Plant Response as a Means of Physiological Investigation

Photonasty and para-heliotropism. — In experimenting with dorsi-ventral shoots, De Vries found that when light was applied to the lower or normally shaded surface of, for example, runners of Lysimachia Nummularia, the result was a concavity of that side ; and when strong light was applied on the upper surface, the result was still the concavity of the normally shaded, and now distal, side. He obtained similar results also with midribs of leaves. Again, in the case of Marchantia thallus, it was found by Frank and Sachs that it was the normally shaded side which became concave, whether light was applied above or below.

De Vries explained the curvature away from light, when the dorsal surface was illuminated, by the assumption that the organ was negatively heliotropic ; but the concave curvature induced when the lower surface of the same organ was illuminated necessitates the description of that surface, at least, as positively heliotropic. Thus we are driven to assume that the two surfaces of the organ are endowed at the same time with two opposite heliotropic properties, the obvious impossibility of which has led to the idea that in this class of phenomena we have not to deal with the directive action of light at all.

In the case of certain pulvinated organs I shall be able to show an exact parallel to these phenomena — that is to say, an induction under light of concavity in the lower surface, whether light be applied from above or below. This being so, it is clear that the only theory of the phenomena which could be satisfactory would be one which would apply equally to both. Closely connected with the same inquiry is the phenomenon of para-heliotropism, or the so-called diurnal sleep. Under the intense illumination of midday, the leaves or leaflets of certain plants take up positions which outwardly resemble those which they adopt at night. In some cases again, in which the normal daylight position is outspread, and the nocturnal one of downward folding, the effect of light at midday on the leaflets is to induce a folding upwards. These phenomena have not yet been satisfactorily explained. Darwin suggested that such habits had been acquired for the special purpose of avoiding too intense an illumination. I shall be able, however, to offer a simple and inclusive ex-

planation, applicable to the phenomena both of photonasty and of para-heliotropism. It will be noticed that in the first case we have to deal with the effect of light on all growing dorsi-ventral organs, and in the second with its effect on mature dorsi-ventral pulvini ; and in dealing with both these classes I shall pass step by step from the consideration of simple to that of more complex types. The response of Tropseolum majus. — Sachs found that when the stem of Tropceolum majus is exposed to intense and long-continued unilateral illumination, a negative heliotropic curvature is induced ; but when the plant is exposed to moderate unilateral illumination it exhibits positive heliotropic movement. The explanation of this difference is made quite clear from the experiments which I have already described, with reference to seedlings of Sinapis nigra and to the tendril of Vitis ; for it has been shown that in these cases moderate unilateral stimulus of light caused a positive heliotropic effect ; whereas, under the action of intense stimulation, a transient anisotropy was induced, on account of which the excitability of the over-stimulated proximal side was diminished. Hence the distal side was rendered the relatively more excitable, and the intense stimulus becoming internally diffused caused a concavity of the distal side, and gave rise to a negative heliotropic effect (p. 609).

The negative effect is thus due to anisotropy, and internal diffusion of excitation from the proximal to the distal sides, this latter factor evidently tending to be facilitated by any agency that increases the conductivity of the tissue (p. 603). We have also seen elsewhere that this power of conductivity is very much augmented in summer (p. 475). In connection with this, it is interesting to note that the negative heliotropic curvature of Tropceolum majus, due to transmitted excitation, has only been observed by Sachs during summer, while in autumn the effect observed by him was always positive, owing to stimulus remaining localised. Another instance of the same kind is furnished by the hypocotyl of Ivy (Hedera helix), which from the normal positive heliotropic curvature, in

autumn, as observed by Darwin, passes into the exhibition of a strongly negative heliotropic condition in summer. Responses of plagiotropic stems. — It is found that under feeble diffuse illumination the Gourd-plant (Cucurbita) for example, grows erect. In the open, however, if, on account of its own weight, or by the action of the wind, it is once made to bend, the stem is brought into a position where its upper surface is constantly acted on by strong light, whilst its lower is shaded. By the continuous action of the stimulus of light the upper surface is now rendered less excitable, and a permanent anisotropy (plagiotropism) is induced, such as we saw transiently exhibited in the hypocotyl of Sinapis (p. 609) under the short experimental exposure of a specimen to intense unilateral illumination. It has already been shown in Chapter VII. that such plagiotropic stems are more excitable on their lower or shaded side, and that under diffuse stimulation response is by concavity of that side. Hence a recumbent plagiotropic stem of this description, acted on by strong vertical light, which becomes internally diffused, will always exhibit concavity of the lower surface, in consequence of which it will be closely pressed against the ground.

Such a plagiotropic organ, then, acted upon dorsally, shows a concavity of the ventral surface, or a negative heliotropic effect ; but if the ventral, or normally shaded, side be itself acted upon directly by light, the result will still be the concavity of the lower or more sensitive surface — that is to say, a positive heliotropic effect. In the former case, then, we have an example of the differential response of an anisotropic organ to diffuse stimulation, by concavity of the more excitable ; and in the latter, the direct contraction and concavity of the surface acted upon, which happens in this case to be also the more excitable. This will explain why, in the midribs of leaves, in plagiotropic shoots of Lysimachia, and in the thallus of Marchantia, we always observe the concavity of the shaded and more excitable side, in response to the action of strong light, whether it is applied from above or below.

0) Mimosa.— This fact, that in plagiotropic stems under the diffuse stimulation of light it is the more excitable shaded side which becomes concave, I have been able to demonstrate by numerous other experiments ; for example, taking four creeping stems of Mimosa, I tied them in such a manner that their free ends should be vertical. The shaded sides of the four specimens were turned so as to face each a different point of the compass — east, west, north, and south. Subjected thus to the diffuse stimulation of light from the sky, they all executed curvatures. The specimen whose under side faced the east became bent towards the east. The same happened to those which faced north, south, and west — that is to say, they became curved towards the north, south, and west respectively. The fundamental responsive action by which all these were determined was the induced concavity of the under, or normally shaded, side, which is the more excitable.

ib) Ipomcea. — Another example was that of the creeping stem of Ipomcea. This I tied up vertically with its end free, at 10 A.M., the normally shaded side being represented in the diagram to the left (fig. 249). For purposes of the record I placed behind it a piece of paper, on which its different positions were traced from time to time. It will be seen that by 1 P.M it had become considerably curved, the normally shaded side being concave. This concavity had become still more

Fig. 251. Response to Diurnal Light and Darkness of Plagiotropic Stem of Ipomcea held vertical The curvature is seen to increase with the progress of day, the shaded side to the left becoming more and more concave. The dotted figures represent positions of gradual recovery at night. marked by 4 P.M. The next record was taken at 7 P.M., when the sun had gone down, and the source of stimulus was therefore removed. The dotted portion of the record shows the partial recovery which had now taken place, and whose extent was still further increased by 10 P.M.

This partial undoing of the induced curvature is mainly due to the recovery which is always observed on cessation of stimulus. It might, however, be urged that geotropic action, absent when the plant was vertical, and coming into increasing effectiveness with its growing horizontally, played a large part in bringing it about. In order, then, to eliminate this element, I next experimented on a stem of Cucurbita. Light and Darkness of Plagiotropic plagiotropic Stem was held Stem of Cucurbita maxima , . , . . . . «

of dorsi-ventral division vertical, the m SUCh a way mat trie ventral surface being here represented plane which divided its to the right. The action of gravitation J is thus practically eliminated. Cumulative effect of day's illumination is seen in progressive concavity of normally lower or shaded surface. Dotted figures side is here represented in (fig. 250). Owing to this particular arrangement of the plant it will be seen that the responsive heliotropic movement must take place in a horizontal plane, and that on it geotropism will have no influence. The free end of the plagiotropic stem had been naturally somewhat curved upwards, and this appeared, when placed sideways, as a slight curve to the left, in its position at 7 A.M. Under the stimulus

of daylight by 10 A.M. the stem is seen to have taken up a strongly curved position, with concavity of the shaded side. This process is seen from the figure to have been progressive, but on the approach of darkness there was recovery, the return being considerable at 7 p.m., and still greater at 10 P.M. It may be stated here that such recoveries are never quite complete, part of the curvature being fixed by growth. Thus in a naturally growing plagiotropic stem, the portion which was one day slightly lifted above the ground is on the next, owing to this residual effect, made to lie closely against it, so that by the action of light the growing stem is pressed progressively closer to the earth.

Daily periodic movement of plagiotropic stem. — From this demonstration of light-curvature and recovery, it will be seen that the free organ has a daily periodicity in virtue of which, in the daytime, it moves gradually downwards, and at night, owing to recovery, upwards. In this plagiotropic stem, then, we see, as will be shown later, the first induction of that nyctitropic movement which is more strikingly displayed in dorsi-ventral leaves. Responsive movement of pulvinated organs. — The demonstration which I have just made of the peculiar responses of anisotropic organs to the stimulus of light, as exemplified in the case of plagiotropic stems, will be found still more strikingly applicable to pronouncedly dorsi-ventral organs like pulvini. In the last-named instance, indeed, as motility is great, the investigation has the advantage of concerning itself with responsive effects which take place very quickly.

From what has already been said, we shall be prepared to meet with two different types of responses, according as the transverse conductivity of the organ is feeble or considerable. In the former of these cases, under unilateral stimulation, there is no internal diffusion of stimulus, and the side acted on, whether above or below, will respond by concavity ; but in the latter case— that is to say, where the transverse conductivity of the tissue is great — it will be the more excit-

able side which, under strong unilateral stimulation of either, will become concave. Supposing the lower side to be the more excitable, then, strong stimulation, whether of dorsal or of ventral, will bring about concavity of the lower ; but here we must bear in mind the possibility of an effect which will be the result of moderate stimulus. The differential effect, which brings about the concavity of the lower, even when it is the dorsal surface that is excited, depends on the internal diffusion of stimulation, and this in turn is dependent not only on the conductivity of the tissue, but also on the intensity of the stimulus. If, then, a feeble or moderate stimulus be applied on the dorsal surface, even of a highly conducting organ, the result will be a concavity of that surface. The long-continued action of moderate stimulus will, however, bring about a gradual percolation, and the first response due to localised, will give place to the differential effect of diffuse, stimulus. If the stimulus again be very much stronger, this reversal will take place much more quickly. We thus see that responsive movements may be positive, neutral, or negative, according to the strength and duration of the stimulus.

Pulvinated organ showing positive heliotropic movement : (a) Terminal leaflet of Desmodium. — We shall now take the case of a pulvinus in which the conductivity is feeble, and in which we should therefore expect to obtain positive response. I have already given response-curves in illustration of this positive response, whether it be the upper or lower surface of the pulvinus which is subjected to light (p. 586). It was also shown that the leaflet moved towards the light, and that in such cases it was the pulvinus, and not the lamina, which was both the perceptive and responding region. •

(b) Response of leaflet of Robinia. — The leaflets of Robinia, under the vertical light of the noonday sun, exhibit what is known as diurnal sleep — that is to say, they fold themselves upwards. This is simply an instance of the positive heliotropic effect common to all those pulvinated organs in which transverse conductivity is feeble. I give here (fig. 253) a response-record obtained with a leaflet of Robinia under the action of sunlight from above. A similar response, but in a downward direction, is obtained when light is made to act from below ;

Fig. 253. Positive Heliotropic Response of Leaflet of Robinia to Sunlight Acting from Above Dotted line represents recovery on cessation of stimulus. Time - marks represent intervals of five minutes. (c) Responsive movements of leaflets of Erythrina indica and of Clitoria ternatea. — For the sake of simplicity I described the movement of Robinia leaflet as upward ; but the actual direction is one which more or less accurately coincides with that of incident sunlight. As further examples of this particular type of diurnal sleep movement, I may mention the leaflets of Erythrina indica and Clitoria ternatea (Indian name Aparajitd). Both of these are so remarkably sensitive that they follow the course of the sun, in such a way that the axis of the cup formed by the folding leaflets at the end of the petiole is coincident with the rays of light, and continues so from about 1 1 A.M. till about 3 P.M.

The negative heliotropic type of response. — We shall now pass on to the second, or negative, heliotropic type of response due to the internal diffusion of stimulus; and in order to show that there is a continuity between these and the Fig. 254. Positive Heliotropic Response of Leaflets of Erythrina indica The leaflets form a hollow cup with direction of incident sunlight as axis. former instances, I shall here refer once more to the case, already mentioned, of the heliotropic response of the terminal leaflet of Desmodiumy which in winter is always positive, but in a given experiment, under the conditions of greater transverse conductivity which are brought on in summer, exhibited, after two hours of continuous exposure to vertical sunlight, a neutralisation of the previous positive effect (p. 604).

{a) Response of pulvinus of Mimosa. — The second type of response will be the better understood if we first study in detail all its characteristics. As I have already said, these negative heliotropic responses result from the transverse diffusion of stimulus across the tissue, which brings about the concavity of the more excitable lower half of the organ. We can easily, in the continuous response-record to be given presently, detect this gradual process of the percolation of stimulus through the tissue, when light is applied from above. As there is in the case of Mimosa a considerable mass of intervening tissue between the upper and lower surfaces of the pulvinus, it follows that unless the stimulus applied be excessive, there will be a certain interval of time required for its passage. We should therefore expect that on the application of light to the dorsal surface there would be a local contraction and concavity of that side, raising the leaf up, and causing a preliminary positive response ; but this movement will be arrested, and gradually reversed, so soon as the stimulus reaches the lower side, and begins to induce antagonistic contraction there ; and after this, the greater excitability of the lower will be manifested by its greater contraction, as seen in the negative response, or depression of the leaf. All this will be clearly understood from the series of records given below.

It is necessary, however, before describing the effect of the continuous application of light from above, to analyse the responsive sensibilities of the two sides of this organ ; and this is the more desirable since, in the case of Mimosa, it is commonly assumed that excitability characterises only the lower half of the organ, the responsive movement of the pulvinus being due to that factor alone. In the course of the present work, however, I have frequently stated that the upper half also was excitable, and that the usual responsive movement was due to the differential excitability of the two. It is now easy, using localised stimulus of light, to submit this question to a crucial test.

First I took a record of the responsive movement of the leaf of Mimosa, when the upper half of the pulvinus alone was subjected to stimulus of sunlight. Fig. 255, a, shows the moderate positive, or in this case upward, movement, which was the result of this stimulation. By means of a properly inclined mirror light was now thrown vertically upwards, so as to strike the lower half of the pulvinus. The consequent positive responsive movement, in this case downwards, is seen to be much stronger (fig. 255, b\ on account of

the greater excitability of the lower half of the organ. The differential character of the responsive movement under externally diffused stimulation is shown in fig. 255, r, which is a record of the response given by the pulvinus when both its upper and lower sides were simultaneously acted upon by light. It will be seen that in this record, then, we have a case of response to externally diffused stimulus. We shall next observe the effect of stimulus which has become internally diffused, owing to conduction from the upper to the lower

Light acting on pulvinus from ahove ; (/>) light acting on pulvinus from below ; (c) light acting simultaneously from above and below. Dotted line represents recovery on cessation of light. half of the pulvinus. We have just seen that in the case of externally diffused stimulation both sides of the organ are acted upon at the same moment, and the differential response is downward from the beginning ; but when continuous stimulus is applied on the dorsal surface, it is at first unilateral, and only afterwards becomes internally diffused. We therefore obtain in this case (fig. 256), as was theoretically

inferred, all the phases of response, positive, passing through neutral, into negative. A very interesting feature of this record is the after-effect, on the cessation of stimulus, which is represented by a dotted line. In Chapter XXXIV., while dealing with the detection of the latent factor, it was explained that there are two distinct aftereffects, positive and negative (P- 457)- In tne former, the movement is simply a continuation of the effect seen when stimulus was acting ; but the latter, or negative, after-effect, being due to the increase of latent energy by absorption of incident stimulus, finds expression in an opposite movement. In the case of the response of growth we saw that the positive after-effect consisted of a persistent retardation, and the negative of an acceleration, of growth. In the case of pulvinated organs, however, the negative after effect is generally indistinguishable from the movement of recovery ; but in the present instance we find it clearly exhibited in the fact that the after-effect not only causes recovery to the original position, but carries the leaf to a distance beyond.

The positive heliotropic response of the leaf, then, persists Fig. 256. Response of Pulvinus of Mimosa to Action of Continuous Light from Above applied at Moment marked with \, Positive heliotropic movement caused by excitation of upper half neutralised by transmission to distal side, and ultimately reversed owing to greater excitability of lower half. Dotted line represents recovery on cessation of light. Note final erection of leaf above original position as negative after-effect by absorbed stimulus.

only as long as it takes the stimulus to reach the distal side of the pulvinus. When strong concentrated sunlight is applied from above, therefore, the duration of this upward movement is reduced, and it appears only as a slight twitch. The same effect is produced under less intense light when, as in thinner pulvinated organs, the distance to be traversed by stimulus is not so great. (&) The diurnal sleep of Oxalis. — In consequence of strong vertical illumination, as at noonday, negative heliotropic response, with downward folding of the leaves, takes place in those pulvinated organs in which there is internal diffusion of stimulus, and in which the lower side of the pulvinus is much more excitable than the upper. As examples of this, we shall take the cases of Oxalis and Biophytum. In the former, if light of moderate intensity, say from a lamp, be applied on the dorsal or upper surface, an up-/ ward, or positive, heliotropic curvature is induced. If the same light be applied below, a positive heliotropic movement, in this case downwards, is again induced ; but the fact that the lower side is much the more excitable is seen when the upper and lower surfaces are excited simultaneously, for there is now a downward movement (fig. 257, a). Thus external diffuse stimulation induces downward movement, and we shall also find that stimulus internally diffused has the same effect. This is seen by throwing a strong beam of sunlight on the upper surface,

(a) Shows greater excitation of lower half and downward response when both upper and lower are simultaneously subjected to stimulus of light ; {&) shows downward or negative response owing to internal diffusion of stimulus when upper surface only is acted on by strong light. Arrows indicate the directions of incident light. when the leaflet is found to respond by depression (fig. 255, b ). Here, then, we have a repetition of those movements which we have already seen in the case of plagiotropic stems, where strong illumination of whichever surface always had the same effect, that is to say the induced concavity in the lower.

if) Diurnal sleep of Biophytum. — In the case of Oxalis and in that of Mimosa the responsive movement is more or less continuous ; but in such a pronouncedly multiplyresponding organ as that of Biophytum, the effect of internally diffused stimulus on the more excitable lower half of the pulvinus is to induce depression by a series of multiple responses. This is well seen in the following record (fig. 258), where under the action of strong sunlight from above the leaflet is undergoing a progressive fall. A record of a similar effect in the case of Biophytum is seen in fig. 123, the fall being in that case represented by an up curve. The specimen there was somewhat sluggish, and there were three pulsations in the course of ten minutes, making an average of 3*3 minutes to each. In the present instance, however, there were six pulsations in the course of fifteen minutes — each, that is to say, having an average period of 2*5 minutes. Similarly in Averrhoa the fall of the leaflet occurs in a pulsating manner, an instance of which is given by Darwin.1