Plant Response as a Means of Physiological Investigation

Coprinus drooped in darkness, and became turgid on restoration to light ; and the further supposition that light diminished growth was held to be negatived by the instance of a dark-rigored plant, in which growth, so far from being retarded, was accelerated or renewed by simple exposure to light. In both these instances we notice the abnormal condition induced in the plant We must bear in mind that that rhythmic activity which is essential to growth depends, like all other rhythmic activities, not only on the turgor of the tissue, but also on the energy which it has absorbed. We have seen that when the sum total of independent stimulating factors present in the plant is adequate to raise its tonic condition above par, then rhythmic activity is initiated or renewed. Among these factors are, as has been shown : (a) a proper condition of turgidity ; (b) favourable temperature ; and (c) that previous absorption of energy of light which determines what we may distinguish as the phototonic condition.

The fact that, besides turgor, a certain amount of energy is also necessary to initiate growth has been fully demonstrated in a previous chapter. Taking now the case of the dark-rigored plant, we see that the arrest of its growth is due to a deficit of absorbed energy— in this instance, phototonus. Under these conditions, the renewed exposure to light would be sufficient, by supplying the missing factor, to re-initiate rhythmic activity and consequent growth. The drooping of the pileus of Coprinus in darkness is another expression of the sub-tonic condition of the plant. It must be remembered that the suctional activity which determines turgor is itself dependent on the rhythmic activity, and therefore on the tonic condition, of the plant. The critical element of this tonic condition may in certain cases be the absorption of light. I have noticed a similar drooping in flowering plants kept in the dark for a long time. Exposure to light, restoring the tonic condition, is in such cases, as also in Coprinus, sufficient to restore the normal turgidity of the plant.

that the after-effect of absorbed stimulus in maintaining a favourable tonic condition is more prolonged in some cases than in others. In Biophytum, for example, the rhythmic activity by which the leaflets are thrown into pulsation is maintained only as long as the stimulating factors are acting. In Desmodium, on the other hand, there is a considerable capacity for storage of energy, and rhythmic activity persists for a long time, even on the removal of external stimulating agencies.

We thus see that the question of turgidity-variation alone is not sufficient to explain the action of light on a plant. We have also to take into account the important element of energy. We have briefly considered the effect of light on tissues in an abnormal condition ; but our main inquiry concerns itself with the precise nature of the stimulatory effect of light on tissues which are in a normal tonic condition. I propose to demonstrate the character of this action by three independent lines of investigation. First, we shall study the action of light on pulvinated organs, in order to see whether or not this stimulus produces the same kind of differential contractile effect as other forms of stimulation. Secondly, discarding those complications which inevitably result from the differentiated structure of the pulvinated organ, I shall proceed to determine the precise nature of the fundamental form-change undergone by a tissue when excited by light. For this purpose I shall subject a radial organ to diffuse stimulation of light, observing whether or not under these circumstances it exhibits longitudinal contraction. And, lastly, I shall study the effect of light in inducing changes in the rate of growth, and shall also try to find out whether the fundamental responsive action discovered in the case of stationary, that is to say of non-growing, organs, might not be capable of explaining the observed variations of growth under the action of light.

Effect of light on pulvinated organs. — If a strong beam of light be applied to the pulvinus of the leaf of Mimosa to the pulvini of the leaflets of Biophytum, it is known that a responsive movement, similar to that evoked by any other form of stimulus, is induced. Now, we have seen that the response of such pulvini is given by means of differential longitudinal contraction. If, then, the effect of stimulus of light in this case is to produce a differential longitudinal contraction, we ought to be able to obtain from a radial organ, subjected to stimulus of light from all sides, a responsive longitudinal contraction.

Effect of diffuse stimulation of light on non-growing radial organs. — In order to demonstrate this, I took a radial style of Datura, and subjected it to the stimulus of light from all sides, by throwing a beam of sunlight which struck it on one side directly, and on others by reflection from properly inclined mirrors. On exposing the specimen to this stimulus for a period of four minutes, a responsive contraction of seven divisions was induced. On the cessation of light, there was recovery in the further course of a period of nine minutes. I next applied stimulus of light for six minutes, and a responsive contraction of eleven divisions was then induced, with a subsequent recovery on the stoppage of light, which was completed in fifteen minutes. We thus see that the effect of stimulus of light in producing responsive contraction is precisely the same as that which is the result of any other form of stimulation ; that a feeble or short-lived stimulus induces a corresponding effect, from which recovery takes place, in a comparatively short time ; and that a strong stimulus — unless it induce fatigue— will bring about a considerable contractile effect, from which the recovery is accomplished in a proportionately longer time. Again, I find that continuous stimulation of light produces a maximum tetanic effect, and that too long-continued action, bringing about fatigue, may induce fatigue-reversal — contraction passing into relaxation — as we found in the case of Mimosa, and of various radial organs which were subjected to too long continued action of stimulus.

thus see that the fundamental contractile effect of stimulus is precisely the same in the case of light as in that of any other! form of stimulation. We know also that the response of a growing organ is the same as that of one which is not growing. It has been shown further, that in a growing organ the contraction due to direct action of stimulus had the effect of retarding growth. Now, from the fact that the effect of light is the same as that of other forms of stimulation, it follows that the result of its direct action on a growing organ should be to produce contraction, and

Fig. 234. Longitudinal Contraction and Retardation of Growth under Light in Hypocotyl of Sinapis nigra The first part of the curve shows the normal rate of growth. Arrow {\) indicates moment of application of diffuse light, which is seen not only to retard growth, but also to induce a marked contraction. The second arrow indicates moment of withdrawal of light, and dotted portion of the curve shows recovery. resultant retardation of growth. Though this conclusion, however, was thus clearly established in theory and by other experiments, using different forms of stimulus, I yet thought it important to test the matter with regard to the specific action of light, under conditions so simple as to bring out the fundamental phenomenon unmistakably.

In order to do this, I took a seedling ol Sinapis nigra in which the hypocotyl was strictly radial, and made a record of its longitudinal growth (fig. 234). Its normal rate of growth, seen in the curve as movement upwards, was at the rate of "015 mm. per minute. On now applying light to the specimen on all sides at once, the growth is seen to undergo rapid diminution till arrested, as seen by the curve becoming horizontal at its highest point. The continued ipplication of light now proceeds to cause a marked contraction, the maximum rate of which is -02 mm. per minute. It is therefore to be noticed that, in this particular experiment, light not only retarded growth, but also produced an actual shortening of the plant. On the cessation of light, growth was slowly recommenced, but the average rate of growth during the sixteen minutes following the stoppage of light was only 001 mm. instead of the normal rate of '015 mm. per minute. This rate was, however, gradually increased in the absence of light, and after a certain interval became normal again.

It must be borne in mind, in connection with this, that though the immediate and after-effects of incident light are here seen as retardation of growth, nevertheless the rate, after the lapse of a certain interval from the cessation of stimulus, may be again increased above the normal in consequence of the enhancement of the tonic condition of the plant, by its absorption of energy of light. In the last experiment we saw that responsive contraction not only arrested growth, but made the tissue actually shorter. In other instances the contraction and resulting retardation are not so great. Thus in a. second experiment with Sinapis, the normal rate of growth was '02 mm. per minute, which during the continuance of light fell to onetenth of this, or '002 mm. per minute. Thus the effect of light on the growing organ is always a contraction, which may in some cases induce a mere retardation, but in others culminates not even in cessation of growth, but in an actual shortening of the responding tissue.

The effects thus described occur in plants which are in normal tonic condition. But we have seen that when the specimen is, on the other hand, in a sub-tonic condition, absorption of enegy in any form from outside will, at first, by increasing the internal energy, serve to accelerate growth ; and that afterwards, when the normal tonic condition has been attained, external stimulus will have the normal effect of retarding growth. In order to verify this inference I took a growing flower-bud of Crinum Lily, which had been previously kept in the dark. And for the further purpose of detecting even the transitory variations, I used the delicate method of balanced record. On now subjecting the specimen to the stimulation of sunlight, acting on it from all sides, I observed a preliminary acceleration of growth, which lasted one

minute (fig. 235). By this time the plant had evidently attained its normal tonic condition, and the continued action of light resulted in a retardation of growth, as seen in the rapid descent of the curve. The light was next shut off, and the after-effect of absorbed energy is seen in the consequent acceleration of the rate of growth above the normal. This acceleration lasted for four minutes, after which the plant returned to almost its normal growth, as seen by the record approximating to the horizontal. The plant may at this point be regarded as in ordinary tonic condition. Light was again applied, and retardation of growth is immediately shown by the descent of the curve. There is now no preliminary acceleration of growth, as in the case when the plant was sub-tonic. Under the long-continued action of light, there is now seen the very interesting phenomenon of the induction of autonomous pulsations of the rate of growth, whose period is about ten minutes.

FiG. 235. Balanced Record of Variation of Growth in Flower-bud of Crinum Lily under Diffuse Stimulation of Light Continuous lines represent the effect during application of light, the dotted line on withdrawal of light. The plant was originally in a sub-tonic condition, and application of light at x , after short latent period, induces preliminary acceleration of growth. After this follows the normal retardation. On withdrawal of light, in the dotted portion of the curve is seen the negative after-effect, followed by return to the normal rate of growth. A second and long-continued application of light induces retardation, followed by oscillacory response.

Phenomenon of oscillation under long-continued stimulation.— We have seen, in Chapter XXIV., that when a tissue is subjected to continuous stimulation, so that it becomes possessed of excess of energy, its response becomes pulsatory in consequence of periodic variations of its excitability. The exhibition of this variation of excitability we have, for the sake of convenience, designated as periodic fatigue. The occurrence of autonomous response is intimately connected with this periodic variation. In some tissues it may be exhibited only once, while in others it may be repeated indefinitely. For example, in Mimosa^ under continuous stimulation, we see a single complete pulsation, consisting of the contractile response, and the subsequent fatigue-relaxation, which looks like recovery (fig. 59). But, in the longitudinal response of the style of Uriclis Lily to continuous stimulus, we observed two pulsations (fig. 58). And in Biophylum and Desmodium we see these repeated indefinitely, constituting what we Fig. 236. Oscillatory Re-

sponse of Arsenic acted know as multiple or autonomous on Continuously by It is very interesting to note, in ^^^^^ connection with this, that even inorganic substances, under continuous stimulation, exhibit this pulsatory response. I give here a record (fig. 236) of the oscillatory response of arsenic when acted on continuously by Hertzian radiation. Or we may view this induction of multiple response in a radial vegetable organ, again, from a different standpoint. By the action of stimulus, it is easy to see that antagonistic hydrostatic actions may be set up as between the excited organ and the rest of the plant. Thus, the direct effect of continuous stimulation is contraction, with increasing expulsion of water from the excited organ into the rest of the plant. The hydrostatic reaction which this will induce in the

rest of the plant will also constantly increase. The occurrence of oscillatory action, under these balanced and opposed forces, is what might be expected. Moreover, in the responding organ itself, we see the action of opposed forces to be induced ; for while the direct effect of local stimulus is to cause contraction, the absorbed stimulus is meanwhile increasing the internal energy, the result of which is the opposite expression in expansion. Thus, from what has been said, it would appear that in all these — namely, the hydrostatic action and reaction between the excited organ and the rest of the plant, the opposed effects of external stimulus and internal energy, and the presence of an excess of latent energy, with periodic variations of excitability — we have so many factors, which would all contribute to a common result, in the oscillatory character of the responsive expression.

But when we come to the case of anisotropic or dorsiventral organs, we find an additional element making for alternation of effects, for when such an organ is diffusely stimulated, we obtain a differential response. But as the constitutions of the two anisotropic halves are different, the fatigue produced on opposite sides will not be simultaneous but alternate. Similar effects will also appear under unilateral stimulation of a radial organ. For here, too, the organ, owing to the relatively greater fatigue of one side, becomes molecularly anisotropic, and the stimulus by its longcontinued action becomes internally diffused.

Similarity of responsive reaction under light and other forms of stimulation. — It has thus been fully demonstrated that the fundamental response of the plant to the stimulus of light takes place, like that to all other forms of stimulation, by contraction, leading, in the case of growing organs, to retardation of growth. Hence the unilateral stimulus of light may be expected to induce curvatures similar to those observed under other forms of stimulation, that is to say :

light on the growing region will be a responsive concavity. 2. The effect of such stimulus of light, acting on the tip of either shoot or root, will be a convexity of the same side of the responding region. 3. Strong or long-continued stimulation of light acting on the growing region will induce a neutral or reversed effect. 4. The response, under certain conditions of continuous stimulation, may be characterised by pulsations. It has been shown in previous chapters that the responsive action of anisotropic or dorsi-ventral organs is not fundamentally different from that of radial organs, the seeming differences being accounted for by differential action. In explaining the various effects of stimulus of light, then, the assumption of various sensibilities in the plant is unjustified.

I shall attempt, therefore, to trace out the manner in which one fundamental effect of responsive contraction is made to find diverse expressions, owing to the anatomicophysiological differentiation of the responding organ. And as the supposed different specific sensibilities do not exist, I shall designate all movements and curvatures induced by light as heliotropic, the signs positive, negative, and diabeing used only for descriptive purposes. An investigation into the action of stimulus of light, then, must apply itself to the following points :

1. The effect of unilateral stimulus, of varying intensity and duration, on the tips and growing regions respectively of radial organs. 2. The induction of autonomous movement by the absorption of energy of light. 3. The action of stimulus of light on molecularly anisotropic and on dorsi-ventral organs. 4. The direct and after-effects of light, in inducing movements of daily periodicity. These are the questions which will be specially dealt with in the course of the following chapters.

The action of the diffused stimulus of light on a radial organ is, like that of other forms of stimulation, to induce a longitudinal contraction. The action on mature does not differ from that on growing organs. In a growing organ the induced contraction has the effect of retarding growth. The response to light may thus consist not merely of a retardation of growth, but sometimes also of an actual shortening of the responding organ. When an organ is in a sub-tonic condition, absorption of energy of light may give rise to a transient acceleration of the rate of growth ; but when the plant has attained the normal tonic condition, response is by the usual retardation of growth.

The after-effect of stimulus of light may consist of a simple continuation of the characteristic responsive contraction or retardation of growth. This constitutes the positive after-effect. There may also be a negative after-effect, consisting of expansion or acceleration of the rate of growth. Under the long-continued action of light, fatigue is induced. There is sometimes an exhibition of periodic fatigue with oscillatory response. The response to light is not different from that evoked by any other form of stimulation. The various responsive movements which occur under the action of light are thus explicable without the assumption of the possession by different organs of different specific sensibilities to light.

Introduction — Theory of de Candolle — Inadequacy of de Candolle's theory — Definition of terms positive and negative — Darwin's theory of modified circumnutation — Response of terminal leaflet of Desmodium — Extreme sensitiveness of some plant-organs to light — Merging of multiple in continuous response- Orientation induced by light — The perceptive region in the terminal leaflet of Desmodium — Heliotropic response in radial organ— Magnetically controlled recorder — Heliotropic response of hypocotyl of Sinapis — Recovery and theory of recti-petality.

Having demonstrated the fact that the stimulus of light induces contraction in mature organs, and retardation of growth in growing organs, in precisely the same manner as any other form of stimulus, we shall now proceed to study in detail the various effects produced by the unilateral application of light. Such application, long continued, is seen to induce movement of the organ, in some cases towards the light, in others away from it, and in still other organs to induce a position at right angles to it. While it is doubtless convenient to distinguish such external effects as positive, negative, and dia-heliotropic, it is nevertheless unfortunate that these terms carry with them an assumption that the movements in question are due to as many distinct sensibilities on the part of the plant-organs. I shall, however, endeavour to show that there is in the plant but one fundamental sensibility to light, as to other forms of stimulation, which finds expression in contraction under its direct action ; and that the resulting movements of the organs are dependent on the question as to whether stimulus acts directly or indirectly, unilaterally or diffusely, and also, on anatomical or physiological peculiarities of structure The complexity of the subject being very great, it has often

been found difficult to resolve a given movement into its components, and this has led to the abandonment of the attempt to relate these various movements to any single basic reaction. So far was this carried that, in spite of the well-known observation that a radial organ, illuminated with different intensities on two different sides simultaneously, bends in the direction of the more intense illumination, Sachs found himself compelled to believe that it was the direction and not the intensity of light that determined the responsive movement.

Theory of de Candolle. — It is appropriate to make here a brief mention of the theory of de Candolle, which has hitherto met with unmerited neglect. De Candolle started from the known fact that light retards growth, and explained growth-curvature as due to the relatively greater growth of the shaded, inducing concavity of the lighted, side. This explanation of the mechanics of such movements constitutes an important advance, though it does not take full account of all the factors of the problem. This theory of de Candolle has, however, been discarded, in consequence of the difficulty which it presented of explaining the action of the negatively heliotropic organs, in which the lighted side is found to be convex. Extending this theory to cases of negative heliotropism, it was regarded as a logical inference that light should here accelerate growth. It is doubtful, however, whether such an inference is justifiable. In any case this was negatived by the researches of Miiller-Thurgau, F. Darwin, and Wiesner, who showed that light retarded general growth in negative as well as in positive heliotropic organs. It will be shown, however, in the course of succeeding chapters, that though the circumstances which modify the response of the plant, so that it is exhibited as negative heliotropism, are somewhat complicated, yet they in no way detract from the theory of de Candolle as applied to positive heliotropism.

Inadequacy of de Candolle's theory. — The flaw in his theory lies rather in the fact that he regarded the normal rate of growth under shade as the active factor in growthcurvature, elongation on the lighted side being retarded, whereas in the case of positive light-curvatures the motive-power really lies in the active responsive contraction of the lighted side, the expelled water from which, reaching the opposite, may further cause an increase of growth above the normal. This fact, that it is the active contraction of the lighted, and not the passive growth of the unlighted, side that is actually the efficient cause of heliotropic curvature, will be made clear by taking an extreme case in which there is no growth. Here, if heliotropic curvature had been due simply to differential growth, the occurrence of curvature would have been an impossibility. But heliotropic curvatures are observed in organs which have come to growth-standstill. Again, grass haulms, in which growth is arrested, exhibit curvatures due to the contraction of the lighted, and the renewal of growth (due to the increased turgescence caused by expelled water) on the unlighted, side, in precisely the same manner as they were observed to do under the action of gravitational stimulus.

I shall now proceed to describe some typical experiments on heliotropic effects as induced by the unilateral application of light. And since it has been shown that the responses of growing organs are not essentially different from those of pulvinated organs, it will be helpful to begin by observing the effect of the unilateral application of light on the latter, especially as these have the advantage of showing relatively rapid reactions. In order to obtain a pulvinus which approximates in character to that of a radial organ, a specimen must be selected in which the difference of excitability, as between the upper and lower halves, is as small as possible. This may be found in the pulvinus of the large terminal leaflet of Desmodium.

Definition of terms ' positive' and ' negative.'— It will be demonstrated, in this and succeeding chapters, that however various in type may be the responsive movements induced by light, they are not due to the different specific sensibilities of different organs, but can all be shown to form only special instances of a single fundamental effect And this fundamental effect is the same in growing, in stationary, in radial, and in anisotropic organs. In describing the responsive actions induced by light, it will, however, be necessary to distinguish the direction of movement, in relation to the stimulating light, by clearly defined terms. I shall therefore designate all movements, of whatever organs, towards light as positively heliotropic, and all movements away from light of whatever organs, as negatively heliotropic.