRATIONAL STIMULI (CAULIFLOWER-STALK) Stimuli applied at intervals of three minutes. Vertical line = 1 volt.]

(_B_) 1. The vibrational stimulus was increased from 25 to 5 to 75 to 10 to 125 in amplitude. It will be observed how the intensity of response tends to approach a limit (fig. 32).

TABLE SHOWING THE INCREASED E.M. VARIATION PRODUCED BY INCREASING STIMULUS

+----------------------------------------+

Angle of Vibration

E.M.F

+----------------------------------------+

25

044 volt

5

075 volt

75

090 volt

10

100 volt

125

106 volt

+----------------------+-----------------+

2. The next figure shows how little variation is produced with low value of stimulus, but with increasing stimulus the response undergoes a rapid increase, after which it tends to approach a limit (fig. 33, _a_).

[Ill.u.s.tration: FIG. 33.--RESPONSES TO INCREASING STIMULI PRODUCED BY INCREASING ANGLE OF VIBRATION (_a_) Record with a specimen of fresh radish. Stimuli applied at intervals of two minutes. The record is taken for one minute.

(_b_) Record for stale radish. There is a reversed response for the feeble stimulus of 5 vibration.]

3. As an extreme instance of the case just cited, I have often come across a curious phenomenon. During the gradual increase of the stimulus from a low value there would be apparently no response. But when a critical value was reached a maximum response would suddenly occur, and would not be exceeded when the stimulus was further increased. Here we have a parallel to what is known in animal physiology as the 'all or none' principle. With the cardiac muscle, for example, there is a certain minimal intensity which is effective in producing response, but further increase of stimulus produces no increase in response.

4. From an inspection of the records of responses which are given, it will be seen that the slope of a curve which shows the relation of stimulus to response will at first be slight, the curve will then ascend rapidly, and at high values of stimulus tend to become horizontal. The curve as a whole becomes, first slightly convex to the abscissa, then straight and ascending, and lastly concave. A far more p.r.o.nounced convexity in the first part is shown in some cases, especially when the specimen is stale. This is due to the fact that under these circ.u.mstances response is apt to begin with an actual reversal of sign, the plant under feebler than a certain critical intensity of stimulus giving positive, instead of the normal negative, response (fig. 33, _b_).

#Diminution of response with excessively strong stimulus.#--It is found that in animal tissues there is sometimes an actual diminution of response with excessive increase of stimulus. Thus Waller finds, in working with retina, that as the intensity of light stimulus is gradually increased, the response at first increases, and then sometimes undergoes a diminution. This phenomenon is unfortunately complicated by fatigue, itself regarded as obscure. It is therefore difficult to say whether the diminution of response is due to fatigue or to some reversing action of an excessively strong stimulus.

From fig. 33, _b_, above, it is seen that there was an actual reversal of response in the lower portion of the curve. It is therefore not improbable that there may be more than one point of reversal.

In physical phenomena we are, however, acquainted with numerous instances of reversals. For example, a common effect of magnetisation is to produce an elongation of an iron rod. But Bidwell finds that as the magnetising force is pushed to an extreme, at a certain point elongation ceases and is succeeded, with further increase of magnetising force, by an actual contraction. Again a photographic plate, when exposed continuously to light, gives at first a negative image. Still longer exposure produces a positive. Then again we have a negative.

There is thus produced a series of recurrent reversals. In photographic prints of flashes of lightning, two kinds of images are observed, one, the positive--when the lightning discharge is moderately intense--and the other, negative, the so-called 'dark lightning'--due to the reversal action of an intensely strong discharge.

In studying the changes of conductivity produced in metallic particles by the stimulus of Hertzian radiation, I have often noticed that whereas feeble radiation produces one effect, strong radiation produces the opposite. Again, under the continuous action of electric radiation, I have frequently found recurrent reversals.[13]

#Diminution of response under strong stimulus traced to fatigue.#--But there are instances in plant response where the diminution effect can be definitely traced to fatigue. The records of these cases are extremely suggestive as to the manner in which the diminution is brought about.

The accompanying figures (fig. 34) give records of responses to increasing stimulus. They were made with specimens of cauliflower-stalks, one of which (_a_) showed little fatigue, while in the other (_b_) fatigue was present. It will be seen that the curves obtained by joining the apices of the successive single responses are very similar.

[Ill.u.s.tration: FIG. 34.--RESPONSES TO INCREASING STIMULUS OBTAINED WITH TWO SPECIMENS OF STALK OF CAULIFLOWER In (_a_) fatigue is absent, in (_b_) it is present.]

In one case there is no fatigue, the recovery from each stimulus being complete. Every response in the series therefore starts from a position of perfect equilibrium, and the height of the single responses increases with increasing stimulation. But in the second case, the strain is not completely removed after any single stimulation of the series. That recovery is partial is seen by the gradual shifting of the base line upwards. In the former case the base line is horizontal and represents a condition of complete equilibrium. Now, however, the base line, or line of modified equilibrium, is tilted upwards. Thus even in this case if we measure the heights of successive responses from the line of absolute equilibrium, they will be found to increase with increasing stimulus.

Ordinarily, however, we make no allowance for the shifting of the base line, measuring response rather from the place of its previous recovery, or from the point of modified equilibrium. Judged in this way, the responses undergo an apparent diminution.

FOOTNOTES:

[13] See 'On Electric Touch,' _Proc. Roy. Soc._ Aug. 1900.

CHAPTER VIII

PLANT RESPONSE--ON THE INFLUENCE OF TEMPERATURE

Effect of very low temperature--Influence of high temperature--Determination of death-point--Increased response as after-effect of temperature variation--Death of plant and abolition of response by the action of steam.

For every plant there is a range of temperature most favourable to its vital activity. Above this optimum, the vital activity diminishes, till a maximum is reached, when it ceases altogether, and if this point be maintained for a long time the plant is apt to be killed. Similarly, the vital activity is diminished if the temperature be lowered below the optimum, and again, at a minimum point it ceases, while below this minimum the plant may be killed. We may regard these maximum and minimum temperatures as the death-points. Some plants can resist these extremes better than others. Length of exposure, it should however be remembered, is also a determining factor in the question as to whether or not the plant shall survive unfavourable conditions of temperature. Thus we have hardy plants, and plants that are affected by excessive variations of temperature. Within the characteristic power of the species, there may be, again, a certain amount of individual difference.

These facts being known, I was anxious to determine whether the undoubted changes induced by temperature in the vital activity of plants would affect electrical response.

#Effect of very low temperature.#--As regards the influence of very low temperature, I had opportunities of studying the question on the sudden appearance of frost. In the previous week, when the temperature was about 10 C., I had obtained strong electric response in radishes whose value varied from 05 to 1 volt. But two or three days later, as the effect of the frost, I found electric response to have practically disappeared. A few radishes were, however, found somewhat resistant, but the electric response had, even in these cases, fallen from the average value of 075 V. under normal temperature to 003 V. after the frost.

That is to say, the average sensitiveness had been reduced to about 1/25th. On warming the frost-bitten radish to 20 C. there was an appreciable revival, as shown by increase in response. In specimens where the effect of frost had been very great, i.e. in those which showed little or no electric response, warming did not restore responsiveness. From this it would appear that frost killed some, which could not be subsequently revived, whereas others were only reduced to a condition of torpidity, from which there was revival on warming.

I now tried the effect of artificial lowering of temperature on various plants. A plant which is very easily affected by cold is a certain species of Eucharis lily. I first obtained responses with the leaf-stalk of this lily at the ordinary temperature of the room (17 C.). I then placed it for fifteen minutes in a cooling chamber, temperature -2 C., for only ten minutes, after which, on trying to obtain response, it was found to have practically disappeared. I now warmed the plant by immersing it for awhile in water at 20 C., and this produced a revival of the response (fig. 35). If the plant be subjected to low temperature for too long a time, there is then no subsequent revival.

[Ill.u.s.tration: FIG. 35.--DIMINUTION OF RESPONSE IN EUCHARIS BY LOWERING OF TEMPERATURE (_a_) Normal response at 17 C.

(_b_) The response almost disappears when plant is subjected to -2 C.

for fifteen minutes.

(_c_) Revival of response on warming to 20 C.]

I obtained a similar marked diminution of response with the flower-stalk of Arum lily, on lowering the temperature to zero.

My next attempt was to compare the sensibility of different plants to the effect of lowered temperatures. For this purpose I chose three specimens: (1) Eucharis lily; (2) Ivy; and (3) Holly. I took their normal response at 17 C., and found that, generally speaking, they attained a fairly constant value after the third or fourth response.

After taking these records of normal response, I placed the specimens in an ice-chamber, temperature 0 C., for twenty-four hours, and afterwards took their records once more at the ordinary temperature of the room. From these it will be seen that while the responsiveness of Eucharis lily, known to be susceptible to the effect of cold, had entirely disappeared, that of the hardier plants, Holly and Ivy, showed very little change (fig. 36).

Another very curious effect that I have noticed is that when a plant approaches its death-point by reason of excessively high or low temperature, not only is its general responsiveness diminished almost to zero, but even the slight response occasionally becomes reversed.

[Ill.u.s.tration: FIG. 36.--AFTER-EFFECT OF COLD ON IVY, HOLLY, AND EUCHARIS LILY _a._ The normal response; _b._ Response after subjection to freezing temperature for twenty-four hours.]

#Influence of high temperature, and determination of death-point.#--I next tried to find out whether a rise of temperature produced a depression of response, and whether the response disappeared at a maximum temperature--the temperature of death-point. For this purpose I took a batch of six radishes and obtained from them responses at gradually increasing temperatures. These specimens were obtained late in the season, and their electric responsiveness was much lower than those obtained earlier. The plant, previously kept for five minutes in water at a definite temperature (say 17 C.), was mounted in the vibration apparatus and responses observed. The plant was then dismounted, and replaced in the water-bath at a higher temperature (say 30 C.) again, for five minutes. A second set of responses was now taken. In this way observations were made with each specimen till the temperature at which response almost or altogether ceased was reached. I give below a table of results obtained with six specimens of radish, from which it would appear that response begins to be abolished in these cases at temperatures varying from 53 to 55 C.

TABLE SHOWING EFFECT OF HIGH TEMPERATURE IN ABOLISHING RESPONSE

Temperature Galvanometric response (100 dns. = 07 V.)

(1) {17 C 70 dns.

{53 " 4 "

(2) {17 " 160 "

{53 " 1 "

(3) {17 " 100 "

{50 " 2 "

(4) {17 " 80 "

{55 " 0 "

(5) {17 " 40 "

{60 " 0 "

(6) {17 " 60 "

{55 " 0 "

[Ill.u.s.tration: FIG. 37.--THE GLa.s.s CHAMBER CONTAINING THE PLANT Amplitude of vibration which determines the intensity of stimulus is measured by the graduated circle seen to the right. Temperature is regulated by the electric heating coil R. For experiments on action of anaesthetics, vapour of chloroform is blown in through the side tube.]

#Electric heating.#--The experiments just described were, however, rather troublesome, inasmuch as, in order to produce each variation of temperature, the specimen had to be taken out of the apparatus, warmed, and remounted. I therefore introduced a modification by which this difficulty was obviated. The specimen was now enclosed in a gla.s.s chamber (fig. 37), which also contained a spiral of German-silver wire, through which electric currents could be sent, for the purpose of heating the chamber. By varying the intensity of the current, the temperature could be regulated at will. The specimen chosen for experiment was the leaf-stalk of celery. It was kept at each given temperature for ten minutes, and two records were taken during that time. It was then raised by 10 C., and the same process was repeated.

It will be noticed from the record (fig. 38) that in this particular case, as the temperature rose from 20 C. to 30 C., there was a marked diminution of response. At the same time, in this case at least, recovery was quicker. At 20 C., for example, the response was 21 dns., and the recovery was not complete in the course of a minute. At 30 C., however, the response had been reduced to 75 divisions, but there was almost complete recovery in twelve seconds. As the temperature was gradually increased, a continuous decrease of response occurred. This diminution of response with increased temperature appears to be universal, but the quickening of recovery may be true of individual cases only.