[Ill.u.s.tration: FIG. 38.--EFFECT OF TEMPERATURE ON RESPONSE The response was abolished at the hot-water temperature of 55 C.]
TABLE SHOWING DIMINUTION OF RESPONSE WITH INCREASING TEMPERATURE
(01 Volt = 35 divisions)
Temperature Response
20 21 30 75 40 55 50 4 65 3
In radishes response disappeared completely at 55 C., but with celery, heated in the manner described, I could not obtain its entire abolition at 60 C. or even higher. A noticeable circ.u.mstance, however, was the prolongation of the period of recovery at these high temperatures. I soon understood the reason of this apparent anomaly. The method adopted in the present case was that of dry heating, whereas the previous experiments had been carried on by the use of hot water. It is well known that one can stand a temperature of 100 C. without ill effects in the hot-air chamber of a Turkish bath, while immersion in water at 100 C. would be fatal.
In order to find out whether subjection to hot water would kill the celery-stalk, I took it out and placed it for five minutes in water at 55 C. This, as will be seen from the record taken afterwards, effectively killed the plant (fig. 38, w).
[Ill.u.s.tration: FIG. 39.--EFFECT OF RISING AND FALLING TEMPERATURE ON THE RESPONSE OF SCOTCH KALE]
#Increased sensitiveness as after-effect of temperature variation.#--A very curious effect of temperature variation is the marked increase of sensitiveness which often appears as its after-effect. I noticed this first in a series of observations where records were taken during the rise of temperature and continued while the temperature was falling (fig. 39). The temperature was adjusted by electric heating. It was found that the responses were markedly enhanced during cooling, as compared with responses given at the same temperatures while warming (see table). Temperature variation thus seems to have a stimulating effect on response, by increasing molecular mobility in some way. The second record (fig. 40) shows the variation of response in Eucharis lily (1) during the rise, and (2) during the fall of temperature. Fig. 41 gives a curve of variation of response during the rise and fall of temperature.
TABLE SHOWING THE VARIATION OF RESPONSE IN SCOTCH KALE DURING THE RISE AND FALL OF TEMPERATURE
Temperature Response Response [Temperature rising] [Temperature falling]
19 C. 47 dns. -- 25 " 24 " -- ^ 30 " 11 " 23 dns.
50 " 8 " 16 "
70 " v 7 " -- ----->
[Ill.u.s.tration: FIG. 40.--RECORDS OF RESPONSES IN EUCHARIS LILY DURING RISE AND FALL OF TEMPERATURE Stimulus constant, applied at intervals of one minute. The temperature of plant-chamber gradually rose on starting current in the heating coil; on breaking current, the temperature fell gradually.
Temperature corresponding to each record is given below.
Temperature rising: (1) 20, (2) 20, (3) 22, (4) 38, (5) 53, (6) 68, (7) 65.
Temperature falling: (8) 60, (9) 51, (10) 45, (11) 40, (12) 38.]
#Point of temperature maximum.#--We have seen how, in cases of lowered temperature, response is abolished earlier in plants like Eucharis, which are affected by cold, than in the hardier plants such as Holly and Ivy. Plants again are unequally affected as regards the upper range. In the case of Scotch kale, for instance, response disappears after ten minutes of water temperature of about 55 C., but with Eucharis fairly marked response can still be obtained after such immersion and does not disappear till it has been subjected for ten minutes to hot water, at a temperature of 65 C. or even higher. The reason of this great power of resistance to heat is probably found in the fact that the Eucharis is a tropical plant, and is grown, in this country, in hot-houses where a comparatively high temperature is maintained.
[Ill.u.s.tration: FIG. 41.--CURVE SHOWING VARIATION OF RESPONSE IN EUCHARIS WITH THE RISE AND FALL OF TEMPERATURE]
#The effect of steam.#--I next wished to obtain a continuous record by which the effects of suddenly increased temperatures, culminating in the death of the plant, might be made evident. For this purpose I mounted the plant in the gla.s.s chamber, into which steam could be introduced. I had chosen a specimen which gave regular response. On the introduction of steam, with the consequent sudden increase of temperature, there was a transitory augmentation of excitability. But this quickly disappeared, and in five minutes the plant was effectively killed, as will be seen graphically ill.u.s.trated in the record (fig. 42).
[Ill.u.s.tration: FIG. 42.--EFFECT OF STEAM IN KILLING RESPONSE The two records to the left exhibit normal response at 17 C. Sudden warming by steam produced at first an increase of response, but five minutes exposure to steam killed the plant (carrot) and abolished the response.
Vibrational stimulus of 30 applied at intervals of one minute; vertical line = 1 volt.]
It will thus be seen that those modifications of vital activity which are produced in plants by temperature variation can be very accurately gauged by electric response. Indeed it may be said that there is no other method by which the moment of cessation of vitality can be so satisfactorily distinguished. Ordinarily, we are able to judge that a plant has died, only after various indirect effects of death, such as withering, have begun to appear. But in the electric response we have an immediate indication of the arrest of vitality, and we are thereby enabled to determine the death-point, which it is impossible to do by any other means.
It may be mentioned here that the explanation suggested by Kunkel, of the response being due to movement of water in the plant, is inadequate.
For in that case we should expect a definite stimulation to be under all conditions followed by a definite electric response, whose intensity and sign should remain invariable. But we find, instead, the response to be profoundly modified by any influence which affects the vitality of the plant. For instance, the response is at its maximum at an optimum temperature, a rise of a few degrees producing a profound depression; the response disappears at the maximum and minimum temperatures, and is revived when brought back to the optimum. Anaesthetics and poisons abolish the response. Again, we have the response undergoing an actual reversal when the tissue is stale. All these facts show that mere movement of water could not be the effective cause of plant response.
CHAPTER IX
PLANT RESPONSE--EFFECT OF ANaeSTHETICS AND POISONS
Effect of anaesthetics, a test of vital character of response--Effect of chloroform--Effect of chloral--Effect of formalin--Method in which response is unaffected by variation of resistance--Advantage of block method--Effect of dose.
The most important test by which vital phenomena are distinguished is the influence on response of narcotics and poisons. For example, a nerve when narcotised by chloroform exhibits a diminishing response as the action of the anaesthetic proceeds. (See below, fig. 43.) Similarly, various poisons have the effect of permanently abolishing all response.
Thus a nerve is killed by strong alkalis and strong acids. I have already shown how plants which previously gave strong response did not, after application of an anaesthetic or poison, give any response at all.
In these cases it was the last stage only that could be observed. But it appeared important to be able to trace the growing effect of anaesthetisation or poisoning throughout the process. There were, however, two conditions which it at first appeared difficult to meet.
First it was necessary to find a specimen which would normally exhibit no fatigue, and give rise for a long time to a uniform series of response. The immediate changes made in the response, in consequence of the application of chemical reagents, could then be demonstrated in a striking manner. And with a little trouble, specimens can be secured in which perfect regularity of response is found. The record given in fig. 16, obtained with a specimen of radish, shows how possible it is to secure plants in which response is absolutely regular. I subjected this to uniform stimulation at intervals of one minute, during half an hour, without detecting the least variation in the responses. But it is of course easier to find others in which the responses as a whole may be taken as regular, though there may be slight rhythmic fluctuations. And even in these cases the effect of reagents is too marked and sudden to escape notice.
[Ill.u.s.tration: FIG. 43.--EFFECT OF CHLOROFORM ON NERVE RESPONSE (WALLER)]
For the obtaining of constant and strong response I found the best materials to be carrot and radish, selected individuals from which gave most satisfactory results. The carrots were at their best in August and September, after which their sensitiveness rapidly declined. Later, being obliged to seek for other specimens, I came upon radish, which gave good results in the early part of November; but the setting-in of the frost had a prejudicial effect on its responsiveness. Less perfect than these, but still serviceable, are the leaf-stalks of turnip and cauliflower. In these the successive responses as a whole may be regarded as regular, though a curious alternation is sometimes noticed, which, however, has a regularity of its own.
My second misgiving was as to whether the action of reagents would be sufficiently rapid to display itself within the time limit of a photographic record. This would of course depend in turn upon the rapidity with which the tissues of the plant could absorb the reagent and be affected by it. It was a surprise to me to find that, with good specimens, the effect was manifested in the course of so short a time as a minute or so.
#Effect of chloroform.#--In studying the effect of chemical reagents in plants, the method is precisely similar to that employed with nerve; that is to say, where vapour of chloroform is used, it is blown into the plant chamber. In cases of liquid reagents, they are applied on the points of contact A and B and their close neighbourhood. The mode of experiment was (1) to obtain a series of normal responses to uniform stimuli, applied at regular intervals of time, say one minute, the record being taken the while on a photographic plate. (2) Without interrupting this procedure, the anaesthetic agent, vapour of chloroform, was blown into the closed chamber containing the plant. It will be seen how rapidly chloroform produces depression of response (fig. 44), and how the effect grows with time. In these experiments with plants, the same curious shifting of the zero line is sometimes noticed as in nerve when subjected similarly to the action of reagents. This is a point of minor importance, the essential point to be noticed being that the responses are rapidly reduced.
[Ill.u.s.tration: FIG. 44.--EFFECT OF CHLOROFORM ON RESPONSES OF CARROT Stimuli of 25 vibration at intervals of one minute.]
#Effects of chloral and formalin.#--I give below (figs. 45, 46) two sets of records, one for the reagent chloral and the other for formalin. The reagents were applied in the form of a solution on the tissue at the two leading contacts, and the contiguous surface. The rhythmic fluctuation in the normal response shown in fig. 45 is interesting. The abrupt decline, within a minute of the application of chloral, is also extremely well marked.
[Ill.u.s.tration: FIG. 45.--ACTION OF CHLORAL HYDRATE ON THE RESPONSES OF LEAF-STALK OF CAULIFLOWER Vibration of 25 at intervals of one minute.]
[Ill.u.s.tration: FIG. 46.--ACTION OF FORMALIN (RADISH)]
#Response unaffected by variation of resistance.#--In order to bring out clearly the main phenomena, I have postponed till now the consideration of a point of some difficulty. To determine the influence of a reagent in modifying the excitability of the tissue, we rely upon its effect in exalting or depressing the responsive E.M. variation. We read this effect by means of galvanometric deflections. And if the resistance of the circuit remained constant, then an increase of galvanometer deflection would accurately indicate a heightened or depressed E.M.
response, due to greater or less excitability of tissue caused by the reagent. But, by the introduction of the chemical reagent, the resistance of the tissue may undergo change, and owing to this cause, modification of response as read by the galvanometer may be produced without any E.M. variation. The observed variation of response may thus be partly owing to some unknown change of resistance, as well as to that of the E.M. variation in response to stimulus.
We may however discriminate as to how much of the observed change is due to variation of resistance by comparing the deflections produced in the galvanometer by the action of a definite small E.M.F. before and after the introduction of the reagent. If the deflections be the same in both cases, we know that the resistance has not varied. If there have been any change, the variation of deflection will show the amount, and we can make allowance accordingly.
I have however adopted another method, by which all necessity of correction is obviated, and the galvanometric deflections simply give E.M. variations, unaffected by any change in the resistance of the tissue. This is done by interposing a very large and constant resistance in the external circuit and thereby making other resistances negligible.
An example will make this point clear. Taking a carrot as the vegetable tissue, I found its resistance plus the resistance of the non-polarisable electrode equal to 20,000 ohms. The introduction of a chemical reagent reduced it to 19,000 ohms. The resistance of the galvanometer is equal to 1,000 ohms. The high external resistance was 1,000,000 ohms. The variation of resistance produced in the circuit would therefore be 1,000 in (1,000,000+19,000+1,000) or one part in 1,020. Therefore the variation of galvanometric deflection due to change of resistance would be less than one part in a thousand (cf. fig. 49).
#The advantage of the block method.#--In these investigations I have used the block method, instead of that of negative variation, and I may here draw attention to the advantages which it offers. In the method of negative variation, one contact being injured, the chemical reagents act on injured and uninjured unequally, and it is conceivable that by this unequal action the resting difference of potential may be altered. But the intensity of response in the method of injury depends on this resting difference. It is thus hypothetically possible that on the method of negative variation there might be changes in the responses caused by variation of the resting difference, and not necessarily due to the stimulating or depressing effect of the reagent on the tissue.
But by the block method the two contacts are made with uninjured surfaces, and the effect of reagents on both is similar. Thus no advantage is given to one contact over the other. The changes now detected in response are therefore due to no advent.i.tious circ.u.mstance, but to the reagent itself. If further verification be desired as to the effect of the reagent, we can obtain it by alternate stimulation of the A and B ends. Both ends will then show the given change. I give below a record of responses given by two ends of leaf-stalk of turnip, stimulated alternately in the manner described. The stalk used was slightly conical, and owing to this difference between the A and B ends the responses given by one end were slightly different from those given by the other, though the stimuli were equal. A few drops of 10 per cent.
solution of NaOH was applied to both the ends. It will be seen how quickly this reagent abolished the response of both ends (fig. 47).
[Ill.u.s.tration: FIG. 47.--ABOLITION OF RESPONSE AT BOTH A AND B ENDS BY THE ACTION OF NaOH Stimuli of 30 vibration were applied at intervals of one minute to A and B alternately. Response was completely abolished twenty-four minutes after application of NaOH.]
#Effect of dose.#--It is sometimes found that while a reagent acts as a poison when given in large quant.i.ties, it may act as a stimulant in small doses. Of the two following records fig. 48 shows the slight stimulating effect of very dilute KOH, and fig. 49 exhibits nearly complete abolition of response by the action of the same reagent when given in stronger doses.
[Ill.u.s.tration: FIG. 48.--STIMULATING ACTION OF VERY DILUTE KOH]
So we see that, judged by the final criterion of the effect produced by anaesthetics and poisons, the plant response fulfils the test of vital phenomenon. In previous chapters we have found that in the matter of response by negative variation, of the presence or absence of fatigue, of the relation between stimulus and response, of modification of response by high and low temperatures, and even in the matter of occasional abnormal variations such as positive response in a modified tissue, they were strictly correspondent to similar phenomena in animal tissues. The remaining test, of the influence of chemical reagents, having now been applied, a complete parallelism may be held to have been established between plant response on the one hand, and that of animal tissue on the other.
[Ill.u.s.tration: FIG. 49.--NEARLY COMPLETE ABOLITION OF RESPONSE BY STRONG KOH The two vertical lines are galvanometer deflections due to 1 volt, before and after the application of reagent. It will be noticed that the total resistance remains unchanged.]