426 CHAP. XXXVII. GENERAL REVIEW 



the want of knowledge of the important part played by 

 direct and indirect stimulation, and by the conduction of 

 excitation across the organ. The directly stimulated 

 proximal side of the organ exhibits excitatory contraction, 

 electromotive change of galvanometric negativity, diminu- 

 tion of turgor and retardation of rate of growth ; the distal 

 side which is indirectly stimulated exhibits, on the other 

 hand, expansion, electromotive change of galvanometric 

 positivity, increase of turgor, and enhancement of the rate of 

 growth. Positive curvature towards the stimulus is thus 

 caused by the joint effects of the contraction of the proximal 

 and expansion of the distal side. The fact that stimulation 

 of one side of the organ causes an increase of turgor at the 

 diametrically opposite distal side has been demonstrated by 

 stimulation of one side of the stem of Mimosa, which caused 

 the erectile movement of the motile leaf on the opposite 

 side, indicative of an enhancement of turgor (p. 121). 



The tropic movement has been vaguely ascribed to 

 change of turgor ; but this cannot take place without a 

 definite and active force which determines the direction of 

 flow of sap, causing differential turgor at the two sides of 

 the organ. I have shown that the law which governs the 

 directive movement of sap is that it follows the stimulation 

 gradient from the stimulated to the unstimulated region. The 

 turgor is thus diminished at the directly stimulated proximal 

 side from which the sap is expelled, and increased at the 

 distal side where it is accumulated (p. 57). 



Transverse conduction of excitation across the organ in- 

 duces further modification of the normal positive curvature. 



Mechanotropism and Twining of Tendrils 



Under unilateral mechanical stimulation the directly 

 stimulated side of a tendril undergoes contraction, while 

 the indirectly stimulated distal side exhibits expansion ; 

 a positive curvature is thus produced with a movement 

 towards the stimulus, which results in the twining of the 



