Plant Response as a Means of Physiological Investigation

mum effect has been attained, that the Response of Plants individual effects of stimuli sometimes {Style oi Datura aOa) become distinguishable by slight oscillatory movements of the curve. In the case of rhythmic cardiac muscle, however, there is no tetanus ; and similarly, as described in Chapter XXVII., we find no tetanus in the rhythmic vegetable tissue of Desmodium. There is a minimal intensity of stimulus necessary to initiate response. A stimulus, singly ineffective, becomes effective on repetition.

Increasing intensity of stimulus produces increasing response, which, however, tends to approach a limit. The effects of rapidly succeeding stimuli, in plant-tissues, as in animal, become fused, individual effects being then indistinguishable. A maximum contractile effect is then produced depending on the intensity of stimulus. In all the above respects, we find that the responses oi plants in general exhibit the closest parallel to the responses of skeletal muscle in animals.

But in the animal a different type of response is exhibits by certain rhythmic tissues like cardiac muscle. The response here is on the ' all or none ' principle, and such a tissu< cannot be tetanised. In the parallel instances of rhythmic vegetable tissues, the same characteristics are present ; that is to say, the responses are on the ' all or none ' principle, and there is no tetanus. Uniform response in plants — Staircase effect — Fatigue due to molecular strain — Fatigue in plant-responses — Periodic fatigue — Fatigue under continuous stimulation — Explanation of anomalous erection of leaf of Mimosa under continuous stimulation — Conductivity and excitability of tissue diminished through incomplete protoplasmic recovery— Relatively greater fatigue in a motile than conducting organ — Disappearance of the motile excitability earlier than conductivity— Refractory period — Absence of responsive effect when stimulus falls within refractory period.

The mechanical response of plants is fundamentally due, as we have seen, to those molecular changes which are the result of stimulus. These changes bring about contractions of the excited cells, in consequence of which water is expelled, and we obtain longitudinal response in radial organs, or lateral movement in dorsi-ventral organs, the latter being simply a special case of differential longitudinal contraction. On the cessation of stimulus the expelled water is reabsorbed, and the organ resumes its original position. In the case, for example, of the leaves of Mimosa, this position of equilibrium is, approximately speaking, at an angle of 45 ° above the horizon, and this, for convenience, may be called the erect position. After a period of rest, then, molecular equilibrium being re-established, the protoplasm recovers its original properties, of which excitability is one, and response takes place on stimulation as before. This resumption by the leaf of its original position may thus be taken as a rough indication of the restoration of its original protoplasmic properties. But this is only true in a general way, for there may be cases, as we shall see, in which the apparent return of the leaf

to its original position is deceptive, and does not really indicate a complete protoplasmic recovery. Uniform responses. — If the motile organ, however, be restored, by an appropriate period of rest, to exactly its original molecular condition, and therefore to its original condition of excitability, it is clear that we ought to be able to obtain uniform responses to uniform stimuli. That this is true has been shown to be the case, with regard to the responses of the leaflet of Biophytum, and the longitudinal contractions of various radial organs (figs. 18 and 38). By taking electromotive instead of mechanical responses, I obtained a similar result, of uniform responses to uniform stimuli, from various species of plants (fig- 51)- I" tne case °f muscle, also, the responses are found to be uniform, if intervening periods of rest be allowed? sufficient for full recovery (fig. 9).

1 Staircase' effect. —It is sometimes found that a tissue falls into a sluggish Mechanical Response condition, and successive stimuli, by Eucharis) ^ & increasing molecular mobility, have the effect of gradually enhancing the responses, which are seen to increase in a ' staircase ' manner. I give here an instance of this effect (fig. 52) in the case of longitudinal response, obtained with a style of Eucharis Lily. Fig. 53. Fatigue in Longitudinal Mechanical Response of Plant (Style of Datura)

In the case of the first pair of responses, a sufficient interval for recovery, namely one minute, was allowed. When the period allowed for recovery was reduced to half a minute, there was rapid fatigue ; the second pair of responses shows this immediate effect ; the third pair of responses are to the tenth and eleventh stimuli. Fatigue. — It has been said that, when sufficient time is allowed for protoplasmic recovery, the responses are uniform, but that, if sufficient time be not allowed, molecular recovery will be incomplete, and the tissue will remain in a strained condition. Under these circumstances, it is obvious that there will not be a complete restoration of the original protoplasmic excitability, hence successive responses will exhibit a diminution or fatigue. The following record (fig. 53) shows this in the case of longitudinal response. Uniform stimuli were first applied at intervals of one minute, by which time the recovery was complete ; and these responses of twenty divisions are seen to be large and uniform. The stimuli were next applied at intervals of half a minute, and the response at once fell to eleven divisions. Now, owing to the effect of cumulative strain, the succeeding responses at this interval underwent

continuous diminution, and Fig. 54. Fatigue shown in Electrical Re- 1 1 r 11 , , sponse, when sufficient Time is not allowed five divisions at the tenth in (^) stimuli were applied at intervals of one Stimulus. In the next minute ; in (^ the intervals were reduced to half a minute ; this caused a diminution figure (fig. 54), fatigue is of response. In (c) the original rhythm is i,,,,^ • .1 «1A««...:„„1 restored, and the response is found to be en-

shown in the electrical hancedt'onearly its original value (Radish). the same conditions, sufficient time for recovery, that is to say, not being allowed. Similar instances of fatigue are well known in the case of muscle. Fatigue being principally due to residual strain, it is to be expected that, other things being equal, strain will be more persistent with stronger stimulus, as has been shown in the last chapter. It was there shown also that the period required for recovery from a strong was more protracted than from a moderate stimulus. From this it follows that the stimulation-frequency which will exactly allow for complete recovery, and so give rise to uniform responses, in the case of a moderate stimulus, will not be sufficient for stronger stimulus. Hence, keeping the intervals constant, we may

FlG. 55. Alternate Fatigue (a) in Electrical Responses of Petiole of Cauliflower ; (b) in Multiple Electric Responses of Peduncle of Biophytum ; (/) in Multiple Mechanical Responses of Leaflet of Biophytum ; and [d] in Autonomous Responses of Desmodiiun obtain uniform responses to moderate, and diminished or fatigue-responses to strong stimulus. There is another curious phenomenon, of alternate or periodic fatigue, which I have often observed in the response of plants. The simplest type of such periodic fatigue is that in which responses wax and wane, in regular alternation. Such alternate fatigue is sometimes seen in the electrical response of plants, the multiple response of Biophytum, and also in the autonomous response of Desmodium (fig. 55). Curiously enough, I have sometimes obtained similar alternate responses with the compound strip of ebonite and indiarubber, previously described. There are other cases of response in plants, where the variations are cyclic in type, that

is to say, they consist of groups of responses, which wax and wane alternately. Fatigue under continuous stimulation. — In connection with the induction of fatigue under long-continued stimulation, I shall now anticipate certain results regarding the character of the responses given by matter universally — results to be described in detail in the next chapter. It will there be shown that if we represent a given molecular change, caused by stimulus, as positive, the continuation of stimulus will at first increase that change, until it has attained a maximum, after which, under the still continued action of the same stimulus, there will be a reversal or change to the negative. In a living tissue, then, where the incidence of stimulus causes contraction, we may be prepared to find that the same stimulus, long continued, will bring about a reversal of this effect, or, that is to say, a relaxation.

In any case, such a reversal is illustrated in the corresponding phenomenon in contractile muscle. It is there found that, whereas the first effect of stimulus is contraction, the same stimulus, when too long continued, brings about relaxation to the original form. In the electrical response of plants under continuous stimulation, I also find this peculiar fatigue-reversal (fig. 56). I have also obtained similar fatigue-reversals in the longitudinal response of radial organs. In fig. 56, (a), a series of tetanic electric shocks was continuously applied during a period of four minutes. The maximum contraction was attained in the course of one minute, after which there was a reversal, or relaxation. Under this condition of fatigue-

Fig. 56. Rapid Fatigue under Continuous Stimulation in (a) Muscle ; (b) Leaf-stalk of Celery (Electrical Response) reversal, the tissue is incapable of the normal excitatory contraction, but after a period of rest of about seven minutes it Fig. 57. Fatigue under long-continued Stimulation in the Contractile Response of Plants (a) Stimulation by tetanising electric shocks ; {b) stimulation by rapidly succeeding thermal shocks. Continuous lines represent action during stimulation ; dotted lines represent after-effect (coronal filament of Passiflora, magnification forty times).

again gives response, which is at first normal, and then, after reaching a maximum, becomes reversed. The second response is, however, seen to be smaller than the first. I obtained parallel results under the action of rapidly succeeding thermal shocks I have already described curious instances of alternating fatigue exhibited in successive single responses to single stimuli (fig. 54). A very curious and interesting effect of this nature occurring under continuous electric stimulation, is shown in the accompanying photographic record (fig. 58) of responses given by the filament of Uriclis Lily. It will

Fig. 58. Photographic Record of Periodic Fatigue under Continuous Stimulation in Contractile Response (Filament of Uriclis Lily) there be noticed that the maximum contraction is attained in the course of three minutes, after which there is a fatiguerelaxation, which continues up to the eleventh minute. There then occurs a second, though much feebler, response, after which comes a slow and continuous reversal action. So-called ' anomalous ' response in Mimosa. — In connection with the subject of fatigue, I shall here enter upon the explanation of certain well-known responsive effects in the case of Mimosa which have hitherto been regarded as anomalous. It is generally found that an erect leaf of this plant is sensitive, that is to say, when stimulated it becomes depressed. In this depressed position it is apparently insensitive, hence we are apt to assume that the erect posture is one of sensitiveness, depression indicating the reverse. It will be found, however, that if a Mimosa leaf be continuously stimulated by successive blows or taps, in the manner of Pfefifer's experiment, the leaf will at first fall. But, though the blows be continued, the petiole will, after a time, return to its normal erect position. In this erect posture, however, further blows prove to have no effect upon it, the leaf being now insensitive.

I give above a photographic record of this effect in Mimosa (lower record, fig. 59), continuous stimulation in this case having been produced by tetanic electric shocks. It will be noticed that after its responsive fall the leaf returns to the erect position, in spite of the fact that stimulus is Fig. 59. Photographic Records of Normal Response of Mimosa to Single Stimulus (upper figure), and to Continuous Stimulation (lower (figure) In the latter case the leaf is erected in spite of continuous stimulation.

being continued. It is important to note that the two records were taken with the same specimen, and in immediate succession to each other. The first, or upper, records the response of the leaf to a single stimulus and its recovery ; the lower gives the response to continuous stimulation. In appearance the two records are singularly alike. But though the leaf at the end of each response occupies the same position, the molecular conditions in the two cases are, as will be shown presently, entirely different, inasmuch as in the first, renewed response was immediately obtained, thus showing its sensitive condition ; while in the second, the organ was insensitive, and could give no response until after a period of rest of about ten minutes.

The explanation of this apparent anomaly is quite clear from the experiments which have already been described, showing that under continuous stimulation the normal longitudinal contraction undergoes reversal and passes into relaxation, as is also the case with continuously excited muscle, the motile response of Mimosa being only an instance of differential longitudinal contraction. We obtain here also the usual sequence of first, normal contraction, and second, the fatigue-relaxation, or posture of erection.

To sum up, then, it is clear that our association of the erect position with sensitiveness is not always correct, for the leaf may assume this posture as the result of fatigue. That its position in this case, however, though outwardly imitating that of sensitiveness, is profoundly different, is at once revealed on application of stimulus. The leaf is now irresponsive. But if we allow it a period of rest — of some eight or nine minutes in summer, or double that time in winter — the internal molecular equilibrium is re-established. But of this internal readjustment the leaf gives no visible indication. It remains in the same unchangingly erect position. During the course of the cycle, then, it has passed from the normal erect to the normal depressed, and thence to the abnormal erect position. It next passes, without any outward change, from this abnormal erect to the normal erect position, after

a period of repose. And this return of the leaf to its normal condition is testified by its once more responding to stimulus. Fatigue of conductivity and excitability. — It has already been pointed out that the protoplasmic properties of a tissue cannot be restored to their original condition after stimulation, without the intervention of a suitable period for the reestablishment of molecular equilibrium. This fact I shall now demonstrate by additional experiments.

The restoration of the normal protoplasmic condition in a tissue may be tested by observing the recovery of some of those properties which are capable of measurement. One of these is its conductivity, measured by determining the speed with which excitation travels through the tissue, the method of which determination will be fully described in Chapter XX. Under normal conditions this velocity is constant If we excite the tissue, and measure the rate at which the excitation travels, and if we then allow a sufficient interval of rest for complete protoplasmic recovery and again determine this velocity, we shall find that the two are the same. But, if the necessary resting interval be not allowed, recovery being incomplete, there will remain a residual strain. The velocity of transmission of excitation will, under these circumstances, be found to be reduced.

Another protoplasmic property which is capable of measurement is the excitatory contraction seen in motile organs, measurable by the amplitude of response. We have seen that this amplitude is constant under normal conditions, and when sufficient intervening periods of rest are allowed. But diminution of the intervening resting period produces diminution of the amplitude of response. From what has been said it follows that, if the intervening resting periods of a tissue be continuously diminished, there will be a continuously increasing residual strain, and this might be detected by the consequent continuous decrease of conductivity and motile excitability. I have been able to verify this deduction by an experiment on a leaf of Biophytum, the details of which will be found in Chapter XX.

It will there be seen that, as the resting period was gradually shortened to half a minute, the conductivity underwent diminution from the normal r88 mm. to 1*54 mm. per second, that is to say, by 18 per cent. The reduction of motile excitability was found, however, to be still more marked, the height of response being reduced from the normal thirtyfour to one division, that is to say, by as much as 97 per cent. On still further reducing the period of rest, from half a minute to ten seconds, it was found that there was no motile response whatever. The tissue is thus seen to be altogether incapable of response during a certain refractory period. The timevalue of this refractory period not only differs in different plants, but also varies with the physiological condition. In Biophytum, normally speaking, it is ten seconds, and in Mimosa about one minute.

Earlier abolition of motile excitability than of conductivity.— We have seen in the last experiment, that while the conductivity of the petiole was reduced by 18 per cent, the motile excitability of the attached leaflet underwent a diminution of 97 per cent. This shows that motile excitability disappears earlier than conductivity. The reason will be apparent if we consider the difference between the two expressions of protoplasmic excitation. We have, in the conduction of \the state of excitation from point to point, a direct expression of the transmission of the molecular change initiated by stimulus. The motile response, however, is a somewhat remote consequence of the series of events which follows on the fundamental molecular change. Intermediate occurrences are the permeability variation and the contraction, in this case differential, which it produces. The movement of the leaf is a result of all of these, and depends for its complete fulfilment on certain favourable circumstances. In any case, there are mechanical obstacles which have to be overcome in forcing the expelled water through channels of escape. That this must involve some degree of waste of force is partly seen in the fact that all excitations do not produce response, it being necessary that the stimulus should

exceed a certain minimum value in order to produce any movement at all. If, again, the escape of water should be resisted, owing to the peculiar condition of the tissue, which has already been described (p. 49), then even a strong stimulus would be unable to bring about movement. Analysis of the different phases in the response of Mimosa under continuous stimulation. — Having now considered in detail some of those changes of protoplasmic properties which are brought about by the action of stimulus, we are enabled to study the effect of long-continued stimulation

Fig. 60. Ineffectiveness of Stimuli, owing to Increasing Fatigue, in Mimosa In the left-hand figure stimuli were applied at intervals of 3-5 minutes. These evoked feeble responses. In the right-hand figure stimuli were applied at intervals of two minutes. Response now became inconspicuous. Where stimuli were applied at intervals of one minute no effect was produced. The leaf was refractory. from a somewhat different point of view. We saw that with incomplete recovery, protoplasmic excitability was progressively diminished. In order to demonstrate this, in the case of Mimosa, I obtained responses at intervals of 3*5 minutes, the uniform stimulus of condenser discharge being employed. The responses, which had been uniform, when stimulus was applied after complete recovery, at intervals of about eight minutes, were now found to be very much reduced (fig. 60). In a second series of experiments on the same specimen, the intervening periods of rest were still further

reduced, to two minutes ; and it will be noticed that, owing to increasing incompleteness of recovery, the responses were here reduced to the merest indications of twitches. When the intervening periods were still further shortened to less than one minute, the stimuli fell within the refractory period of the tissue, and produced no indication whatsoever of their effect. As an extreme instance, we have the effect of continuous stimulation, already described on p. 109. To be precise, we must remember that in this gradual abolition of response, under quickening stimulation, we not only see the action of diminished excitability, but also of diminished conductivity.

The erection of the leaf of Mimosa, by the relaxing action of fatigue, may also be assisted by the later contraction of the upper half of the pulvinus. For we have seen that in an anisotropic organ, the less excitable half responds subsequently to the more excitable. The contraction of the upper half of the pulvinus in Mimosa would produce erection of the leaf, and that this might sometimes happen appears probable from the fact that in the erection of the leaf under continuous stimulation it is occasionally found to be lifted above its normal position.

Stimulus, by causing molecular derangement, brings about mechanical response ; and by molecular transmission of disturbance from point to point, the excitation is conducted to a distance. Excitatory mechanical response and conduction of excitation are different expressions of the effect of stimulus After a period of rest from the action of stimulus, there is a restoration of molecular equilibrium. The protoplasmic properties of excitability and conductivity are then completely restored. Under such normal conditions responses are uniform.

A tissue in a sluggish condition has its molecular mobility increased by the action of successive stimuli. This produces the ' staircase ' effect, of gradually enhanced responses. molecular strain. The conductivity and excitability of an organ are thus diminished, and the responses undergo diminution, in consequence of cumulative residual strain. Fatigue is greater in a motile than in a conducting organ. Motile excitability disappears earlier than conductivity.