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 passing through blue glass 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 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.
DETERMINATION OF THE DIRECTION OF STIMULUS.
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.
SUMMARY.
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.