Plant Response as a Means of Physiological Investigation

(J?) Intermediate or neutral response. — Though the transverse conductivity of a tissue may be feeble, yet under somewhat strong stimulation the true excitatory effect is transversely conducted from proximal to distal. The result is that when the two opposite sides are equally excited, there is a neutralisation, or disappearance of responsive curvature ; or, by alternate fatigue of the two sides again, the organ may be made to oscillate to and fro about a more or less mean position.

(c) Negative response. — When stimulus is very strong and long continued, we obtain not only the transverse conduction of effect, but also temporary induction of anisotropy of the organ. The proximal side is now, owing to fatigue brought about by the direct impact of excessive stimulus, the less excitable ; and the internally diffused stimulus, causing greater contraction of the more excitable, induces concavity of the distal, or a negative responsive movement.

Besides this we have organs which are characterised by a permanent anisotropy or dorsi-ventrality, and we then obtain two classes of effects, according as the transverse conductivity is very feeble or moderately strong. Owing to the dorsi-ventral structure, the responsive movement can only take place at right angles to the plane which separates the anisotropic halves of the organ. These effects are the same in growing organs, such as plagiotropic shoots and dorsi-ventral petioles, and in mature dorsi-ventral organs,

conductivity is feeble, the stimulus remains localised on the side of the organ acted upon. Thus the stimulation of either upper or lower side induces a positive response, or movement towards stimulus. (e) Dorsi-ventral response which may become negative. — When the transverse conductivity of the organ is considerable, and the excitability of the lower half relatively great, then the strong stimulation of the upper side will, by internal diffusion, cause contraction in, and concavity of, the more excitable lower. The responsive movement will then be negative, or away from stimulus ; but feeble or moderate stimulation of the upper half, not being transmitted to the lower half, causes a positive response. Direct excitation of the more excitable lower half will always give rise to a movement towards stimulus, or positive response.

2. Indirect effect of unilateral stimulation : (a) Negative response. — When moderate stimulus is unilaterally applied at a distance from the responding organ, it is the indirect effect that is transmitted to that region, causing convexity of the same side, with consequent movement away from stimulus, or negative response. This is very well illustrated when the tip of either shoot or root is subjected to moderate unilateral stimulation. (b) Positive response. — But when the unilateral stimulus at the distant point is strong or long continued, the excitatory effect is transmitted by conduction, and induces a contraction and concavity of the same side, resulting in a movement towards stimulus, or positive response.

From what has been said it will be understood that moderate unilateral stimulation of the tip of root or shoot induces negative, and excessive stimulation positive, while between these two extreme cases there may be intermediate or neutral, response of the responding region. These effects are induced by stimulation of all forms, and it is thus clear that there is no specific sensitiveness of the dorsi-ventral as distinguished from the radial organ, nor is there any polar difference between the response of root or

shoot, the tips of both organs behaving alike. The one universal law which applies in every case is, that the direct effect of stimulus is to induce contraction, and its indirect effect to cause expansion. On taking a general survey of the responsive movements which are induced by the unilateral action of stimulus, we find that moderate stimulation of the growing region induces a positive movement. Or negative movement, again, may be induced in either of two ways — that is to say, by moderate stimulation of the tip, or by very strong stimulation of the growing region.

Responsive action under stimulus of gravity.— I have shown that the unilateral application of pressure of particles is efficient to cause responsive contraction. An experiment was described in which it was shown that the unilateral pressure of magnetically attracted particles would induce concavity of the side acted on (p. 497). The weight of statolithic particles may thus be the efficient cause of stimulation by gravity. It is to be borne in mind, however, that stimulation caused by such means as the weight-effect of these minute particles can only be moderate. We have therefore in the case of geotropic stimulation to deal only with the direct and indirect effects of unilateral stimulus of moderate intensity. In the case of the stem the growing region is directly stimulated. A horizontally laid stem thus curves upwards to meet the lines of force, or rays of gravity, just as it would bend upwards under heliotropic action to meet the rays of incident light. It is supposed that the curvature of the stem under gravity is mainly due to an active growth of the convex side ; but I have shown that it is due, on the contrary, to an excitatory response, which consists, like all other forms of response to external stimulus, of a contraction. The active element in the induced responsive curvature is thus the contraction of the upper side of the organ, aided subsidiarily by that expansion of the under side which is brought about by the indirect effect of stimulus on the distal. That this is the case is sec n

from the fact that, on localised cooling of the upper side, the movement of the organ in response to gravity is abolished, whereas cooling of the lower side has little or no effect on the responsive movement. This experiment incidentally supports the view that it is the inner tangential wall of the cells which is relatively effective in responding to the stimulus of gravity. In turning to the geotropic response of the root, on the other hand, we find that it is the distant tip which is the perceptive region for gravitational stimulus. Hence it is only the indirect effect of stimulus which acts on the responding growing region. But we have seen that moderate stimulation of the tip, by any form of stimulus whatsoever, always induces a movement at the responding region, of opposite sign to that which is the result of direct stimulation, and from this the opposite geotropic responses of shoot and root follow as a matter of course. This fact entirely negatives the assumption that shoot and root are possessed of any polar difference of sensibility, or that any specific geotropic sensibility has been evolved in the radicle for the advantage of the plant.

Heliotropic action in radial organs. — We shall find similarly, in studying the various movements of the plant in response to heliotropic stimulus, that, diverse as they seem, they are characterised by an underlying unity, being in fact but so many expressions of the universal law that response takes place by the contraction and concavity of the more excited. The fundamental effect of light was demonstrated by showing that, in a growing organ, diffuse stimulation induces a contraction and retardation of the rate of growth. This was also shown to be true of all other forms of stimulation, including those of thermal and electrical radiation. The incidence of radiation may, it is true, cause a rise of temperature ; and this would, as we know, have the effect of enhancing the rate of growth. In order, therefore, to discriminate the effect of radiation as such from that of temperature, an experiment was described in which the circumstances were so arranged that no rise of temperature

could take place while the effect induced by radiation as such was being observed. Under these crucial conditions it was demonstrated that the effect of radiation is to induce responsive contraction. case of gravity, two definite and distinct effects were observed, according as stimulus was applied directly on the responding region or on the distant tip. In the former case we obtained a positive, and in the latter a negative, responsive movement.

Up to this point, then, the actions of light and of gravitation heliotropic movement of the stem corresponds to the so-called positive geotropic. Looked at in relation to the direction of stimulus, however, it may be said that the response which is commonly known as ' negative geotropic ' is actually positive, and vice versa ; for, accepting the theory of statolithic or hydrostatic pressure as to the effective cause of stimulation, the direction of the excitatory pressure is in the direction of

then, and acted on by vertical lines of gravitational force, or by vertical rays of light, we obtain the same directive response to these similar directive stimuli, by the bending upwards of confusion is therefore inevitable when one of these responses is designated as positive, and the other as negative, for the of directly opposite terms. This difficulty might perhaps be the stem as positive phototropic and positive gravitropic, or pro-gravitropic, and that of the root as negative phototropic

We next turn to the differences between the effects of heliotropic and geotropic action. Such differences arise from the two facts that : ( 1 ) only in the root is the region of the perception of gravitational stimulus separated from that of response ; and that (2) geotropic stimulus is always of moderate intensity. As regards the first of these two differences, it has been shown that, on applying unilateral heliotropic stimulus of moderate intensity to the tip of the shoot, we obtained the same negative response of indirect stimulation as is given by the root-tip. In the case of geotropic stimulus, however, there can be no phenomenon corresponding to this, inasmuch as in the stem the statolithic particles appear to be diffused, instead of being localised at the tip. The second point of difference between the two responses arises from the fact that heliotropic stimulus may be of any degree of intensity. Hence the direct excitatory effect of strong unilateral stimulation of the root-tip may in the case of light be transmitted to the growing region, and there induce a positive response, or movement towards stimulus. This accounts for the fact that while roots in general give one kind of gravitational response of so-called positive sign (but really negative), some roots give negative response to light, and others positive.

Turning next to the direct action of unilateral heliotropic stimulus on the growing region, we find, as explained in the summary of responsive action in general (p. 535), that the effect is modified by the intensity of stimulus, by the transverse conductivity of the organ, and by its existing anisotropy. Thus in the case of a radial organ, such as the hypocotyl of Sinapis, moderate stimulus, its effect remaining localised on the proximal side, has been shown to evoke a positive responsive movement. Stronger or long-continued stimulus, reaching the distal side by transverse conduction, neutralises this first effect, and the organ thus remains at right angles to the incident light, or in a dia:heliotropic position, apparently unaffected by it. In other instances, again, owing to the alternate excitation of the two sides, the organ may oscillate to and fro about a mean position. With still stronger stimulus, however, an anisotropy is induced, by which the proximal side becomes, through fatigue, the less excitable, and the internally diffused stimulus causes greater contraction and resultant concavity of the distal side ; that is

to say, a negative response. It was thus made clear that the three types of response— positive, negative, and dia-heliotropic— are not due to three different specific sensibilities. It has been pointed out, further, that these considerations explain why it happens in many cases that, while moderate stimulation induces a considerable responsive movement, stronger stimulation, instead of increasing this, actually neutralises it. It is due, as we have seen, to the transverse conduction of stimulus by the tissue, that the positive effect is counteracted or reversed. . This explanation has been shown to account satisfactorily for various cases apparently anomalous.

Certain tendrils are regarded as heliotropically insensitive. For example, the tendril of Passiflora when acted on by sunlight shows little or no responsive movement. On artificially diminishing the transverse conduction, however, by the application of cold, I have shown that it exhibits the ordinary positive responsive movement. The tendril of Vitis, again, which is supposed to be endowed with a specific sensibility of negative character, has also been shown to exhibit the normal positive response under light of moderate intensity. The modifications of transverse conductivity which are brought about by age and season, with their consequent appropriate variations of response, are seen in Tropceolum. A very young tissue, as a general rule, owing to the fact that the fibro-vascular elements are not fully developed, is a bad conductor of stimulus, which therefore remains localised at the point of application. Hence young plants exhibit movements of positive response, whereas older plants, owing to transverse conduction, with its effect of neutralisation, appear to be little affected by light. In connection with this it must also be borne in mind that the power of contraction declines with age. The characteristic effect of season, again, results from the fact that the conducting power of a tissue is at its feeblest in autumn and winter, and correspondingly greater in spring and summer. In autumn, therefore, stimulus remains localised, and Tropceolum and Ivy during that season respond to heliotropic

stimulus by positive curvature ; whereas in summer strong unilateral stimulation is transversely conducted, and induces negative responsive curvature. Heliotropic response in plagiotropic and dorsi-ventral organs. — We have seen that negative response is brought about in a radial organ by induced anisotropy, and transverse conduction of stimulus. Effects fundamentally similar are seen in organs which are characterised by a natural anisotropy. A connecting link between this transient induced anisotropy of radial organs, and the permanent anisotropy of a dorsi-ventral pulvinus, is afforded by plagiotropic stems, in which anisotropy has become more or less permanent, owing to the long-continued unilateral action of vertical light. Two different types of response are exhibited by anisotropic organs, depending on their transverse conductivity and on the relative excitabilities of their two sides. In the first of these, transverse conductivity being feeble, vertical illumination remains localised, and induces positive response. This is the true explanation of the so-cailed diurnal sleep, with upward folding of the leaflets, of Robinia, Erythrina indica, and Clitoria ternatea (p. 629). In the second type, the stimulus of vertical illumination is transmitted to the more excitable distal side, inducing concavity of that side, and consequent negative response. The different stages of this effect are well seen in Mifnosa, when the stimulus of light acts on the dorsal or upper side of the pulvinus. Here the immediate effect is a positive response or erection of the leaf; and as the stimulus percolates to the distal side, this effect is neutralised and converted into an increasingly negative response. The greatest degree of negativity or fall in nature is thus attained by the cumulative action of the whole day's illumination. Such is the response which is characteristic of the second type. The action of strong vertical light in such cases induces movement downwards.

And this is seen in plagiotropic stems like those of Cucurbita and Ipomcea ; in the thallus of Marchantia, and the midribs of various leaves ; and in the so-called diurnal sleep, with downward folding of the leaflets, of such leaves as those of Oxalis, Biophytum, and Averrhoa. In the heliotropic responses of ordinary leaves, again, we have exactly similar classes of phenomenal In these cases it has been shown that there is no specific diaheliotropic sensitiveness, such as that by which the upper surface of the leaf was supposed to place itself at right angles to the light, for the purpose of absorbing the largest possible amount of stimulus. It was shown, moreover, that with regard to that response by which the ultimate position of the leaf is determined, the lamina was not the perceptive organ. In ordinary, as in pulvinated, leaves we find response to be of two extreme types, connected by innumerable gradations. First, we have leaves like that of Mangifera indica, in which on account of the feeble conductivity of the pulvinoid vertical illumination induces a positive response, or movement upwards ; and as an example of the second type we saw that negative response, or movement downwards, was given by the leaf of Artocarpus under vertical light. These responsive movements induced by light, although, generally speaking, predominant, are modified by the presence of other subsidiary factors, which all contribute in various degrees to bring about the variety of attitudes ultimately assumed by the leaves. These subsidiary factors were enumerated as: (1) the epinastic or hyponastic tendency; (2) the general turgescent condition of the plant ; (3) the characteristic limits of flexibility of the motile organ ; and (4) the differential geotropic excitability of the organ. When a petiole is acted on laterally by light a torsion is induced, by which the upper surface of the leaf is made to face the incident stimulus.

It has been shown that this movement is not due to any specific dia-heliotropic sensibility ; for any form of lateral stimulation, say chemical or thermal, will induce a similar response by torsion, the result being always that the less excitable surface is made to face the stimulus. Similar effects are also observed in compound strips made of such unequally contractile substances as ebonite and indiarubber. The fact that it is the differential excitability of the organ which under lateral stimulation causes this torsional movement was further demonstrated, when the difference was artificially increased by the local application of chloroform to the upper half of the pulvinus. The torsional response was then found to take place, with a corresponding enhancement of rate, in the same direction as before. But when this natural difference of excitability was reversed, by the abolition through local application of chloroform of the excitability of the lower half of the pulvinus, the direction of the responsive torsion was found to undergo reversal.

By carrying out a similar series of experiments, with special reference to the lateral action of gravitational stimulus on a dorsi-ventral organ, it was shown that such an organ as a whole exhibited neither a positive nor a negative, but a differential geotropic action. The investigation showed that the upper half of a pulvinus was less excitable than the lower half under geotropic stimulus. An artificial increase of the existing difference between the excitabilities of the two halves enhanced the rate of the normal torsional response, and the reversal of these natural excitabilities reversed the direction of the torsional response to geotropic stimulus (p. 664).

Phototactic movements. — A leaflet of Desmodium in a state of standstill resumes its pulsatory beats when stimulated by light. Owing to the anisotropy of the motile organs, one half of the beat is more rapid than the other. Too strong an intensity of light, however, by causing greater fatigue of the more excitable half of the organ, may cause a reversal of the relative rapidities of the up and down beats. In Desmodium, under the continuous stimulation of strong light, these reversals are often recurrent. The downstroke, which was at first the quicker, becomes less quick than the upstroke, and this reversal may take place again and again, in alternation with its opposite. These effects, seen in a pair of anisotropic motile organs in Desmodium, afford an explanation of the swimming movements of certain ciliated

organisms. These swimming movements are brought about by the rhythmic beats of the anisotropic cilia under unilateral stimulus of light, either the up or down stroke of each such beat being relatively quicker than the other. For reasons which have been explained, moderate stimulation, initiating these rhythmic responses, causes movement in one direction, and stronger stimulation movement in the opposite direction ; or, as in the case of Desmodium leaflets, there may be recurrent reversals, causing alternate progressions or retrogressions to and fro. Similar forms of response with similar variations are brought about by forms of stimulation other than light ; there are thus thermotactic, galvanotactic, and chemotactic swimming movements.

Nyctitropic movement. — The nyctitropic movement has been shown to be the result of heliotropic action, the fall of the leaf of Mimosa at evening being due not to the action of on-coming darkness, but to the cumulative stimulus of the whole day's illumination. Taking this plant as the type, it was shown that the diurnal movement was caused by the action of two different periodic factors, namely: (1) the differential effect of light on the pulvinus itself during the day, alternating with the cessation of stimulus at night ; and (2) a periodic inflow and outflow of water, which takes place in the plant as a whole by the recurrent action of light and darkness. By the first of these factors the leaf is progressively depressed during the day, the reverse process taking place during the night, as a result of natural recovery, aided by the conserved internal energy which gives an impulse opposite to that of external stimulus. These two periodic factors, of the effect on the pulvinus itself, and of that on the plant as a whole, act concordantly, and give rise to periodic movements of the leaf which are of large amplitude. Such forced diurnal vibrations, by long repetition, give rise to periodic after-effects which persist for a time, even on the cessation of the periodically exciting cause.

Vitalism — Fundamental unity of physiological response in plant and animal — Theory of Darwin — Variation as induced by external forces. We have reviewed, in the last two chapters, the various phenomena of plant response. We shall now turn our attention to the consideration of irritability,* or the capacity of responding to stimulus, in general. Vitalism. — We have seen that when a tissue is rendered molecularly sluggish by any physical means, such as cooling, its irritability is found to be temporarily abolished. Irritability is thus ultimately due to molecular responsiveness, and excitatory response is brought about by the molecular derangement consequent on stimulus, with the subsequent selfrecovery. We have seen further that the state of excitation is exhibited, either by a mechanical or by the electrical mode of response, and that even where mechanical indications are not available, the electrical sign of excitation is unfailing. We have also seen briefly in the course of the present work, and I have demonstrated in full elsewhere,1 the fact that similar excitatory response is given, even by inorganic matter, under stimulation ; and in such cases also we have been able to observe and record not only the phenomenon of response itself, but also its numerous appropriate modifications under varying conditions. Thus fatigue brings about diminution of inorganic, as of organic response. Amongst chemical reagents, again, some induce exaltation and others depression ; and many so-called poisons act here, as in the case

of the plant or animal, by inducing the abolition of response. Irritability or molecular responsiveness, therefore, must be regarded not as characteristic of organic substances alone, but as the universal property of matter. In the case of what is commonly known as the living, we have merely higher complexities, with greater instabilities, of molecular structure. External stimulus is here liable to induce greater derangement, and the irreversible molecular change known as death takes place the more easily, the more highly organised the complexus may be. Bacteria, for example, will survive conditions which would immediately prove fatal to more complex organisms.

In studying the responsive phenomena of living organisms, therefore, we must fix our attention on their molecular aspect, and try to follow out the physico-chemical changes which are consequent on the molecular derangement induced by stimulus ; and we are more likely to succeed in obtaining a growing insight into the various phenomena of life, when we approach the subject from this point of view, than when we permit ourselves to evade each difficulty as it arises by referring it to the inexplicable action of a mystical vital force. Physical and mechanical considerations at first appear to us to be inadequate to the explanation of the complex movements of the living machine, just as the similarly complex movements of a wind-motor, connected with a hidden electrical apparatus, would at first sight be inexplicable to an inexperienced observer (fig. 278). Let such an observer be brought face to face for the first time with such a windmill. Its movements under the action of wind will arouse his wonder, and this will be increased when sometimes even in the absence of wind he sees the vanes revolving still, but now in an opposite direction. He may again notice oscillatory rotations, now in one way and then in the other. Failing to find any rational explanation of these movements in this first stage of his inquiry, he will be driven to attribute them to an unknown power, whose characteristic it is to manifest itself by such erratic actions,

now in one direction and again in the opposite, and whose mystery lies mainly in this caprice. But the observer, in the course of his further inquiry, finds that the vanes, whose rotation under the impact of the external stimulus of wind first attracted his attention, are but a part of a complex machine, the interior of which had been hidden from his view. He finds that the energy supplied from outside is being transformed by a dynamo inside, and stored up in an accumulator. When the external force is not acting, the reverse movement is caused by the internal energy thus stored up. This very movement, being apparently without a cause, he would formerly have designated as automatic. When the storedup energy is exhausted, the seemingly autonomous movement comes to a standstill, and only by the accession of fresh external stimulus, causing renewed storage, can it be resumed. At a given moment, moreover, the responsive movement of the vanes is determined by the opposing actions of the external and internal factors. As long as the wind is sufficiently strong, movement takes place in one direction, and when there is a pause the internal energy begins to find expression, by causing movement in the opposite direction. If the circumstances were such that the rise of the wind were synchronous with day, and its fall with night,

Fig. 278. Diagrammatic Representation of a Windmill with Attached Dynamo, D, and Accumulator* s Wind acting on vanes, v, from right, represented by arrow (<-), causes responsive rotation in direction opposite to that taken by the hands of a watch. This external energy also causes electrical storage. On the cessation of the wind the accumulator begins to part with its stored-up energy, and, the dynamo now acting as a motor, causes a responsive rotation of the vanes in the other direction, as shown by the arrow (••>).

the windmill, with its alternate movements, would afford a very excellent illustration of the alternate day and night phases in the nyctitropic movement of the plant. Now, with regard to the living machine, similarly, a full insight into its action can only be obtained if we are able to disentangle the two opposite factors, of internal energy and external stimulus, and follow them into their responsive expressions, at the same time recognising that the principle of the conservation of energy must hold good in the living system as in the non-living. External energy acting on the plant performs work on it, a part of this incident energy being taken and held latent ; and, in virtue of the latent energy so conserved, work is performed by the plant. Of this internal work, besides the potential chemical energy which is accumulated, the maintenance of suction, growth, and autonomous movements may be cited as examples.