Fig. 177.Fig. 178.Fig. 177.Fig. 178.Fig. 177.—Curve of geo-electric excitation in different layers ofNymphæa. Ordinate represents geo-electric excitation; abscissa, distance from upper surface of flower stalk. The diagrammatic section underneath shows the position of geo-perceptive layer (starch-sheath) corresponding to maximum induced galvanometric negativity and positivity on the two sides.Fig. 178.—The curve of geo-electric excitation in different layers ofBryophyllum.
Fig. 177.
Fig. 178.
Fig. 177.Fig. 178.
Fig. 177.—Curve of geo-electric excitation in different layers ofNymphæa. Ordinate represents geo-electric excitation; abscissa, distance from upper surface of flower stalk. The diagrammatic section underneath shows the position of geo-perceptive layer (starch-sheath) corresponding to maximum induced galvanometric negativity and positivity on the two sides.
Fig. 178.—The curve of geo-electric excitation in different layers ofBryophyllum.
A curve constructed from the data given above is seen in figure 177. The diameter of the flower stalk was 6·8 mm. The negative geo-electric reaction is seen to undergo an increase till it attains a climax at the depth of 1·4 mm. It then undergoes a continuous diminution till it becomes zero at the depth of 3 mm.; this neutral zone extends through 1 mm. When the probe enters a depth of 4·2 mm. measured from the upper side, it enters a region affected by the perceptive layer situated on the under side, the opposite physiological reaction being indicated by induced electric change of galvanometric positivity. This positivity reaches a climax at a depth of 5·4 mm. measured from the upper side, and 1·4 mm. when measuredfrom the lower side. The points of maximum positivity and negativity are situated symmetrically on the opposite sides of the organ. The electric variation of maximum positivity on the lower side is comparatively feeble, less than half the corresponding maximum negativity on the upper side. Microscopic section showed that the geo-perceptive layers were the same as the starch-crescents.
Experiment 192.—I carried out similar experiments with the shoot ofBryophyllum. The results are given in Table XLV; the curve of the electric distribution along the diameter is seen in figure 178. The characteristics of this curve are the same as that ofNymphæa. The maximum galvanometric negativity occurred at the depth of 0·6 mm., and of positivity at a corresponding point on the opposite side.
TABLE XLV.—SHOWING INDUCED GEO-ELECTRIC DISTRIBUTION ACROSS THE STEM OFBryophyllum(diameter = 3·6 mm.).
Position of probe.Galvanometric deflection.Surface0 divisions.0·2 mm.-24 "0·4 "-45 "0·6 "-63"0·8 "-21 "1·0 "- 9 "1·2 "- 6 "1·4 "- 3 "1·6 "0 "1·8 "0 "2·0 "0 "2·2 "0 "2·4 "+ 3 "2·6 "+ 4 "2·8 "+ 9 "3·0 "+36"3·2 "+21 "3·4 "+ 9 "3·6 "0 "
Microscopic examination showed that the electric maxima inBryophyllumcoincided with the diametrically opposite points in the continuous endodermic ring. InBryophyllumas inNymphæa, the excitatory galvanometric negativity of the upper geo-perceptive layer is greater than the inducedpositivity of the lower layer in the ratio of about 2:1. But in a depressed condition of the tissue, the excitatory reaction is the first to disappear and the positive reaction persists, though with diminished intensity.
The geo-electric distribution in vigorous specimens seems to indicate that under the stimulus of gravity a marked excitatory reaction (contraction) takes place in the layer of cells contiguous to the upper geo-perceptive layer, and, a less marked positive reaction (expansion) occurs in layers contiguous to the lower perceptive layer.
It is remarkable that physiological reaction of opposite kinds should occur on the upper and lower sides of an organ under the identical stimulus of gravity. The difference of reaction may conceivably be connected with the fact that the vertical lines of gravity enter by the upper, and leave by the lower side of the organ. The statolithic particles rest on the inner tangential walls of the perceptive cells of the upper layer, and on the outer tangential walls of the lower layer. Similar difference of physiological reactions of a polar character are also known in responses of plants under the action of an identical electric current; here with different ionic distributions, contraction takes place at the kathode, and expansion at the anode.
The geo-electric reactions that have been described were obtained under unfavourable conditions of climate and of temperature. But under better conditions the reaction becomes very greatly enhanced, as would appear from the following account of results which I obtained on two separate occasions in the beginning of August. The season had not become quite as unfavourable as towards the end of the month, but the prevailing sultry weather had caused great depression of the geo-electricexcitability. On the first occasion referred to, thunderstorm had broken out at night, and it was refreshingly cool in the morning. It was with the utmost surprise that I noted the astonishing violence of the geo-electric response which the plants gave that morning; the maximum response hitherto obtained was about 100 divisions of the galvanometer scale; but on the present occasion the displacement of the plant, from vertical to horizontal position, induced responsive deflection so great that the galvanometer spot of light flew off the scale of 3,000 divisions. I was at first incredulous of the results and wasted the valuable occasion in trying to discover some hidden source of error. Subsequent tests showed that my misgivings were groundless, and that the extraordinary large deflection was really due to geo-electric reaction. On the second favourable occasion, which lasted for three hours (during the cool hours of the morning), I was able to secure a number of important observations. Thus displacement of the flower stalk ofNymphæathrough +90° was immediately followed by geo-electric response, the deflection being about 3,000 divisions of the scale. The latent period hardly exceeded a second; the return of the plant to the vertical position was quickly followed by electric recovery which was complete. The above results were obtained with the same specimen time after time without a single failure. The successive responses showed no sign of fatigue. Another remarkable effect was noticed during gradual increase of the angle of inclination. Nothing happened till a critical angle was reached, which was roughly estimated to be about 33°; when this critical angle was exceeded by a single degree, there was a sudden precipitation of geo-electric response. The experiments were repeated time after time with the identical result. It appeared as if some frictional resistance obstructed the displacement of the geotropic particles accumulated at the basal end of the cell, and it was nottill the organ had been tilted beyond 33° that this resistance to sliding was overcome.
The electric distribution induced in an organ under the stimulus of gravity may be mapped out by means of an exploring Electric Probe.
The induced galvanometric negativity of the upper side of an organ (indicative of excitation) undergoes variation in different layers of the organ. The excitatory reaction attains a maximum value at a definite layer, beyond which there is a decline.
The geo-perceptive layer is experimentally localised by measuring the depth of intrusion of the probe for maximum deflection of galvanometric negativity.
The geo-perceptive layer thus determined is found to be the starch sheath which contains a number of large-sized starch grains.
The power of geo-perception undergoes seasonal variation. It is also lowered by high temperature.
The geo-electric response undergoes decline with growing sub-tonicity of the specimen; such specimens exhibit abnormal positive electric response under the stimulus of prick and feeble curvature under geotropic stimulus. The large-sized starch-grains, normally observed in the endodermis, are found to disappear in specimens which have become geo-electrically insensitive.
The electric response of the lower side of the organ to gravitational stimulus is of opposite sign to that of the upper side. The electric distribution on the lower side exhibits variations in different layers, the maximumpositivity occurring at the perceptive layer. In vigorous specimens the excitatory negative electric change on the upper side is greater than the positive electric change on the lower side. Depressed condition of the tissue is attended by a relatively greater decline of the negative in comparison with the positive.
The induced electric variation on the upper and on the lower side indicates that the layers of tissue contiguous to the upper perceptive layer undergoes contraction, while those contiguous to the lower perceptive layer undergoes expansion.
[38]Haberlandt—Ibid, p. 597.
[38]Haberlandt—Ibid, p. 597.
I have explained that in a dorsiventral organ, lateral application of various stimuli induces a responsive torsion by which the less excitable side is made to face the stimulus (p. 403). I shall in this chapter show that the effect of stimulus of gravity is in every respect similar to other forms of stimulation.
Fig. 179.Fig. 179.—Diagram of arrangement for torsional response under geotropic stimulus. The less excitable upper half of pulvinus is, in the above figure, to the left and the torsional response is clockwise.
Fig. 179.—Diagram of arrangement for torsional response under geotropic stimulus. The less excitable upper half of pulvinus is, in the above figure, to the left and the torsional response is clockwise.
The direction of force of gravity is fixed, and we have to arrange matters in such a way that the geotropic stimulus should act on the dorsiventral organ in a lateral direction. In the following experiments the pulvinus ofMimosais taken as the typical dorsiventral organ. For lateral stimulation, the plant is placed on its side, so that the vertical lines of gravity impinge on one of the two flanks of the organ. In regard to this, I shall distinguish two different positions,aandb. In thea-position, the apex of the stem and the upper half of the pulvinus are to the left of the observer, and inb-position, the apex of the stem and the less excitable upper half of the pulvinus are to the right. The arrangement for obtaining record of the torsional response undera-positionis shown in figure 179.
Torsional response in a- and b-positions: Experiment 193.—When the leaf is ina-position, the geotropic torsion is found to be with the movement of the hands of a clock. In theb-position, on the other hand, the torsion is against the hands of a clock. In both these cases thegeotropic torsion makes the less excitable upper half of the pulvinus face the vertical lines of gravity. The incident stimulus is vertical, and it is the upper flank, consisting of the upper and lower halves of the pulvinus (on which the vertical lines of gravity impinge) that undergoes effective stimulation.
Algebraical summation of geotropic and phototropic effects: Experiment 194.—We are, however, able to adduce further tests in confirmation of the above. If the direction of the incident geotropic stimulus is vertical, and should it act more effectively on the upper flank, it follows that stimulus of light acting from above would enhance the previous torsional response due to geotropism. In the above case, the lines of gravity and the rays of light coincide. The effect of rays of light acting from below should, on the other hand, oppose the geotropic torsion. The additive effect of stimulus of light and gravity is seen illustrated in figure 180. The first part of the curve is the record of pure geotropic torsional movement. Light from above isapplied at L; the rate of movement is seen to become greatly enhanced. Light is next cut off, and the enhanced rate induced by it is also found to disappear, the response-curve being now due solely to geotropic action. The effect of geotropism in opposition to phototropism will be found in the following experiments, where the opposing action of light of different intensities is seen to give rise to a partial, to an exact, or to an over-balance.
Fig. 180.Fig. 181.Fig. 180.Fig. 181.Fig. 180.—Additive effect of stimulus of gravity G, and of light L. Application of light at—L increases torsional response. Removal of light restores original geotropic torsion.Fig. 181.—Algebraical summation of geotropic and phototropic actions. Light applied below at -L, opposes geotropic action. Cessation of light restores geotropic torsion. Cessation of light is indicated by L within a circle.
Fig. 180.
Fig. 181.
Fig. 180.Fig. 181.
Fig. 180.—Additive effect of stimulus of gravity G, and of light L. Application of light at—L increases torsional response. Removal of light restores original geotropic torsion.
Fig. 181.—Algebraical summation of geotropic and phototropic actions. Light applied below at -L, opposes geotropic action. Cessation of light restores geotropic torsion. Cessation of light is indicated by L within a circle.
Fig. 182.Fig. 182.—Application of white light at -L in opposition causes reversal of torsion. Red light R, is ineffective, and geotropic torsion is restored. Reapplication of white light causes once more the reversal of torsion.
Fig. 182.—Application of white light at -L in opposition causes reversal of torsion. Red light R, is ineffective, and geotropic torsion is restored. Reapplication of white light causes once more the reversal of torsion.
Photo-geotropic balance: Experiment 195.—I shall here describe in detail the procedure for obtaining an exact balance. A parallel beam of light from a small arc lamp is reflected by means of an inclined mirror, so as to act on the pulvinus below. An iris diaphragm regulates the intensityof incident light. The first part of the curve is the record of geotropic torsional movement. Light of a given intensity was applied below at a point marked -L (Fig. 181); this is seen to produce an over-balance, the phototropic effect being slightly in excess. The intensity of incident light was continuously diminished by regulation of the diaphragm till an exact balance was obtained as seen in the horizontal part of the record. It is with great surprise that one comes to realise the fact that the effect of one form of stimulus can be so exactly balanced by that of another, so entirely different, and that the stimulus of gravity could be measured, as it were, in candle powers of light! After securing the balance, light was cut off, and the geotropic torsion became renewed on the cessation of the counteracting phototropic action.
Fig. 183.Fig. 183.—Effect of coal gas on photo geotropic balance. Geotropic torsion, G, is exactly balanced by opposing action of light -L. Application of coal gas at C, at first caused enhancement of phototropic action with resulting reversal. Prolonged application induced depression of phototropic reaction, geotropic action thus becoming predominant.
Fig. 183.—Effect of coal gas on photo geotropic balance. Geotropic torsion, G, is exactly balanced by opposing action of light -L. Application of coal gas at C, at first caused enhancement of phototropic action with resulting reversal. Prolonged application induced depression of phototropic reaction, geotropic action thus becoming predominant.
Comparative balancing effects of white and red lights: Experiment 196.—White light was at first applied at -L in opposition to geotropic movement. The intensity of light was stronger than what was necessary for exact balance, and its effect was at first to retard and then reverse the torsional response due to geotropism. When thus overbalanced, red glass was interposed on the path of light at R. As the phototropic effect of this light is feeble or absent, the geotropic torsion became predominant as seen in the subsequent up-curve. The red glass was next removed substituting white light at -L to act once more in opposition; the result is seen in the final over-balance, and reversal of torsion (Fig. 182).
Effect of coal gas on the balance: Experiment 197.—The method of balance described above opens out new possibilities in regard to investigations on the relative modifications of geotropic and phototropic excitabilities by a given external change. Traces of coal gas are known to enhance the phototropic excitability of an organ while continued absence of oxygen is found to depress it. The experiment I am going to describe shows: (1) the enhancement of phototropic excitability on the introduction of coal gas, and (2) the depressing effect of excess of coal gas and of the absence of oxygen. After obtaining the normal curve of geotropic torsion, light was applied below at -L, and exact balance was obtained in the course of two minutes as seen in the top of the curve becoming horizontal. Coal gas was now introduced in the plant-chamber at C. This induced an enhancement of phototropic effect with resulting over-balance seen in the reversal of torsion. This enhancement persisted for more than three minutes. By this time the plant-chamber was completely filled with coal gas, and the resulting depression of phototropic action is seen in the second upset of the balance, this time in favour of geotropic torsion (Fig. 183). It would seem that the cells which respond to light are situated nearer the surface of the organ than those which react to geotropic stimulus. Hence an agent which acts on the organfrom outside, induces phototropic change earlier than variation in geotropism.
Under lateral action of geotropic stimulus, a dorsiventral organ undergoes torsional response by which the less excitable half of the organ is made to face the stimulus.
The direction of incident geotropic stimulus is the same as the direction of vertical lines of gravity. Under geotropic stimulus it is the upper side of the organ that undergoes effective stimulation.
The effects of gravity and of light become algebraically summated under their simultaneous action. Light may be made to act in opposition to the stimulus of gravity. By suitable adjustment of the intensity of light, the two torsions become exactly balanced.
This state of balance is upset by any slight variation in one of the opposing stimuli.
The relative modification of geotropic and phototropic excitabilities by an external agent, is determined by the resulting upset of the photo-geotropic balance.
I shall in this chapter investigate the effect of variation of temperature on geotropic response. We have to bear in mind in this connection, that for the exhibition of geotropic curvature two conditions are necessary: (1) the presence of a perceptive organ to undergo excitation under the stimulus of gravity, and (2) the motility of the organ. A motile organ, including both the pulvinated and growing, will exhibit no geotropic effect on account of the depression of the power of perception through seasonal or other changes, or in the entire absence of the perceptive organ. The organ may, on the other hand, possess the geo-perceptive apparatus, but no visible movement can take place in the absence of motility of the tissue.
As regards the modifying influence of temperature on geotropic curvature, the effect will depend on two factors:
(1) the influence of variation of temperature on geo-perception by the sensitive layer, and(2) the modifying effect of temperature variation on the motile reaction.
(1) the influence of variation of temperature on geo-perception by the sensitive layer, and
(2) the modifying effect of temperature variation on the motile reaction.
Fig. 184.Fig. 184.—Magnet M causes deflection of the needlen s, suspended by a thin wire. Increase of magnetisation of M increases deflection, while decrease of magnetisation diminishes the deflection.
Fig. 184.—Magnet M causes deflection of the needlen s, suspended by a thin wire. Increase of magnetisation of M increases deflection, while decrease of magnetisation diminishes the deflection.
I have in Chapter XLIII adduced facts which appear to show that the power of geo-perception declines at high temperatures. As regards motile reaction, we have seen that inMimosait increases from a minimum to an optimum temperature beyond which there is a depression (p. 55). As the optimum temperature for geo-perception is not necessarily the same as that for responsive curvature, the result is likely to be very complex.
The case becomes simpler after the attainment of maximum curvature. Enhanced temperature has a tendency to diminish the tropic curvature, as we found in the arrest and reversal of phototropic curvature under the application of warmth (p. 393); it appears as if rise of temperature induced a relatively greater expansion of the contracted side of the organ.
I shall now describe the effect of rising temperature on geotropic curvature in general, including torsion. A horizontally laid shoot curves upwards under geotropic action; a dorsiventral organ, owing to the differential excitabilities of its upper and lower sides, places itself in the so-called dia-geotropic position. A dorsiventral organ, moreover, exhibits a torsional movement under lateral stimulus of gravity.
In the geotropic movements we are able, as stated before, to distinguish three different phases (cf. Fig. 161). In the first, the movement initiated undergoes an increase; in the second, the rate of movement becomes more or less uniform; and in the last phase, a balance takes place between the tropic reaction, and the increasing resistance of the curved or twisted organ to further distortion.
The question now arises whether this position of geotropic equilibrium is permanent, or whether it undergoes modification in a definite way by variation of temperature. I shall proceed to show that the position of equilibrium undergoes a change in one direction by a rise, and in the opposite direction by a fall of temperature. I shall use the termthermo-geotropismas a convenient phrase to indicate the effect of temperature in modification of geotropic curvature and torsion.
I shall first deal with the effect of variation of temperature on geotropic torsion. Under the continued action of stimulus of gravity the torsion increases till it reachesa limit; for the twisted organ resists further distortion and a balance is struck when the twisting and untwisting forces are equal and opposite. In this state of equilibrium the effect of an external agent, say of variation of temperature, will bring about an upset of the balance. The torsion will be increased if the external agent induces an enhancement of geotropic action; it will, on the other hand, be decreased when it induces a diminished reaction.
A physical analogy will make this point clear; imagine a small magnetic needle suspended by a thin wire; the earth's directive force is supposed to be annulled by the well known device of a compensating magnet. A second and larger magnet M is now placed at right angles to the suspended needle; N will repelnand attracts, and a deflection will be produced, the deflecting force of the magnet M being balanced by the force of torsion of suspending wire (Fig. 184).
The state of equilibrium will however be disturbed by variation of the magnetic force of M. It is known that a rise of temperature diminishes magnetisation while lowering of temperature increases it. Hence the deflectingforce of the magnet will be diminished under rise of temperature with concomitant diminution of deflection of the needle and the torsion of the wire. Fall of temperature, on the other hand, will cause an increase of deflection and of torsion. The physical illustration given above will help us to understand how the physiological effect of variation of temperature may bring about changes in geotropic curvature and torsion.
The following experiment will show that the position of tropic equilibrium is not fixed but subject to variation under changes of effective stimulation.
Fig. 185.Fig. 185.—Effect of variation of intensity of light on phototropic equilibrium. Increase of intensity of light from L to L' produces an increased positive curvature and a new state of balance. Diminished intensity of lightlbrings about a new balance at a lower level. The cessation of light (lwithin a circle) restores the normal position of the organ.
Fig. 185.—Effect of variation of intensity of light on phototropic equilibrium. Increase of intensity of light from L to L' produces an increased positive curvature and a new state of balance. Diminished intensity of lightlbrings about a new balance at a lower level. The cessation of light (lwithin a circle) restores the normal position of the organ.
Experiment 198.—I have explained how a maximum tropic curvature is induced under continued action of light. Employing the pulvinus ofErythrina indicaI applied light on the upper half of the pulvinus: (1) of medium intensity L, (2) of strong intensity L', and (3) of feeble intensityl. The source of light was an arc lamp; theintensity of light was varied by means of a focussing lens, which gave a parallel, a convergent or a divergent beam, with corresponding increase or diminution of intensity of light. Light was in each case continued till equilibrium was reached. Inspection of figure 185 shows that the position of equilibrium depends on the intensity of stimulation; the balance is 'raised' under increased and 'lowered' under decreased intensity.
In the case of geotropism the stimulus is constant, but its tropic effect, we shall presently see, undergoes variation with changing temperature.
Fig. 186.Fig. 186.—Effect of variation of temperature on geotropic torsion. Application of warmth at H diminishes the geotropic torsion; return to normal temperature (H) restores the original torsion; cooling at C, increases the geotropic torsion.
Fig. 186.—Effect of variation of temperature on geotropic torsion. Application of warmth at H diminishes the geotropic torsion; return to normal temperature (H) restores the original torsion; cooling at C, increases the geotropic torsion.
Modification of geotropic torsion: Experiment 199.—TheMimosaplant was placed on its side, so that the pulvinus was subjected to lateral geotropic action. In response to this it underwent torsion, the upper half of the pulvinus tending to place itself so as to face the vertical lines of gravity. This torsional response was recorded as an up-movement; on the attainment of equilibrium the record became horizontal. The plant was now subjected to a cyclic variation of temperature, and the resulting variation of torsion recorded at the same time. The temperature of the plant chamber was gradually raised from the normal 30° to 34° C. and then allowed to return to the normal; finally the temperature was lowered to 26°C. Rise of temperature was effected by means of an electrical heater placed inside the chamber with a vessel of water placed above it. Care has to be taken that the rise of temperature is gradual, since a sudden variation often acts as a stimulus. The water in the vessel not only keeps the chamber in a humid condition but also prevents sudden fluctuation of temperature. After the temperature had been raised to 34°C., the heating current was stopped and thedoor of the plant chamber gradually opened, so as to allow the temperature to be restored to the normal. Cooled air was next introduced into the chamber till the temperature fell to 26°C. Figure 186 exhibits clearly the effect of variation of temperature on geotropic torsion. The maximum torsion had been attained at 30°C. and the first part of the record is therefore horizontal. Warmth was applied at H, and after a latent period of ten minutes, the geotropic torsion underwent a continuous diminution till a new state of equilibrium was reached at 34°C. This took place shortly after the stoppage of the heating current at (H). On return to normal temperature the torsional balance was restored to its original position of equilibrium. Application of cold at C, is seen to bring about a new state of balance with an increase of geotropic torsion.
The position of geotropic equilibrium is thus seen to be modified by variation of temperature, the tropic effect being diminished with the rise, and enhanced with the fall of temperature.
It may be thought that the phenomenon just described may not be different from ordinary thermonasty, exhibited by the perianth leaves ofCrocusandTulipin which a rise of temperature induces a movement of unfolding, and a fall of temperature brings about the opposite movement of closure. In these cases the movement is determined solely by the natural anisotropy of the organ, and not by the paratonic action of a directive external force. Thus the inner side of the perianth leaves undergoes an expansion with rise of temperature attended by the opening of the flower; this movement of opening does not undergo any change on holding the flower in an inverted position.
But the torsional movement of the leaf ofMimosa, and the induced variation of torsion under change of temperature are not solely determined by the natural anisotropy of the organ; it is, on the contrary, regulated by the directive action of the stimulus of gravity. The pulvinus in normal position does not exhibit any geotropic torsion and in the absence of an antecedent torsion change of temperature cannot induce any variation in it. It is only after the pulvinus had become torsioned under the lateral action of geotropic stimulus that a responsive variation is induced in it by the action of changing temperature.
The change in torsion is, moreover, determined in reference to the paratonic action of incident geotropic stimulus. This will be clearly understood from the tabular statement given below.
TABLE XLVI.—SHOWING THE EFFECT OF RISE OF TEMPERATURE ON GEOTROPIC TORSION.
Position of the organ.Geotropic effect.Effect of rise of temperature.Right flank above:Right-handed torsion.Left-handed torsional movement (untwist).(a) position.Left flank above:Left-handed torsion.Right-handed torsional movement (untwist).(b) position.
By right flank in the above table is meant the side of the pulvinus to the right of the observer facing the leaf of the plant held in the normal position. When the plantis laid on its left side in thea-position, the right flank will be above and the responsive torsion under geotropic stimulus becomes right handed or with the hands of a clock (Cf. Fig. 179). When the plant is laid on its right side, the left flank will be above and the geotropic torsion becomes left handed or against the hands of the clock.
It will be seen from the above that in whatever way the experimental condition may be varied, the movement in response to variation of temperature is determined in relation to the antecedent geotropic torsion. The geotropic effect whether left-handed or right-handed torsion is always diminished by the rise of temperature, and enhanced by the fall of temperature.
I shall now proceed to show that variation of temperature not merely induces variation of geotropic torsion but also of geotropic curvature. I shall first demonstrate the effect of thermal change on geotropic curvature of the shoot, and then demonstrate its effect on dia-geotropic curvature of leaves.
Experiment 200.—A specimen ofTropæolum majusgrown in a small flower pot, is laid on its side. Under geotropic action the shoot becomes curved, the upper side becoming concave and the lower side convex. The end of the stem is attached to the recording apparatus; when the plant is subjected to a rise of temperature, the movement induced shows that the geotropic effect has undergone a diminution, the curvature exhibiting a flattening; lowering of temperature, on the other hand, increases the geotropic curvature. Other instances of this will be found in a subsequent chapter. The diurnal movement of the 'Praying Palm' is a striking example of the effect of variation of temperature in modification of geotropic curvature (p. 30).Rise of temperature is thus shown to diminish geotropic torsion of dorsiventral organs, and the apo-geotropic curvature of radial organs. We have next to study the effect of temperature variation on the dia-geotropic equilibrium of leaves.
In the normal position of the plant, the leaf ofMimosaassumes, under geotropic action, an equilibrium position which is approximately horizontal. I shall proceed to show that this position of equilibrium also undergoes appropriate variation under changing temperature, the leaf undergoing a fall during rise, and an erection during fall of temperature.
I stated that the torsional response is one of the means of recording geotropic effect and its variations. In the ordinary position of the plant, the geotropic variation will be indicated by the responsive up or down movement of the leaf in a vertical plane. Taking the leaf ofMimosa, we have thus the means of studying the effect of variation of temperature by two independent means of inquiry, namely, by record of ordinary responsive movement in a vertical plane, and also by record of torsional response. The variation of temperature which induces these movements may be simultaneously recorded by means of a differential metallic thermometer. The Multiplex Recorder employed for this research consists of three recording levers. A photographic reproduction of the apparatus will be found in a subsequent chapter (see Fig. 190). The first lever is attached to the leaf ofMimosaplaced in the normal position; the second lever records the torsional response ofMimosaleaf, the plant being placed on its side; the third lever attached to the differential metallic thermometer gives a continuous record of variation of temperature.
Fig. 187.Fig. 187.—Simultaneous record (a) of variation of temperature, (b) of up or down movement of leaf ofMimosa, and (c) of variation of torsion. Rise of temperature is attended by fall of leaf and diminution of torsion, fall of temperature inducing the opposite effect.
Fig. 187.—Simultaneous record (a) of variation of temperature, (b) of up or down movement of leaf ofMimosa, and (c) of variation of torsion. Rise of temperature is attended by fall of leaf and diminution of torsion, fall of temperature inducing the opposite effect.
Effect of variation of temperature: Experiment 201.—Special arrangement was made for gradual variation of temperature in the plant chamber. Two rectangular metallic vessels each 50 × 30 × 6 cm. were placed on opposite sidesof the plant chamber, and warm water was made to circulate through them; this device ensured a steady rise of temperature. The flow of warm water was then stopped and the plant chamber was allowed to cool down; the fall of temperature was at first moderately rapid, but later on the rate of cooling became extremely slow; on account of this the temperature of the plant chamber, towards the end of the experiment remained higher than the normal temperature outside. The rate of rise and fall of temperature during the entire course is illustrated in the thermo-graphic (a) tracing (Fig. 187); the record (b) exhibits the movement of the leaf in a vertical plane, rise of temperature being attended by a diminution of geotropic curvature resulting in the fall of the leaf, the fall of temperature inducing the opposite effect. In record (c) is seen the responsive variation of geotropic torsion, rise of temperature inducing a diminution and fall of temperature causing an enhancement of torsion. The results obtained by diverse methods thus prove that the geotropic effect is diminished under rise, and increased under fall of temperature.
The position of equilibrium under geotropic action is not fixed but undergoes change with variation of temperature.
The geotropic curvature and torsion are increased by lowering of temperature, and decreased by rise of temperature. This is equally true of apo-geotropic and dia-geotropic curvatures.
The subject has long been a perplexing one, and its literature is copious. After a good many years of experimental investigation, I have succeeded in analysing the main factors concerned in the many phenomena which have been described as Nyctitropism. The results of the researches are given in a sequence of five papers, which may be read separately, yet will be seen as so many chapters of what has been a single though varied investigation.
The different chapters are:
1. Daily movements in relation to Light and Darkness.2. Daily movements due to Variation of Temperature affecting Growth.3. Daily movements due to Variation of Temperature affecting Geotropic Curvature.4. The Immediate and After-effect of Light.5. Diurnal Movement of the leaf ofMimosadue to combined effects of various factors.
1. Daily movements in relation to Light and Darkness.
2. Daily movements due to Variation of Temperature affecting Growth.
3. Daily movements due to Variation of Temperature affecting Geotropic Curvature.
4. The Immediate and After-effect of Light.
5. Diurnal Movement of the leaf ofMimosadue to combined effects of various factors.
Nyctitropic movements are thus described by Jost[39]:
"Many plant organs, especially foliage and floral leaves take up, towards evening, positions other than those they occupy by day. Petals and perianth leaves, for example, bend outwards by day so as to open the flower, and inwards at night so as to close it.... Many foliage leaves also may be said to exhibit opening and closing movements, notmerely when they open and close in the bud but also when arranged in pairs on an axis, they exhibit movements towards and away from each other. In other cases, speaking generally, we may employ the termsnight positionandday positionfor the closed and open conditions respectively. The night position may also be described as thesleep position." After reviewing the various theories proposed, he proceeds to say "that a completely satisfactory theory of nyctitropic pulvinus movements is not yet forthcoming. Such a theory can only be established after new and exhaustive experimental research."
The difficulties of the experimental reinvestigation here called for towards clearing up and explanation of the subject are sufficiently great; they are further increased by the fact that these diurnal movements may be brought about by different agencies independent of each other. Thus inCrocusand inTulip, the movement of opening during rise of temperature has been shown by Pfeffer to be due to differential growth in the inner and outer halves of the perianth. I shall in this connection show that a precisely opposite movement of closing is induced inNymphæaunder similar rise of temperature. I shall for convenience distinguish the differential growth under temperature variation asThermonastyproper. Again certain leaflets open in light, and close in darkness in the so-called sleep position. Intense light, however, produces the 'midday sleep'—an effect which is apparently similar to that of darkness. The determining factor of these movements is the variation of light.
There are other instances of diurnal movement, far more numerous, which cannot be explained from considerations given above. It has therefore been suggested that the "Day and night positions may arise by the combined action of geotropism and heliotropism. Thus Vochting (1888) observed in the case ofMalva verticillatta, thatthe leaves, when illuminated from below, turned their laminæ downwards during the day, but during the night became erect geotropically. The sleep movements in leaves and flowers, referred to above, cannot however be explained by assuming such a combination of heliotropism and geotropism."[40]
I commenced my investigation on nyctitropism five years ago, after having perfected an apparatus for continuous record of the movements of plants throughout day and night. A contrivance, described further on, has been devised for obtaining a record of diurnal variation of temperature. I have also succeeded recently, in perfecting a device for automatic record of variation of intensity of light. It has thus been possible not only to obtain a continuous record of the diurnal movement of the plant, but also obtain simultaneous record of those changes in the environment which might have an influence on the daily movement. I have in this way collected several hundred autographs of different plants throughout all seasons of the year. The records thus obtained were extremely diverse, and it was at first impossible to discover any fundamental reaction which would explain the phenomenon. While in this perplexity my attention was directed two years ago to the extraordinary performances of the "Praying Palm" of Faridpore, in which the geotropic curvature of the tree underwent an accentuation during fall of temperature, and a diminution during rise of temperature.
The discovery of this new phenomenon led me to the inquiry whether Thermo-geotropic reaction, as I may call it, was exerted only on Palm trees, or whether it was aphenomenon of universal occurrence. I therefore extended my investigation on various geotropically curved procumbent stems ofIpœmia,Basella, and ofTropæolum majus. Here also I found that diurnal variation of temperature induced a periodic movement exactly similar to that in Palm trees.
I next wished to find whether the Thermo-geotropic reaction observed in stems was also exhibited by lateral organs such as leaves, which being spread out in a horizontal direction are subjected to the stimulus of gravity. I found that in a large number of typical cases, a periodic movement took place which was exactly similar to that given by rigid trees and trailing stems. A standard curve was thus obtained which was found to be characteristic not only of trees and herbs, but also of leaves. The stem and leavesfellcontinuously with the rise of temperature, from the minimum at about 6 in the morning to the maximum at about 2 p.m. They erected themselves with falling temperature from 2 p.m. to 6 a.m. next morning.
In the diurnal record ofMimosaI met, however, with an unaccountable deviation from the standard curve, for which I could not for a long time find an adequate explanation. Subsequent investigations showed that the deviation was due to the introduction of additional factors of variation, namely of immediate and after-effects of light.
I have already referred to the great difficulty of explanation of nyctitropism from the fact that the diurnal movements may be brought about by different agencies independent of each other. It is, moreover, not easy to discriminate the effect of one agency from that of the other.
The combined effects of different factors will evidently be very numerous. This will be understood from consideration of the number of possible combinations with only two variables, geotropism and phototropism. The effect of geotropism may be strongG, or feeble,g. Similarly we may have strong effect of lightL, or feeble effect of lightl. Light may exert positive phototropic action +Lor negative action -L. Thus from two variables we obtain the following eight combinations:
G+L;G-L;G+l;G-l;g+L;g-L;g+l;g-l.
The number of possible variables are, however, far more numerous as will be seen from the following:
Geotropism.—The effect of geotropic stimulus on horizontally placed organs is one of erection. But this stimulus, which is constant, cannot by itself give rise to periodic movements. It has however been shown that variation of temperature has a modifying influence on geotropic curvature (p. 519).
Phototropism.—The action of unilateral light is to induce a tropic curvature, which in some cases is positive, in others negative (p. 386). In addition to these effects induced during the incidence of light, we have to take account of the after-effects on the cessation of light.
After-effects of light.—I find two very different effects, depending on the intensity and duration of previous illumination. Of these the most important is the phenomenon of 'overshooting' which occurs on the cessation of light of long duration. This particular reaction, to be fully described, will be found to offer an explanation of certain anomalous effects in diurnal movement.
Periodic variation of turgor.—I have shown (p. 39) that artificial enhancement of turgor in the plant inducesan erectile movement of the leaf ofMimosa, diminution of turgor inducing the opposite movement of fall. Kraus and Millardet have shown that a diurnal variation of tension takes place in the shoot of all plants, which is presumably indicative of variation of turgor. This variation of turgor in the shoot must have some effect on the lateral leaves. But the leaves are subjected to conditions which are absent in the stem. The erect stem is, for example, free from geotropic action, whereas the lateral leaf is subject to it. The effect of turgor variation in the shoot on the movement of leaves may be, and often is, overpowered by the predominant geotropic action. I shall, later on, refer to this question in greater detail.