Plant Response as a Means of Physiological Investigation

changes which are liable to occur during the lapse of long intervals of time. The action of light is, generally speaking, so predominant over that of the subsidiary factors, that a high magnification of record is not necessary. Under these circumstances, that is to say under low magnification, the record before the application of light is practically a horizontal line. Under vertical illumination of the upper side of the leaf, then, deviation above this horizontal represents a positive heliotropic movement, while deflection in the opposite direction signifies the negative.

But before I proceed to give details of my expedients, I shall adduce facts rhich will show that there is io inherent tendency in the leaves to move in such a lanner as to place themselves it right angles to the light. Fig. 261. Curve showing Timerelations of the Responsive Angular Movement of Terminal Leaflet of Desmodium under Light, as represented in the last figure The original angle of 450 between the surface of the leaflet and the direction of incident light was reduced to 130 in the course ot 25 minutes.

Fig. 260. Movement of Terminal Leaflet of Desmodium placing itself Parallel to Incident Horizontal Light ie terminal leaflet being at an angle of 450 with the horizon, iunlight was then made to strike it horizontally (fig. 260). It iow the leaflet had a dia-heliotropic tendency, it is clear that ts movement would be such as to increase its angle with the lorizon to 900, thus bringing the light to strike it at right angles. Movement in the other direction — that is to say, towards the decrease of the angle — would, on the other hand, indicate that the effect of light was to induce the normal positive heliotropic response. From the record (fig. 261) it will be seen that the latter was the case — that is to say, the leaflet moved continuously, tending to become parallel to the light, having in the course of twenty-five minutes moved from its position at an angle of 450 to one at 130 to the horizon — that is to say, at an average rate of angular movement of about 1*3° per minute.

The lamina not the perceptive organ.— If, again, the object of the so-called dia-heliotropic movement had been the absorption by the upper surface of the lamina of the largest possible amount: of light, it would have been necessary for the lamina to be the perceptive organ, determining its movement according to the direction of stimulus ; but that this is not the case may be demonstrated by subjecting the lamina alone to the action of light, and covering the pulvinus with a small opaque shield of black paper. On doing so all movement is found to be arrested. If now, on the other hand, the leaflet be covered with opaque paper, and the pulvinus be left exposed, the usual heliotropic movement is found to take place. This conclusively proves that, as regards the response of the leaf to light, the lamina is not the perceptive organ, and therefore that a supposed advantage to itself cannot be the important factor in determining its movement. That the lamina is not the perceptive organ has indeed been shown already in the case of ordinary leaves, when subjected to forms of stimulus other than light (p. 60). We saw this, for example, in the case of a leaf of Artocarpus when subjected to electrical and thermal stimulation. It was in that case shown that since the lamina consisted of a mass of non-conducting parenchymatous tissue, local excitation might be caused by the incidence of stimulus, but could not be effectively transmitted to the distant pulvinus or pulvinoid by which the movement of the leaf was brought about. The only case in which such transmission is possible to some

extent in leaves is in the parallel-veined leaves of monocotyledons. This fact, that the lamina is not in general the perceptive organ with regard to stimulus, say of light, is still further demonstrated in the following experiment, performed on the leaflet of Erythrina indica. Sunlight was first applied locally to the lamina of this leaflet by means of a suitably inclined mirror, at the point in the record which is marked x (fig. 262). It will be noticed that during ten minutes of such application not the slightest responsive movement was induced. Keeping everything else the same, the light was

Fig. 262. Record showing that Lamina is not the Perceptive Organ Light applied vertically at x on lamina of Erythrina indica, and continued for ten minutes, produces no response ; but when applied on the pulvinus at the arrow ( \ ) there is immediate positive response. Dotted line shows positive after-effect and recovery on cessation of light. now shifted by a movement of the mirror, and thrown directly on the pulvinus, at the point in the record which is marked with an arrow (t). It will be observed that the leaf began to respond immediately by a movement towards the light, and in the course of seventeen minutes' exposure its tip moved through a distance of 20 mm. or at an average rate of a little over 1 mm. per minute. On the withdrawal of light the movement persisted, owing to the positive aftereffect, for a period of seven minutes, after which recovery began. The marked difference between the quiescent con-

dition of the leaflet during the ten minutes' exposure of the lamina to the action of light, and its energetic movement immediately on the application of light to the pulvinus, shows once more, in a striking manner, that it is the latterl organ and not the lamina whose perception of light is effective in initiating the responsive action. Principal types of the response of leaves to stimulus | of light. — I shall now proceed to show that the directive! effect of light on leaves is very definite. In studying the [ response of anisotropic organs — that is to say, plagiotropic I shoots and dorsi-ventral pulvinated leaves — we have seen that there are two extreme types of response, of which the I first is exhibited by organs possessing feeble transverse conductivity, and the second by those in which the transverse | conductivity is great, and the lower side the more excitable. In the first of these types, light acting on the organ from above gives rise to a positive heliotropic response. In the latter, long-continued application of strong light from above gives rise to internal diffusion of stimulus, causing concavity of the more excitable lower half, with the result of negative heliotropic response. Intermediate between these we have seen that there are cases of the equal excitation ot the upper and lower halves of the organ, bringing about the so-called dia-heliotropic position. As examples of the two extreme types — within which lie the responses of all ordinary leaves — I shall here give records of the movements of leaves of Mango (Mangifera indica) and of Artocarpus.

Positive type of response : Mango. — The leaves of this plant when young are bent abruptly downwards by the sharp curvature of the short petiole. In the course of a week or so, however, they rise, and gradually attain a position at or above the horizontal, a process in which the action of light is an important agent. This will be seen from the following record of the movement of a Mango leaf when acted upon * by sunlight from above (fig. 263). The record before the application of light was practically horizontal, showing that there was little natural movement ; but the

that is to say, a positive heliotropic response, the tip of the Negative type of response : Artocarpus. — As an example of negative response by the internal diffusion of stimulus, due to the high transverse conductivity of the tissue, and the greater excitability of the lower half of the pulvinoid, I give here two records, from leaves of different ages, borne on the same plant, under the action of strong sunlight from above (fig. 264). The upper of these two records was taken from a young leaf, which was second in order from the top of the shoot, and the lower from the fourth. It is generally found that motile sensitiveness is at its greatest in leaves which are neither too young nor too old. In the present experiment the upper of the two leaves, which was very young, gave a negative heliotropic response, the tip moving through 15 mm. in the course of eighty minutes, or at an average rate of almost '2 mm. per minute. In the case of the lower leaf the rate of the responsive movement was more rapid, being on an average *5 mm. per minute.

As regards leaves, then, their responsive movements under light have been shown to be very definite, and simply Fig. 263. Positive Heliotropic Movement of Leaf of Mangifera indica under Sunlight acting from Above determined by the fact that it is always the more excited! side of the motile organ that undergoes contraction and concavity, That various types of this effect arise is due only to the unequal excitabilities of the two halves, and to the fact

that the stimulus remains ! in some cases localised, ; while in others, owing to the better transverse con- j ductivity of the tissue, it becomes internally diffused. The position ultimately taken up by the leaves is thus determined primarily by the action of light, and secondarily by that of various subsidiary factors, which are : (i) the natural movement of the organ, due to hyponasty or epinasty ; (2) the differential excitability of the two halves of the dorsiventral organ under the stimulus of gravity ; (3) the turgescent condition of the plant, as determined by its suctional activity ; and (4) the limits of flexibility of the organ, as determined by its anatomical peculiarities. It is out of the innumerable possible combinations of these factors that the variety of attitudes ultimately assumed by the leaves directly arises.

With regard to the nature of geotropic action on leaves, it will be shown in the next chapter that here also, as in the case of dia-heliotropism, the assumption of a dia-geotropic property is unnecessary ; for the observed effects are explained by the fact, which I shall demonstrate, that a dorsiventral organ possesses differential geotropic sensibility. Fig. 264. Negative Heliotropic Response of Leaves of A rtocarpus under Sunlight acting from Above

The upper record exhibits the response of a very young leaf, second in order from the top of the shoot. The lower record shows that of an older leaf, fourth in order on the same shoot. The position ultimately assumed by a leaf is determined by heliotropic action and other subsidiary factors. These subsidiary factors are: (1) the natural movement of the organ, due to epinasty or hyponasty ; (2) the differential excitability to gravitational stimulus of the two halves of the organ ; (3) the effect of suctional activity and of turgescence ; and (4) the modification of effect by the characteristic limits of flexibility of the organ.

As regards the action of light, the lamina is not the perceptive organ, and there is no specific dia-heliotropic sensitiveness possessed by the leaves. The sensibility of the pulvinoid of a leaf is essentially similar to that of a pulvinus. There are two main types of response given by pulvinoids : (1) that exhibited when the conductivity of the organ is feeble, and vertical light induces movement upwards, ox positive response ; and (2) that exhibited when conductivity is great, and the lower half is the more excitable, inducing movement downwards, or negative response.

The various attitudes assumed by the leaves are the joint effects of these responsive actions, and their modification by epinasty or hyponasty, the differential action of gravity, the turgescent condition of the plant, and the limits of flexibility of the pulvinus or pulvinoid. Torsional effect — Method of recording torsional response — Torsional response under the lateral action of light — Torsional response to other forms of lateral stimulation — Torsional response of compound strip of ebonite and stretched india-rubber — Modification of torsional response by artificial variation of the relative excitabilities of the two halves — Laws of torsional response — Demonstration of differential geotropic excitation in a dorsi ventral organ — Torsional response to lateral geotropic stimulation — Modification of torsional geotropic response by artificial variation of differential excitability — Autonomous torsion : effect of temperature — Effect of light— Effect of electrical current — Effect of gravity — The twining of stems.

It has been shown in the course of the last chapter that the movement of the leaf in response to light constitutes a simple instance of that general reaction of plant tissues to stimulus with which we have now become familiar, and that no specific sensibility requires to be postulated in order to account for it. Torsional effect. — There is one effect, however, which has hitherto appeared to be inexplicable, except on the supposition that some such specific sensibility had actually been acquired by the leaf for the definite purpose of subserving the advantage of the plant by placing the upper surface of the lamina at right angles to incident light. A leaf when struck laterally by light undergoes a torsion, which carries the upper surface of the lamina into such a position that it faces the light. No result could seem at first sight more conclusively to support the theory of dia-heliotropic sensibility. Before going further into this question, I shall give records of the actual effect observed. In fig. 265 is shown a curve which exhibits the increasing torsion induced

by lateral application of sunlight in the terminal leaflet of Desmodium. A light index was attached transversely to the lamina, by means of which, and with the help of a protractor, the "gradually increasing torsion was measured at definite intervals of time. It will be seen from the curve Fig. 265. Torsional Response of Terminal Leaflet of Desmodium under the Action of Lateral Sunlight Abscissa represents time in minutes, and ordinate angular movement in degrees.

thus obtained, of which the abscissa represents time, and the ordinate the induced angular torsion, that in this case, within twenty-five minutes, a torsion of 140 had been induced. Method of recording" torsional response. — For the purpose of certain investigations, presently to be described, in connection with this torsion, it became necessary to devise special experimental arrangements by which a continuous record of the rate of torsion and its variations could be obtained. The mode in which this was accomplished will be understood from the illustration in fig. 266. A mirror, carried on a light spring-clip made of aluminium, is slipped over and attached to the petiole, at a short distance from the pulvinus or pulvinoid, which is the seat of the torsional movement. As the object is to record only the torsion, the

vertical up and down movement, if there be any, is prevented by a lateral support, which has at its end a smooth bent rod of glass, in the concavity of which the free petiole rests. If there be any responsive torsional movement, a spot of light reflected from the mirror will now undergo a vertical deflection. This is, for convenience of the record, converted into lateral by reflection from a second mirror suitably inclined. The response record is then taken in the usual manner on

M, mirror slipped over petiole by light aluminium clip behind ; G, bent glass piece supporting petiole to prevent vertical movement. a revolving drum. The absolute value of the angular movement shown in the record can be determined from a knowledge of the distance from the mirror of the recording drum. Torsional response under the lateral action of light. — The pulvinus of the leaf, say of Mimosa, is now stimulated by throwing upon it a horizontal beam of light, which strikes it laterally. By the word lateral is here meant either the right or left flank of the pulvinus or pulvinoid, consisting of part of the upper and part of the lower anisotropic halves. This causes a responsive torsion, by which the petiole is rotated, the tendency being for the upper, or less excitable, half to face the stimulating agent. The up curve, then, in this and the following records will represent a torsional movement by which the less excitable upper side is made to

Torsional response to other forms of lateral stimulation.— The supposition that this torsional response is due to a specific sensibility to light, evolved for the advantage of the plant, will be found entirely untenable if it can be shown that the same movement is manifested under the same conditions in response to other forms of stimulus. Thus, on bringing a heated wire near that side which was previously excited by light, I obtained exactly the same torsional response of the same sign. Still more interesting is the excitation of the same lateral side by chemical stimulus, say a strong solution of common salt. In the record given (fig. 268) we see how similar in every way are this response and that evoked by light. The upward arrow (t) indicates the application of chemical stimulus to the same flank as had previously been stimulated by light, of which the record is

Fig. 267. Torsional Re- sponse of Leaf of Mimosa when Laterally Stimulated by Sunlight The ordinate gives the absolute angular movement in degrees. The dotted line shows positive aftereffect and recovery on stoppage of light. given in fig, 267. The downward arrow (1) indicates the application of the chemical solution to the opposite flank. It will be noticed that we have in consequence, in the dotted portion of the record, a reversal of the first torsional response. Torsional response of compound strip of ebonite and stretched india-rubber. — Enough has already been said to demonstrate the fact that the torsion induced in leaves by the lateral application of light is not due to any specific sensibility to light as such. I shall next therefore proceed to show that the same torsion is the mechanical result of the differential

Fig. 268. Torsional Response of Leaf of Mimosa to Lateral Chemical Stimulus Upward arrow ( \ ) indicates moment of application of stimulus to one flank ; (!) indicates application of stimulus to opposite flank, with effect of reversing torsional movement. Fig. 269. Torsional Re- sponse of Complex Strip of Ebonite and Indiarubber under Lateral Action of Strong Radiation contraction of a complex organ, which is fixed at one end, and subjected to lateral stimulation. I have in a former chapter described an artificial model of the pulvinus of Mimosa, which consisted of a compound strip, the upper half of which was ebonite, and the lower the more contractile stretched india-rubber. If such a strip be held securely at one end in a clamp, and if the lateral flank, consisting half of ebonite and half of india-rubber, be subjected to the strong action of light, records being taken in the usual manner, it will be found that a torsional response takes place which is in every respect similar to that of the pulvinus of a

leaf, the less contractile ebonite being turned so as to face the light (fig. 269). Modification of torsional response by artificial variation of the relative excitabilities of the two halves.— I find it necessary to go still further into this subject of torsional response, as by its means 1 have been enabled to solve a question of very great importance, that, namely, of the different excitabilities of the two halves of the anisotropic organ to geotropic stimulus. The fact that it is the differential character of the excitabilities of these two halves that brings about the torsion of a dorsi-ventral organ under lateral stimulation may be still further established in a very interesting manner by inducing artificial variation in the existing excitabilities. The torsion depends, as said before, on the difference of excitability as between the two. If, then, we could render the lower and more excitable half gradually less and less excitable, till its differential excitability had disappeared, the organ would thus have been rendered virtually radial. The torsional effect might then be expected to vanish, and a simple curvature towards stimulus to result, without torsion, as in the case of other radial organs. Let us next suppose the reduction of excitability to be carried still further, till the lower half of the pulvinus have been rendered less excitable than the upper. On the theory of torsional response which has just been advanced, it is the less excitable lower half which should now twist round to face the stimulating light. In other words, there would then be a reversal of the original torsion.

Thus, as the excitability of the lower half becomes gradually reduced, the intensity of the normal positive torsional response by which the upper half was made to face the stimulus would be gradually first decreased to zero, and then reversed to negative, as the excitability of the lower became first equal to, and then less than, that of the upper half. If, on the other hand, the excitability of the upper half be reduced, the existing differential excitability as between upper and lower would then be still further increased, and the intensity of the positive torsional response would be

enhanced. All these conclusions will be found exactly verified in the records given in fig. 270. In the first part of the left-hand figure we see the normal positive torsional response of Mimosa under the lateral action of light. The excitability of the lower half of the pulvinus was then reduced by the local application of chloroform, at the moment represented by the arrow from below (t). It will be seen that the response now undergoes reversal, owing to the fact that it is the upper side that is the relatively more excitable. In the figure to the right, again, is shown the effect on the torsional response of the leaf, of increasing the already existing difference as between the excitabilities of the two halves, when that of the upper is reduced by the local application of chloroform. It will be seen that this application as marked by the arrow from above (4,) caused an enhancement of torsional response, as seen in the greater steepness of the curve.

Laws of torsional response. — From these experiments we arrive at the following laws : 1. An anisotropic organ, when laterally excited, undergoes torsion, by which the less excitable side is made to face the stimulus. 2. Stimulus remaining constant, the intensity of torsional response increases with the differential excitability. But when the original difference is in any way reduced or reversed, the torsional response undergoes concomitant diminution or reversal.

P'lG. 270. Records showing Modification of Torsional Response under Induced Variations of Differential Excitabilities in Pulvinus of Mimosa In left-hand figure the normal torsion is seen to be reversed by application of chloroform to the lower half (f), reversing the differential excitability of the organ. In the right-hand record the normal responsive torsion is enhanced by application of chloroform to the upper half (j), increasing the natural differential excitability of the organ.

3. An organ which, under lateral stimulation, responds by torsion, is always physiologically anisotropic, and the side which is made to face the stimulus is the less excitable. Demonstration of differential geotropic excitation in a dorsi-ventral organ. — From this experimental demonstration, then, we have obtained a new means of discriminating the relative excitabilities of the two halves of an organ, since that side which is turned by the responsive movement to face the given stimulus is relatively the less excitable to it. It will be remembered that we found that the reason why certain dorsi-ventral organs showed a tendency to assume a horizontal position under the action of vertical light was not that they possessed dia-heliotropic sensibility, but was due to the differential excitability of the two halves under stimulus in general, including that of light. Again, in the case of the action of gravity, it is found that such organs exhibit a similar tendency to place themselves horizontally ; and the assumption of a specific dia-geotropic sensibility is not necessary, if it can be shown that the upper and lower sides are unequally sensitive to this stimulus also. We may first take an ideally simple case. We have seen that in a radial apogeotropic stem, when laid horizontally, it is only the upper half that is effectively stimulated by geotropic action ; and there was reason to believe that this was due to the fact that it was the inner tangential wall of the upper side — in contrast to the less excitable outer wall of the lower side — that was excited by the statolithic or other influence of weight (pp. 495, 503). If now, for any reason, the excitability of the upper half of the horizontally placed radial organ be abolished, the geotropic response of that effective half will disappear, and the organ will remain horizontal, as if unaffected by stimulus of gravity.

This state of things we have already realised, when the excitability of the upper half was artificially diminished by the local application of cold, and geotropic response was seen to be arrested (p. 504). Now, a horizontally placed radial organ which has been rendered unequally excitable in the two halves, by the reduction of excitability of the upper, is virtually a plagiotropic or dorsi-ventral organ, and we can thus see why a true plagiotropic organ, laid horizontally, has a tendency to remain in that position. It would remain in that position absolutely if the excitability of the outer tangential wall of the lower side were actually zero, and the general excitability of the upper half of the dorsi-ventral organ had also completely vanished ; but if these two values were not both zero, various effects would occur, according to the relative differential excitabilities of the two halves.