Dr. Bose then proceeded to investigate whether the responsive effects which he had shown to occur in ordinary plants might not be further exhibited by means of _visible mechanical response_, thus fully removing the distinction commonly assumed to exist between the 'sensitive' and supposed 'non-sensitive.' Dr. Bose invented 'special apparatus of extreme delicacy,' which detected infinitesimal tremors, and showed that ordinary plants, usually regarded as insensitive, gave _motile responses_, which had hitherto passed unnoticed. His later investigation shows that "all plants, even the trees, are fully alive to changes of environment; they respond visibly to all stimuli, even to the slight fluctuations of light by a drifting cloud."[17] 'TROPIC' MOVEMENTS Finding that the plants give, not only _electric_ but _motile_ response as well, to stimulus, Dr. Bose proceeded to study the nature of responses evoked in plants by the _stimuli of the natural forces_. He found that plants respond visibly, by movements, to _environmental stimuli_. But the movements induced--'tropic' movements--are extremely diverse. Light, for example, induces sometimes positive curvature, sometimes negative. Gravitation, again, induces one movement in the root, and the opposition in the shoot. Dr. Bose applied himself to find out whether the movements in response to external stimuli, though apparently so diverse, could not be ultimately reduced to a fundamental unity of reaction. As a result of a very deep and penetrating study of the effects of various environmental stimuli, on different plant organs, he showed that the cells on two sides are unequally influenced, on account of different external conditions, and contract unequally, and hence the various movements are produced--that the many anomalous effects, hitherto ascribed to 'specific sensibilities,' are due to the 'differential sensibilities'--differential excitability of anisotropic structures and to the opposite effects of external and internal stimuli--that all varieties of plant movements are capable of a consistent mechanical explanation. Dr. Bose's "latest investigations recently communicated to the Royal Society have established the single fundamental reaction which underlies all these effects so extremely diverse."[18] EXTENDED APPLICATION OF MECHANICAL THEORY With an extended application of his mechanical theory, Dr. Bose has gradually removed the veil of obscurity from many a phenomenon in plant life. The 'autonomous' movements of plants, for example, which remained enveloped in mystery, received a satisfactory solution at his hands. 'AUTONOMOUS' MOVEMENTS