Fig. 146.Fig. 146.—Effect of rise of temperature on phototropic curvature. (a) normal positive curvature followed by recovery, (b) reversal of positive into negative curvature by rise of temperature at (H). (Pea seedling).
Fig. 146.—Effect of rise of temperature on phototropic curvature. (a) normal positive curvature followed by recovery, (b) reversal of positive into negative curvature by rise of temperature at (H). (Pea seedling).
After-effect of rise of temperature: Experiment 147.—The after-effect of rise of temperature exhibited by this specimen was extremely curious. The temperature of the chamber was allowed to return to the normal, and the experiment repeated after an hour; the response was now found to be negative (Fig. 147a). It appeared probable that the temperature in the interior of the tissue had not yet returned to the normal, and an interval of four hours was therefore allowed for the restoration of the tissue to the normal temperature of the room. The response still persisted to be negative, as seen in the series of records obtained under successive stimulations of light of short duration; these negative responses exhibited recovery on thecessation of light (Fig. 147b). This reversal of response as an after-effect of rise of temperature was in this case found to persist for several hours. I experimented with the same specimen next day when the response was found restored to the normal positive.
Fig. 147.Fig. 147.—After-effect of rise of temperature, persistent negative curvature: (a) response one hour after rise of temperature; (b) series of negative responses after 4 hours (successive stimuli applied at vertical lines).
Fig. 147.—After-effect of rise of temperature, persistent negative curvature: (a) response one hour after rise of temperature; (b) series of negative responses after 4 hours (successive stimuli applied at vertical lines).
Rise of temperature, within limits, enhances the general excitability of the organ. This has the effect of increasing positive phototropic curvature. But the physiological expansion induced by rise of temperature exerts an antagonistic effect.
The transverse conductivity is increased with the rise of temperature; this favours neutralisation and reversal of phototropic curvature.
Tendrils ofPassiflora, supposed to be phototropically insensitive, exhibit positive curvature at low, and negative curvature at a moderately high temperature.
The change of phototropic curvature exhibited byTropæolum majusand Ivy, from positive in autumn to negative in summer, is probably due to the effect of temperature. Higher temperature with enhanced transverse conductivity in summer, may thus convert positive into negative curvature.
The physiological effects of rise of temperature and the stimulus of light are antagonistic to each other.
Rise of temperature tends to neutralise or reverse the positive phototropic curvature. The after-effect of temperature is often very persistent.
In addition to positive or negative curvatures light induces a responsive torsion. With regard to this Jost says:—
"The mechanics of the torsions have not as yet been fully explained. It has long been believed that these torsions were occasioned only by the action of a series of external factors, such as light, gravity, weight of the organ which individually led to curvatures, but in combination induced torsions; but later investigations have shown that torsions might appear when light only was the functional external factor.... If the torsions cannot generally be regarded as due to the combination of two curvatures, we are completely in the dark as to the mechanics of their production."[23]
A leaf when struck laterally by light undergoes a twist, so that the upper surface is placed, more or less, at right angles to the incident rays; as no explanation was available for this movement, the suggestion has been made that the particular reaction is for the advantage of the plant. I shall presently show, that it is possible to reverse this normal torsion and thus make the upper surface of the leaf move away from light.
The experiments which I shall presently describe will, it is hoped, throw light on the obscure phenomenon. I shall be able to show:
(1) that the torsional response is not dependent on the combination of two curvatures,(2) that it is also independent of the effect of weight,(3) that it may be induced not merely by stimulus of light but by all forms of stimulation,(4) that the direction of the torsional response depends on the direction of the incident stimulus and the differential excitability of the organ, and(5) that there is a definite law which determines the torsional movement.
(1) that the torsional response is not dependent on the combination of two curvatures,
(2) that it is also independent of the effect of weight,
(3) that it may be induced not merely by stimulus of light but by all forms of stimulation,
(4) that the direction of the torsional response depends on the direction of the incident stimulus and the differential excitability of the organ, and
(5) that there is a definite law which determines the torsional movement.
I shall first describe a typical experiment on the responsive torsion under the action of light. We have seen that in the pulvinus ofMimosa, light of moderate intensity and of short duration applied on the upper half induces a slow up-movement, while the stimulus of light applied below induces a more rapid down-movement. The difference is due to the fact that the lower half of the pulvinus is relatively the more excitable. Vertical light thus induces a movement in a vertical plane. But an interesting variation is induced in the response under the action of lateral light. A stimulus will be calledlateralwhen it acts on either the right or left flank of adorsiventralorgan. We shall presently find that a dorsiventral organ responds to lateral stimulus by torsion.
The present series of experiments were carried out with the leaf ofMimosa, and in order to eliminate the effect of weight and also for obtaining record of pure torsion, I employed the following device. The petiole was enclosed in a hooked support made of thin rod of glass, the petiole resting on the concavity of the smooth surface. Friction and theeffect of weight is thus practically eliminated; the looped support prevented up or down movements, and yet allowed perfect freedom for torsional response. This latter is magnified by a piece of stout aluminium wire fixed at right angles to the petiole (Fig. 148). The end of the aluminium wire is attached to the short arm of a recording lever; there is thus a compound magnification of the torsional movement. The Oscillating Recorder gave successive dots at intervals which could be varied from 20 seconds to 2 minutes. Time-relations of the response may thus be obtained from the dotted record.
Fig. 148.Fig.148.—Diagrammatic representation for record of torsional response. H, thin glass hook: A, aluminium wire attached to petiole for magnification of torsional movement. T, silk thread for attachment to recording lever.
Fig.148.—Diagrammatic representation for record of torsional response. H, thin glass hook: A, aluminium wire attached to petiole for magnification of torsional movement. T, silk thread for attachment to recording lever.
With the experimental device just described, we shall be in a position to study the effect of various stimuli applied at one flank of the pulvinus—at the junction of the upper and lower halves of the organ. The observer standing in front of the leaf is supposed to look at the stem. Torsional response will then appear as a movement either with or against the hands of the clock. The torsional response, right-handed or left-handed,will presently be shown to depend on the direction of incident stimulus. In figure 149, anti-clockwise torsion is recorded as an up-curve; clockwise rotation is recorded as a down-curve.
Experiment 148.—The pulvinus of the leaf was stimulated by a horizontal beam of light thrown in a lateral direction; the areas contiguous to line of junction of the upper and lower halves of the anisotropic organ thus underwent differential excitation. When light struck on the left flank, the responsive torsion was anti-clockwise; the responsive reaction thus madethe upper and the less excitable half of the pulvinus face the stimulus. Figure 149 gives a record of the torsional movement when light struck the left flank of the organ; on the cessation of stimulus the response is followed by recovery.
Fig. 149.Fig. 149.—Record of torsional response of pulvinus ofMimosa pudica.
Fig. 149.—Record of torsional response of pulvinus ofMimosa pudica.
Experiment 149.—If now the direction of stimulus be changed so that light strikes on the right flank insteadof the left, the responsive torsion is found to be reversed, the direction of movement being clockwise. Here also the responsive movement is such that it is the less excitable upper half of the organ that is made to face the stimulus. It will thus be seen that the torsion, anti-clockwise or clockwise, depends on two factors, namely the direction of stimulus, and the differential excitability of the organ.
I shall now proceed to show that the torsional response is induced not merely by the action of light, but by all forms of stimulation.
Effect of chemical stimulation: Experiment 150.—Dilute hydrochloric acid was at first applied on the left flank of the pulvinus along the narrow strip of junction of the upper and lower halves. This gave rise to a responsive torsion against the hands of a clock. Chemical stimulation of the right flank induced, on the other hand, a torsional movement with the hands of a clock. Here also the direction of stimulus is found to determine the direction of responsive torsion.
Effect of thermal radiation: Experiment 151.—I next employed thermal radiation as the stimulus; the source of radiation was a length of electrically heated platinum wire. It is advisable to interpose a narrow horizontal slit, so as to localise the stimulus at the junction of the upper and lower halves of the pulvinus. Stimulus applied at the left flank induced left-handed or anti-clockwise torsion; application at the right flank gave rise to right-handed torsion.
Geotropic stimulus.—The stimulus of gravity induces, as I shall show in a subsequent chapter, a similarresponsive torsion, the direction of which is determined by the direction of the incident stimulus.
Under normal conditions, the torsional response under light places the upper surface of the leaf or leaflets at right angles to light. That this movement is not due to some specific sensibility to light is shown by the fact that all modes of stimulation, chemical, thermal or gravitational, induce similar responsive torsion. The torsional response is, moreover, shown to be determined by the direction of incident stimulus, and the differential excitability of the organ. This latter may be reversed by the local application of various depressing agents on the normally more excitable lower half of the pulvinus. Under this treatment, the lower half of the pulvinus may be rendered relatively the less excitable. Lateral application of light now induces a torsional movement which is the reverse of the normal, so that the upper surface of the leaf moves away from light. The advantage of the plant cannot, therefore, be the factor which determines the directive movement; the teleological argument often advanced is, in any case, no real explanation of the phenomenon.
In all the instances given above, and under every mode of stimulation, the responsive movement makes the less excitable half of the pulvinus face the stimulus. The torsional response is, in reality, the mechanical result of the differential contraction of a complex organ, which is fixed at one end and subjected to lateral stimulation. I have been able to verify this, by the construction of an artificial pulvinus consisting of a compound strip, the upper half of which is ebonite, and lower half the morecontractile stretched India-rubber; if such a strip be held securely at one end in a clamp, and if the lateral flank, consisting half of ebonite and half of India-rubber, be subjected to radiation, and record taken in the usual manner, it will be found that a torsional response takes place which is similar to that of the pulvinus ofMimosa. The above experiment was devised to offer an explanation of the mechanics of the movement. It should, however, be borne in mind in this connection that the torsional response of pulvinus is brought about by differentialphysiologicalcontraction of the organ, the movement being abolished at death.
From the results given above, we arrive at the following:—
1. AN ANISOTROPIC ORGAN, WHEN LATERALLY EXCITED BY ANY STIMULUS, UNDERGOES TORSION BY WHICH THE LESS EXCITABLE SIDE IS MADE TO FACE THE STIMULUS.
2. THE INTENSITY OF TORSIONAL RESPONSE INCREASES WITH THE DIFFERENTIAL EXCITABILITY; WHEN THE ORIGINAL DIFFERENCE IS REDUCED, OR REVERSED, THE TORSIONAL RESPONSE UNDERGOES CONCOMITANT DIMINUTION OR REVERSAL.
Having thus established the laws that guide torsional response, I shall try to explain certain related phenomena which are regarded as highly obscure. I shall also describe the application of the method of torsional response in various investigations.
The leaves of the so-called "Compass plants" exhibit very complex movements, these being modified according tothe intensity of incident light. Thus in compass plants the leaves, under moderate intensity of light in the morning or in the evening, turn themselves so as to expose their surfaces to the incident rays. But under intense sun light, the leaves perform bendings and twistings so that they stand at profile at midday.
I have not yet been able to secure "Compass plants" at Calcutta. I shall, however, describe my investigations on the complicated torsional movements exhibited by certain leaflets by the action of vertical light. The results obtained from these will show that torsional movements, even the most complex, are capable of explanation from the general laws that have been established.
Torsional movement of leaflet ofCassia alata:Experiment 152.—These leaflets are closed laterally at night but place themselves in an outspread position at day time. The character of the movement is, however, modified by the intensity of light. With moderate light in the morning the leaflets open out laterally. But under more intense light, the pulvinules of the leaflets exhibit a torsion by which the formerly infolded surfaces of the leaflets are exposed at right angles to light from above (Fig. 150). Such complicated movements, in two directions of space, are also exhibited by other leaflets which are closed at night in a lateral direction.
Fig. 150.Fig. 150.—Leaflets ofCassia alata: open in daytime, and closed in evening.
Fig. 150.—Leaflets ofCassia alata: open in daytime, and closed in evening.
For obtaining an explanation of these complex movements under different intensities of light, we have first to discover the particular disposition of the two halves of the pulvinule which are unequally excitable; we have next to explain the responsive movements under the directive action of moderate and of intense light.
Determination of differential excitabilities of the organ: Experiment 153.—In the leaflet ofCassiathe movement of opening under diffuse stimulation of light can only be brought about by the contraction of the outer half, which must therefore be the more excitable. This is independently demonstrated by the reaction to an electric-shock. On subjecting the half closed leaflets to diffuse electric stimulation, they open outwards in alateraldirection. The disposition of the unequally excitable halves of the pulvinule is thus different from that of the main pulvinus ofMimosa. In the latter, the plane that divides the two halves is horizontal, the lower half being the more excitable. Thus in the pulvinule ofCassiathe plane that separates the two unequally excitable halves is vertical, the outer half being the more excitable than the inner. By inner half is here meant that half which is inside when the leaflets are closed.
Effect of strong vertical light: Experiment 154.—When the plant is placed in a moderately lighted room, the leaflets open out laterally to the outmost. This is brought about by the contraction of the more excitable outer half of the organ. If strong light be thrown down from above, a new movement is superposed, namely, of torsion by which the leaflets undergo a twist and thus place their inner surface at right angles to the vertical light. In order to investigate this phenomenon in greater detail I placed the plant in a well lighted room, the leaflets being three quarters open under the diffuse light. A verylight index was attached to the leaflet for magnifying the subsequent torsional movement. A strong beam of parallel light from an arc lamp was thrown down on the pulvinule from above; this fell at the junction of the more excitable outer with the less excitable inner half of the organ, the plane of separation of the two unequally excitable halves being, as previously explained, vertical. I have shown that under lateral stimulation, a differentially excitable organ undergoes torsion by which the less excitable half is made to face the stimulus. Since it is the inner half of the organ that is the less excitable, the attached leaflet becomes twisted so as to expose its (former infolded) surface upwards, at right angles to the incident light.
As a confirmatory test, strong light was made to strike the pulvinule frombelowwith the result that the leaflets exhibited an opposite torsion by which their surfaces faced downwards, so as to be at right angles to light that struck them from below.
Under normal conditions sunlight comes from above; stimulation thus takes place at the junction of the two differentially excitable halves of the organ, the plane of separation of which is vertical. The torsion induced makes the less excitable inner half turn in such a way that the inner surfaces of the leaflets are placed perpendicular to the incident light.
The torsional response not only affords a new method of enquiry on the reaction of various stimuli, but it also possesses certain advantages. For instance in studying the response of the leaf ofMimosaunder light, the records weretaken of the movement of the leaf in a vertical plane. But the responsive up-movement, induced by light acting from above, is opposed by the weight of the leaf. But in the torsional response, the leaf rests on the hooked glass support and the movement is thus free from the complicating factor of the weight of the leaf. Again the pulvinus ofMimosa, for example, is sometimes subject to spontaneous variation of turgor, on account of which it exhibits an autonomous up or down movement. In the ordinary method of record the true response to external stimulus may thus be modified by natural movement of the leaf. But in the torsional method, the autonomous up or down movement is restrained by the hooked support, and the response to lateral stimulus is unaffected by the spontaneous movement of the leaf. The torsional method, moreover, opens out possibilities of inquiry in new directions, such as the comparison of the excitatory effects of different stimuli by the Method of Balance, and the determination of the effective direction of geotropic stimulus.
A beam of light falling on the left flank of the pulvinus ofMimosainduces a torsion against the hands of the clock. A second beam falling on the right flank opposes the first movement; the resultant effect is therefore determined by the effective stimulation of the two flanks. The pulvinus thus becomes a delicate index by which two stimuli may be compared with each other. The following experiment is cited as an example of the application of the method of phototropic balance.
Experiment 155.—Parallel beam of light from a small arc lamp passing through blue glass falls on the left flank of the pulvinus; a beam of blue light also strikes thepulvinus from the right side, and the intensity of the latter is so adjusted that the resultant torsion is zero. Blue glass is now removed from the left side, the unobstructed white light being allowed to fall on the left flank of the pulvinus. This was found to upset the balance, the resultant torsion being anti-clockwise. This showed that white light induced greater excitation than blue light. We next interpose a red glass on the left side, with the result that the balance is upset in the opposite direction. This is because the phototropic effect of red light is comparatively feeble. We may thus compare the tropic effect of one form of stimulus against a totally different form, phototropic against geotropic action for example. It is enough here to draw attention to the various investigations rendered possible by the method of balance. Concrete examples of some of these will be given in a subsequent chapter.
I have shown that the torsion, clockwise or anti-clockwise, is determined by the direction of incident stimulus. Hence it would be possible to determine the direction of incident stimulus from the observed torsional movement. In the case of light, the direction of incident stimulus is quite apparent. But it is difficult to determine the direction of stimulus which is itself invisible. In such cases, the torsional movement gives us infallible indication of the effective direction of stimulus. The application of this principle will be found in a later chapter.
Lateral stimulus induces a torsional response in a dorsiventral organ. This is true of all modes of stimulation.
The responsive torsion is determined by the direction of incident stimulus, and the differential excitability of two halves of the organ, the torsion being such that the less excitable half of the organ is made to face the stimulus.
The twist exhibited by various leaves and leaflets under light finds its explanation from the demonstrated laws of torsional response.
The direction of incident stimulus may be determined from the responsive torsion of a dorsiventral organ.
The Method of Torsional Balance enables us to compare the excitatory efficiencies of two different stimuli which act simultaneously on the two flanks of the organ.
[23]Jost—Ibid—p. 465.
[23]Jost—Ibid—p. 465.
We have studied the tropic curvature induced by different rays of light. We saw that while the more refrangible rays of the spectrum were most effective, the less refrangible rays were ineffective. Below the red, there are the thermal rays about whose tropic effect very little is definitely known.
The intricacies of the problem are very great owing to the difficulty of discriminating the effect of temperature from that of radiation; to this must be ascribed the contradictory results that have been obtained by different observers, of which Pfeffer gives the following summary:[24]
"In addition to the action of ultra-red rays which are associated with the visible part of the spectrum, dark heat-rays of still greater wave length, as well as differences of temperature may produce a thermotropic curvature in certain cases. Wortmann observed that seedlings ofLepidium sativumandZea Mays, as well as sporangiphores ofPhycomycescurved towards a hot iron plate emitting dark heat-rays. Steyer has, however, shown that the sporangiphore ofPhycomyceshas no power of thermotropic reaction.... Wortmann observed that the seedling shoot ofZea Mayswas positively, but that ofLepidiumnegatively, thermotropic.... Steyer, however, found that both plants were positively thermotropic. Wortmann has also investigated the radicles of seedlings by growing them in boxes of saw-dust, one side being kept hot, the other cold."
It will be noted that in the investigations described above, thermotropic reaction has been assumed to be the same under variation of temperature (as in experiments with unequally heated saw-dust), and under radiation from heated plate of metal. With reference to this Jost maintains that "so far as we know, thermotropism due toradiantheat cannot be distinguished from thermotropism due toconduction."[25]
The effect of temperature, within optimum limits, is a physiological expansion and enhancement of the rate of growth. The effect of visible radiation is, on the other hand, a contraction and retardation of growth. Should radiant heat act like light, the various tropic effects in the two cases would be similar; the temperature effect would in that case be opposite to the radiation effect. In order to find whether the thermal radiation produces tropic curvature similar to that of light, we have to devise a crucial experiment in which the complicating factor of rise of temperature on the responding organ is eliminated.
Experiment 156.—I have described the effect of light applied unilaterally to the stem ofMimosa, at a point diametrically opposite to the indicating leaf (Expt.104). It was shown that the effect of indirect stimulus induced at first an erectile movement of the leaf, and that this was followed by a fall of the leaf on account of transverse transmission of excitation. In the present experiment I applied thermal radiation instead of light. The source of radiation was a spiral of platinum wire heated short of incandescence bymeans of electric current. The intensity of incident radiation could thus be maintained constant, or increased or decreased by approach or recession of the radiating spiral. The effect of unilateral stimulus of heat-rays was found exactly similar to that of light;i.e., there was at first an erectile movement due to indirect stimulation, followed by the fall of the leaf due to transmitted excitation. It will be noticed that under the particular condition of the experiment, the responding pulvinus was completely shielded from temperature-variation. The reaction to thermal radiation is thus similar to that of light.
As regards the effects of rise of temperature and radiation I have shown that they are antagonistic to each other (pp. 211, 308). Thus in positive types of thermonastic organs like the flower ofZephyranthes, while rise of temperature induces a movement of opening, radiation causes the opposite movement of closure. Again, in the negative type exemplified byNymphæa, rise of temperature induces a movement of closure; radiation on the other hand, brings about the opposite movement of opening. The tropic effect of thermal radiation thus takes place in opposition to that of rise of temperature, and the resultant effect is therefore liable to undergo some modification, depending on the relative sensibility of the organ to radiation and to variation of temperature.
The facts that have been given above prove that infra-red radiation is as effective a mode of stimulation as the more refrangible rays of the spectrum. Phototropic and radio-thermotropic reactions would therefore prove to be essentially similar. The following experiments fully confirm the similarity of the two reactions.
Experiment 157.—I shall now describe the normal reaction of a growing organ to the unilateral stimulus ofthermal radiation. Figure 151 gives a record of response of the stem ofDregeato stimulus of short duration; the induced curvature is positive or towards the source of heat. On the cessation of stimulus, there is a recovery which is practically complete, and which takes place at a slower rate than the excitatory positive curvature. Repetition of stimulus gives rise to responses similar to the first.Successive stimuli of moderate intensity thus give rise to repeated responses of growth curvature.An arbitrary distinction has been made between the responses of pulvinated and of growing organs. The former is distinguished as a movement of variation, with its supposed characteristic of repeated response. But the experiment described shows that this is also met with in the response by growth curvature. It is only under long continued stimulation that the curvature is fixed by growth.
Fig. 151.Fig. 151.—Positive response to short exposure to thermal radiation. Successive dots at intervals of 5 seconds. (Dregea volubilis.)
Fig. 151.—Positive response to short exposure to thermal radiation. Successive dots at intervals of 5 seconds. (Dregea volubilis.)
The positive curvature is induced by retardation of growth at the proximal side, and enhancement of growthat the distal side. This latter effect is, as we have seen, brought about by the effect of indirect stimulation.
But under long continued action of stimulus, the negative or excitatory impulse reaches the distal side, inducing diminution of turgor and retardation of the rate of growth. This leads to neutralisation, the organ placing itself at right angles to the orienting stimulus.
Fig. 152.Fig. 152.—Record of positive, neutral and reversed negative curvature under continued action of thermal radiation. The negative response went off the plate. Successive dots at intervals of 5 seconds. (Dregea volubilis).
Fig. 152.—Record of positive, neutral and reversed negative curvature under continued action of thermal radiation. The negative response went off the plate. Successive dots at intervals of 5 seconds. (Dregea volubilis).
Experiment 158.—This neutralisation is seen in the record given in figure 152, where under continuous unilateral stimulation, the growing organ exhibited its maximum positive curvature, after which the movement becamearrested by the arrival of the excitatory impulse at the distal side, on account of which the first positive curvature became neutralised. Further continuation of stimulus caused a reversal into negative in the course of 7 minutes. It will thus be seen that in inducing phototropic curvature, the heat rays in sunlight play as important a part as the more refrangible rays of the spectrum.
The effects of rise of temperature and of radiation are antagonistic to each other.
Under unilateral action of thermal radiation a positive curvature is induced by the retardation of growth at the proximal, and acceleration of growth at the distal side of the organ.
There is a complete recovery on the cessation of stimulus of moderate intensity and short duration. Repeated responses may thus be obtained similar to repeated responses in pulvinated organs. In certain tissues the power of conduction in a transverse direction is wanting; excitation remains localised at the proximal side, and the responsive curvature remains positive.
In other cases, there is a slow conduction of excitation to the distal side. The result of this under different circumstances is dia-radio-thermotropic neutralization, or a reversed negative curvature.
In inducing phototropic curvature, the heat rays in sunlight play as important a part as the more refrangible rays of the spectrum.
[24]Pfeffer—Ibid—Vol. III, p. 776.
[24]Pfeffer—Ibid—Vol. III, p. 776.
[25]Jost—Ibid—p. 480.
[25]Jost—Ibid—p. 480.
A growing plant bends towards light, and this is true not only of the main stem but also of its branches and attached leaves and leaflets. Light affects growth, the effect being modified by the intensity of radiation. Strong stimulus of light causes a diminution of the rate of growth, but very feeble stimulus induces an acceleration. The tropic effect is very strong in the ultra-violet region of the spectrum with its extremely short wave length, but the effect declines practically to zero as we move towards the less refrangible rays—the yellow and the red with their comparatively long wave length. As we proceed beyond the infra-red region, we come across the vast range of electric radiation, the wave lengths of which vary from 0·6 cm., the shortest wave I have been able to produce, to others which may be miles in length. There thus arises the very interesting question, whether plants perceive and respond to the long ether waves including those employed in signalling through space.
At first sight this would appear to be very unlikely; for the most effective rays are in the ultra-violet region with wave length as short as 20 × 10-6cm.; but with electric waves used in wireless signalling we have to deal with waves 50 million times as long. The perceptive power of our retina is confined within the very narrow range ofa single octave, the wave lengths of which lie between 70 × 10-6cm. and 35 × 10-6cm. It is difficult to imagine that plants could perceive radiations so widely separated from each other as the visible light and the invisible electric radiation.
But the subject assumes a different aspect, when we take into consideration thetotaleffect of radiation on the plant. Light induces two different effects which may broadly be distinguished as external and internal. The former gives rise to movement; the latter finds no outward manifestation, but consists of an 'up' or assimilatory chemical change, with concomitant increase of potential energy. Of the two reactions then, one is dynamic attended by dissimilatory 'down' change; the other is potential, associated with the opposite 'up' change. In reality the two effects take place simultaneously; but one of these becomes predominant under definite conditions.
The modifying condition is thequalityof light; with reference to this I quote the following from Pfeffer: "So far as is at present known, the action of different rays of the spectrum gives similar curves in regard to heliotropic and phototactic movements, to protoplasmic streaming and movements of the chloroplastids as well as the photonastic movements produced by growth or by changes of turgor. On the other hand, it is the less refrangible rays which are most active in photo-synthesis."[26]The dynamic and potential manifestations are thus seen to be complementary to each other, the rays which induce photo-synthesis being relatively ineffective for tropic reaction andvice versâ.
Returning to the action of electric waves, since they exert no photo-synthetic action they might conceivably induce the complementary tropic effect. These considerations led me to the investigation of the subject fourteenyears ago, and my results showed that very short electric waves induce a retardation of rate of growth; they also produce responsive movements of the leaf ofMimosa, when the plant was in a highly sensitive condition.[27]The energy of the short electric waves is very feeble, and undergoes great diminution at a distance; hence the necessity of employment of a specimen of plant in a highly sensitive condition.
I resumed my investigations on the subject at the beginning of this year. I wished to find out whether plants in general perceived and responded to the long ether waves which reached it from a distance. The perception of the wireless stimulation was to be tested not merely by the responsive movement of sensitive plants, but by diverse modes of response given by all kinds of plants.
Stimulus induces, as we have seen, three different types of response in plants. It causes excitation in sensitive plants likeMimosa, in consequence of which the leaf undergoes a fall; this is the mechanical response to stimulus. Stimulus also induces electric response in plants, both sensitive and ordinary, the excited tissue undergoing an electric change of galvanometric negativity. Finally, the effect of stimulus on growing plants is a variation in the rate of growth, an acceleration under feeble, and a retardation under strong stimulus. I undertook to investigate the effect of electric waves on plants by the methods of mechanical and of electrical responses, and also by that of induced variation of growth.
Fig. 153.Fig. 153.—Diagrammatic representation of method employed for obtaining response to wireless stimulation. Transmitting apparatus seen to the right. Receiving aerial connected to upper part of plant, the lower part of the plant or the flower-pot being connected with the earth.
Fig. 153.—Diagrammatic representation of method employed for obtaining response to wireless stimulation. Transmitting apparatus seen to the right. Receiving aerial connected to upper part of plant, the lower part of the plant or the flower-pot being connected with the earth.
For sending wireless signals, I had to improvise the following arrangement, more powerful means not being available. The secondary terminals of a moderate sized Ruhmkorff's coil were connected with two cylinders of brass, each 20 cm. in length; the sparking took place between two small spheres of steel attached to the cylinders. One of the two cylinders was earthed, and the other connected with the aerial 10 meters in height. The receiving aerial was also 10 meters in height and its lower terminal led to the laboratory, and connected by means of a thin wire to the experimental plant growing in a pot; this latter was put in electric connectionwith the earth (Fig. 153). The distance between the transmitting and receiving aerial was about 200 meters, the maximum length permitted by the grounds of the Institute.
Experiment 159.—One of the leaves ofMimosawas connected with the aerial by means of a thin tinsel of loose wire, which did not interfere with the free movement of the leaf. This latter was attached to the recording lever. Wireless signals induced a responsive fall of the leaf (Fig. 154) which was gradual as under action of light, and not so abrupt as under a mechanical blow.
Fig. 154.Fig. 155.Fig. 154.Fig. 155.Fig. 154.—Mechanical response of leaf ofMimosato electric wave.Fig. 155.—Electric response ofMimosa pudicato wireless stimulation.
Fig. 154.
Fig. 155.
Fig. 154.Fig. 155.Fig. 154.—Mechanical response of leaf ofMimosato electric wave.Fig. 155.—Electric response ofMimosa pudicato wireless stimulation.
Experiment 160.—The leaf ofMimosawas in this experiment held securely, and two electrical connections made, one with the less excitable upper and the other with the more excitable lower half of the pulvinus. Theincident ether-wave induced an electric response in the pulvinus, the more excitable lower half exhibiting galvanometric negativity. On the cessation of stimulus there was a recovery (Fig. 155).
It is not at all necessary to employ the sensitiveMimosafor exhibition of electric response; for this is universally exhibited by all plants. The only condition for electric response is that the points of electric contacts should be made with two unequally excitable areas in the plant. This may be secured by artificial means as by causing 'injury' to one point of contact.[28]It is however much better to take advantage of the natural difference of excitability of two different areas in the organ as in the pulvinus ofMimosa. This difference of excitability is also found between the inner and outer sides of a hollow tubular organ as in the peduncles of various lilies. I was thus able to secure specimens which were far more sensitive to the action of electric waves than the pulvinus ofMimosa.
There now remains the very interesting question as to whether the effect of long ether waves induce any variation of growth. The results given below show that growing plants not only perceive but respond to the stimulus of electric waves. The effects to be presently described are exhibited by all plants.
I shall, however, content myself in describing a typical experiment carried with the seedling of wheat. The specimen was mounted on the Balanced Crescograph, and the growth exactly balanced. This gives a horizontal record; an acceleration of growth above the normal is, in thefollowing records, represented by a down curve, and a retardation by an up-curve.
Effect of feeble stimulus: Experiment 161.—I first studied the effect of feeble stimulus. This was secured by decreasing the energy of sparks of the radiator. The response was an acceleration of rate of growth as seen in figure 156a. The analogy of this with the accelerating effect of sub-minimal intensity of light (p. 224) is very remarkable.