Part 16 (1/2)

LAWS OF TORSIONAL RESPONSE.

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.

COMPLEX TORSION UNDER LIGHT.

The leaves of the so-called ”Compa.s.s plants” exhibit very complex movements, these being modified according to the intensity of incident light. Thus in compa.s.s 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 ”Compa.s.s 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 of_ Ca.s.sia 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 s.p.a.ce, are also exhibited by other leaflets which are closed at night in a lateral direction.

[Ill.u.s.tration: FIG. 150.--Leaflets of _Ca.s.sia 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 of _Ca.s.sia_ the 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 a _lateral_ direction. The disposition of the unequally excitable halves of the pulvinule is thus different from that of the main pulvinus of _Mimosa_. In the latter, the plane that divides the two halves is horizontal, the lower half being the more excitable.

Thus in the pulvinule of _Ca.s.sia_ the 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 very light 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 from _below_ with 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.

ADVANTAGES OF THE METHOD OF TORSIONAL RESPONSE.

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 of _Mimosa_ under light, the records were taken 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 gla.s.s support and the movement is thus free from the complicating factor of the weight of the leaf. Again the pulvinus of _Mimosa_, 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.

THE TORSIONAL BALANCE.

A beam of light falling on the left flank of the pulvinus of _Mimosa_ induces 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 pa.s.sing through blue gla.s.s falls on the left flank of the pulvinus; a beam of blue light also strikes the pulvinus from the right side, and the intensity of the latter is so adjusted that the resultant torsion is zero. Blue gla.s.s is now removed from the left side, the un.o.bstructed 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 gla.s.s 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.

DETERMINATION OF THE DIRECTION OF STIMULUS.