(1) The response to unilateral stimulation of the tip of the seedling.(2) The response of growing hypocotyl to direct stimulation.(3) Summated effects of direct and indirect stimulation.
(1) The response to unilateral stimulation of the tip of the seedling.
(2) The response of growing hypocotyl to direct stimulation.
(3) Summated effects of direct and indirect stimulation.
The Recorder.—The pull exerted by the tropic curvature of the seedling is very feeble; it was therefore necessary to construct a very light and nearly balancedrecording lever. A long glass fibre is supported by lateral pivots on jewel bearings. The seedling is attached to the short arm of the lever by means of a cocoon thread. The recording plate oscillates to and fro once in a minute; the successive dots give therefore the time relations of the responsive movement. The positive curvature towards light is recorded as an up-curve, the negative curvature being represented by a down-curve.
Fig. 135.Fig. 135.—Arrangement for local application of light to the tip and the growing region. O, O', apertures on a metallic screen. Light is focussed by a lens on the tip, and on the growing region at o, o'. Figure to the right shows front view of the shutter resting on a pivot and worked by string, T.
Fig. 135.—Arrangement for local application of light to the tip and the growing region. O, O', apertures on a metallic screen. Light is focussed by a lens on the tip, and on the growing region at o, o'. Figure to the right shows front view of the shutter resting on a pivot and worked by string, T.
Arrangement for local stimulation by light.—The device of placing tin foil caps on the tip employed by some observers labours under the disadvantage, that it causes mechanical irritation of the sensitive tip. The appliance seen in figure 135 is free from this objection and offers many advantages. A metallic screen has two holes O and O'; these apertures are illuminated by a parallel beam of light from an arc lamp. A lens focusses the light from O, on the hypocotyl, and that from O', on the tip of the cotyledon. A rectangular pivoted shutter S, lies between the apertures O and O'. In the intermediate position of the shutter, light acts on both the tip and the growing region. The shutter is tilted up by a pull on the thread T, thus cutting off light from the growing region; release of the thread cuts off light from the tip. Thus by proper manipulation of the shutter, the tip or the growing hypocotyl, or both of them, may be subjected tothe stimulus of light. The experiment was carried out in a dark room, special precaution being taken that light was screened off from the plant except at points of localised stimulation.
Fig. 136.Fig. 136.—Response of seedling ofSetariato unilateral stimulation of the tip applied at dotted arrow.Note preliminary negative curvature reversed later into positive.
Fig. 136.—Response of seedling ofSetariato unilateral stimulation of the tip applied at dotted arrow.
Note preliminary negative curvature reversed later into positive.
Experiment 137.—If the tip of the seedling ofSetariabe illuminated on one side, it is found that apositivecurvature (i.e., towards light) is induced in the course of an hour or more. But in obtaining record of the seedling by unilateral stimulation of the tip, I found that the immediate response was not towards, but away from light (negative curvature). The latent period was about 30 seconds and the negative movement continued to increase for 25 minutes (Fig. 136). This result, hithertounsuspected, is not so anomalous as would appear at first sight. Indirect stimulus, unilaterally applied, has been shown to give rise to two impulses: a quicker positive and a slower excitatory negative. The former induces a convexity on the same side, and a movement away from stimulus (negative curvature); the excitatory negative, on the other hand, is conducted slowly and induces contraction and concavity, and a movement towards the stimulus (positive curvature). In semi-conducting or non-conducting tissues, the excitatory negative is weakened to extinction during transit, and the positive reaction with negative curvature persists as the initial and final effect.
But inSetariathe excitatory negative impulse is transmitted along the parenchyma which is moderately conducting; the speed of transmission of heliotropic excitation is, according to Pfeffer, one or two mm. in five minutes or about 0·4 mm. per minute. Thus under the continued action of light, the excitatory impulse will reach the growing region, and by its predominant reaction neutralise and reverse the previous negative curvature.
Inspection of figure 136 shows that this is what actually took place; the intervening distance between the tip of the cotyledon and the growing region in hypocotyl was about 20 mm., and the beginning of reversal from negative to positive curvature occurred 29 minutes after application of light. The velocity of transmission of excitatory impulse under strong light is thus 0·7 mm. per minute. The positive curvature continued to increase for a very long time and became comparatively large. This is for two reasons: (1) because the sensibility of the tip of the cotyledon is very great, and (2) because the positive curvature induced by longitudinally transmittedexcitation is not neutralised by transverse conduction (see below).
Fig. 137.Fig. 137.—Effect of application of light to the growing hypocotyl at arrow induced positive phototropic curvature followed by neutralisation. Application of indirect stimulus at dotted arrow on the tip gave rise at first to negative, subsequently to positive curvature. (Seedling ofSetaria).
Fig. 137.—Effect of application of light to the growing hypocotyl at arrow induced positive phototropic curvature followed by neutralisation. Application of indirect stimulus at dotted arrow on the tip gave rise at first to negative, subsequently to positive curvature. (Seedling ofSetaria).
Experiment 138.—The growing region of the hypocotyl ofSetariais supposed to be totally devoid of the power of perception. In order to subject the question to experimental test, I applied unilateral light on the growing region of the same specimen, after it had recovered from the effect of previous stimulation. The response now obtained was vigorous and wasab-initiopositive. Direct stimulus has thus induced the normal effect of contraction and concavity of the excited side. The belief that the hypocotyl ofSetariais incapable of perceiving stimulus is thus without any foundation. The further experiment which I shall presently describe will, however, offer an explanation of the prevailing error. On continuing theaction of unilateral light, the positive curvature after attaining a maximum in the course of 15 minutes, underwent a diminution and final neutralisation (Fig. 137). On account of this neutralisation the seedling became erect after an exposure of 30 minutes; in contrast with this is the increasing positive curvature under unilateral illumination of the tip (Fig. 136) which continues for several hours. The explanation of this neutralisation under direct stimulation of the growing region is found in the fact that transverse conduction of excitation induces contraction at the distal side of the organ and thus nullifies the positive curvature. The seeming absence of tropic effect under direct stimulation is thus not due to want of perception, but to balanced antagonistic reactions on opposite sides of the organ.
Though stimulation of the hypocotyl results in neutralisation, yet the illumination of one side of the organ including the tip and hypocotyl is found to give rise to positive curvature. This will be understood from the following experiment.
After the neutralisation in the last experiment light was also applied to the tip from the right side at the dotted arrow (Fig. 137). The record shows that this gave rise at first to a negative curvature (away from light); under the continued action of light, however, the negative was subsequently reversed to a positive curvature, towards light. Inspection of the curve shows another interesting fact. The positive curvature induced by direct stimulation is very much less than that brought out by indirect stimulation. This is due to two reasons: (1) the sensitiveness of the tip of the organ is, as is well known, greater than thatof the hypocotyl, (2) the positive curvature under direct stimulation cannot proceed very far, since it is neutralised by transverse conduction of excitation.
It will be seen from the above that the illumination of the tip practically inhibits the neutralisation and thus restores the normal positive curvature. The question now arises as to how this particular inhibition is brought about.
An instance of inhibition, though of a different kind, was noticed in the response of the tendril ofPassiflora(p. 296); the under side of the organ is highly sensitive, while the upper side is almost insensitive. Stimulation of the under side of the tendril induces a marked curvature, but simultaneous stimulation of the diametrically opposite side inhibits the response. This neutralisation could not be due to the antagonistic contraction of the upper side since the irritability of that side is very slight. I have shown that the inhibition results from the two antagonistic reactions, contraction at the proximal side due to direct stimulation and expansion caused by the positive impulse from the indirectly stimulated distal side.
We have in the above an algebraical summation of the effects of direct and indirect stimulations. The longitudinally transmitted effect of indirect stimulus inSetariamay, likewise, be summated with the effect of direct stimulus. The phenomenon of algebraical summation is demonstrated in a very striking and convincing manner in the following experiment, which I have been successful in devising.
Experiment 139.—I have explained, (Expt. 126) that unilateral application of stimulus of light on the upper half of the responding pulvinus ofMimosainduces an up or positive curvature, followed by a neutralisation and even a reversal into negative, the last two effects being brought about by transverse conduction of excitation to the distal side. When the incident light is of moderate intensity, the transmitted excitation only suffices to induce neutralisation without further reversal into negative; while in this state of balanced neutralisation let us apply indirect stimulus by throwing light on the stem at a point directly opposite to the leaf (Fig. 138).
Fig. 138.Fig. 138.—(a) Diagrammatic representation of direct application of light (↓) on the pulvinus and the indirect application on the stem (→) (b) Record of effect of direct stimulus, positive curvature followed by neutralisation. Superposition of the positive reaction of indirect stimulus induces erectile up-response followed by down movement due to transmitted excitatory impulse (Mimosa).
Fig. 138.—(a) Diagrammatic representation of direct application of light (↓) on the pulvinus and the indirect application on the stem (→) (b) Record of effect of direct stimulus, positive curvature followed by neutralisation. Superposition of the positive reaction of indirect stimulus induces erectile up-response followed by down movement due to transmitted excitatory impulse (Mimosa).
Two different impulses are thus initiated from the effect of indirect stimulus. In the present case the positivereached the responding pulvinus after 30 seconds and induced an erectile movement of the leaf; the excitatory negative impulse reached the organ 4 minutes later and caused a rapid fall of the leaf. The record (Fig. 138) shows further that the previous action of direct stimulus which brought about neutralisation, does not interfere with the effects of indirect stimulus. The individual effects of direct and indirect stimulus are practically independent of each other; hence their joint effects exhibit algebraical summation.
Fig. 139.Fig. 139.—Diagrammatic representation of the effects of direct and indirect stimulus on the response ofSetaria. 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).
Fig. 139.—Diagrammatic representation of the effects of direct and indirect stimulus on the response ofSetaria. 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).
We are now in a position to have a complete understanding of the characteristic response of Paniceae to transmitted phototropic excitation.
(1) Local stimulation of the tip gives rise to two impulses, positive and negative. The former induces a transient negative movement (away from light); the latter causes a permanent and increasing positive curvature towards light.
(2) Local stimulation of the growing hypocotyl gives rise to positive curvature, subsequently neutralised by the transverse conduction of excitation to the distal side. The absence of tropic effect in the growing region is thus due not to lack of power of perception, but to balanced antagonistic reactions of two opposite sides of the organ.
(3) The effects of direct and indirect stimulations are independent of each other; hence, on simultaneous stimulations of the tip and the growing hypocotyl, the effects of indirect stimulus are algebraically summated with the effect of direct stimulus (neutralisation). The indirect stimulation of the tip on the right side gives rise to two impulses, of which the expansive positive reached the right side of the responding region earlier, inducingconvexity and movement away from stimulus (negative curvature). This is diagrammatically shown in Fig. 139. Had the intervening tissue been non-conducting, the slow excitatory negative impulse would have failed to reach the responding region, and the negative curvature induced by the positive impulse would prove to be the initial as well as the final effect. In the case ofSetaria, however, the excitatory impulse reaches the right side of the organ after the positive impulse; the final effect is therefore an induced concavity and positive curvature (movement towards stimulus).
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 anegativecurvature.
Type B.—If the intervening tissue be conducting the transmission of the excitatory impulse will finally give rise to apositivecurvature.
Type B is exemplified by the seedling ofSetariawhere 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 ofSinapis.
Negative phototropism of root ofSinapis:Experiment 140.—For investigation of the negative phototropism of the root ofSinapis nigraI 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.
Thus in the root ofSinapis, the expansive effect of indirect stimulation of the tip is superposed on that of direct stimulation of the growing region (neutral or slightlynegative). The final result is thus a movement away from light or anegativephototropic curvature.
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.
[20]Jost—Ibid—p. 468.
[20]Jost—Ibid—p. 468.
[21]"Response in the Living and Non-Living"—p. 17.
[21]"Response in the Living and Non-Living"—p. 17.
[22]"Plant Response"—p. 604.
[22]"Plant Response"—p. 604.
Phototropic response, positive or negative, is determined by the directive action of light. But photonastic reaction is supposed to belong to a different class 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 ofmovement will further be shown to be distorted by the lateral action of light.
The different sides of a radial organ, such as the young stem ofMimosa, 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 ofMimosathe lower or the shaded side is, generally speaking, found to be the more excitable. In fact such anisotropic stem ofMimosaacts 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 ofMimosain 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 compass—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.
Fig. 140.Fig. 140.—Photonastic curvature of creeping stem ofMimosa 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).
Fig. 140.—Photonastic curvature of creeping stem ofMimosa 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 ofLysimachia, all of which, in response to the action of strong light actingfrom above, exhibit concavity of the shaded and more excitable side.
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 termedpara-heliotropismby 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 aspositivelypara-heliotropic, and those which fold downwards asnegativelypara-heliotropic. This is merely for convenience of description. There is no specific irritability which distinguishes one from the other.
Para-heliotropic response ofErythrina indicaand ofClitoria 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). Therewas no neutralisation or reversal, demonstrating the absence of transverse conduction (cf.Fig. 132).
Fig. 141.Fig. 141.—Positive para-heliotropic response of leaflets ofErythrina indica.
Fig. 141.—Positive para-heliotropic response of leaflets ofErythrina indica.
Para-heliotropic movement of leaflets ofMimosa 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 pronounced 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 passed unnoticed. Application of light from above gave, on account of the greater excitability of the upper half of the pulvinule, a pronounced positive response or movement towards light. The anomaly of an identical organ appearing as positively heliotropic when acted by light from above, and negativelyheliotropic 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.
Fig. 142.Fig. 143.Fig. 142.Fig. 143.Fig. 142.—Response of leaflet ofMimosato light applied below: transient positive followed by pronounced negative curvature.Fig. 143.—Response of leaflet ofAverrhoa, to light applied above: transient positive followed by pronounced negative curvature.Up-curve represents up-movement, and down-curve, down-movement.
Fig. 142.
Fig. 143.
Fig. 142.Fig. 143.
Fig. 142.—Response of leaflet ofMimosato light applied below: transient positive followed by pronounced negative curvature.
Fig. 143.—Response of leaflet ofAverrhoa, to light applied above: transient positive followed by pronounced negative curvature.
Up-curve represents up-movement, and down-curve, down-movement.
Response of leaflet ofAverrhoa carambola:Experiment 144.—The leaflets of this plant, and also those ofBiophytum sensitivumfold 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 analysis of reaction under light is rendered possible fromthe record of response of leaflet ofAverrhoa, 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 inAverrhoathe effect of continuous light applied above or below is a downward movement; inMimosathe movement is upwards. The explanation of this difference lies in the fact, that inMimosaleaflet it is the upper half of the pulvinule that is more excitable; while inAverrhoaand inBiophytumthe lower is the more excitable half of the organ.
Fig. 144.Fig. 144.—Diagrammatic representation of different types of phototropic response. (See text.)
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, withpositive and negative signs, represents the amplitude and sign of curvature.
a.Radial thick organ, in which transverse conduction is absent. Curvature ispositive,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, andnegative. Reversal of direction of light gives rise to similar sequence of responses as before (e.g., seedling ofSinapis).c.Anisotropic thick organ; transverse conduction possible. Thick line represents the more excitable distal side. Sequence of curvature: positive, neutral and pronounced negative. When light strikes from opposite direction on the more excitable side the curvature will remain positive, since the pronounced reaction of the more excitable side cannot be neutralised or reversed by transmitted excitation to the less excitable distal side (e.g., leaf ofMimosa).In the absence of transverse conduction, the curvature remains positive (e.g., leaflet ofErythrina).d.Anisotropic thin organ with high transverse conductivity. Sequence of curvature: transient positive, quickly masked by predominant negative. Light striking on the more excitable side will give rise only topositive. The response in relation to the plant, will apparently be in the same direction whether light strikes the organ on one side or the opposite (e.g., leaflets ofMimosa,AverrhoaandBiophytum).
a.Radial thick organ, in which transverse conduction is absent. Curvature ispositive,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, andnegative. Reversal of direction of light gives rise to similar sequence of responses as before (e.g., seedling ofSinapis).
c.Anisotropic thick organ; transverse conduction possible. Thick line represents the more excitable distal side. Sequence of curvature: positive, neutral and pronounced negative. When light strikes from opposite direction on the more excitable side the curvature will remain positive, since the pronounced reaction of the more excitable side cannot be neutralised or reversed by transmitted excitation to the less excitable distal side (e.g., leaf ofMimosa).
In the absence of transverse conduction, the curvature remains positive (e.g., leaflet ofErythrina).
d.Anisotropic thin organ with high transverse conductivity. Sequence of curvature: transient positive, quickly masked by predominant negative. Light striking on the more excitable side will give rise only topositive. The response in relation to the plant, will apparently be in the same direction whether light strikes the organ on one side or the opposite (e.g., leaflets ofMimosa,AverrhoaandBiophytum).
I have shown that tissues in sub-tonic condition exhibit an acceleration of the rate of growth under stimulus (p. 224)the corresponding tropic reaction would therefore be away from stimulus ornegativecurvature. The tonic condition is, however, raised to the normal by the action of stimulus itself, and the tropic curvature becomes positive.
I give below a table which will show at a glance all possible variations of phototropic reaction.
TABLE XXXI.—MECHANICAL RESPONSE OF PULVINATED AND GROWING ORGANS UNDER LIGHT.
Description of tissue.Action.Effect observed.I Tissue sub-tonic.Stimulus causes increase of internal energy.Expansion or enhanced rate of growth,e.g.,PileusofCoprinusdrooping in darkness, made re-turgid by light. Renewed growth of dark rigored plant exposed to light.II Normally excitable organ under unilateral light.A 1. Moderate light, causing excitatory contraction of proximal and positive expansion of distal.1. Curvature towards light,e.g., flower bud ofCrinum.A. Organ radial.A 2. Strong light. Excitatory effect transmitted to distal, neutralising first.2. Neutralisations,e.g., seedling ofSetaria.A 3. Intense and long-continued light. Fatigue of proximal and excitatory contraction of distal.3. Reversed or negative response,e.g., seedling ofZea Mays.B. Dorsiventral organ.B 1. Excitatory contraction of proximal predominant, owing either to greater excitability of proximal or feeble transverse conductivity of tissue.1. Positive response,e.g., upward folding of leaflets in so-called "diurnal sleep" ofErythrina indicaandClitoria ternatea.B 2. Transmission of excitation through highly conducting tissue to more excitable lower or distal. Greater contraction of distal.2. Negative response,e.g., downward folding of leaflets in so-called "diurnal sleep" ofBiophytumandAverrhoa.III Rhythmic tissue.Considerable absorption of energy, immediate or prior.Initiation of multiple response inDesmodium gyranspreviously at standstill; multiple response under continuous action of light inBiophytum.
There is no line of demarcation between tropic and nastic movements.
In a differentially excitable organ the effect of strong unilateral stimulus becomes internally diffused, and causes greater contraction of the more excitable side of the organ.
In the absence of transverse conduction, the positive curvature reaches a maximum without neutralisation or reversal. The leaflets ofErythrina indicaand ofClitoria ternateathus fold upwards, the apices of the leaflets pointing towards the sun.
Internally diffused excitation under strong light induces greater contraction of the more excitable half of the pulvinule, causing upward folding ofMimosaleaflet, and downward folding of the leaflets ofBiophytumandAverrhoa.
I shall in this chapter deal with certain anomalies in phototropic curvature, brought about by variation of temperature and by seasonal change; certain organs again are apparently erratic in their phototropic response.
Sachs observed a positive phototropic curvature in the stems ofTropæolum majusin autumn; but this was reversed into negative in summer; similarly in the hypocotyl of Ivy, the positive curvature in autumn is converted into negative curvature in summer.
Certain organs are apparently insensitive to the action of light. Thus no phototropic response is found in the tendril ofPassifloraeven under the action of strong light. The tendrils ofVitisandAmpelopsisexhibit, according to Wiesner, positive phototropism under feeble, and negative phototropism under strong light.
The anomalies referred to above may be explained by taking into consideration the modifying influence oftemperature on the excitability, and the conductivity of the organ.
The excitability of an organ is abolished at a low temperature; it is enhanced by a rise of temperature up to an optimum. The temperature minimum and optimum varies in different tissues. The following table shows the enhancement of excitability ofMimosaat different temperatures, the testing stimulus being the same.
TABLE XXXII—SHOWING VARIATION OF EXCITABILITY OF PULVINUS OFMimosaAT DIFFERENT TEMPERATURES.
Temperature.Amplitude of response.22°C.2 divisions.27°C.16 "32°C.36 "
Below 20°C. the excitability of the pulvinus ofMimosais practically abolished. The excitability increases till an optimum temperature is reached, above which it undergoes a decline.
Though rise of temperature enhances excitability up to an optimum, there is an antagonistic reaction induced by it which opposes the excitatory contraction. The physiological reaction of a rise of temperature, within normal range, is expansion and this must oppose the contraction induced by stimulus. Hence the effect of rise of temperature is complex; it enhances the excitability which favours contraction, while tending to oppose this contraction by the induced physiological expansion. As a result of these opposite reactions there will be a critical temperature, below which the contractile effectwill relatively be greater than expansion; above the critical point, expansion will be the predominant effect. The critical temperature will obviously be different in different organs. The positive curvature may thus be increased by a slight rise, while it may be neutralised, or even reversed by a greater rise of temperature.
The induced variation of excitability due to change of temperature is not the only factor in modifying tropic curvature, for variation of conductivity also exerts a marked effect.
The conducting power of an organ is greatly enhanced with rise of temperature. Thus inMimosathe velocity of transmission of excitation is doubled by a rise of temperature through 9°C. (p. 100). An organ which is practically non-conducting at a low temperature will become conducting at a higher temperature.
Thus at a low temperature the organ may be non-conducting, and the excitatory contraction under unilateral stimulus will remain localised at the proximal side; this will give rise to a positive curvature. But under rising temperature, the power of transverse conduction will be increased and the excitation will be conducted to the distal side. The result of this will be a neutralisation or reversal into negative curvature (p. 139). A positive curvature is thus reversed into negative by change of excitability and conductivity, induced by rise of temperature; examples of this will be given presently.
I shall here adduce considerations which will show that the apparent anomalies regarding the response of tendrilsto light is due to the variation of transverse conductivity of the organ. In a semi-conducting tissue, while the excitatory effect of feeble stimulus remains localised at the proximal side, the effect of stronger stimulus is conducted to the distal side. This explains the positive phototropic curvature of tendrils ofVitisandAmpelopsisunder feeble light, and its reversal into negative curvature under intense light.
As the conducting power is increased with rise of temperature it is evident that at a certain temperature the tropic effect will be exactly neutralised by transverse conduction. Lowering of temperature, by reducing the transmission of excitation to the distal side, will restore the positive curvature. Enhancement of conduction under rise of temperature will, on the other hand, increase the antagonistic reaction of the distal side and give rise to a negative curvature.
I shall in verification of the above, describe experiments which I have carried out on the phototropic response of the tendril ofPassiflora, supposed to be insensitive to the action of light.
Phototropic response of the tendril ofPassiflora:Experiment 145.—The tendril was cooled by keeping it for a long time in a cold chamber, maintained at 15°C. The effect of unilateral light on the cooled specimen was found to be positive; the tendril was next allowed to assume the temperature of the room which was 30°C. The response was now found to have undergone a change into negative. The positive and negative phototropic curvatures of an identical organ at different temperatures is seen in the two records given in figure 145. Neutralisationtakes place at an intermediate temperature, and the organ thus appears insensitive to light.
Fig. 145.Fig. 145.—(a) Positive curvature of tendril ofPassifloraat 15°C.; (b) negative phototropic curvature at 30°C.
Fig. 145.—(a) Positive curvature of tendril ofPassifloraat 15°C.; (b) negative phototropic curvature at 30°C.
Reference has been made of the phototropic curvature ofTropæolumand of Ivy undergoing a change from positive in autumn to negative in summer. The experiment described above shows that rise of temperature, by enhancing transverse conductivity, transforms the positive into negative heliotropic curvature. The reversal of the phototropic curvature ofTropæolumand Ivy, from positive in autumn to negative in summer, finds a probable explanation in the higher temperature condition of the latter season. This inference finds independent support from the fact previously described (p. 100) that while the velocity ofconduction of excitation in the petiole ofMimosais as high as 30 mm. per second in summer, it is reduced to about 4 mm. in late autumn and early winter.
I have explained the complex effect of rise of temperature on phototropic curvature. Rise of temperature, within limits, enhances the excitability, and therefore the positive curvature under light. Its expansive reaction, on the other hand, opposes the contraction of the proximal side, which produces the normal positive curvature. Rise of temperature, as previously stated, introduces another element of variation by its effect on conductivity. Transverse conduction favoured by rise of temperature promotes neutralisation and reversal; the resultant effect will thus be very complicated. I give below account of an experiment where the induced positive curvature under light underwent a reversal during rise of temperature.
Reversal of tropic curvature under rise of temperature: Experiment 146.—The specimen employed for this experiment was a seedling of pea, enclosed in a glass chamber, the temperature of which could be gradually raised by means of an electric heater. Provisions were made to maintain the chamber in a humid condition. The temperature of the chamber was originally at 29°C., and application of light on one side of the organ gave rise to positive curvature, followed by complete recovery on the cessation of light (Fig. 146a). The next experiment was carried out with the same specimen; while the plant was undergoing increasing positive curvature under the continued action of light, the temperature of thechamber was gradually raised from 29° to 33°C. at the point marked with arrow. It will be seen that the positive curvature became arrested, neutralised, and finally reversed into negative (Fig. 146b).