+------------------------------------------------------------------+ |Stimulation. |Nature of the tissue. |Final effect. | +---------------+----------------------------+---------------------+ |Direct (Feeble)|Semi-conducting tissue. |Positive curvature. | |Indirect " | " " |Negative curvature. | |Direct (Strong)|Better conducting tissue. |Neutral or negative | | | |curvature. | |Indirect " | " " " |Negative followed by | | | |positive curvature. | +------------------------------------------------------------------+ The results of investigations already described, enable us to formulate the general laws of tropic curvature applicable to all forms of stimuli, and to all types of responding organs, pulvinated or growing. LAWS OF TROPIC CURVATURE. 1. (_a_) DIRECT APPLICATION OF UNILATERAL STIMULUS OF MODERATE INTENSITY, INDUCES A POSITIVE OR CONCAVE CURVATURE, BY THE CONTRACTION OF THE PROXIMAL AND EXPANSION OF THE DISTAL SIDE. (_b_) UNDER STRONG OR LONG-CONTINUED STIMULATION, THE POSITIVE CURVATURE IS NEUTRALISED OR REVERSED, BY TRANSVERSE CONDUCTION OF EXCITATION; THIS EFFECT IS ACCENTUATED BY THE DIFFERENTIAL EXCITABILITY OF THE TWO SIDES OF THE ORGAN. 2. (_a_) INDIRECT APPLICATION OF UNILATERAL STIMULUS OF FEEBLE INTENSITY INDUCES A NEGATIVE CURVATURE. (_b_) IN A CONDUCTING TISSUE THE EXCITATORY EFFECT BEING TRANSMITTED UNDER STRONG AND LONG CONTINUED STIMULATION, INDUCES A POSITIVE CURVATURE. It will thus be seen that the tropic effect is modified by: (1) the point of application of stimulus, (2) the intensity and duration of stimulus, (3) the conducting power of tissue in the transverse direction, (4) the relative excitabilities of the proximal and distal sides of the organ. In the following series of Papers the tropic effects of various forms of stimuli will be studied in detail. SUMMARY. In a semi-conducting tissue Direct stimulation induces a diminution of turgor and contraction, Indirect stimulation inducing the opposite effect of increase of turgor and expansion. Unilateral stimulation thus induces a positive curvature by the joint effects of contraction at the proximal, and expansion at the distal side. Under strong and long continued unilateral stimulation, the excitation at the proximal side is transmitted to the distal side. Transverse conduction thus neutralises or reverses the normal positive curvature. XXVI.--MECHANOTROPISM: TWINING OF TENDRILS _By_ SIR J. C. BOSE, _Assisted by_ GURUPRASANNA DAS.