Life Movements in Plants Part 7

COMPLEXITY OF PROBLEM OF PHOTOTROPIC REACTION.

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.

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.

ACTION OF LIGHT.

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 r.e.t.a.r.dation 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 contraction under 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 a.n.a.logies 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 a.s.sumption that there are phototropic particles which function like statoliths in geotropic organs. There is, however, certain outstanding difference between the two cla.s.ses 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.

POSITIVE PHOTOTROPIC CURVATURE.

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 leaflet of the bean plant. The source of illumination was 32 c.p. electric lamp, enclosed in a metallic tube with circular aperture for pa.s.sage 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).

[Ill.u.s.tration: FIG. 112.--Successive positive responses of the terminal leaflet of bean. Light applied from above for 20 seconds; complete recovery in 8 minutes.]

POSITIVE PHOTOTROPIC CURVATURE OF GROWING ORGANS.

_Effect of moderate stimulation: Experiment 118._--I shall presently show that the intensity of phototropic reaction depends 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 of _Dregea 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).

[Ill.u.s.tration: FIG. 113.--Positive curvature under moderate phototropic stimulation. Note complete recovery (_Dregea_).]

[Ill.u.s.tration: 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 stronger stimulation 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.

PHENOMENON OF RECOVERY.

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 a.s.sumes the action of an unknown regulating power by which the organ is brought back to a straight line; but beyond the a.s.sumption 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.

IMMEDIATE AND AFTER-EFFECT OF LIGHT ON GROWTH.

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 r.e.t.a.r.dation (p. 225),

(2) that a stimulus of moderate intensity induces the normal r.e.t.a.r.dation of the rate of growth.

It is evident that there is a _critical intensity_ of stimulus, above which there is a r.e.t.a.r.dation, 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 r.e.t.a.r.dation of growth in specimens the tonic condition of which is _above par_, and an acceleration in others, in which it is _below par_. The following experiments will demonstrate the immediate and after-effect of light of increasing intensity and duration.

[Ill.u.s.tration: 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 r.e.t.a.r.dation (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 r.e.t.a.r.dation; 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 was acted 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 r.e.t.a.r.dation, 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 the _intensity of phototropic effect is proportional to the quant.i.ty of incident light_. This quant.i.ty at the beginning proved to be sub-minimal, and hence there was an acceleration at the beginning.

Continued action induced the normal effect of r.e.t.a.r.dation, 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 r.e.t.a.r.dation 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 r.e.t.a.r.dation to the normal rate being gradual. In the next experiment (the result of which is not given in the record) the intensity of light was increased still further; the r.e.t.a.r.dation 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 r.e.t.a.r.dation. 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 r.e.t.a.r.dation 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 r.e.t.a.r.dation is very great, and this persists for a long time even on the removal of light.