Fig. 28. Abnormal Positive passing into Normal Negative in a Stale Specimen of Leaf-stalk of CauliflowerFig. 28. Abnormal Positive passing into Normal Negative in a Stale Specimen of Leaf-stalk of CauliflowerStimulus was gradually increased from 1 to 10, by means of spring-tapper. When the stimulus intensity was 10, the response became reversed into normal negative. (Parts of 8 and 9 are out of the plate.)
Fig. 28. Abnormal Positive passing into Normal Negative in a Stale Specimen of Leaf-stalk of Cauliflower
Stimulus was gradually increased from 1 to 10, by means of spring-tapper. When the stimulus intensity was 10, the response became reversed into normal negative. (Parts of 8 and 9 are out of the plate.)
The plant thus gives a reversed response under abnormal conditions of staleness. I have sometimes found similar reversal of response when the plant is subjected to the abnormal conditions of excessively high or low temperature.
Radial E.M. variation.—We have seen that a current of response flows in the plant from the relatively more to the relatively less excited. A theoretically important experiment is the following: A thick stem of plant stalk was taken and a hole bored so as to make one contact with the interior of the tissue, the other beingon the surface. After a while the current of injury was found to disappear. On exciting the stem by taps or torsional vibration, a responsive current was observed which flowed inwards from the more disturbed outer surface to the shielded core inside (fig. 29). Hence it is seen that when a wave of disturbance is propagated along the plant, there is a concomitant wave of radial E.M. variation. The swaying of a tree by the wind would thus appear to give rise to a radial E.M.F.
Fig. 29.—Radial E.M. VariationFig. 29.—Radial E.M. Variation
Fig. 29.—Radial E.M. Variation
FOOTNOTES:[12]For general purposes it is immaterial whether the responses are recorded up or down. For convenience of inspection they are in general recordedup. But in cases where it is necessary to discriminate the sign of response, positive response will be recorded up, and negative down.
[12]For general purposes it is immaterial whether the responses are recorded up or down. For convenience of inspection they are in general recordedup. But in cases where it is necessary to discriminate the sign of response, positive response will be recorded up, and negative down.
[12]For general purposes it is immaterial whether the responses are recorded up or down. For convenience of inspection they are in general recordedup. But in cases where it is necessary to discriminate the sign of response, positive response will be recorded up, and negative down.
As already said, in the living tissue, molecular disturbance induced by stimulus is accompanied by an electric disturbance, which gradually disappears with the return of the disturbed molecules to their position of equilibrium. The greater the molecular distortion produced by the stimulus, the greater is the electric variation produced. The electric response is thus an outward expression of a molecular disturbance produced by an external agency, the stimulus.
Curve of relation between stimulus and response.—In the curve showing the relation between stimulus and response in nerve and muscle, it is found that the molecular effect as exhibited either by contraction or E.M. variation in muscle, or simply by E.M. variation in nerve, is at first slight. In the second part, there is a rapidly increasing effect with increased stimulus. Finally, a tendency shows itself to approach a limit of response. Thus we find the curve at first slightly convex, then straight and ascending, and lastly, concave to the abscissa (fig. 30).
In muscle the limit of response is reached much sooner than in nerve. As will be seen, the range of variation of stimulus in these curves is not verygreat. When the stimulus is carried beyond moderate limits, the response, owing to fatigue or other causes, may sometimes undergo an actual diminution.
Fig. 30.—Curves Showing the Relation Between the Intensity of Stimulus and ResponseFig. 30.—Curves Showing the Relation Between the Intensity of Stimulus and ResponseAbscissæ indicate increasing intensity of stimulus. Ordinates indicate magnitude of response. (Waller.)
Fig. 30.—Curves Showing the Relation Between the Intensity of Stimulus and Response
Abscissæ indicate increasing intensity of stimulus. Ordinates indicate magnitude of response. (Waller.)
Fig. 31Fig. 31Taps of increasing strength 1:2:3:4 producing increased response in leaf stalk of turnip.
Fig. 31
Taps of increasing strength 1:2:3:4 producing increased response in leaf stalk of turnip.
I have obtained very interesting results, with reference to the relation between stimulus and response, when experimenting with plants. These results are suggestive of various types of response met with in animal tissues.
1. In order to obtain the simplest type of effects, not complicated by secondary phenomena, one has to choose specimens which exhibit little fatigue. Having procured these, I undertook two series of experiments. In the first (A) the stimulus was applied by means of the spring-tapper, and in the second (B) by torsional vibration.
(A) The first stimulus was given by a fall of the lever throughh, the second through 2h, and so on. The response-curves clearly show increasing effect with increased stimulus (fig. 31).
Fig. 32.—Increased Response with Increasing Vibrational Stimuli (Cauliflower-stalk)Fig. 32.—Increased Response with Increasing Vibrational Stimuli (Cauliflower-stalk)Stimuli applied at intervals of three minutes. Vertical line = ·1 volt.
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 2·5° to 5° to 7·5° to 10° to 12·5° in amplitude. It will be observed how the intensity of response tends to approach a limit (fig. 32).
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).
Fig. 33.—Responses to Increasing Stimuli produced by Increasing Angle of VibrationFig. 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.
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 responseswhich 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 pronounced 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 circumstances 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.
Fromfig. 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.
Fig. 34.—Responses to Increasing Stimulus obtained with Two Specimens of Stalk of CauliflowerFig. 34.—Responses to Increasing Stimulus obtained with Two Specimens of Stalk of CauliflowerIn (a) fatigue is absent, in (b) it is present.
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.
[13]See ‘On Electric Touch,’Proc. Roy. Soc.Aug. 1900.
[13]See ‘On Electric Touch,’Proc. Roy. Soc.Aug. 1900.
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.
Fig. 35.—Diminution of Response in Eucharis by Lowering of TemperatureFig. 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.
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 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.
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.
Fig. 36.—After-effect of Cold on Ivy, Holly, and Eucharis LilyFig. 36.—After-effect of Cold on Ivy, Holly, and Eucharis Lilya. The normal response;b. Response after subjection to freezing temperature for twenty-four hours.
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.
Fig. 37.—The Glass Chamber containing the PlantFig. 37.—The Glass Chamber containing the PlantAmplitude 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 anæsthetics, vapour of chloroform is blown in through the side tube.
Fig. 37.—The Glass 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 anæsthetics, 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 glass 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 forten 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 atleast, 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 7·5 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.
Fig. 38.—Effect of Temperature on ResponseFig. 38.—Effect of Temperature on ResponseThe response was abolished at the hot-water temperature of 55° C.
Fig. 38.—Effect of Temperature on Response
The response was abolished at the hot-water temperature of 55° C.
(·01 Volt = 35 divisions)
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 circumstance, 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 itfor five minutes in water at 55° C. This, as will be seen from the record taken afterwards, effectively killed the plant (fig. 38, w).
Fig. 39.—Effect of Rising and Falling Temperature on the Response Of Scotch KaleFig. 39.—Effect of Rising and Falling Temperature on the Response Of Scotch Kale
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, ascompared 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 fallof temperature.Fig. 41gives a curve of variation of response during the rise and fall of temperature.
Fig. 40.—Records of Responses in Eucharis Lily during Rise and Fall of TemperatureFig. 40.—Records of Responses in Eucharis Lily during Rise and Fall of TemperatureStimulus 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°.
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.
Fig. 41.—Curve showing Variation of Response in Eucharis with the Rise and Fall of TemperatureFig. 41.—Curve showing Variation of Response in Eucharis with the Rise and Fall of Temperature
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 glass chamber, into which steamcould 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 illustrated in the record (fig. 42).
Fig. 42.—Effect of Steam in Killing ResponseFig. 42.—Effect of Steam in Killing ResponseThe 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.
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, weare 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. Anæsthetics 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.
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 anæsthetic 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 anæsthetic 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 anæsthetisation 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 seriesof 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 infig. 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.
Fig. 43.—Effect of Chloroform on Nerve Response (Waller)Fig. 43.—Effect of Chloroform on Nerve Response (Waller)
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 contactAandBand 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 anæsthetic 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.
Fig. 44.—Effect of Chloroform on Responses of CarrotFig. 44.—Effect of Chloroform on Responses of CarrotStimuli of 25° vibration at intervals of one minute.
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 infig. 45is interesting. The abrupt decline, within aminute of the application of chloral, is also extremely well marked.
Fig. 45.—Action of Chloral Hydrate on the Responses of Leaf-stalk of CauliflowerFig. 45.—Action of Chloral Hydrate on the Responses of Leaf-stalk of CauliflowerVibration of 25° at intervals of one minute.
Fig. 45.—Action of Chloral Hydrate on the Responses of Leaf-stalk of Cauliflower
Vibration of 25° at intervals of one minute.
Fig. 46.—Action of Formalin (Radish)Fig. 46.—Action of Formalin (Radish)
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 adventitious circumstance, but to the reagent itself. If further verificationbe desired as to the effect of the reagent, we can obtain it by alternate stimulation of theAandBends. 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 theAandBends 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).