[Ill.u.s.tration: FIG. 139.--Diagrammatic representation of the effects of direct and indirect stimulus on the response of _Setaria_. Direct stimulation, represented by thick arrow gives rise to antagonistic concavities of opposite sides of responding hypocotyl, resulting in neutralisation.

Indirect stimulus represented by dotted arrow gives rise to two impulses, the quick positive impulse represented by a circle, and the slower negative impulse represented by crescent (concave).]

The results given above enable us to draw the following generalisations:--

1. In an organ, the tip of which is highly excitable, the balanced state of neutralisation, induced by direct stimulation of the responding region, is upset in two different ways by two impulses generated in consequence of indirect stimulation at the tip. Hence arises two types of resultant response:--

Type A.--If the intervening tissue be semi-conducting, the positive impulse alone will reach the growing region and induce convexity of the same side of the organ giving rise to a _negative_ curvature.

Type B.--If the intervening tissue be conducting the transmission of the excitatory impulse will finally give rise to a _positive_ curvature.

Type B is exemplified by the seedling of _Setaria_ where the transmission of excitatory impulse from the tip upsets the neutral balance and induces the final positive curvature.

Example of type A is found in the negative phototropism of the root of _Sinapis_.

_Negative phototropism of root of_ Sinapis: _Experiment 140._--For investigation of the negative phototropism of the root of _Sinapis nigra_ I took record of its movement under unilateral action of light by means of a Recording Microscope, devised for the purpose.[22] When the root-tip alone was stimulated by unilateral light, the root moved away from the source of light. This was due to the longitudinal transmission of positive impulse to the growing region at some distance from the tip.

The intervening distance between the tip and the growing region is practically non-conducting, hence the excitatory impulse could not be conducted from the tip. After a period of rest in darkness, I next took record of its movement under direct unilateral illumination of the growing region; the result was at first a positive movement; but this, on account of transverse conduction of excitation under continued stimulation, underwent a neutralisation and slight reversal. In taking a third record, in which both the tip and growing region were simultaneously subjected to unilateral stimulation of light, I found that a resultant responsive movement was induced which was away from light.

[22] "Plant Response"--p. 604.

Thus in the root of _Sinapis_, the expansive effect of indirect stimulation of the tip is superposed on that of direct stimulation of the growing region (neutral or slightly negative). The final result is thus a movement away from light or a _negative_ phototropic curvature.

SUMMARY.

The effect induced by stimulus of light is transmitted to a distance, in a manner precisely the same as in other modes of stimulation.

In the Paniceae, the local unilateral stimulation of the tip of the cotyledon induces positive curvature in the growing hypocotyl, at some distance from the tip. This is due to transmitted excitatory effect of indirect stimulation; the earlier positive impulse induces a preliminary negative curvature, which is reversed later by the excitatory negative impulse into positive curvature.

Contrary to generally accepted view the hypocotyl not only perceives but responds to light. The positive curvature induced by direct stimulation is, however, neutralised by transverse conduction of excitation.

The effects of direct and indirect stimulus are independent of each other; the final effect is determined by their algebraical summation.

x.x.xIV.--ON PHOTONASTIC CURVATURES

_By_

SIR J. C. BOSE,

_a.s.sisted by_

GURUPRASANNA DAS.

Phototropic response, positive or negative, is determined by the directive action of light. But photonastic reaction is supposed to belong to a different cla.s.s of phenomenon, where the movement is independent of the directive action of light. I shall, however, be able to establish a continuity between the tropic response of a radial and the nastic movement of a dorsiventral organ. The intermediate link is supplied by organs originally radial, but subsequently rendered anisotropic by the unilateral action of stimulus of the environment. In a dorsiventral organ, owing to anatomico-physiological differentiation, the responsive movement is constrained to take place in a direction perpendicular to the plane of separation of the two unequally excitable halves of the organ. Even in such a case, it will be shown, that light does exert a directive action; the direction of movement will further be shown to be distorted by the lateral action of light.

PHOTOTROPIC RESPONSE OF ANISOTROPIC ORGANS.

The different sides of a radial organ, such as the young stem of _Mimosa_, are equally excitable. The response to unilateral light of moderate intensity is therefore positive; owing to equal excitabilities of the two sides the response of the opposite sides are alike. Diffuse stimulation therefore induces no resultant curvature. If, however, the plant is allowed to form a creeping habit, the excitabilities of the dorsal and ventral sides will no longer remain the same. Thus in the creeping stem of _Mimosa_ the lower or the shaded side is, generally speaking, found to be the more excitable. In fact such anisotropic stem of _Mimosa_ acts somewhat like the pulvinus of the same plant. Diffuse stimulation induces, in both, a concavity of the more excitable lower half with the down movement of the leaf or the stem.

_Experiment 141._--I took four creeping stems of _Mimosa_ in vigorous condition and tied them in such a manner that their free ends should be vertical. The shaded sides of the four specimens were so turned that each faced a different point of the compa.s.s--east, west, north and south. Subjected thus to diffuse stimulus 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 curved towards the north, south, and west respectively (Fig. 140). The fundamental action by which all these were determined was the induced concavity of the under or normally shaded side, which was the more excitable. I obtained similar results with various other creeping stems.

[Ill.u.s.tration: FIG. 140.--Photonastic curvature of creeping stem of _Mimosa pudica_: in the central figure the stem is seen to be vertical: action of diffuse light induced appropriate curvatures by greater contraction and concavity of the more excitable lower or shaded side, as seen in figures to the right (_b_) and left (_c_).]

It has been shown that under prolonged unilateral stimulation, excitation becomes internally diffused; this gives rise to an effect similar to that of external diffuse stimulus. Under strong light the shaded side becomes concave, and thus press against the ground or the support; this will be the characteristic response of creeping stems in which the shaded side is the more excitable. The facts given above will probably explain the response of midribs of leaves, of the creeping stem of _Lysimachia_, all of which, in response to the action of strong light acting from above, exhibit concavity of the shaded and more excitable side.

PARA-HELIOTROPISM.

Under strong sunlight, the leaflets of various plants move sometimes upwards, at other times downwards, so as to place the blades of leaflets parallel to incident light. This 'midday sleep' has been termed _para-heliotropism_ by Darwin. It has been thought that para-heliotropic action has nothing to do with the directive action of light, since many leaflets either fold upwards or downwards, irrespective of the direction of incident light. I shall for convenience distinguish the leaflets which fold upwards under light as _positively_ para-heliotropic, and those which fold downwards as _negatively_ para-heliotropic. This is merely for convenience of description. There is no specific irritability which distinguishes one from the other.

POSITIVE PARA-HELIOTROPISM.

_Para-heliotropic response of_ Erythrina indica _and of_ c.l.i.toria ternatea: _Experiment 142._--For the purpose of simplicity I have described the type of movement of these leaflets as upwards; but the actual direction in which the leaflets point their apices is towards the sun. Both the plants mentioned here are so remarkably sensitive that the leaflets follow the course of the sun, in such a way that the axis of the cup, formed by the folding leaflets at the end and the sides of the petiole, is coincident with the rays of light. The pulvinus makes a sharp curvature which is concave to light, the blade of the leaflet being parallel to light. I have taken record of continuous action of strong light acting on the responding pulvinus of the leaflets from above. The result is an increasing positive curvature which reached a limit (Fig. 141). There was no neutralisation or reversal, demonstrating the absence of transverse conduction (_cf._ Fig. 132).

[Ill.u.s.tration: FIG. 141.--Positive para-heliotropic response of leaflets of _Erythrina indica_.]

_Para-heliotropic movement of leaflets of_ Mimosa pudica: _Experiment 143._--These leaflets, as previously stated, fold themselves upwards, when strongly illuminated either from above or below. Diffuse electric stimulation also induce a closing movement upwards; hence the upper half of the pulvinule of these leaflets are the more excitable. In order to obtain a continuous record of the leaflet under the action of unilateral light, I constructed a very delicate recording lever magnifying about 150 times. Light of moderate intensity from a 100 candle-power incandescent lamp was applied on the less excitable lower side of the pulvinule. The record (Fig. 142) shows that the immediate response is positive, or a movement towards the light. But owing to transverse conduction, through the thin and highly conducting pulvinule, the response was quickly reversed into a very p.r.o.nounced negative, or movement away from light. Had a delicate means of obtaining magnified record not been available, the slight positive twitch, and the gradual transition from positive to negative phototropic curvature would have pa.s.sed unnoticed. Application of light from above gave, on account of the greater excitability of the upper half of the pulvinule, a p.r.o.nounced positive response or movement towards light. The anomaly of an identical organ appearing as positively heliotropic when acted by light from above, and negatively heliotropic when acted from below, is now fully removed. The response of the leaflets is also seen to be determined by the directive action of light, though the short-lived response of the less excitable lower side is quickly masked by the predominant reaction of the more excitable upper side of the organ.

[Ill.u.s.tration: FIG. 142.--Response of leaflet of _Mimosa_ to light applied below: transient positive followed by p.r.o.nounced negative curvature.]

[Ill.u.s.tration: FIG. 143.--Response of leaflet of _Averrhoa_, to light applied above: transient positive followed by p.r.o.nounced negative curvature.

Up-curve represents up-movement, and down-curve, down-movement.]

NEGATIVE PARA-HELIOTROPISM.

_Response of leaflet of_ Averrhoa carambola: _Experiment 144._--The leaflets of this plant, and also those of _Biophytum sensitivum_ fold downwards under action of strong light, applied above or below. In these leaflets diffuse electric stimulation induce a fall of the leaflets demonstrating the greater excitability of the lower half of the pulvinule. The a.n.a.lysis of reaction under light is rendered possible from the record of response of leaflet of _Averrhoa_, given in Fig.

143. Light of moderate intensity from an incandescent electric lamp acted from above: the result was a feeble and short-lived positive response, quickly reversed to strong negative by transmission of excitation to the more excitable lower side. Illumination from below gave rise only to strong positive response. Thus in _Averrhoa_ the effect of continuous light applied above or below is a downward movement; in _Mimosa_ the movement is upwards. The explanation of this difference lies in the fact, that in _Mimosa_ leaflet it is the upper half of the pulvinule that is more excitable; while in _Averrhoa_ and in _Biophytum_ the lower is the more excitable half of the organ.

[Ill.u.s.tration: FIG. 144.--Diagrammatic representation of different types of phototropic response. (See text.)]

As a summary of the tropic action of light I shall give diagrammatic representations of various types of phototropic response, including the photonastic (Fig. 144). The direction of the arrow indicates the direction of incident light. Dotted specimens are those which possess transverse conductivity. Thick lines represent the more excitable side of an anisotropic or dorsiventral organ. The size of the circles, with positive and negative signs, represents the amplitude and sign of curvature.

_a._ Radial thick organ, in which transverse conduction is absent. Curvature is _positive_, _i.e._, movement towards light. The result will be similar when light strikes in an opposite direction, _i.e._, from right to left.

_b._ Radial thin organ. There is here a possibility of transverse conduction. Sequence of curvature: _positive_, _neutral_, and _negative_. Reversal of direction of light gives rise to similar sequence of responses as before (_e.g._, seedling of _Sinapis_).

_c._ Anisotropic thick organ; transverse conduction possible.

Thick line represents the more excitable distal side.

Sequence of curvature: positive, neutral and p.r.o.nounced negative. When light strikes from opposite direction on the more excitable side the curvature will remain positive, since the p.r.o.nounced reaction of the more excitable side cannot be neutralised or reversed by transmitted excitation to the less excitable distal side (_e.g._, leaf of _Mimosa_).