Plant Response as a Means of Physiological Investigation

Fundamental unity of physiological response in plant and animal. — Having now seen the intimate connection which exists between the physical and the physiological, and having also seen that the molecular response of the inorganic is not altogether different from that of the plant, it remains only to glance at the physiological continuity of response as between plant and animal ; and here we shall find that there is hardly any phenomenon of irritability, observed in the case of animal tissues, which is not also to be discovered in some simple form in the case of the plant. These resemblances, moreover, are so numerous and so detailed as to lead us inevitably to the conclusion that we have to deal in the two cases with a single identical phenomenon.

In the longitudinal response of a radial vegetable organ we have seen how similar is responsive contraction in animal and plant ; and that this similarity extends even to characteristic details is seen when we compare the records of the Kunchangraph in the case of the plant, with those of the Myograph in that of the animal. The two are alike in the exhibition of a latent period, which is prolonged by cold and reduced by warmth. The staircase increase of the respon-

sive effect, its diminution by fatigue, and the induction of tetanus under rapidly succeeding stimuli, in either case corresponds to a like phenomenon in the other. Fatiguerelaxations, moreover, under strong and long-continued stimulation, are the same in both. And, turning to the electrical mode of response, we find that the excitatory condition of a tissue is indicated by its induced galvanometric negativity, whether the tissue be animal or vegetable ; and further, similar physiological modifications, as induced by the action of various external agents, are manifested by similar changes in the two cases in the electrical response.

As by the nerves of the animal, so also by certain conducting channels in the plant-tissue, the state of excitation is, in the two cases alike, transmitted to a distance ; and this conduction takes place in both by propagation of protoplasmic changes. In both alike, cold reduces, and warmth accelerates, the velocity of the transmission. In both alike, the stronger the stimulus, the greater is the velocity with which it is transmitted. In both, this velocity is diminished with fatigue. Anaesthetics cause the temporary abolition of conduction in both. The anode, again, blocks the transmission of the excitatory wave in both. And, lastly, the velocity of the transmission of excitation in the plant is comparable to that of its transmission in the nerves of some of the lower animals. Thus, in a certain specimen of the sensitive Mimosa, excitation was found to be transmitted with a velocity of 14 mm. per second ; and in the ordinary plant Ficus religiosa this velocity was determined at 9*4 mm. per second ; while in the nerve of Anodon a value of 10 mm. per second has been recorded. If, then, the characteristic of nerve be to conduct excitation, it must be admitted that the plant, like the animal, is provided with a nervous system.

In the matter of the excitation induced by the electrical current, there is an equally remarkable similarity of effects in animal and vegetable. Under normal conditions in both cases, the kathode excites at make and the anode at break ; and that reversal of these normal effects which is liable to occur under fatigue, or under excessively strong electromotive force, is the same in the one case as in the other. In addition, moreover, to the contractile effects of kathode-make and anode-break, the plant exhibits a responsive expansion at anode-make and kathode-break, which is also seen in animal tissues.

Turning next to such tissues as are characterised by the property of rhythmicity in a marked manner, we have seen that this effect as exhibited by the plant cannot be distinguished from that in the animal. The rhythmic tissues of the plants Biophytum and Desmodium are characterised by the possession of relatively long refractory periods, a peculiarity which also marks the rhythmic cardiac muscle of the animal. The response of rhythmic tissues in both plant and animal is found to be on the ' all or none ' principle. In both alike the rhythmic tissue is incapable of tetanus ; and in both, when at standstill, a single moderate stimulus gives rise to a single response, and stronger stimulus to a multiple series of responses. In Desmodium, again, as in the cardiac muscle, increased internal hydrostatic pressure renews pulsation in a tissue at standstill. In rhythmic tissues, again, whether animal or vegetable, under favourable tonic conditions, persistent pulsatory movements take place which are apparently automatic ; and these rhythmic pulses are found to exhibit the same types of cyclic variation in the plant as in the animal. The effects of temperature on both are exactly the same — that is to say, its rise increases the frequency and diminishes the amplitude of pulsation. And still more striking, finally, is the identity of the modifications induced by drugs in the rhythmic responses of animal and vegetable.

But this unity of rhythmic responses in plant and animal is not merely a question of their fundamental characteristics. They sometimes appear also to subserve functions somewhat similar. Thus the rhythmic cardiac tissue of the animal maintains the circulation, and we have seen that the rhythmic tissue of the plant maintains the ascent of the sap. From such considerations it may perhaps appear not very far-fetched to regard a plant as possessed of a diffuse heart.

With special reference to the effect of drugs on plant and animal tissues, we find the identity of phenomena similarly impressive. This is exemplified in both cases by their action not only on ordinary contractile tissues, but on rhythmic tissues also. Thus it is only necessary to mention such facts as that anaesthetics, like the vapours of ether and chloroform, induce a transient abolition of excitability, with abolition of response, in both animal and vegetable ; that this excitability, with its concomitant response, is gradually restored in either case on blowing off the applied vapour ; and that poisonous reagents, on the other hand, induce a permanent abolition of all response. The action of these and other chemical agents has already been described in some detail.

In the case of rhythmic response, again, a like parallelism was found to exist, as between the effects of drugs on plant and animal tissues respectively ; and this parallelism was further shown to extend through a wide range of phenomena. A remarkable instance was seen in the antagonistic effects on responsive rhythmic tissues of the actions of acid and alkali. Acid, when applied to cardiac muscle, induces a diastolic standstill, whereas the effect of alkali is exactly the opposite, a standstill, namely, of systolic contraction ; and the standstill induced by either of these is found to be counteracted by the application of the other. Now, I have shown that the effects of acid and alkali on the rhythmic tissues of Desmodium are similarly antagonistic. Thus, dilute hydrochloric acid induced arrest of pulsation in the diastolic, or relaxed, position, in the motile organ of Desmodium ; whereas, with solution of sodium hydrate, the induced arrest was of systolic contraction. Moreover, when arrest in its own particular position had been brought about by either of these reagents, its effect was neutralised by the application of the other (p. 353). The same effects were found again curiously reproduced in the case of the autonomous response of growth. Here, the application of acid induced an arrest

of growth, but only after an abnormal relaxation or expansion. Alkali, on the other hand, induced arrest, but after contraction. And, finally, the arrest induced by one was counteracted by the effect of the application of the other (p. 484). These facts, and others which have already been fully described, afford a conclusive demonstration of the essential unity of the physiological effects of drugs on plant and animal tissues. The existence of such a unity having been established, it is evident that much may be gathered from investigations carried out on plants, as to the obscure question known to medical practice as the modification of the effect of drugs by individual constitutions ; for while a given individual will succumb quickly to the action of a certain poison, another, as is well known, will throw off its influence and survive. Again, a particular dose of a given drug may have the effect of producing excitation in one case, and in another profound depression. The effect on a tissue of any given reagent, then, does not merely represent the action of that reagent as such, but is further determined also by the reacting power of the responding tissue itself. And this reacting power is modified by what is known as the individual constitution of the organism. Thus no result can be definitely predicted of a reagent, unless we have a precise knowledge of the action of the same drug on various definite constitutions. This problem, of the variations induced in the effects of drugs by the different reacting powers of different constitutions, may now be attacked, therefore, through the study of the plant, in which, as I have shown, it is possible to induce known differences of constitution by artificial means.

It was shown, for example, in one case that, while a 5 per cent, solution of the poisonous reagent, copper sulphate, produced an immediate depression, quickly followed by death, another similar plant, whose tonic condition had been raised to the optimum, was found to withstand the action of this poison for a considerable time, the immediate effect being an actual exaltation of its response. The opposite effects of the same dose on different constitutions were shown, again, in the fact that, while a I per cent, solution of copper sulphate caused depression, and subsequent death, of a plant under normal tonic conditions, the same dose in the case of a similar specimen, which had been raised artificially to the optimum condition, brought about exaltation of response, which was found to continue for a fairly long period, after which the effect of the poison was completely overcome (p. 487).

The generalisation which has thus been established will be found to be of great significance. A unity of phenomena, as between animal and plant, so fundamental, so detailed, as has been shown to exist, points unmistakably to a basic property of responsiveness common to the two, and manifested in both alike by the same effects and modification of effects under stimulation. Nor is the power of response something which makes its appearance suddenly in organic substances only, for it has been demonstrated as existing even in the inorganic. Thus inorganic and organic are held together in a linked continuity. All are responsive, all are depressed by fatigue, all are made excitable by stimulants and rendered irresponsive by ' poisons.' Again, with regard to plants, it may be said that there is hardly a responsive physiological peculiarity in the highest animal that may not be found foreshadowed here. Thus the serial development of the physiological functions in these two cases has been more or less parallel.

In the establishment of this generalisation, further, it has been made possible to solve many of the most obscure and difficult problems of Animal Physiology, by studying them under the simple and more manageable conditions of vegetable life. That this is the case has already been seen in many instances, such as that of the polar effects of currents and their reversal under given conditions ; in the light shed on the nature of automatism ; in the different parts played by external stimulus and internal energy, and their mutual relation ; in the bifurcated expression of incident stimulus as external and internal work ; and finally, in the

similar action of chemical reagents on plant and animal. It must be remembered, moreover, that one of the greatest advantages to be derived from such a use of the plant as a means of physiological investigation, lies in the fact that this study can be carried on with intact and growing specimens under normal circumstances. The experimental conditions are in this case, therefore, better than those which correspond to them in regard to animal tissues, since the latter specimen will generally be found to be suffering from the effects of injury, and may therefore have been rendered abnormal to an unknown degree. The study of the responsive phenomena in plants must thus form an integral part of physiological investigation into the various problems relating to the irritability of living tissues, and without such study that investigation must in future be regarded as incomplete.

In thus reviewing the movements of plants as a whole, in the light of the investigations which have been described in the course of the present work, we cannot have failed to be struck by the fact that all alike represent, under changing conditions, a single fundamental responsive phenomenon ; and this response of the plant offers us, as we have seen, a means of tracing the process by which physiological differentiation from simple to complex has taken place, under the action of stimulus itself. It also enables us to refer a specific differentiation back to definite forces, which have acted asymmetrically upon the organism and induced such a change. We are thus able to see that responsive movements apparently opposite in kind, are nevertheless traceable, not to different specific sensibilities, but to a single universal sensibility, finding different expressions by reason of these induced physiological differentiations.

Theory of Darwin.— It will be remembered that, according to the theory of Darwin, a given individual variation which might be in any way advantageous to the organism was perpetuated by the process of natural selection. Thus, in the struggle for existence, only those could continue to live that were best fitted to their external conditions. Going back a step, however, to the question of the origin of these variations themselves, we find that by some writers they are held to be due to spontaneous unknown causes, inherent in the organism. Now it would obviously be more satisfactory, since no effect can occur without a cause, if we could assign at least some of these variations to something more definite than this. And Darwin himself was of opinion that variability of every kind was due, directly or indirectly, to changes in the conditions of life. It was difficult, however, to distinguish clearly how much of any given variation was due ' to the accumulative action of natural selection, and how much to the definite action of the conditions of life.' l

It was this difficulty which, for example, compelled him to ascribe the movements of plants to specific heliotropic or geotropic sensibilities, acquired as the result of natural selection. The particular reaction or reply to stimulus, manifested by the plant in any special case, was thus to be regarded, not as the direct and necessary result of changes in the environment, but as an adaptive act forced on the species by the struggle for life. Variation as induced by external forces. - With the delicate modes of investigation, however, which are now at the disposal of observers, it has become possible to demonstrate a direct connection between some of the differentiations induced in plant-organs and the conditions of the environment. The factors which are conceivable as bringing about variation may be classed under two heads, first, internal or spontaneous, and second, those which arise from external stimulus ; but with regard to the first of these we have seen that, as far as experiment has carried us, there is no such thing as spontaneity, in the sense of an effect which occurs without antecedent cause, for the internal energy to which such seeming actions have hitherto been vaguely ascribed has been shown to be itself traceable to external stimulus. And as regards the effect of external stimulus, on

the other hand, we have seen that under its action in nature, heterogeneity is evolved out of homogeneity. Thus it is the unequal action of an external force which, for example, causes a radial organ to become anisotropic, with corresponding physiological complexity, culminating in dorsiventrality. One instance of an organ in which, owing to this differentiation, a movement of apparent advantage to the plant has been induced, is found in the pulvinus of Oxalis. Here, by the greater excitability of the lower half, the leaflets are made to fold downwards, with the consequence of avoiding too intense illumination, in the responsive movement known as diurnal sleep. But the differentiation which we find here is not unique or suddenly evolved, for we find a similar anisotropy even in the pulvinoids of ordinary leaves such as those of Artocarpus. And such physiological differentiation can be traced still further back, to the case of organs which were originally radial. Thus, a long stem, such as that of Cucurbita, happening to become recumbent, becomes also dorsi-ventral, by the unequal action of sunlight on the two sides, the too long excited upper side being now the relatively less excitable. There is again no fixed line of demarcation between this more or less permanent and a transient differentiation ; for when a radial stem of Cucurbita is acted on by a transient unilateral stimulus, a temporary anisotropy is induced as between the excited and the unexcited sides, the latter, which is fresh, being now the more excitable ; but this anisotropy immediately disappears on the recovery from excitation of the excited side.

We may thus have in the same organ at different times a transient anisotropy, lasting for a minute or so, under moderate unilateral stimulation ; a more prolonged anisotropy, lasting for an hour or so, under stronger stimulation ; and a permanent differentiation under still stronger and longer continued unilateral stimulation. The difference between the first and the last of these is simply a question of whether the limit of elasticity has been exceeded or not. In a torsioned wire, similarly, on the cessation of moderate

stress, there is complete recovery ; and the same wire, when torsioned beyond the limits of elasticity, does not recover, but remains permanently strained. In the same way, we have seen that the effect of stimulus on a living tissue is to induce a molecular derangement, from which there is complete recovery, with a concomitant recovery of all the physiological properties, if the derangement have been not too great ; but, with excessive or long-continued stimulus, the limit of physiological elasticity is exceeded, and a permanent physiological differentiation is thus induced.

The same causes, moreover, which initiated the primitive differentiation may now act to induce further a series of complex movements. Thus we first see plagiotropic dorsiventrality induced in the creeping stem of Cucurbita, by the unilateral action of sunlight ; and then the diurnal periodicity of light and darkness acting on this already differentiated organ to cause a periodic swing, which increases with repetition. In this we have, as has been pointed out, the first stage in the evolution of the nyctitropic movement. Now* it is quite possible that this nyctitropic movement may be found, at least in some cases, to subserve the advantage of the plant ; yet it would not be true to say that it was evolved for the purpose of such advantage. Indeed, we have to guard ourselves carefully against being led, by this theory of the final advantage of the plant, into an argument in a circle. Assuming any given movement to be advantageous to the plant, we have first to determine the nature of the mechanism by which it is produced, and secondly to find out what was the exciting cause, and what the character of the conditions under which it first arose. If we are not guided in our inquiry by such considerations, we are liable to be misled in our inferences. One such example was seen in the general belief that a certain specific sensibility resides in the root-tip, by which it is endowed with the faculty of moving away from rough surfaces, which might have been injurious to it. So far from its having any such peculiar faculty, however, we found that when a red-hot wire was presented to it

the tip moved towards it, and was thus destroyed. The sensitive reaction of the root-tip is thus seen to be, not an adaptive act evolved for the advantage of the plant, but merely an example of the general law, that under moderate stimulation it is the indirect effect of stimulus that reaches the growing organ and causes movement away from, while under strong stimulation the direct effect determines movement towards, the source of stimulus. Now, it is perfectly true that had any given reaction been such as, under normal conditions, to bring about the self-destruction of large numbers of organisms, we should not have witnessed the survival of organisms characterised by that particular movement. The plant lives because the physiological differentiations induced in it under natural conditions, and the movements induced under periodic changes of those conditions, are in harmony with the fluctuating forces of the environment. Thus the forced rhythm becomes more deeply impressed with repetition, and the greater is the harmony between this rhythm and the environment, the greater will be its stability under given conditions ; the plant persists, that is to say, because it is perpetually in tune, instead of perpetually at war, with its surroundings. We may take it, therefore, in the case of any particular movement, that it constitutes an expression of this stable relation of the plant to its environment, but not that it represents any deliberate adaptation to such an end.

Reverting to the nyctitropic movement in particular, we find it adduced by Darwin as to a certain extent furnishing an example of the influence of heredity on the individual organism, a view which has been questioned. But if we are prepared to give a sufficiently extended and consistent meaning to the word, we must accept this, for heredity is essentially the repetition of a past cycle, the persistence of after-effects ; and there are innumerable degrees of such persistence, between that of a transitory after-effect and the phenomenon of absolute persistence, if such occurs. The diurnal periodicity of Mimosa, for example, is maintained for several days under unchanging conditions of illumination or of darkness,

after which the impressed periodicity is lost. The autumnal periodicity of trees which grow in temperate climates, where the leaves are habitually shed at the approach of winter, persists for a few or for numerous years when the plant is transferred to a warm climate. Certain species of bacteria, again, can be made to exhibit an artificial change of characteristics which will persist during many generations, even on the return of the organism to normal conditions ; but even after this degree of hereditary persistence of effect has been exhibited, these induced varieties are liable to undergo reversion to their original type. It is thus seen that the persistence of hereditary characteristics is merely relative. Had it been absolute, the species itself would have been immutable. A given rhythm only persists so long as those circumstances which originally occasioned it persist, and during a certain period afterwards ; but this after- or hereditary-effect must, in the non-continuance of the original periodically exciting cause, prove, however long maintained, to be but transitory, like the ultimate arrest of a pendulum when its vibrational energy is exhausted.

The phenomenon of life, then, introduces no mystical power, such as would in any way thwart, or place in abeyance, the action of forces already operative. In the machinery of the living, as in that of the non-living, we merely see their transformation, in obedience to the same principle of conservation of energy as obtains elsewhere ; and it may be expected that, in proportion as our power of investigation grows, the origin of each variation of the living organism will be found more and more traceable to the direct or indirect action upon it of external forces, the element of chance being thus progressively eliminated, as the definite sequence of cause and effect comes to be perceived with an increasing clearness ; and only, I venture to think, as this is worked out, can we learn to apprehend fully the true significance of the great Theory of Evolution.

3. Determination of latent period and its variation by cold . . . 268 9. Response to indirect stimulation longitudinally transmitted . . 532 10. Response to indirect stimulation transversely transmitted . . . 532 1 1 . Response under favourable tonic condition when chemically stimulated 12. Response to salt solution in ordinary tonic condition . . . . 552 13. Response to transmitted stimulation with preliminary erectile twitch 24 19. Demonstration of factor of internal energy in the process of recovery 401

43. Indirect hydrostatic and true excitatory response .... 37 45. Response to moderate stimulation of tip of growing organ . 517 47. Response to direct stimulation of growing region . . . .518 52. Determination of death-point by spasmodic lateral response 53. Determination of death-point by spasmodic movement of uncurling 54. Determination of death-point by sudden volumetric contraction 55. Determination of death-point from critical point of thermo-mechanical

57. Modification of thermo-mechanical curve showing effect of age . 59. Duplication of rigor-point in Afimosa by chemical agents 60. Translocation of death-point by unfavourable weather . .17 71. Multiple response to constant chemical stimulus in Biophytum 76. Initiation of autonomous response in Biophytum above thermo-tonic 77- Resumption of autonomous response in Desmodium by accession of energy ........... 78. Resumption of autonomous response in Desmodium under increased

79. Localisation of seat of autonomous excitation in Desmodium 80. ' Systolic ' and ' diastolic ' positions in Desmodium pulsation . . 82. Measurement of period and amplitude of Desmodium pulsation from 83. Measurement of period and amplitude of Desmodium pulsation from 84. Periodic variation in Desmodium pulsation . . . 341, 470 See also Periodic variation of growth and Suctional response 472, 473, 474 86. Shoshungraphic record under Hydrostatic Balance . . . . 376

89. Periodic variation of transpiration in Cucurbita .... 472 See also Mechanical, Growth, and Torsional responses to suctional 94. Resumption of growth in drought-rigored Cucurbita after supply 95. Growth in response to hydraulic transmission of energy . . . 430 96. Effect of decreased suctional activity on growth .... 429 97. Effect of increased hydrostatic pressure on growth of Balsam . . 428 98. Effect of increased hydrostatic pressure on growth of Crinum Lily . 428