Fig. 12.—Response in Plant to Mechanical Tap or VibrationFig. 12.—Response in Plant to Mechanical Tap or VibrationThe end B is injured. A tap was given between A and B and this gave the response-curvea. A stronger tap gave the responseb. By means of the handle H, a torsional vibration of 45° was now imparted, this gave the responsec. Vibration through 67° gaved.
Fig. 12.—Response in Plant to Mechanical Tap or Vibration
The end B is injured. A tap was given between A and B and this gave the response-curvea. A stronger tap gave the responseb. By means of the handle H, a torsional vibration of 45° was now imparted, this gave the responsec. Vibration through 67° gaved.
Effectiveness of stimulus dependent on rapidity also.In order that successive stimuli may be equally effectiveanother point has to be borne in mind. In all cases of stimulation of living tissue it is found that the effectiveness of a stimulus to arouse response depends on the rapidity of the onset of the disturbance. It is thus found that the stimulus of the ‘break’ induction shock, on a muscle for example, is more effective, by reason of its greater rapidity, than the ‘make’ shock. So also with the torsional vibrations of plants, I find response depending on the quickness with which the vibration is effected. I give below records of successive stimuli, given by vibrations through the same amplitude, but delivered with increasing rapidity (fig. 13).
Fig. 13.—Influence of Suddenness on the Efficiency of StimulusFig. 13.—Influence of Suddenness on the Efficiency of StimulusThe curvesa,b,c,d, are responses to vibrations of the same amplitude, 30°. Inathe vibration was very slow; inbit was less slow; it was rapid inc, and very rapid ind.
Fig. 13.—Influence of Suddenness on the Efficiency of Stimulus
The curvesa,b,c,d, are responses to vibrations of the same amplitude, 30°. Inathe vibration was very slow; inbit was less slow; it was rapid inc, and very rapid ind.
Thus if we wish to maintain the effective intensity of stimulus constant we must meet two conditions: (1) The amplitude of vibration must be kept the same. This is done by means of the graduated circle. (2) The vibration period must be kept the same. With a little practice, this requirement is easily fulfilled.
The uniformity of stimulation which is thus attained solves the great difficulty of obtaining reliable quantitative values, by whose means alone can rigorous demonstration of the phenomena we are studying become possible.
FOOTNOTES:[8]A preliminary account of Electric Response in Plants was given at the end of my paper on ‘Electric Response of Inorganic Substances’ read before the Royal Society on June 6, 1901; also at the Friday Evening Discourse, Royal Institution, May 10, 1901. A more complete account is given in my paper on ‘Electric Response in Ordinary Plants under Mechanical Stimulus’ read before the Linnean Society March 20, 1902.I thank the Royal Society and the Linnean Society for permission to reproduce some of my diagrams published in theirProceedings.—J. C. B.[9]By this is meant a rapid to-and-fro or complete vibration. In order that successive responses should be uniform it is essential that there should be no resultant twist, i.e. the plant at the end of vibration should be in exactly the same condition as at the beginning.
[8]A preliminary account of Electric Response in Plants was given at the end of my paper on ‘Electric Response of Inorganic Substances’ read before the Royal Society on June 6, 1901; also at the Friday Evening Discourse, Royal Institution, May 10, 1901. A more complete account is given in my paper on ‘Electric Response in Ordinary Plants under Mechanical Stimulus’ read before the Linnean Society March 20, 1902.I thank the Royal Society and the Linnean Society for permission to reproduce some of my diagrams published in theirProceedings.—J. C. B.
[8]A preliminary account of Electric Response in Plants was given at the end of my paper on ‘Electric Response of Inorganic Substances’ read before the Royal Society on June 6, 1901; also at the Friday Evening Discourse, Royal Institution, May 10, 1901. A more complete account is given in my paper on ‘Electric Response in Ordinary Plants under Mechanical Stimulus’ read before the Linnean Society March 20, 1902.
I thank the Royal Society and the Linnean Society for permission to reproduce some of my diagrams published in theirProceedings.—J. C. B.
[9]By this is meant a rapid to-and-fro or complete vibration. In order that successive responses should be uniform it is essential that there should be no resultant twist, i.e. the plant at the end of vibration should be in exactly the same condition as at the beginning.
[9]By this is meant a rapid to-and-fro or complete vibration. In order that successive responses should be uniform it is essential that there should be no resultant twist, i.e. the plant at the end of vibration should be in exactly the same condition as at the beginning.
I shall now proceed to describe another and independent method which I devised for obtaining plant response. It has the advantage of offering us a complementary means of verifying the results found by the method of negative variation. As it is also, in itself, for reasons which will be shown later, a more perfect mode of inquiry, it enables us to investigate problems which would otherwise have been difficult to attempt.
When electrolytic contacts are made on the uninjured surfaces of the stalk atAandB, the two points, being practically similar in every way, are iso-electric, and little or no current will flow in the galvanometer. If now the whole stalk be uniformly stimulated, and if both endsAandBbe equally excited at the same moment, it is clear that there will still be no responsive current, owing to balancing action at the two ends. This difficulty as regards the obtaining of response was overcome in the method of negative variation, where the excitability of one end was depressed by chemical reagents or injury, or abolished by excessive temperature. On stimulating the stalk there was produced a greater excitation atAthan atB, and a current of action was then observed to flow in the stalk from the more excitedAto the less excitedB(fig. 6).
But we can cause this differential action to become evident by another means. For example, if we produce a block, by clamping atCbetweenAandB(fig. 14,a), so that the disturbance made atAby tapping or vibration is prevented from reachingB, we shall then haveAthrown into a relatively greater excitatory condition thanB. It will now be found that a current of action flows in the stalk fromAtoB, that is to say, from the excited to the less excited. When theBend is stimulated, there will be a reverse current (fig. 14,b).
Fig. 14.—The Method of BlockFig. 14.—The Method of Block(a) The plant is clamped at C, between A and B.(b) Responses obtained by alternately stimulating the two ends. Stimulation of A produces upward response; of B gives downward response.
Fig. 14.—The Method of Block
(a) The plant is clamped at C, between A and B.
(b) Responses obtained by alternately stimulating the two ends. Stimulation of A produces upward response; of B gives downward response.
We have in this method a great advantage over that of negative variation, for we can always verify any set of results by making corroborative reversal experiments.
By the method of injury again, one end is made initially abnormal, i.e. different from the condition which it maintains when intact. Further, inevitable changes will proceed unequally at the injured and uninjured ends, and the conditions of the experiment may thus undergo unknown variations. But by theblock method which has just been described, there is no injury, the plant is normal throughout, and any physiological change (which in plants will be exceedingly small during the time of the experiment) will affect it as a whole.
Fig. 15.—Response in Plant (from the Stimulated A to Unstimulated B) Completely Immersed Under WaterFig. 15.—Response in Plant (from the Stimulated A to Unstimulated B) Completely Immersed Under WaterThe leaf-stalk is clamped securely in the middle with the cork C, inside the tube T, which is filled with water, the plant being completely immersed. Moistened threads in connection with the two non-polarisable electrodes are led to the side tubes t t′. One end of the stalk is held in ebonite forceps and vibrated. A current of response is found to flow in the stalk from the excited A to the unexcited B, and outside, through the liquid, from B to A. A portion of this current, flowing through the side tubes t t′, produces deflection in the galvanometer.
Fig. 15.—Response in Plant (from the Stimulated A to Unstimulated B) Completely Immersed Under Water
The leaf-stalk is clamped securely in the middle with the cork C, inside the tube T, which is filled with water, the plant being completely immersed. Moistened threads in connection with the two non-polarisable electrodes are led to the side tubes t t′. One end of the stalk is held in ebonite forceps and vibrated. A current of response is found to flow in the stalk from the excited A to the unexcited B, and outside, through the liquid, from B to A. A portion of this current, flowing through the side tubes t t′, produces deflection in the galvanometer.
Plant response a physiological or vital response.—I now proceed to a demonstration of the fact that whatever be the mechanism by which they are brought about, these plant responses are physiological in their character. As the investigations described in the next few chapters will show, they furnish an accurate index of physiological activity. For it will be found that, other things being equal, whatever tends to exalt or depress the vitality of the plant tends also to increase or diminish its electric response. These E.M. effects are well marked, and attain considerable value, rising sometimes, as has been said before, to as much as ·1 volt or more. They are proportional to the intensity of stimulus.
It need hardly be added that special precautions are taken to avoid shifting of contacts. Variation of contact, however, could not in any case account for repeated transient responses to repeated stimuli, when contact is made on iso-electric surfaces. Nor could itin any way explain the reversible nature of these responses, whenAandBare stimulated alternately. These responses are obtained in the plants even when completely immersed in water, as in the experimental arrangement (fig. 15). It will be seen that in this case, where there could be no possibility of shifting of contact, or variation of surface, there is still the usual current of response.
I shall describe here a few crucial experiments only, in proof of the physiological character of electric response. The test applied by physiologists, in order to discriminate as to the physiological nature of response, consists in finding out whether the response is diminished or abolished by the action of anæsthetics, poisons, and excessively high temperature, which are known to depress or destroy vitality.
I shall therefore apply these same tests to plant responses.
Effect of anæsthetics and poisons.—Ordinary anæsthetics, like chloroform, and poisons, like mercuric chloride, are known to produce a profound depression or abolish all signs of response in the living tissue. For the purpose of experiment, I took two groups of stalks, with leaves attached, exactly similar to each other in every respect. In order that the leaf-stalks might absorb chloroform I dipped their cut ends in chloroform-water, a certain amount of which they absorbed, the process being helped by the transpiration from the leaves. The second group of stalks was placed simply in water, in order to serve for control experiment. The narcotic action of chloroform, finallyculminating in death, soon became visually evident. The leaves began to droop, a peculiar death-discolouration began to spread from the mid rib along the venation of the leaves. Another peculiarity was also observed. The aphides feeding on the leaves died even before the appearance of the discoloured patches, whereas on the leaves of the stalks placed in water these little creatures maintained their accustomed activity, nor did any discolouration occur. In order to study the effect of poison, another set was placed in water containing a small quantity of mercuric chloride. The leaves here underwent the same change of appearance, and the aphides met with the same untimely fate, as in the case of those subjected to the action of chloroform. There was hardly any visible change in the appearance of the stalks themselves, which were to all outer seeming as living as ever, indications of death being apparent only on the leaf surfaces. I give below the results of several sets of experiments, from which it would appear that whereas there was strong normal response in the group of stalks kept in water, there was practically a total abolition of all response in those anæsthetised or poisoned.
Experiments on the effect of anæsthetics and poisons.A batch of ten leaf-stalks of plane-tree was placed with the cut ends in water, and leaves in air; an equal number was immersed in chloroform-water; a third batch was placed in 5 per cent. solution of mercuric chloride.
Similarly a batch of three horse-chestnut leaf-stalks was put in water, another batch in chloroform-water, and a third batch in mercuric chloride solution.
These results conclusively prove the physiological nature of the response.
I shall in a succeeding chapter give a continuous series of response-curves showing how, owing to progressive death from the action of poison, the responses undergo steady diminution till they are completely abolished.
Effect of high temperature.—It is well known that plants are killed when subjected to high temperatures. I took a stalk, and, using the block method, with torsionalvibration as the stimulus, obtained strong responses at both endsAandB. I then immersed the same stalk for a short time in hot water at about 65° C., and again stimulated it as before. But at neitherAnorBcould any response now be evoked. As all the external conditions were the same in the first and second parts of this experiment, the only difference being that in one the stalk was alive and in the other killed, we have here further and conclusive proof of the physiological character of electric response in plants.
The same facts may be demonstrated in a still more striking manner by first obtaining two similar but opposite responses in a fresh stalk, atAandB, and then killing one half, sayB, by immersing only that half of the stalk in hot water. The stalk is replaced in the apparatus, and it is now found that whereas theAhalf gives strong response, the endBgives none.
In the experiments on negative variation, it was tacitly assumed that the variation is due to a differential action, stimulus producing a greater excitation at the uninjured than at the injured end. The block method enables us to test the correctness of this assumption. TheBend of the stalk is injured or killed by a few drops of strong potash, the other end being uninjured. There is a clamp betweenAandB. The endAis stimulated and a strong response is obtained. The endBis now stimulated, and there is little or no response. The block is now removed and the plant stimulated throughout its length. Though the stimulus now acts on both ends, yet, owing to the irresponsive condition ofB, there is a resultant response, which from its direction is foundto be due to the responsive action ofA. This would not have been the case if the endBhad been uninjured. We have thus experimentally verified the assumption that in the same tissue an uninjured portion will be thrown into a greater excitatory state than an injured, by the action of the same stimulus.
Effect of single stimulus.—In a muscle a single stimulus gives rise to a single twitch which may be recorded either mechanically or electrically. If there is no fatigue, the successive responses to uniform stimuli are exactly similar. Muscle when strongly stimulated often exhibits fatigue, and successive responses therefore become feebler and feebler. In nerves, however, there is practically no fatigue and successive records are alike. Similarly, in plants, we shall find some exhibiting marked fatigue and others very little.
Fig. 16.—Uniform Responses (Radish)Fig. 16.—Uniform Responses (Radish)
Fig. 16.—Uniform Responses (Radish)
Fig. 17.—Fusion of Effect of Rapidly Succeeding StimuliFig. 17.—Fusion of Effect of Rapidly Succeeding Stimuli(a) in muscle; (b) in carrot.
Fig. 17.—Fusion of Effect of Rapidly Succeeding Stimuli
(a) in muscle; (b) in carrot.
Superposition of stimuli.—If instead of a single stimulus a succession of stimuli be superposed, it happens that a second shock is received before recovery from the first has taken place. Individual effects will then become more or less fused. When the frequency is sufficiently increased, the intermittent effects are fused, and we find an almost unbroken curve. When for example the muscle attains its maximum contraction (corresponding to the frequency and strength of stimuli) itis thrown into a state of completetetanus, in which it appears to be held rigid. If the rapidity be not sufficient for this, we have the jagged curve of incomplete tetanus. If there is not much fatigue, the upper part of the tetanic curve is approximately horizontal, but in cases where fatigue sets in quickly, the fact is shown by the rapid decline of the curve. With regard to all these points we find strict parallels in plant response. In cases where there is no fatigue, the successive responses are identical (fig. 16). With superposition of stimuli we have fusion of effects, analogous to the tetanus of muscle (fig. 17). And lastly, the influence of fatigue in plants is to produce a modification of response-curve exactly similar to that of muscle (see below). One effect of superposition of stimuli may be mentioned here.
Fig. 18.—Additive EffectFig. 18.—Additive Effect(a) A single stimulus of 3° vibration produced little or no effect, but the same stimulus when rapidly superposed thirty times, produced the large effect (b). (Leaf-stalk of turnip.)
Fig. 18.—Additive Effect
(a) A single stimulus of 3° vibration produced little or no effect, but the same stimulus when rapidly superposed thirty times, produced the large effect (b). (Leaf-stalk of turnip.)
Additive effect.—It is found in animal responses that there is a minimum intensity of stimulus, below which no response can be evoked. But even a sub-minimal stimulus will, though singly ineffective, become effective by the summation of several. In plants, too, we obtain a similar effect, i.e. the summation of single ineffective stimuli produces effective response (fig. 18).
Staircase effect.—Animal tissues sometimes exhibit what is known as the ‘staircase effect,’ that is to say, the heights of successive responses are gradually increased, though the stimuli are maintained constant. This is exhibited typically by cardiac muscle, though it is not unknown even in nerve. The cause is obscure, but it seems to depend on the condition of the tissue. It appears as if the molecular sluggishness of tissue were in these cases only gradually removed under stimulation, and the increased effects were due to increased molecular mobility. Whatever be the explanation, I have sometimes observed the same staircase effect in plants (fig. 19).
Fig. 19.—‘Staircase Effect’ in PlantFig. 19.—‘Staircase Effect’ in Plant
Fig. 19.—‘Staircase Effect’ in Plant
Fatigue.—It is assumed that in living substances like muscle, fatigue is caused by the break down ordissimilation of tissue by stimulus. And till this waste is repaired by the process of building-up or assimilation, the functional activity of the tissue will remain below par. There may also be an accumulation of the products of dissimilation—‘the fatigue stuffs’—and these latter may act as poisons or chemical depressants.
In an animal it is supposed that the nutritive blood supply performs the two-fold task of bringing material for assimilation and removing the fatigue products, thus causing the disappearance of fatigue. This explanation, however, is shown to be insufficient by the fact that an excised bloodless muscle recovers from fatigue after a short period of rest. It is obvious that here the fatigue has been removed by means other than that of renewed assimilation and removal of fatigue products by the circulating blood. It may therefore be instructive to study certain phases of fatigue exhibited under simpler conditions in vegetable tissue, where the constructive processes are in abeyance, and there is no active circulation for the removal of fatigue products.
It has been said before that the E.M. variation caused by stimulus is the concomitant of a disturbance of the molecules of the responsive tissues from their normal equilibrium, and that the curve of recovery exhibits the restoration of the tissue to equilibrium.
No fatigue when sufficient interval between successive stimuli.—We may thus gather from a study of the response-curve some indication of the molecular distortion experienced by the excited tissue. Let us first take the case of an experiment whose record is giveninfig. 20,a. It will be seen from that curve that one minute after the application of stimulus there is a complete recovery of the tissue; the molecular condition is exactly the same at the end of recovery as in the beginning of stimulation. The second and succeeding response-curves therefore are exactly similar to the first,provided a sufficient interval has been allowed in each case for complete recovery. There is, in such a case, no diminution in intensity of response, that is to say, no fatigue.
We have an exactly parallel case in muscles.‘In muscle with normal circulation and nutrition there is always an interval between each pair of stimuli, in which the height of twitch does not diminish even after protracted excitation, and no fatigue appears.’[10]
Fig. 20.—Record Showing Diminution of Response when Sufficient Time is not Allowed for Full RecoveryFig. 20.—Record Showing Diminution of Response when Sufficient Time is not Allowed for Full RecoveryIn (a) stimuli were applied at intervals of one minute; in (b) the intervals were reduced to half a minute; this caused a diminution of response. In (c) the original rhythm is restored, and the response is found to be enhanced. (Radish.)
Fig. 20.—Record Showing Diminution of Response when Sufficient Time is not Allowed for Full Recovery
In (a) stimuli were applied at intervals of one minute; in (b) the intervals were reduced to half a minute; this caused a diminution of response. In (c) the original rhythm is restored, and the response is found to be enhanced. (Radish.)
Apparent fatigue when stimulation frequency increased.—If the rhythm of stimulation frequency be now changed, and made quicker, certain remarkable modifications will appear in the response-curves. Infig. 20, the first part shows the responses at one minute interval, by which time the individual recovery was complete.
Fig. 21.—Fatigue in CeleryFig. 21.—Fatigue in CeleryVibration of 30° at intervals of half a minute.
Fig. 21.—Fatigue in Celery
Vibration of 30° at intervals of half a minute.
The rhythm was now changed to intervals of halfa minute, instead of one, while the stimuli were maintained at the same intensity as before. It will be noticed (fig. 20,b) that these responses appear much feebler than the first set, in spite of the equality of stimulus. An inspection of the figure may perhaps throw some light on the subject. It will be seen that when greater frequency of stimulation was introduced, the tissue had not yet had time to effect complete recovery from previous strain. The molecular swing towards equilibrium had not yet abated, when the new stimulus, with its opposing impulse, was received. There is thus a diminution of height in the resultant response. The original rhythm of one minute was now restored, and the succeeding curves (fig. 20,c) at once show increased response. An analogous instance may be cited in the case of muscle response, where ‘the height of twitch diminishes more rapidly in proportion as the excitation interval is shorter.’[11]
Fig. 22.—Fatigue in Leaf-stalk of CauliflowerFig. 22.—Fatigue in Leaf-stalk of CauliflowerStimulus: 30° vibration at intervals of one minute.
Fig. 22.—Fatigue in Leaf-stalk of Cauliflower
Stimulus: 30° vibration at intervals of one minute.
From what has just been said it would appear that one of the causes of diminution of response, or fatigue, is the residual strain. This is clearly seen infig. 21, in a record which I obtained with celery-stalk. It will be noticed there that, owing to the imperfect molecular recovery during the time allowed, the succeeding heights of the responses have undergone a continuous diminution.Fig. 22gives aphotographic record of fatigue in the leaf-stalk of cauliflower.
It is evident that residual strain, other things being equal, will be greater if the stimuli have been excessive. This is well seen infig. 23, where the set of first three curvesAis for stimulus intensity of 45° vibration, and the second setB, with an augmented response, for stimulus intensity of 90° vibration. On reverting inCto stimulus intensity of 45°, the responses are seen to have undergone a great diminution as compared with the first setA. Here is seen marked fatigue, the result of overstrain from excessive stimulation.
Fig. 23.—Effect of Overstrain in Producing FatigueFig. 23.—Effect of Overstrain in Producing FatigueSuccessive stimuli applied at intervals of one minute. The intensity of stimulus in C is the same as that of A, but response is feebler owing to previous over-stimulation. Fatigue is to a great extent removed after fifteen minutes’ rest, and the responses in D are stronger than those in C. The vertical line between arrows represents ·05 volt. (Turnip leaf-stalk.)
Fig. 23.—Effect of Overstrain in Producing Fatigue
Successive stimuli applied at intervals of one minute. The intensity of stimulus in C is the same as that of A, but response is feebler owing to previous over-stimulation. Fatigue is to a great extent removed after fifteen minutes’ rest, and the responses in D are stronger than those in C. The vertical line between arrows represents ·05 volt. (Turnip leaf-stalk.)
If this fatigue be really due to residual strain effect, then, as strain disappears with time, we may expect the responses to regain their former height after a period of rest. In order to verify this, therefore, I renewed the stimulation (at intensity 45°) after fifteen minutes. Itwill at once be seen from recordDhow far the fatigue had been removed.
One peculiarity that will be noticed in these curves is that, owing to the presence of comparatively little residual strain, the first response of each set is relatively large. The succeeding responses are approximately equal where the residual strains are similar. The first response ofAshows this because it had had long previous rest. The first ofBshows it because we are there passing for the first time to increased stimulation. The first ofCdoesnotshow it, because there is now a strong residual strain.Dagain shows it because the strain has been removed by fifteen minutes’ rest.
Fatigue under continuous stimulation.—The effect of fatigue is exhibited in marked degree when a tissue is subjected to continuous stimulation. In cases where there is marked fatigue, as for instance in certain muscles, the top of the tetanic curve undergoes rapid decline. A similar effect is obtained also with plants (fig. 24).
Fig. 24.—Rapid Fatigue under Continuous Stimulation in (a) Muscle; (b) in Leaf-stalk of CeleryFig. 24.—Rapid Fatigue under Continuous Stimulation in (a) Muscle; (b) in Leaf-stalk of Celery
Fig. 24.—Rapid Fatigue under Continuous Stimulation in (a) Muscle; (b) in Leaf-stalk of Celery
The effect of rest in producing molecular recovery, and hence in the removal of fatigue, is well illustrated in the following set of photographic records (fig. 25). The first shows the curve obtained with a fresh plant.The effect is seen to be very large. Two minutes were allowed for recovery, and then stimulation was repeated during another two minutes. The response in this case is seen to be decidedly smaller. A third case is somewhat similar to the second. A period of rest of five minutes was now allowed, and the curve obtained subsequently, owing to partial removal of residual strain, is found to exhibit greater response.
Fig. 25.—Effect of Continuous Vibration (through 50°) in CarrotFig. 25.—Effect of Continuous Vibration (through 50°) in CarrotIn the first three records, two minutes’ stimulation is followed by two minutes’ recovery. The last record was taken after the specimen had a rest of five minutes. The response, owing to removal of fatigue by rest, is stronger.
Fig. 25.—Effect of Continuous Vibration (through 50°) in Carrot
In the first three records, two minutes’ stimulation is followed by two minutes’ recovery. The last record was taken after the specimen had a rest of five minutes. The response, owing to removal of fatigue by rest, is stronger.
The results thus arrived at, under the simple conditions of vegetable life, free as they are from all possible complications and uncertainties, may perhaps throw some light on the obscure phenomena of fatigue in animal tissues.
FOOTNOTES:[10]Biedermann,Electro-physiology, p. 86.[11]Biedermann,loc. cit.
[10]Biedermann,Electro-physiology, p. 86.
[10]Biedermann,Electro-physiology, p. 86.
[11]Biedermann,loc. cit.
[11]Biedermann,loc. cit.
When a plant is stimulated at any point, a molecular disturbance—the excitatory wave—is propagated outwards from the point of its initiation.
Diphasic variation.—This wave of molecular disturbance is attended by a wave of electrical disturbance. (Usually speaking, the electrical relation between disturbed and less disturbed is that of copper to zinc.) It takes some time for a disturbance to travel from one point to another, and its intensity may undergo a diminution as it recedes further from its point of origin. Suppose a disturbance originated atC; if two points are taken near each other, asAandB, the disturbance will reach them almost at the same time, and with the same intensity. The electric disturbance will be the same in both. The effect produced atAandBwill balance each other and there will be no resultant current.
By killing or otherwise reducing the sensibility ofBas is done in the method of injury, there is no response atB, and we obtain the unbalanced response, due to disturbance atA; the same effect is obtained by puttinga clamp betweenAandB, so that the disturbance may not reachB. But we may get response even without injury or block. If we have the contacts atAandB, and if we give a tapnearerAthanB(fig. 26,a), then we have (1) the disturbance reachingAearlier thanB. (2) The disturbance reachingAis much stronger than atB. The disturbance atBmay be so comparatively feeble as to be negligible.
It will thus be seen that we might obtain responses even without injury or block, in cases where the disturbance is enfeebled in reaching a distant point. In such a case on giving a tap nearAa responsive current would be produced in one direction, and in the opposite direction when the tap is given nearB(fig. 26,b). Theoretically, then, we might find a neutral point betweenAandB, so that, on originating the disturbance there, the waves of disturbance would reachAandBat the same instant and with the same intensity. If, further, the rate of recovery be the same for both points, then the electric disturbances produced atAandBwill continue to balance each other, and the galvanometer will show no current. On taking a cylindrical root of radish I have sometimes succeeded in finding a neutral point, which, being disturbed, did not give rise to any resultant current. But disturbing a point to the right or to the left gave rise to opposite currents.
It is, however, difficult to obtain an absolutely cylindrical specimen, as it always tapers in one direction. The conductivity towards the tip of the root is not exactly the same as that in the ascending direction. Itis therefore difficult to fix an absolutely neutral point, but a point may be found which approaches this very nearly, and on stimulating the stalk near this, a very interesting diphasic variation has been observed. In a specimen of cauliflower-stalk, (1) stimulus was applied very much nearerAthanB(the feeble disturbance reachingBwas negligible). The resulting response was upward and the recovery took place in about sixty seconds.
Fig. 26.—Diphasic VariationFig. 26.—DiphasicVariation
Fig. 26.—DiphasicVariation
(2) Stimulus was next applied nearB. The resulting response was now downward (fig. 26,b).
(3) The stimulus was now applied near the approximately neutral pointN. In this case, owing to a slight difference in the rates of propagation in the two directions, a very interesting diphasic variation was produced (fig. 26,c). From the record it will be seen that the disturbance arrived earlier atAthan atB. This produced an upward response. But during thesubsidence of the disturbance atA, the wave reachedB. The effect of this was to produce a current in the opposite direction. This apparently hastened the recovery ofA(from 60 seconds to 12 seconds). The excitation of A now disappeared, and the second phase of response, that due to excitation ofB, was fully displayed.
Positive after-effect.—If we regard the response due to excitation ofAas negative, the later effect onBwould appear as a subsequent positive variation.
In the response of nerve, for example, where contacts are made at two surfaces, injured and uninjured, there is sometimes observed, first a negative variation, and then a positive after-effect. This may sometimes at least be due to the proximal uninjured contact first giving the usual negative variation, and the more distant contact of injury giving rise, later, to the opposite, that is to say, apparently positive, response. There is always a chance of an after-effect due to this cause, unless (1) the injured end be completely killed and rendered quite irresponsive, or (2) there be an effective block betweenAandB, so that the disturbance due to stimulus can only act on one, and not on the other.
I have found cases where, even when there was a perfect block, a positive after-effect occurred. It would thus appear that if molecular distortion from stimulus give rise to a negative variation, then during the process of molecular recovery there may be over-shooting of the equilibrium position, which may be exhibited as a positive variation.
Positive variation.—The responses given by muscle or nerve are, normally speaking, negative. But that of retina is positive. The sign of response, however, is apt to be reversed if there be any molecular modification of the tissue from changes of external circumstances. Thus it is often found that nerve in a stale condition gives positive, instead of the normal negative variation, and stale retina often gives negative, instead of the usual positive.
Fig. 27.—Abnormal Positive Responses in Stale Leaf-stalk of Turnip converted into Normal Negative under Strong StimulationFig. 27.—Abnormal Positive Responses in Stale Leaf-stalk of Turnip converted into Normal Negative under Strong Stimulation[12]The relative intensities of stimuli in the two cases are in the ratio of 1:7.
Fig. 27.—Abnormal Positive Responses in Stale Leaf-stalk of Turnip converted into Normal Negative under Strong Stimulation[12]
The relative intensities of stimuli in the two cases are in the ratio of 1:7.
Curiously enough, I have on many occasions found exactly parallel instances in the response of plants. Plants when fresh, as stated, give negative responses as a rule. But when somewhat faded they sometimes give rise to positive response. Again, just as in the modified nerve the abnormal positive response gives place to the normal negative under strong and long-continued stimulation, so also in the modified plant the abnormal positive response passes into negative(fig. 27) under strong stimulation. I was able in some cases to trace this process of gradual reversal, by continuously increasing the intensity of stimulus. It was then found that as the stimulus was increased, the positive at a certain point underwent a reversal into the normal negative response (fig. 28).