Plant Response as a Means of Physiological Investigation

The directive versus non-directive action of light. — It will be well at this point to enter into the question of arbitrary distinction which is commonly made as between heliotropic action in radial organs and the same action in the anisotropic or dorsi-ventral. It is clear, however, that such a distinction Fig. 258. Negative Multiple Response in Biophytum when Acted on from Above by Strong Sunlight is no longer possible when the fundamental unity of effects in the two cases has been perceived.

But it is customary to make a further distinction between these effects, on the ground that the movement of the radial is determined by, while that of the anisotropic is independent of, the directive action of light. In this matter again I shall be able to show further that no such line of demarcation can be drawn. It will then be seen that the differences between the two classes of phenomena are only apparent. In the case of radial organs, we saw that the direct action of moderate stimulus of light, inducing concavity of the proximal, gave rise to positive heliotropic movement. The same is true of the similar direct action of a moderate intensity of light on, say, the pulvinus of Mimosa, where the upper and lower halves of the organ both exhibit positive response. With stronger light, again, the radial organ develops an induced anisotropy, by which the proximal side, owing to fatigue, becomes the relatively less excitable. Under this condition, the proximal side of the radial organ corresponds to the less excitable upper half of the pulvinus of Mimosa, and the distal to the more excitable lower half. This differentiation is seen to be continuous throughout the three cases of the radial organ, the plagiotropic stem, and the pulvinus, although in the first of these it is transient, lasting only during the action of stimulus. In all these cases alike, strong unilateral stimulus, acting on the less excitable proximal or upper surface, becomes internally diffused, and causes movement away from stimulus, or negative heliotropic response, in a direction which is perpendicular to the plane of separation of the two anisotropic halves. Thus there is no difference between the negative heliotropic responses of a radial and a pulvinated organ.

The distinction between the two cases of the permanent and temporary differentiation of the organ has been illustrated by Sachs by a reference to the induction of polarity in steel and in soft iron respectively. According to this analogy, Marchantia behaves to intense light like steel to a magnet, retaining permanently the induced polarity. Tropceolum majus, on the other hand, behaves like soft iron, assuming a definite but temporary polarity, which disappears when the influence of light, like that of the magnet, is withdrawn. This illustration, however,, though vivid, is likely to be misleading, for it suggests that, under strong stimulus, the normal properties of the organ pass into a polar or opposite condition. But there is no such change. Light merely induces a difference of excitability in the two sides. That there is no reversal of heliotropic sensibility is shown by the fact that either side, when excited by moderate intensity of stimulus, gives positive response. The necessary condition for the exhibition of negative heliotropic response is not only the differential excitability of the two sides of the organ, but also the internal diffusion of stimulus.

Owing, however, to the permanence of this differentiation in a pulvinated organ, there will be certain conditions under which the action of light will appear to be non-directive, for it is the diffuse stimulus, however produced, that brings about the responsive concavity of the more excitable lower half of the pulvinus, and this internal diffusion will take place just the same, whatever be the flank of the organ on which the strong external stimulus may have been applied. In the case of the radial organ, on the other hand, the differentiation between the less excitable proximal and more excitable distal is not fixed, but changes according to the side acted upon at the time by light.

The different responsive movements induced by light — positive, negative, dia-heliotropic, and para-heliotropic — have hitherto been ascribed, as I have already had occasion to point out, to as many specific sensibilities possessed by the plant. They have now, however, been demonstrated to be but so many examples of the general law that plant-organs respond to stimulus, by the induced concavity of the relatively more excited. There is thus no fundamental discontinuity between the responses of radial and of dorsi-ventral organs, or between

positive heliotropic and negative heliotropic response. This will be seen still more clearly in the following statement, applying to the two extreme cases of radial and of dorsiventral organs, the plagiotropic constituting an intermediate link between the two. General view of responsive curvatures induced in different organs by unilateral application of light 1. Radial organs : (a) Positive response \ stimulus localised, the proximal side more excited and concave ; e.g. Sinapis under moderate unilateral light (p. 609).

(b) Intermediate or neutral response : stimulus internally diffused, causing equal excitation of proximal and distal sides ; the organ takes up a neutral (so-called dia-heliotropic) position, at right angles to light ; eg. Sinapis under moderately strong unilateral light (p. 609) ; certain tendrils apparently insensitive. (c) Negative response : strong stimulus internally diffused, which also induces anisotropy, the distal being then the more excitable ; concavity of the more excited distal ; e.g. seedling of Sinapis, and tendril of Vitis under strong unilateral sunlight (pp. 609, 611).

2. Dorsi-ventral organs : (a) Positive response : stimulus localised by relative non-conductivity of organ : the more excited proximal side concave ; e.g. positive movement of terminal leaflet of Desmodium ; the diurnal sleep movements of Robinia, Erythrina indica, and Clitoria ternatea (p. 629). (b) Intermediate or neutral response : stimulus internally diffused ; upper and lower halves equally excited ; e.g. terminal leaflet of Desmodium, under several hours of vertically acting sunlight (p. 604).

(c) Negative response : strong stimulus acting from above on highly conducting organ, of which lower half is considerably more excitable ; concavity of the more excitable lower half ; e.g. fall of Mimosa leaf under strong illumination from above ; diurnal sleep movements of Oxalis, Biophytutn, -and Averrhoa. The responsive action of growing organs being not essentially different from that of pulvinated organs, a similar explanation must be applicable to the two classes of phenomena.

Owing to long-continued unilateral excitation by light, the upper side of a plagiotropic stem is rendered the relatively less excitable. Hence such organs may be regarded as equivalent to diffuse pulvinoids, of which the upper side is the less excitable. In both these cases, of pulvini and diffuse pulvinoids, the local application of moderate stimulus on either side induces normal positive response. But with long-continued application of strong stimulus two types of response may be obtained, according as the organ is characterised by a feeble or high power of transverse conductivity. In the former of these cases the stimulus will remain localised, and the response will be positive. In the second, the stimulus will become internally diffused, and if the lower side be the more excitable, stimulation of the upper will give rise to the concavity of the lower, constituting negative response.

As examples of the first of these classes may be mentioned the diurnal sleep movements of Robim'a, Erythrina indica, and Clitoria ternatea, in which vertical illumination induces upward folding of the leaflets. As representing the second class, we have both plagiotropic and pulvinated organs. The plagiotropic stems of Mimosa, Ipomcea, and Cucurbita exhibit increasing concavity of the lower side with the duration of the day's illumination. A periodic downward movement is thus induced in them, which reaches its maximum at the end of the day. The reverse movement, of gradual erection, occurs during the night.

downward folding of the leaflets, is exhibited in the diurnal sleep movements of Oxalis and Biophylum. In Mimosa the application of moderate intensity of light from above induces upward or positive heliotropic movement. Strong and long-continued application of light on the same upper surface, however, owing to the internal diffusion of excitation, induces concavity of the lower and more excitable side, or negative heliotropic movement. Both these responses may be regarded as cases of the directive action of light ; but if the organ be excited by stimulus which is externally diffuse, the responsive movement is still downwards. The directive action of light has thus passed into non-directive.

Difficulty of distinguishing between effect of light and other reactions— Theories of Frank and De Vries— Subsidiary factors : (i) Epinasty and hyponasty ; (2) Effect of gravity ; (3) Effect of suctional activity and of turgescence ; (4) Modification of effect by characteristic limits of flexibility — Discrimination of the part played by heliotropism in the movement of the leaf — Proof of absence of any specific dia-heliotropic tendency in leaves— The lamina not the perceptive organ— Principal types of the response of leaves to stimulus of light— Positive type of response : mango leaf— Negative type of response : leaf of Artocarpus.

We shall now take up the question of the effects of illumination on ordinary leaves, which are generally supposed, on account of some special dia-heliotropic sensibility, to have the habit of placing themselves at right angles to incident light. We have seen in the last chapter that the movement of anisotropic organs under heliotropic stimulation is not the result of any specific sensitiveness, but constitutes a simple instance of the response of plant-organs to all forms of stimulus, the response being appropriately modified in this particular case by the anatomical and physiological peculiarities of the responding organ. We have been able to demonstrate the continuity of this responsive phenomenon by analysing it in two extreme cases of the anisotropic differentiation, those namely of the plagiotropic stem, in which we see one of its earlier phases, and of dorsi-ventral pulvinated organs, in which it attains its highest development. In the movements of ordinary leaves we have what is merely an intermediate stage between these, hence any explanation which would elucidate their action must be one which is applicable to both the extremes.

Difficulty of distinguishing between effect of light and other reactions. — In the case of the response of pulvinated organs, we had the advantage, owing to their great motility, of being able to refer each particular responsive movement to the immediate stimulating action of incident light. In ordinary leaves, however, the movements being very sluggish, it takes so long a time for any given responsive action to attain the requisite magnitude for ordinary observation, that other factors of variation intervene, and it becomes difficult to know how much of the resultant response is due to heliotropic stimulus as such. It is this great difficulty of disentangling the response due to light, which is the proper subject of the inquiry, from numerous other subsidiary factors that has led to the existing divergence of views among observers.

In the course of the present chapter, therefore, I shall shortly enumerate those various agencies which are subsidiarily instrumental in bringing about the ultimate position assumed by the leaf. I shall then describe a method by which heliotropic action proper can be discriminated with certainty from other influences. The relation between the fundamental action in response to light, which has already been demonstrated, and the heliotropic response of the leaves will thus be made apparent. But before entering upon these questions I shall briefly allude to the principal existing theories on this subject.

Theory of Frank. — There are at present two main types of opinion with regard to the question of the effect of light on leaves. Frank and his school account for that action of leaves by which they place themselves with their flat surfaces perpendicular to the direction of incidence of the rays of light, or of the action of gravity, by assuming that dorsi-ventral organs possess a peculiar property of sensitiveness to the directive action of light and gravity. This they designate as Transverse or Dia-heliotropism, and Transverse or Dia-geotropism. It is supposed that the habit has been acquired for the advantage of the plant, inasmuch as the leaves, by placing themselves

in this particular position, are enabled to absorb the largest amount of sunlight. Such arguments, however, do not throw any light on the mechanism by which the movement is brought about. It is even somewhat difficult to understand how this generalisation, that leaves place themselves at right angles to light, has come to be accepted as a universal fact ; for it is only necessary to make a visit to the open forest in order to see that, so far from this being the case, many leaves place themselves vertically upwards, others downwards, and the rest in all possible intermediate angles between the two. All that can be claimed on behalf of the dia-heliotropic position is that no deviation from it is greater than plus or minus 900.

Theory of De Vries. — De Vries, however, is in disagreement with this theory. He establishes the importance of the unequal rates of growth in the upper and lower halves of the leaves — i.e. epinasty or hyponasty — as a factor in bringing about their ultimate attitude. He next assumes, in accordance with the generally accepted view, the existence of two opposite responsive reactions, positive and negative, in regard to light and gravity. He then proceeds to show that there can be no necessity for the further assumption of a third or dia-heliotropic tendency in the leaves, since epinasty and hyponasty, their different combinations with either of the opposite actions of positive and negative heliotropism and geotropism, and considerations of the weight and balance of parts, are all factors which take a share in determining the position ultimately to be assumed by the dorsi-ventral organ with regard | to light.

I shall now attempt to demonstrate the fact that the responses of ordinary leaves are in every way similar to those of the pulvinated, the mechanics of whose movements have already been fully described. It will further be shown how and under what circumstances the normal positive passes: into negative response, through certain intermediate phases. Subsidiary factors.— But before undertaking either of! these inquiries, it will be well to enumerate briefly the ;

subsidiary factors which combine with the heliotropic action proper in determining the final position of the leaf. 1. Epinasty and hyponasty. — These unequal growths of one side or the other, De Vries believed to be brought about by some spontaneous unknown cause in the plant itself. Detmer, however, came to the conclusion that, as regarded epinasty, it was not spontaneous, but induced by the action of light. This conclusion was based on the observations (1) that cotyledons of Cucurbita remained closed up, in continuous darkness, but opened out when subjected to light ; and (2) that the primordial leaves of P/iaseolus, kept in darkness, remained folded, and only opened out on illumination. The investigations of Vines, however, though he supports the contention of Detmer with regard to Phaseolus, have led him to a different view in the case of Cucurbita, where he finds the epinastic movement to be induced even in the absence of light. He has therefore come to the conclusion that the phenomena of epinasty and hyponasty are spontaneous, and not directly due to the action of illumination.

These differences of opinion, however, have arisen from the obscurity in which autonomous or spontaneous movement has been involved, and they may be expected to disappear when its true nature is clearly perceived. These alternate movements of growth are in fact only another example of multiple or autonomous response, the difference between them and those other forms with which we are already acquainted lying in the greater slowness of their period, and in the relative fewness of the pulsations that can be exhibited. In the case of autonomous pulsation or circumnutation of stems, since the growth on which they depend is indefinitely prolonged, we have an indefinite continuance of these pulsations. In the case of leaves, however, the organ usually completes only half its swing, whether epinastic or hyponastic, and by that time further pulsation is arrested, owing to the cessation of growth ; yet in some cases there is more than a semi-pulsation, as where hyponastic movement is followed

by epinastic. It has already been shown that autonomous response in general can take place only when the internal energy or the sum total of the latent stimulating factors is above par. This is equally true of the autonomous response of growth itself. In the case of autonomous epi- or hyponastic pulsations, therefore, a certain amount of internal energy is essential for their initiation. This is in some cases supplied by other forms of stimulus, but there may be others in which light is the critical factor. In order, then, to study the directive action of light on dorsi-ventral organs under normal tonic conditions, we must be able to determine the characteristic influence which will be exerted by the stimulus of incident light in modification of the already existing movement of the organ. This inquiry is therefore exactly parallel to our previous study of the action of light in modifying the existing growth-movements of radial organs. In that case the variations induced in the ordinary rate of movement afforded us a measure of the effect of the stimulus of light. In the case of dorsi-ventral organs such as leaves, similarly, the effect of light can be correctly inferred only by observing the variations which it induces in their existing movements. The manner in which this is done will be described presently.

2. Effect of gravity. — In long-continued experiments on the curvatures induced by light, the observed movement is also modified in part by the influence of geotropism. This geotropic action in leaves is by some investigators believed to be of two types, negative and positive. Others, again, regard it as dia-geotropic. I shall, however, adduce considerations which will show that the upper and lower halves of dorsi-ventral organs exhibit differential excitability to geotropic as to other forms of stimulus. In order to neutralise the geotropic action, and thus study the heliotropic effect alone, Francis Darwin mounted the plant on a rotating klinostat. It is true that in a strictly radial organ the geotropic effect is successfully neutralised by rotation on the klinostat, since in this case the geotropic sensitiveness of the

different flanks is the same; but this, as will appear from certain experiments to be described later, cannot be said of dorsi-ventral organs, for in these geotropic sensitiveness is different in the upper and lower halves. In my own experiments on the action of light, however, I shall be able to give results which are but little affected by geotropism. 3. Effect of suctional activity and of turgescence. — There are two other factors not hitherto taken into account which exert considerable influence in determining the attitude finally assumed by the leaf. These are the general condition of turgescence of the plant, and the limits of flexibility which characterise the particular responding organ. The first of these is, as we have seen, dependent on the suctional activity. Now, the mechanical response of a plant-organ is the result, as we already know, of the expulsion of water from the excited tissue ; but if the tissue be over-turgid expulsion of water is opposed, and the responsive movement is thereby reduced or abolished, as we have seen in the case of Mimosa in a condition of excessive turgor. The same phenomenon I have again seen manifested, in a remarkable manner, in the difference of the movements made by the leaves in response to light according as they were normally- or superturgid. Thus, in a small plant of Artocarpus, grown in a pot, and in the autumn season, when the suctional activity was not great, the leaves responded to light by making a progressive angle with the vertical, so that, under the long-continued action of light, they first reached the horizontal position and then fell many degrees below it ; but in the rainy season, when they held themselves abruptly vertical in consequence of excessive turgor, the action of light produced little or no responsive movement. In the autumn season, again, the limited system of roots and rootlets possessed by this plant, when grown in a pot, allows it only a moderate degree of turgescence, and in this condition it readily responds to light ; but a large tree of the same species, growing in the open and possessed of a highly ramified and extensive root-system, will during the same

season, its turgescence being great, maintain its leaves in a vertical position, but little affected by the action of light. From this it will also become clear that any influence, such as long maintenance under darkness, which modifies the suctional activity, is liable to render the response of the plant abnormal. 4. Modification of effect by characteristic limits of flexibility.— I shall here deal with another important factor in the determination of the final position assumed by the leaf — that is to say, with the anatomical peculiarities which determine the limits of flexibility. Let us take the petiole bearing the terminal leaflet of a leaf of Desmodium. We now suppose this leaflet to be outspread, in such a position that its midrib is in a continuous straight line with the petiole. This upper line we shall know as the dorsal line. It consists of two parts or components, the laminal and the petiolar. In this particular position they form a straight line ; but the movement of the leaflet takes place with the point of junction as the hinge. We shall then distinguish that particular position of the dorsal line in which the laminal and petiolar halves are continuous and straight as neutral, and angular movements above or below will be measured accordingly. In the case of Desmodium, when the terminal leaflet has reached the neutral position, it cannot, owing to the anatomical peculiarity of the joint, be bent further upwards ; but it can be bent in the opposite direction, that is downwards, until the leaflet lies along the under side of the petiole, the curvature being then through 1800. Thus the limits of flexibility of this leaflet may be expressed by the

itself, in the case of Desmodium, for example, may, and does, above, the leaflet will move continuously upwards till the lamina has reached the neutral position— that is to say, till the midrib constitutes a straight prolongation of the petiole ; but if light act on the leaflet from below, it will bend downwards (fig. 259, a). If the petiole, however, at the beginning of the observation be horizontal instead of vertical, and the lamina be in the neutral position, then vertical light cannot, owing to the anatomical peculiarities of the pulvinar joint, carry the leaflet further above the dorsal line (fig. 259, b). Or we may

Vertical light on terminal leaflet of Desmodium causes (a) up movement till the dorsal line is a continuous straight line ; light applied below causes movement downwards below this neutral line through 1800. In b is shown neutral position, after reaching which there is no further movement upwards. In c is shown movement of terminal leaflet of Erythrina indica upwards, under vertical illumination, through 1800 above the neutral line. again take as an example the terminal leaflet of Erythrina indica (fig. 259, c). The limit of flexibility is in this case represented by an angle of almost 1800 above the neutral, whereas downwards its limit is about 900. The formula is i8oc

from above, it may become almost doubled upon the petiole upwards, just as we found the terminal leaflet of Desmodium to be almost doubled downwards. Hence we see that though the heliotropic effect of light is always the same, yet the position of the leaflet in space that is to say, its relation to the vertical line — is largely modified not only by the limits which the anatomical structure of the laminal pulvinus or pulvinoid imposes upon its flexibility, but also by the angle which the petiole makes with the stem ; for there is, generally speaking, a second pulvinus or pulvinoid at the junction of the petiole and the stem, and this petiolar pulvinus is, in its turn, more or less sensitive to stimulation (p. 59). We have thus obtained some idea of the anatomical elements, regarding the petiole and its joints, which enter into the complex question of the final position assumed by the leaf in space.

Discrimination of the part played by heliotropism in the movement of the leaf. —In studying the heliotropic effect we are concerned only with the action of light itself, and not with the resultant effect, due to various co-operating factors. When the action of each of these is definitely understood, it becomes a simple problem to understand the effects due to their combination. The difficulty of this investigation has hitherto lain, as already said, in the fact that, owing to the generally sluggish nature of responsive movements in ordinary leaves, a long time must be allowed to elapse before they become measurable, and during this long period other factors may become operative in unknown ways. The effect of light, however, can easily be discriminated by the use of the Optic Lever for record, for this allows us any degree of magnification which may be desired. Thus the natural curve gives us the resultant effect of all the pre-existing factors under normal tonic conditions, and its subsequent variations under the incidence of light at once exhibits the distinctive action of that stimulus. On the withdrawal of light, again, the recovery from the induced variation affords an additional corroboration of the inference that the variation itself had been due to the action of light. In consequence, moreover, of the delicacy of this means of continuous record, the characteristic effect can be detected in the course of a few minutes, thus eliminating the unknown