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. XXXVII.--RADIO-THERMOTROPISM _By_ SIR J. C. BOSE, _Assisted by_ GURUPRASANNA DAS. 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] [24] Pfeffer--_Ibid_--Vol. III, p. 776. "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 of _Lepidium sativum_ and _Zea Mays_, as well as sporangiphores of _Phycomyces_ curved towards a hot iron plate emitting dark heat-rays. Steyer has, however, shown that the sporangiphore of _Phycomyces_ has no power of thermotropic reaction.... Wortmann observed that the seedling shoot of _Zea Mays_ was positively, but that of _Lepidium_ negatively, 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."