Plant Response as a Means of Physiological Investigation

The anomalous erection of the Mimosa leaf is brought about by the combined effects of diminution of conductivity, abolition of excitability, and possibly, in some instances, by the subsequent excitation of the upper half of the pulvinus also. The chemical theory of response — Insufficiency of the theory of assimilation and dissimilation to explain fatigue and staircase effects — Similar responsive effects seen in inorganic substances — Molecular theory — When molecular recovery is complete, responses uniform : when incomplete, fatigue brought about by residual strain — Fatigue under continuous stimulation, in inorganic substance, in plant, and in muscle — Staircase effect brought about by increased molecular mobility : examples seen in inorganic substance, and in living tissues — No sharp line of demarcation in the borderland between physical and chemical phenomena — Molecular changes attended by changes of chemical activity — Unequal molecular strain gives rise to a secondary series of chemical actions— Voltachemical effect and by-products — Supposition that response always disproportionately larger than stimulus, not justified — Existence of three types : (i) response proportionate to stimulus ; (2) response disproportionately greater than stimulus ; (3) response disproportionately less than stimulus — Instances of stimulus partially held latent : staircase and additive effects ; multiple response ; renewed growth.

It has already been shown, in previous chapters, that the various types of response met with in animal tissues are exactly paralleled, even in detail, in the response of plants ; and numerous further instances of this fact will be met with in the course of the present work. It would thus appear that the theoretical explanation of either class of responses must be applicable to the other also. Existing theories regarding animal response, however, have not yet been found sufficient to meet all the difficulties of the case, and it is probable that the larger data now made available by the inclusion of response in plants, may go far to throw light on certain obscurities which are at present regarded as perplexing.

Chemical theory of assimilation and dissimilation. — The theory which is generally accepted at present may be referred to briefly as chemical. According to it, living matter is maintained in a state of equilibrium by the two opposed chemical processes of building up or assimilation, and breakdown or dissimilation. Stimulus causes a down or dissimilatory change, which is again compensated, during recovery, by the building up, or assimilative change. In the case of uniform responses, the two processes exactly balance each other. But on occasions when the down change is the greater of the two, the potential energy of the system falls below par, for the building-up process cannot then sufficiently repair the chemical depreciation caused by the downward change. Hence occurs diminution of response, or fatigue, which is supposed to be further accentuated by the production and accumulation of deleterious ' fatigue-stuffs.' The disappearance of fatigue, after a period of rest, is explained by the renovating action of the blood supply, which is also regarded as the means of carrying away the fatigue-stuffs.

A serious objection to these explanations, however, lies in the fact that even excised and bloodless muscles exhibit recovery from fatigue, after a period of rest. In isolated vegetable tissues, again, where there is no active circulation of renovating material, the same effect, and its removal after a period of rest, are observed. Thus the difficulties met with in explaining fatigue according to a purely chemical theory are great enough. But still greater are those which we encounter when we come to deal with the staircase effect — typically shown in cardiac muscle — in which successive responses to uniform stimuli exhibit a gradual enhancement of amplitude. Here the result obtained is in direct opposition to the theory described ; for in this particular case, we have to assume that the same stimulus which is usually supposed to cause a chemical break-down becomes efficient to produce an effect exactly the reverse. It is true that the heart, usually speaking, is charged with blood ; but this particular staircase increment of response, under uniform stimulation, is observed even in the initial twitches of bloodless muscle (fig. 64), and here there can be no question of a supply of renovating blood.

Parallel types of response in living organic, and in inorganic matter. — Such being the difficulties involved in the explanation of a single class of phenomena, on the chemical hypothesis of assimilation and dissimilation, it may be well next to turn our attention to the conclusions suggested by the observation of response in matter generally. And for this purpose it is best to take the responses obtained from inorganic matter in particular, the hypothetical assimilation and dissimilation being in that case out of the question. With regard to the mode of observation, I have already explained how the molecular derangement consequent on stimulus may be studied, either (i) by recording the change of form ; or (2) by recording the variation of conductivity ; or (3) by recording the electromotive variation.

As an example in the first place of responsive contraction in inorganic matter, we may select for our investigation the response of india-rubber, under thermal stimulation. In this case, chemical changes, either up or down, are impossible. The second, or conductivity variation method, may be used in the case of metallic powders, the stimulus being that of Hertzian radiation. In this case also chemical action may be excluded, the experimental material being usually placed in naphtha. In the third case, again, where response is obtained by means of the electromotive variation, under mechanical stimulus, the substance used is platinum, the most chemically inactive of metals, electrolytic contacts being made by water.1 The possibility of chemical action is thus reduced to a minimum, and the assimilatory change entirely excluded.

It will be found, however, that in all these cases of inorganic response, in which substances, physically and chemically widely unlike, are subjected to diverse forms of stimulation, and have their responses tested and recorded by absolutely different methods, the results obtained are exactly parallel. All alike, when sufficient intervening periods of rest are 1 For details of these investigations and results, see Bose, Response in the Living and Non-Living.

restoration of molecular equilibrium, and for this purpose it not essential. When sufficient time, however, is not allowed that in the case of Mimosa, as in that sents the duration of of muscle, a complete reversal of £-{*-■ O™* response is brought about by extreme variation method. fatigue, under continuous stimulation. The following record shows a similar reversal in Arsenic, under the continuous stimulation of electric radiation (fig. 61). It was only after a sufficient interval of rest that this substance gave renewed normal response. It may be added that these fatigue-reversals, as in the longitudinal response of the Uriclis Lily, are sometimes found to be recurrent.

This curve of fatigue-reversal in Arsenic under continuous stimulation was obtained by recording the changes of electric conductivity in the substance. A still more striking analogy with the mechanical records of fatigue in plants and animals is afforded, however, by the automatic record given in fig. 62 of contractile responses in india-rubber. When this substance is excited by rapidly succeeding thermal shocks, we obtain first, the normal contractile effect, and secondly, the relaxation due to fatigue, in a manner exactly similar to that which characterises the fatigue-reversals of Mimosa and of skeletal muscles. In the present case, the india-rubber attained its maximum contraction in the course of two minutes, after which there was a continuous relaxation.

In this response of india-rubber, and in its fatigue-reversal, we have an analogy with the response of living animal tissues, such as muscle, so close as to compel us to the conclusion that both alike are phenomena of molecular response. A mere contraction of the indiarubber might have been supposed to be due to the specific action of heat on that substance. And had this been all, successive thermal shocks would have had the effect of continuously increasing the contraction, till a limit was reached. But if, on the other hand, molecular excitability be a factor in the process of response, we might then expect, under certain conditions of fatigue, a loss of molecular excitability. That this is actually the case is shown by the fatigue-reversal, attended with relaxation, which is seen in the figure.

A muscle, again, in this condition of fatigue-relaxation, after a period of rest — during which the molecules have time for recovery from their state of strain — becomes once more excitable without any visible change. We found a like phenomenon occurring in the case of fatigued and relaxed Mimosa, under continuous stimulation. In that case it was, as we saw, a period of rest of about eight or ten Fig. 62. Automatic Record of Fatigue in the Contractile Response of Indiarubber under Rapidly Succeeding Thermal Shocks

The first effect of stimulus is contraction, but this passes into relaxation under continued stimulation. The time-marks represent intervals of half a minute. minutes, which made the fatigued Mimosa once more contractile. Returning to india-rubber, we find that here also a period of rest of ten minutes enables it to recover its excitability, and once more exhibit its responsive contraction. From these facts it would appear that in order to explain the phenomenon of response, and its various modifications by fatigue and other factors, we have no option but to regard it as an expression of the molecular responsiveness of matter in general.

We next turn to the converse phenomenon of staircase response. Since response is an expression of molecular Fig. 63. Preliminary Staircase In" crease, followed by Fatigue, in the Response of Galena to Hertzian Radiation Fig. 64. Preliminary Staircase Increase, followed by Fatigue, in the Response of Style of Eucharis derangement, it follows that its extent will, other things being equal, depend on the degree of molecular mobility. This being so, it is conceivable that a substance, at first in a sluggish condition, may, by the impact of successive stimuli, have its molecular mobility gradually increased, with a corresponding enhancement of its response. In fig. 63 we have an example of this staircase effect, in the responses of Galena. It is to be noticed that this effect occurs at the beginning of the series of responses, as we should expect. After the attainment of maximum mobility, the phenomenon of overstrain is seen, with its accompanying diminution of response, or fatigue. I have obtained similar results in the longitudinal

response of plants (fig. 64). It need only be mentioned, further, that exactly the same preliminary staircase effect, reaching a maximum, and followed by fatigue, is found in the responses of muscle (fig. 65). The inference that the occurrence of the staircase effect is due to the gradual removal of molecular sluggishness, receives further support from the following experiment on a plant where sluggishness is brought on artificially, and made to disappear gradually. Sluggishness may be induced in a tissue by cooling, this condition being made to disappear, by the gradual return of the substance to the ordinary temperature of the room. On carrying out such an experiment,

Fig, 65. Preliminary Staircase, followed by Fatigue, in the Responses of Muscle (Brodie) Fig. 66. Staircase Increase in Electrical Response of Petiole of Bryophyllum, rendered sluggish by cooling and recording the successive electrical responses to successive uniform stimuli, I obtained, as I had expected, a marked staircase effect (fig. 66). Merging of physical into chemical phenomena.— I have explained that responsive phenomena are primarily due to the molecular distortion caused by stimulus, that is to say, they are of a physical character. Fatigue has also been shown to be primarily due to residual strain. But it must be borne in mind that in the borderland between physics and chemistry there is no sharp line of demarcation. For example, yellow phosphorus under the stimulus of light is

converted into the molecular red, or allotropic variety. This molecular change, however, is also attended by a concomitant change in the chemical activity, phosphorus in the red condition being less active chemically than in the yellow. Under certain circumstances, further, it is possible to have a secondary series of chemical events following upon a condition of unequal molecular strain. We have seen that a homogeneous living tissue, when unstimulated, is iso-electric. When stimulated, however, an electromotive difference is induced, as between the more and the less acted parts of the tissue. The result is an electrical current attended by chemical changes. As a consequence of such volta-chemical action, when prolonged, by-products (fatigue-stuffs?) may be accumulated, and these may have a depressing effect on the activity of the tissue. Hence, just as after very prolonged activity of a voltaic element it is necessary to renew the active element and change the electrolyte surcharged with by-products, so, after sustained activity of a living tissue, the process of renewal, or renovation, will be necessary. Thus we see how, upon the fundamental molecular derangement, a chain of very various chemical events may follow as its after-effect. And it is only by going in this way to the very root of the phenomenon, that we can avoid the many contradictions with which we are confronted by the chemical theory.

I have therefore aimed at demonstrating the universal existence in matter of the property of responsiveness, and by taking the simplest cases and excluding as many complicating factors as possible, have attempted to show further how this power of response is modified by those conditions which occasion fatigue, or its converse, the staircase effect. Having thus cleared the ground, it is possible to take up cases of greater complexity. Different modes of transformation of stimulus. — It has been assumed that response is brought about by a sudden explosive chemical change, the stimulus acting as if on a trigger, for the release and run-down of potential chemical

energy. This implies that response is disproportionately greater than stimulus, and that the responsive change is attended by an evolution of heat and chemical by-products. It would follow, however, from such rapid depreciation, that fatigue must be the invariable consequence of any series of responses. But it is notorious that in the responses of nerve, not only is there no fatigue, but neither is there any evolution of heat, nor occurrence of chemical change, that can be detected.

As the simplest case, we have hitherto considered the substance acted on to be neutral in its character, that is to say, not active in the sense of being able to absorb stimulus and hold it latent. But if we regard the living organism as a machine, three different cases are conceivable. These are : first, that in which the responding substance simply converts the energy received as stimulus into response ; secondly, that in which the responding substance possesses a large amount of energy, some of which is set free by the action of the stimulus ; and lastly, that in which the responding substance is capable of absorbing and holding latent, to a greater or less extent, the stimulus which it receives.

(i) Response proportionate to stimulus. — The first of these types may be illustrated by a responding circuit which contains a magnetic motor— say a galvanometer — translating current into motion. The source of stimulus may be an external battery periodically closed by a tapping key. The waste of energy by the production of heat may be supposed to be brought down to a minimum in this circuit by using a feeble current and reducing the resistance. Uniform stimuli will now cause uniform responses of the responder. Response will be proportionate to stimulus, and there will be no chemical, and no appreciable thermal, change in the responder. This is a state of things which may be said to approximate closely to the responsive peculiarities of the nerve.

(2) Response disproportionately greater than stimulus : responding substance reduced below par. — As an example of the second type, we have to imagine a responding system which contains a large amount of energy. We may suppose it to consist of a storage battery and a galvanometer, the circuit being normally incomplete. External stimulus may, by some easily arranged mechanism, close the responding circuit periodically. In this case, the responses might be made disproportionately larger than the stimulus. There will then be a progressive run-down of the latent energy of the system, and the responses will show diminution, or true fatigue. The system, at the end of the experiment, will be found to be below par. This case may be paralleled by that of highly excitable tissue which is wasted under excessive and longcontinued stimulation.

(3) Stimulus wholly or partially absorbed. — The third type of substance we have supposed to be one which is capable of absorbing and holding latent, to a greater or less extent, the stimulus which it receives, and under this we may have several important sub-cases. (a) Staircase and additive effects. — The absorbed stimulus may gradually enhance the molecular mobility, with gradual enhancement of response. This is seen exemplified in the staircase effect. Another example of the same thing is probably to be found in the singly ineffective stimulus which becomes effective on repetition. Evidently in this case, the energy of the first few stimuli is held latent in the tissue and added up until it reaches the threshold of response.

[b) Multiple response. — We can next see the possibility of a very interesting case of stimulus becoming latent. A spring which is immersed in a viscid fluid may, on receiving a feeble blow, give a single vibrational response. But if the blow be powerful, this single strong stimulus will give rise to a multiple series of vibrational responses. Again, a phosphorescent substance acted on by light absorbs it, and, on the cessation of incident stimulus, continues to give up the excess of latent energy thus acquired in the form of luminous vibration. A selenium cell again, when acted on by a single strong flash of light, I have found to give what may be regarded as two responses, one strong and the other feeble.

From certain metallic particles, again, when exposed to a single strong flash of electric radiation, I have obtained pulsatory responses. From the consideration of all these cases, I was led to investigate the question whether a planttissue, when acted on by a single strong stimulus, could be found to give similar repeated responses. From this I was led to the discovery of Multiple Response in plants, a phenomenon which I find to be very prevalent, and which I shall describe fully in Chapter XX.

(c) Response disproportionately less than stimulus : responding substance raised above par. — It is the last of these subcases, however, which we could least easily have foreseen. It is as if here the active expression of the incident stimulus were bifurcated. Returning once more to the mechanical model, we may imagine the responding portion of the system to contain, besides the mechanically responding galvanometer, two plates of lead, immersed in dilute sulphuric acid, by which the energy of the stimulating current is partially stored up. The external stimulating current has now to do two things, first, to cause mechanical response in the galvanometer, and secondly, to store up an increasing amount of latent energy in the second part of the responding system. It is evident here that, owing to increasing storage, the mechanical response of the galvanometer may become progressively less. We are, perhaps, too apt to ascribe to 'fatigue' all cases of diminution of responses. For in this case we shall have an appearance of fatigue which is not due to the run-down of energy, but to its actual increase, in the responding system.

I have been able to discover a parallel case to this in the response of plants, a case, that is to say, in which the response is disproportionately smaller than stimulus, successive stimuli nevertheless causing an increase of the energy of the system. I took for my experiment a straight tendril of Passiflora in which growth had undergone arrest. It may be pointed out here that a certain tonic condition is necessary to the continuation of growth. This tonic condition is determined, as

will be shown later, by the sum total of energy latent in the tissue. From the curve of response given in fig. 6yy it will be seen that at the beginning of the experiment, the length of the tendril being constant, the first part of the record was horizontal, instead of descending, as would have been the case had the tendril been growing. The contractile shortening brought about by stimulus is here represented upwards. The stimulus of induction shock was now applied at intervals of one minute, and the record shows that the recovery, instead of stopping short at the level of the horizontal line, has proceeded beneath, thus indicating that stimulus has been effective not only in producing contractile response, but also in afterwards initiating growth! (Cf. fig. 178.) And further, as the stimulus goes on causing not only mechanical response, but also accelerated growth, we see that the successive mechanical responses are undergoing diminution. The application of stimulus ceased at the end of the fourth response, and we observe active growth proceeding from this point as a result of the energy absorbed from the previous stimuli. We have thus demonstrated the very curious case in which the building up process is attended by an actual diminution of response, and where, after stimulation, the energy of the responding organ, instead of being reduced, is raised above par. This experiment explains why an athlete in constant practice does not waste away, but actually increases, muscle.

Fig. 67. Mechanical Response shown upwards in Tendril of Passiflora in which Growth was originally at Standstill Stimulation, besides producing mechanical response, initiated the opposite movement of growth, as shown by the slope of the base-line downwards. The principal types of response seen in animal tissues are found also in the responses of plants and of inorganic substances. Three types of responses are possible: (i) that in which response is proportionate to stimulus ; (2) that in which response is disproportionately greater than stimulus ; and (3) that in which all or part of the stimulus is, for a longer or shorter time, absorbed by the tissue and held latent.

The subsequent effect of stimulus which is held latent may sometimes be seen in singly ineffective stimulus which becomes effective on repetition ; or in the staircase response, consequent on the enhancing of molecular mobility by the partial absorption of previous stimulus. The fact that stimulus may be held latent for a time, and subsequently find expression, is strikingly shown in the occurrence of multiple response, in answer to a single strong stimulus.

It is also possible for the incident stimulus to become divided in its expression, part of it finding an outlet directly in mechanical response, and part becoming latent, and causing accelerated growth. The gradual augmentation of the latter of these may cause a corresponding diminution of the former. An appearance of fatigue is thus brought about, which is misleading, as the decrease of mechanical response is in this case due. not to a diminution, but to an increase of latent energy in the responding substance, as shown by its capacity for renewed growth.