XXIX.—ON PHOTOTROPISM

[7]Pfeffer—Ibid, Vol. III, p. 112.

[8]Pfeffer—Ibid, Vol. III, p. 177.

In different organs of plants the stimulus of light induces movements of an extremely varied character. Radial organs exhibit tropic movements in which the position of equilibrium is definitely related to the direction of incident stimulus. Nastic movements under the action of light are, on the other hand, regarded as curvatures of the organ which show "no relation to the stimulus but is determined by the activity of the plant itself".[9]There are thus two classes of response to light which seem to be unrelated to each other. Returning to the directive action of light, radial stems often bend towards the light, while certain roots bend away from it. It may be thought that this difference is due to specific difference of irritability between shoot and root, the irritability of the former being of a positive, and of the latter, of a negative character. But there are numerous exceptions to this generalisation. Certain roots bend towards the light, while a stem, under different circumstances, moves towards light or away from it. Again an identical organ may exhibit a positive or a negative curvature. Thus the leaflets ofMimosa pudicaacted on by light from above fold upwards, the phototropic effect beingpositive. But the same leaflets acted on by light from below exhibit a folding upwards, the phototropic effect being nownegative. Effects precisely the opposite are found with the leaflets ofBiophytumandAverrhoa. They fold downwards whether light acts fromabove or below. Finally, a radial organ in found to exhibit under light of increasing intensity or duration, a positive, a dia-phototropic, or a negative phototropic curvature.

In these circumstances the theory of specific positive and negative irritabilities is untenable; in any case, it throws no light on the phenomenon of movement. The difficulties of the problem are thus clearly stated by Pfeffer: "When we say that an organ curves towards a source of illumination, because of its heliotropic irritability and we are simply expressing an ascertained fact in a conveniently abbreviated form, without explaining why such curvature is possible or how it is produced.... Many observers have unfortunately devoted their attention to artificially classifying the phenomenon observed, and have entirely neglected the explanation of causes underlying them."[10]

The complexity of phototropic reaction arises from the summated effects of numerous factors; for explanation of the resultant response it is therefore necessary to take full account of the individual effect of each of them.

Among these operative factors in phototropic reaction may be mentioned:—

(1) The difference of effects induced by light at the proximal and distal sides of the organ.(2) The modification of the latent period with the intensity of stimulus.(3) The after-effect of stimulus.(4) The modifying influence of tonic condition on response.(5) The effect of direction of light.(6) The effect of intensity of light.(7) The effect of duration of stimulation.(8) The transmitted effect of light.(9) The effect of unequal excitability in different zones of the organ.(10) The effect of transverse conduction in modification of the sign of response.(11) The effect of temperature on phototropic action.(12) The modification of response due to differential excitability of the organ.(13) Nastic and tropic reactions.(14) The torsional effect of light.

(1) The difference of effects induced by light at the proximal and distal sides of the organ.

(2) The modification of the latent period with the intensity of stimulus.

(3) The after-effect of stimulus.

(4) The modifying influence of tonic condition on response.

(5) The effect of direction of light.

(6) The effect of intensity of light.

(7) The effect of duration of stimulation.

(8) The transmitted effect of light.

(9) The effect of unequal excitability in different zones of the organ.

(10) The effect of transverse conduction in modification of the sign of response.

(11) The effect of temperature on phototropic action.

(12) The modification of response due to differential excitability of the organ.

(13) Nastic and tropic reactions.

(14) The torsional effect of light.

The sketch given above will give us some idea of the complexity of the problem. In this and in the following papers I shall describe the investigations I have carried out on the subjects detailed above.

I have shown that there is no essential difference between the responses of pulvinated and growing organs, that diminution of turgor induced by stimulus brings about contraction in the one, and retardation of the rate of growth in the other. Indirect stimulation, on the other hand, induces an expansion and acceleration of the rate of growth. The experimental investigation on the tropic effect of light may therefore be carried out both with pulvinated and growing organs.

As regards the effect of direct stimulus of light on growing organs we found (p. 208) that it induces an incipient contraction, seen in diminution of the rate of growth; this incipient contraction culminates in an actual contraction under increasing intensity of light. The contractionunder direct stimulation is also observed in pulvinated organs. When light acts from above the upper half of the pulvinus undergoes contraction, resulting in erection of the motile leaf or leaflets. As regards the effect of indirect unilateral stimulus of light on the distal side of the organ, we found that its effect is an enhancement of turgor (p. 281). Hence the positive tropic curvature under light is brought about, as in the case of other forms of stimuli, by the contraction of the proximal, and expansion of the distal sides of the organ.

Various analogies have been noticed between phototropic and geotropic reactions, and it has been supposed that the two phenomena are closely related to each other. This has even led to assumption that there are phototropic particles which function like statoliths in geotropic organs. There is, however, certain outstanding difference between the two classes of phenomena. In the case of light, the incident energy is entirely derived from the outside. But in geotropism, the force of gravity by itself is ineffective without the intervention of the weight of cell-contents to exert pressure on the sensitive ectoplasm, and thus induce stimulation. This aspect of the subject will be treated in greater detail in a subsequent chapter.

I shall now describe the phototropic effect of unilateral light in pulvinated, and in growing organs. From the explanation that has already been given, it will be understood that the side of the organ directly acted on by light undergoes contraction and concavity.

Tropic curvature of pulvinated organs: Experiment 117.—For this experiment I employed the terminal leafletof the bean plant. The source of illumination was 32 c.p. electric lamp, enclosed in a metallic tube with circular aperture for passage of light. The leaflet was attached to an Oscillating Recorder. Light was applied on the upper half of the pulvinus for 20 seconds; this induced an up-movement of the leaflet, due to the contraction of the upper half of the organ. Recovery took place in course of 8 minutes (Fig. 112).

Fig. 112.Fig. 112.—Successive positive responses of the terminal leaflet of bean. Light applied from above for 20 seconds; complete recovery in 8 minutes.

Fig. 112.—Successive positive responses of the terminal leaflet of bean. Light applied from above for 20 seconds; complete recovery in 8 minutes.

Effect of moderate stimulation: Experiment 118.—I shall presently show that the intensity of phototropic reactiondepends on the intensity and duration of the incident light. A moderate and effective stimulation may thus be produced by short exposure to strong light. For my present experiment I took a stem ofDregea volubilis, and applied light from a small arc lamp to one side of the organ for 1 minute; this induced a positive curvature followed by complete recovery on the cessation of light (Fig. 113).

Fig. 113.Fig. 114.Fig. 113.Fig. 114.Fig. 113.—Positive curvature under moderate phototropic stimulation. Note complete recovery (Dregea).Fig. 114.—Persistent positive curvature under stronger stimulation (Dregea).

Fig. 113.

Fig. 114.

Fig. 113.Fig. 114.

Fig. 113.—Positive curvature under moderate phototropic stimulation. Note complete recovery (Dregea).

Fig. 114.—Persistent positive curvature under stronger stimulation (Dregea).

Effect of strong stimulation: Experiment 119.—After recovery of the stem of the last experiment, the same light was applied for 5 minutes. It is seen that the curvature is greatly increased (Fig. 114). Thus the phototropic curvature increases, within limits, with the duration of stimulation. The curvature induced under strongerstimulation remained more or less persistent. In certain instances there was a partial recovery after a considerable length of time; in others curvature was fixed by growth.

On the cessation of stimulus of moderate intensity the heliotropically curved organ straightens itself; similar effects are also found in other tropic curvatures. Thus a tendril straightens itself after curvature induced by contact of short duration. The theory of rectipitality has been proposed to account for the recovery, which assumes the action of an unknown regulating power by which the organ is brought back to a straight line; but beyond the assumption of an unknown specific power, the theory affords no explanation of the mechanism by which this is brought about.

The problem before us is to find out the means by which the organ straightens itself after brief stimulation. It will also be necessary to find out why there is no recovery after prolonged stimulation. We have thus to investigate the after-effect of stimulus of various intensities on growth, and the Balanced Method of recording Growth offers us an unique opportunity of studying the characteristic after-effects.

As regards the effect of light I have already shown:

(1) that a sub-minimal stimulus induces an acceleration of growth, but under long continued action the acceleration is converted into normal retardation (p. 225),(2) that a stimulus of moderate intensity induces the normal retardation of the rate of growth.

(1) that a sub-minimal stimulus induces an acceleration of growth, but under long continued action the acceleration is converted into normal retardation (p. 225),

(2) that a stimulus of moderate intensity induces the normal retardation of the rate of growth.

It is evident that there is acritical intensityof stimulus, above which there is a retardation, and below which there is the opposite reaction of acceleration. This critical intensity, I have found to be low in vigorous specimens, and high in sub-tonic specimens. Thus the same intensity of stimulus may induce a retardation of growth in specimens the tonic condition of which isabove par, and an acceleration in others, in which it isbelow par. The following experiments will demonstrate the immediate and after-effect of light of increasing intensity and duration.

Fig. 115.Fig. 115.—Immediate and after-effect of stimulus of light on growth. (a) shows immediate effect of moderate light to be a transitory acceleration (down-curve) followed by retardation (up-curve). The after-effect on cessation of light is an acceleration (down-curve) followed by restoration to normal. (b) Immediate and after-effect of stronger light: immediate effect, a retardation; after-effect, recovery to normal rate without acceleration.

Fig. 115.—Immediate and after-effect of stimulus of light on growth. (a) shows immediate effect of moderate light to be a transitory acceleration (down-curve) followed by retardation (up-curve). The after-effect on cessation of light is an acceleration (down-curve) followed by restoration to normal. (b) Immediate and after-effect of stronger light: immediate effect, a retardation; after-effect, recovery to normal rate without acceleration.

Effect of light of moderate intensity: Experiment 120.—The source of light was a small arc lamp placed at a distance of 50 cm., the intensity of incident light was increased or decreased by bringing the source of light nearer or further away from the plant. Two inclined mirrors were placed behind the plant so that the specimen wasacted on by light from all sides. A seedling of wheat was mounted on the Balanced Crescograph, and record was first taken under exact balance; this gives a horizontal record. The up-curve represents retardation, and down-curve acceleration of rate of growth. The source of light was at first placed at a distance of 50 cm. from the plant, and exposure was given for 4 minutes at the point marked with an arrow (Fig. 115a). We shall find in the next chapter that theintensity of phototropic effect is proportional to the quantity of incident light. This quantity at the beginning proved to be sub-minimal, and hence there was an acceleration at the beginning. Continued action induced the normal effect of retardation, as seen in the subsequent resulting up-curve. On the cessation of light, the balance was upset in an opposite direction, the resulting down-curve showing an acceleration of the rate of growth above the normal. This acceleration persisted for a time, after which the normal rate of growth was restored, as seen in the curve becoming once more horizontal.The after-effect of light of moderate intensity is thus a temporary acceleration of rate of growth above the normal.

Effect of strong light: Experiment 121.—The same specimen was used as in the last experiment. By bringing the source of light to a distance of 25 cm. the intensity of light was increased fourfold; the duration of exposure was kept the same as before. The record (Fig. 115b) shows that a retardation of rate of growth occurred from the very beginning without the preliminary acceleration. This is for two reasons: (1) the increased intensity was now above the critical minimum, and (2) the tone of the organ had become improved by previous stimulation. On the cessation of light, the after-effect showed no enhancement of rate of growth, the recovery from retardation to the normal rate being gradual. In the nextexperiment (the result of which is not given in the record) the intensity of light was increased still further; the retardation now became very marked, and it persisted for a long time even on the cessation of light.

We thus find that:

(1) The immediate effect of light of moderate intensity is a preliminary acceleration, followed by normal retardation. The acceleration is the effect of sub-minimal stimulation. The immediate after-effect is an acceleration above the normal.(2) The immediate effect of strong light is a retardation from the beginning; the immediate after-effect shows no acceleration, the growth rate being gradually restored to the normal.(3) Under very strong light the induced retardation is very great, and this persists for a long time even on the removal of light.

(2) The immediate effect of strong light is a retardation from the beginning; the immediate after-effect shows no acceleration, the growth rate being gradually restored to the normal.

(3) Under very strong light the induced retardation is very great, and this persists for a long time even on the removal of light.

The experiments described explains the reasons of complete recovery after moderate stimulation, and also the absence of recovery after strong stimulation. The immediate after-effect of moderate stimulation is shown to be an acceleration of rate above the normal. Returning to tropic curvature, the contraction at the proximal side induced by unilateral light is thus compensated by the accelerated rate of growth on the cessation of light. There is no such compensation in the case of strong and long continued action of light; for the after-effect of strong light shows no such acceleration as the immediate after-effect.

We may perhaps go a step further in explaining this difference. Stimulus was found to induce at the same time two physico-chemical reactions of opposite signs (p. 144).One is the 'up' or A-change, associated with increase of potential energy of the system, and the other is associated with 'down' or D-change, by which there is a run-down or depletion of energy. With moderate stimulation the A-and-D effects are more or less comparable to each other. But under strong stimulation the down-change is relatively greater. Hence on cessation of moderate stimulation the increase of potential energy, associated with A-change, finds expression in enhancement of the rate of growth. The depletion of energy under strong stimulation is, however, too great to be compensated by the A-change.

With reference to the latent period Jost thus summarizes the known results:[11]"The latent period of the heliotropic stimulus has already been determined. According to Czapek it amounts to 7 minutes in the cotyledons ofAvenaand inPhycomyces; 10 minutes in hypocotyls ofSinapis albaandBeta vulgaris, 20 minutes in the hypocotyl ofHelianthus, and 50 minutes in the epicotyl ofPhaseolus. If one of these organs be unilaterally illuminated for the specified time, heliotropic curvature ensues afterwards in the dark, that is to say, we meet with an after-effect in this case as in geotropism. We are quite ignorant, however, as to whether and how the latent period is dependent on the intensity of light."

With regard to the question of relation of the latent period to the intensity of stimulus I have shown (p. 166) that the latent period is shortened under increasing intensity of stimulus. In the case of tropic curvature induced by light, I find that the latent period is reduced underincreasing intensity of light. The shortest latent period found by Czapek, as stated before, was 7 minutes. But by employing high magnification for record, I find that the latent period of phototropic action under strong light to be a question of seconds.

Fig. 116.Fig. 116.—Latent period for photic stimulation at vertical line. Successive dots at intervals of 2 seconds. (Erythrina indica).

Fig. 116.—Latent period for photic stimulation at vertical line. Successive dots at intervals of 2 seconds. (Erythrina indica).

Determination of the latent period: Experiment 122.—I give a record of response (Fig. 116) of the terminal leaflet ofErythrina inidcato light acting from above. The recording plate was made to move at a fast rate, the successive dots being at intervals of 2 seconds. The latent period in this case is seen to be 35 seconds. By the employment of stronger light I have obtained latent period which is very much shorter.

The term latent period is used in two different sense. It may mean the interval between the application of stimulus and the initiation of response. In the experiment described above, the latent period is to be understood in this sense. But in the extract given above, Jost uses the term latent period as the shortest period of exposure necessary to induce phototropic reaction as an after-effect. What then is the shortest exposure that will induce aretardation of growth? For this investigation I employed the very sensitive method of the Balanced Crescograph.

Fig. 117.Fig. 117.—Effect of a single electric spark on variation of growth. Record taken by Balanced Crescograph. Up-curve shows induced retardation of growth; the after-effect is an acceleration (down-curve) followed by restoration to normal.

Fig. 117.—Effect of a single electric spark on variation of growth. Record taken by Balanced Crescograph. Up-curve shows induced retardation of growth; the after-effect is an acceleration (down-curve) followed by restoration to normal.

Experiment 123.—I stated that the more intense is the light, the shorter is the latent period. The duration of a single spark discharge from a Leyden jar is almost instantaneous, the duration of discharge being of the order of1⁄100,000th of a second. The single discharge was made to take place between two small steel spheres, the light given out by the spark being rich in effective ultra-violet rays. The plant used for the experiment was a seedling of wheat. It was mounted on the Balanced Crescograph, and its normal growth was exactly compensated as seen in the first part of the record. The spark gap was placed at a distance of 10 cm. from the plant; therewas the usual arrangement of inclined mirrors for illumination of the plant. The flash of light from a single spark is seen to induce a sudden retardation of rate of growth which lasted for one and half minutes. The record (Fig. 117) shows another interesting peculiarity of acceleration as an after-effect of moderate stimulation. After the retardation which lasted for 90 seconds, there is an acceleration of growth above the normal, which persisted for 6 minutes, after which the rate of growth returned to the normal.

In order to show that the induced variation is due to the action of light and not to any other disturbance, I interposed a sheet of ebonite between the spark-gap and the plant. The production of spark produced no effect, but the removal of the ebonite screen was at once followed by the characteristic response.

The positive curvature is, as we have seen, due to the contraction of the proximal side and expansion of the distal side. The curvature will increase with growing contraction of the proximal side; a maximum curvature is however reached since:

(1) the contraction of the cells must have a limit,(2) the bending organ offers increasing resistance to curvature, and(3) the induced curvature tends to place the organ parallel to the direction of light when the tropic effect is reduced to a minimum.

(1) the contraction of the cells must have a limit,

(2) the bending organ offers increasing resistance to curvature, and

(3) the induced curvature tends to place the organ parallel to the direction of light when the tropic effect is reduced to a minimum.

The pulvinus ofErythrinaexemplifies the type of reaction in which the positive curvature reaches a maximum, (see below Fig. 132) beyond which there is no furtherchange. This is due to absence of transverse conductivity in the organ. The modifying effect of transverse conductivity on response will be dealt with in the next chapter.

The positive phototropic curvature is brought about by the joint effects of the directly stimulated proximal, and indirectly stimulated distal side.

The phototropically curved organ undergoes recovery after brief stimulation.

The recovery after moderate stimulation is hastened by the previously stimulated side exhibiting an acceleration of the rate of growth above the normal. The after-effects of photic and mechanical stimulation are similar.

The latent period of photic reaction is shortened with the increasing intensity of light. The seedling of wheat responds to a flash of light from an electric spark, the duration of which is about a hundred thousandth part of a second.

Tissues in which the power of transverse conduction is negligible, the positive phototropic curvature under continued action of light attains a maximum without subsequent neutralisation or reversal.

[9]Jost—Ibid, p. 428.

[10]Pfeffer—Ibid, Vol. II, p. 74.

[11]Jost—Ibid, p. 473.

I have explained how under the action of unilateral light the positive curvature attains a maximum. There are, however, cases where under the continued action of strong light the tropic movement undergoes a reversal. Thus to quote Jost: "Each organism may be found in one of the three different conditions determined by the light intensity,viz.(1) a condition of positive heliotropism, (2) a condition of indifference, (3) a condition of negative heliotropism"[12]. No explanation has however been offered as to why the same organ should exhibit at different times, a positive, a neutral, and a negative irritability. These changing effects exhibited by an identical organ is thus incompatible with the theory of specific sensibility, assumed in explanation of characteristic differences in phototropic response.

In regard to this I would draw attention to an important factor which modifies the tropic response, namely, the effect of transverse conduction of excitation. I shall presently describe in detail a typical experiment of theeffect of unilateral stimulus of light on the responsive movement of main pulvinus ofMimosa pudica. The results will be found of much theoretical interest, since a single experiment will give an insight to all possible types of phototropic response. Before describing the experiment I shall demonstrate the tropic reactions of the two halves of the pulvinus ofMimosa.

I have by method of selective amputation shown that as regards electric stimulation the excitability of the upper half of the pulvinus is very much less than that of the lower half (p. 85). I have obtained similar results with photic stimulation.

Tropic effect of light acting from above: Experiment 124.—Light of moderate intensity from an incandescent electric lamp was applied on the upper half of the pulvinus ofMimosafor 4 minutes; this induced a contraction of the stimulated upper half and gave rise to an up or erectile response. On the stoppage of light recovery took place in the course of ten minutes. The phototropic curvature is thus seen to be positive. A series of such positive responses of the upper half of the pulvinus is given in figure 118.

Effect of light acting from below: Experiment 125.—Light was now applied from below; this also induced a contraction of the lower half of the pulvinus, causing a down-movement (Fig. 119). As the responsive movement is towards light, the phototropic effect must be regarded as positive. The greater excitability of the lower half of thepulvinus is shown by the fact that the response of the lower half of the pulvinus to ten seconds' exposure is even larger than that given by the upper half under the prolonged exposure of 240 seconds.

Fig. 118.Fig. 119.Fig. 118.Fig. 119.Fig. 118.—Series of up-responses ofMimosaleaf to light applied on upper half of pulvinus.Fig. 119.—Down-responses given by the same plant on application of light from below.

Fig. 118.

Fig. 119.

Fig. 118.Fig. 119.

Fig. 118.—Series of up-responses ofMimosaleaf to light applied on upper half of pulvinus.

Fig. 119.—Down-responses given by the same plant on application of light from below.

Experiment 126.—A beam of light from a small arc lamp was thrown on the upper half of the pulvinus. After a latent period of 5 seconds, a positive curvature was initiated, by the contraction of the upper and expansion of the lower side of the organ. But under continued action of light, the excitatory impulse reached the lower half of the organ, causing a rapid fall of the leaf, and anegativecurvature. The arrival of transmitted excitation at the more excitable distal half of the organ is clearly demonstrated by the very rapid down-movement, seen as the up-curve in the record (Fig. 120). In sensitive specimens this movement is so abrupt and rapid, that the writing lever is jerked off above the recording plate before making a dot on it. The thickness of thepulvinus was 1·5 mm., the distance which the excitatory impulse has to traverse to reach the lower half would thus be about 0·75 mm. The period for transverse transmission of excitation under strong light was found to vary in different cases from 50 to 80 seconds. The velocity of transmission of excitation in a transverse direction through the pulvinus is about 0·011 mm. per second, which is not very different from 0·010 mm. per second in the stem (p. 282).

Fig. 120.Fig. 120.—Record of effect of continuous application of light on upper half of pulvinus ofMimosaleaf. Note erectile response (positive curvature) followed by neutralisation and pronounced reversal into negative due to transverse conduction of excitation. Up-movement shown by down curve, andvice versâ.

Fig. 120.—Record of effect of continuous application of light on upper half of pulvinus ofMimosaleaf. Note erectile response (positive curvature) followed by neutralisation and pronounced reversal into negative due to transverse conduction of excitation. Up-movement shown by down curve, andvice versâ.

Returning to the main experiment we find that:

(1) As a result of unilateral action of light, there was positive phototropic curvature which lasted for 50 seconds.(2) Owing to the internal conduction of excitation the positive effect underwent neutralisation by the excitatory contraction of the distal side. This neutralisation depends on four factors: (a) on the intensity of the stimulus, (b) on the conductivity of the organ in a transverse direction, (c) on the thickness of the intervening tissue, and (d) on the relative excitability of the distal as compared to the proximal side. The extent of positive curvature also depends on the pliability of the organ.(3) In anisotropic organs where the distal side is physiologically the more excitable than the proximal, the internally diffused excitation brings about a greater contraction of the distal, and thepositivephototropic curvature becomes reversed to a very pronouncednegative. The effect of the internally diffused stimulus is thus the same as that of external diffuse stimulus.(4) When the stimulus is applied on the more excitable half of the organ, the result is a predominant contraction of that half, which cannot be neutralised by the excitation conducted to the less excitable half of the organ. As the curvature is towards the stimulus, the phototropic curvature thus remains positive, even under continued stimulation.

(1) As a result of unilateral action of light, there was positive phototropic curvature which lasted for 50 seconds.

(2) Owing to the internal conduction of excitation the positive effect underwent neutralisation by the excitatory contraction of the distal side. This neutralisation depends on four factors: (a) on the intensity of the stimulus, (b) on the conductivity of the organ in a transverse direction, (c) on the thickness of the intervening tissue, and (d) on the relative excitability of the distal as compared to the proximal side. The extent of positive curvature also depends on the pliability of the organ.

(3) In anisotropic organs where the distal side is physiologically the more excitable than the proximal, the internally diffused excitation brings about a greater contraction of the distal, and thepositivephototropic curvature becomes reversed to a very pronouncednegative. The effect of the internally diffused stimulus is thus the same as that of external diffuse stimulus.

(4) When the stimulus is applied on the more excitable half of the organ, the result is a predominant contraction of that half, which cannot be neutralised by the excitation conducted to the less excitable half of the organ. As the curvature is towards the stimulus, the phototropic curvature thus remains positive, even under continued stimulation.

The positive curvature is due to the differential action of unilateral stimulus on the proximal and distal sides. But when a strong light is made to act continuously on one side of an organ, the excitation becomes internally diffused, and the differential effect on the two sides is reduced in amount or vanishes altogether. Owing to the weak transverse conductivity of the tissue, while the effect of a feeble stimulus remains localised, that of a stronger stimulus is conducted across it.

Oltmanns found that the seedling ofLepidium sativumassumed a transverse or dia-phototropic position under intense and long continued action of light of 600,000 Hefner lamps. He regards this as the indifferent position. But the neutralisation of curvature is not, as explained before, due to a condition of indifference, but to the antagonistic effects of the two opposite sides of the organ, the proximal being stimulated by the direct, and the distal by the transversely conducted excitation. I obtained such neutralisation withDregea volubilisunder the prolonged unilateral action of arc-light. The first effect was positive; this was gradually and continuously neutralised under exposure for two hours; even then the neutralisation was not complete. I shall presently adduce instances where the neutralisation was not merely complete, but the final effect was an actual reversal into negative response.

I may here consider the remarkable fact that has been observed, but for which no explanation has been forthcoming, that "direct sunlight is too bright to bring about heliotropic curvature, only diffuse, not direct sunlight has the power of inducing heliotropic movements."[13]But we cannot conceive of light suddenly losing its phototropic effect by an increase of intensity. The experiment just described will offer full explanation for this apparent anomaly. Feeble or moderate stimulus remains, as we have seen, localised, hence the contraction of the proximal side gives rise to positive curvature. But the intense excitation caused by sunlight would be transmitted to the distal side and thus bring about neutralisation. It is the observation of the final result that has misled observers as to the inefficiency of direct sunlight. A continuous record of the response of the organ shows, onthe other hand, that the first effect of strong light is a positive curvature, and that under its continuous action the positive effect becomes neutralised (cf. Fig. 121). In the study of phototropic action, the employment of strong light has many advantages, since the period of experiment is, by this means, materially shortened. The continuous record then gives an epitome of the various phases of reaction.

I shall next show the continuity of responsive phototropic effects, from the positive curvature to the negative, through the intermediate phase of neutralisation. I have in the preceding paragraph described an experiment where under a given intensity and duration of exposure the excitations of the proximal and distal sides bring about neutralisation, the organ assuming a dia-phototropic position. If the intensity or duration of the stimulating light be further increased, it is easy to see that while excitation transmitted to the distal side is being increased, the excitatory contraction on the proximal side may, at the same time, be decreased owing to fatigue brought on by over-stimulation.

Fig. 121.Fig. 121.—Positive and negative phototropic responses ofOryzaunder continued unilateral stimulus of intense light from arc lamp.

Fig. 121.—Positive and negative phototropic responses ofOryzaunder continued unilateral stimulus of intense light from arc lamp.

In connection with this it should be borne in mind that the pulvinus ofMimosaexhibits under continuous stimulation, a fatigue relaxation instead of normal contraction. Similar effects are known to take place in animal muscles. The effect of relatively greater excitation will thus give rise to negative phototropic curvature. The transverse conductivity of organs of diverse plants will necessarily be different. The neutralisation and reversal into negative will thus depend on three factors: the transverse conductivity of the organ, the intensity, and duration of stimulus.

Neutralisation and reversal under increased intensity of light: Experiment 127.—It is advisable to employ thin specimens (in which the transverse distance is small) forthe exhibition of reversal effect. I took a hypocotyl ofSinapis nigraand subjected it to unilateral action of light from a 16 candle-power incandescent electric lamp placed at a distance of 10 cm. A maximum positive curvature was induced in the course of 50 minutes. The intensity of light was afterwards increased by bringing the lamp nearer to a distance of 6 cm. This resulted in a process of neutralisation of the preceding response; after an exposure of 70 minutes the specimen assumed a dia-phototropic position in which it remained in equilibrium. Sunlight was next applied, and in the further course of 30 minutes there was a pronounced reversal into negative phototropic curvature.

Neutralisation and reversal under continuous stimulation: Experiment 128.—In the last experiment the different changes in the response were brought about by successive increase in the intensity of light. In the present experiment, very strong light was applied from the beginning, and continuous record was taken of the change in the response. In order to reduce the period of experiment I employed a mercury vapour lamp which emits the most effective violet and ultra-violet rays. The specimen used was a seedling of the rice plant (Oryza sativa). The first effect of light was a positive curvature which attained its maximum; after this there was a neutralisation in less than six minutes after the application of light. The furthercontinuation of light induced a pronounced negative curvature (Fig. 121).

I shall in the next chapter give other instances which will show that all organs (pulvinated and growing) possessed of power of transverse conduction, exhibit a transformation of response from positive to negative under continued action of strong light.

Thus an identical organ, under different conditions of intensity and duration of stimulus, exhibitspositivephototropic,dia-phototropic, andnegativephototropic curvatures, proving conclusively that the three effects are not due to three distinct irritabilities. The responsive movements are, on the other hand, traced to a fundamental excitatory reaction, remaining either localised or increasingly transmitted to the distal side.

From the analogy of opposite responses of shoot and root to stimulus of gravity, it was surmised that the root would respond to light by a negative curvature. This was apparently confirmed by the negative phototropic curvature of the root ofSinapis. The supposed analogy is however false; for while the stimulus of gravity acts, in the case of root, only on a restricted area of the tip, the stimulus of light is not necessarily restricted in the area of its action. That there is no true analogy between the action of light and gravitation is seen from the fact that while gravitation induces in the root a movement opposite to that in the stem, in the case of light, this is not always so; for though a few roots turn away from light, others move towards the light.

As regards negative phototropic response of the root ofSinapis, it will be shown (p. 376) to be brought about byalgebraical summation of the effects of direct and indirect photic stimulus.

The normal positive phototropic curvature is modified by transverse conduction of true excitation to the distal side of the organ.

The extent of neutralisation or reversal due to internal conduction of excitation from the proximal to the distal side of the organ depends: (a) on the intensity of the incident stimulus, (b) on the conductivity of the organ in a transverse direction, (c) on the thickness of the intervening tissue, and (d) on the relative excitability of the distal as compared to the proximal side.

The dia-phototropic position is not one of indifference, but of balanced antagonistic reactions of two opposite sides of the organ.

The supposition that direct sunlight is phototropically ineffective is unfounded. The response is fully vigorous, but the first positive curvature may in certain cases be neutralised by the transmission of excitation to the distal side.

Under light of strong intensity and long duration, the transmitted excitation to the distal side neutralises, and finally reverses the positive into negative curvature.

Thepositive-phototropic, thedia-phototropic, and thenegativephototropic curvatures are not due to three distinct irritabilities but are brought about by a fundamental excitatory reaction remaining localised or increasingly transmitted to the distal side.

[12]Jost—Ibid—p. 462.

[13]Jost—Ibid—p. 464.

I shall in this chapter describe experiments in support of the important proposition thatthe intensity of phototropic action is dependent on the quantity of incident light. The proportionality of the tropic effect to the quantity of light will be found to hold good for the median range of stimulation; the deviation from this proportionality at the two ends of the range of stimulation—the sub-minimal and supramaximal—is, as we shall find, capable of explanation, and will be fully dealt with in the next chapter.

The quantity of light incident on the responding organ depends: (1) on the intensity of light, (2) on the angle of inclination orthe directive angle,[14]and (3) on the durationof exposure. I shall give a detailed account of the investigation relating to the individual effects of each of these factors on the tropic reactions not merely in pulvinated but also in growing organs.

Fig. 122.Fig. 122.—Leaf ofDesmodium gyrans, with the terminal large, and two lateral small leaflets. These latter exhibit automatic pulsations.

Fig. 122.—Leaf ofDesmodium gyrans, with the terminal large, and two lateral small leaflets. These latter exhibit automatic pulsations.

The intensity of light was increased in successive experiments, in arithmetical progression 1:2:3 by suitably diminishing the distance between the plant and the source of light, and the resulting tropic curvatures recorded.

Effect of increasing intensity of light on the pulvinus ofDesmodium gyrans:Experiment 129.—The source of light was a 50 candle-power incandescent lamp, and the duration of exposure was 1 minute. The specimen employed was a terminal leaflet ofDesmodium gyrans(Fig. 122) the pulvinus of which is very sensitive to light. It is more convenient to manipulate a cut specimen of the leaf, instead of the whole plant. The petiole is placed in water contained in a U-tube; the depressing effect of wound passes off in the course of an hour or so. Light of increasing intensity is applied from above; this induces a contraction of the upper half of the pulvinus, andthe resulting response is recorded by means of the Oscillating Recorder (Fig. 123).

The first record was obtained under a given intensity, and the second, under an intensity twice as great. The tropic effects are seen to increase with the intensity (Fig. 124). If the tropic curvature increased proportionately to the intensity, the two responses should have been in the ratio of 1:2; the actual ratio was however slightly greater,viz.1:2·6. In this connection it will be shown in the next chapter, that strict proportionality holds good only in the median range, and that the susceptibility for excitation undergoes an increase at the beginning of the phototropic curve.

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