Life Movements in Plants Part 26

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.

XLIV.--ON GEOTROPIC TORSION

_By_

SIR J. C. BOSE,

_a.s.sisted by_

GURUPRASANNA DAS.

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.

[Ill.u.s.tration: 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 of _Mimosa_ is 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, _a_ and _b_. In the _a-position_, the apex of the stem and the upper half of the pulvinus are to the left of the observer, and in _b-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 under _a-position_ is shown in figure 179.

_Torsional response in a- and b-positions: Experiment 193._--When the leaf is in _a-position_, the geotropic torsion is found to be with the movement of the hands of a clock. In the _b-position_, on the other hand, the torsion is against the hands of a clock. In both these cases the _geotropic 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 ill.u.s.trated in figure 180. The first part of the curve is the record of pure geotropic torsional movement. Light from above is applied 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.

[Ill.u.s.tration: 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.]

[Ill.u.s.tration: 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.]

BALANCE OF GEOTROPIC BY PHOTOTROPIC ACTION.

_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 intensity of 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.

[Ill.u.s.tration: 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.]

_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 r.e.t.a.r.d and then reverse the torsional response due to geotropism. When thus overbalanced, red gla.s.s 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 gla.s.s was next removed subst.i.tuting 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).

[Ill.u.s.tration: 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.]

_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 organ from outside, induces phototropic change earlier than variation in geotropism.

SUMMARY.

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.

XLV.--ON THERMO-GEOTROPISM

_By_

SIR J. C. BOSE.

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.

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 in _Mimosa_ it 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 term _thermo-geotropism_ as 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 reaches a 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.

[Ill.u.s.tration: FIG. 184.--Magnet M causes deflection of the needle _n s_, suspended by a thin wire. Increase of magnetisation of M increases deflection, while decrease of magnetisation diminishes the deflection.]

A physical a.n.a.logy 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 repel _n_ and attract _s_, 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 deflecting force 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 ill.u.s.tration given above will help us to understand how the physiological effect of variation of temperature may bring about changes in geotropic curvature and torsion.

TROPIC EQUILIBRIUM UNDER VARYING INTENSITIES OF STIMULUS.