SIR J. C. BOSE, _Assisted by_ SATYENDRA CHANDRA GUHA, M.Sc. The experiments that have been described in the preceding chapter show that the upper side of a horizontally laid shoot undergoes excitatory contraction, in consequence of which the organ bends upwards. The fundamental geotropic reaction is, therefore, not expansion, but contraction which results from all modes of stimulation. In confirmation of the above, I wished to discover and employ new means of detecting excitatory reaction under geotropic stimulus. In regard to this, I would refer to the fact which I have fully established that the state of excitation can be detected by the induced electromotive change of galvanometric negativity. This electrical indication of excitation may be observed even in plants physically restrained from exhibiting response by mechanical movement.[34] [34] "Comparative Electro-Physiology," p. 20. ELECTRIC RESPONSE TO STIMULUS. Before giving account of the results of investigations on the detection of geotropic excitation by means of electric response, I shall describe a few typical experiments which will fully explain the method of the electrical investigation, and show the correspondence of mechanical and electric responses. I have explained how tropic curvatures are brought about by the joint effects, of contraction of the directly excited proximal side A, and the expansion of the distal side B. In the diagram of mechanical response to stimulus (Fig. 164a) the excitatory contraction is indicated by - sign, and the expansion, by + sign. The resulting movement is, therefore, towards the stimulus as shown by the curved arrow. I shall now describe the corresponding electric effects in response to unilateral stimulus. We have to determine the induced electrical variation at the proximal side A, and at the distal side B. [Illustration: Fig. 164.--Diagrammatic representation of the mechanical and electrical response to direct unilateral stimulation indicated by arrow:-- (_a_) Positive mechanical response (curved arrow) due to contraction of directly stimulated A, and expansion of indirectly stimulated B. (_b_) Electric response of induced galvanometric negativity of A under direct stimulation. (_c_) Electric response of induced galvanometric positivity at the distal point B. (_d_) Additive effects of direct and indirect stimulations; galvanometric negativity of the directly stimulated proximal A, and galvanometric positivity of the indirectly stimulated distal point B.]