TRANSVERSE TRANSMISSION OF EFFECT OF STIMULUS 2H.-5 



enhancement of turgor ; the excitatory negative impulse 

 then reached the distal point and caused a su(hlen fall of 

 the leaf (Fig. lOl). 



The experiments that have just been described are of 

 much significance. An organ like the stem of Mimosa, 

 since it exhibits no contraction, may appear insen- 

 sitive to stimulation ; but its perception of stimulus is 

 shown by its power of transmitting two characteristic 

 impulses, one of which is the positive, giving rise to an 

 enhancement of turgor, and the other, the true excitatory 

 negative, inducing the opposite reaction or tiiminution of 

 turgor. Unilateral stimulation gives rise to both these 

 effects in all organs : pulvinated, growang, and non-growing. 

 It was the fortunate circumstance of the insertion of the 

 motile leaf on one side of the Mimosa stem that enabled 

 us to demonstrate the important facts given above. 



The und^-rlying reactions, which give rise to tropic 

 curvature, could have been foretold from the Laws of 

 effects of Direct and Indirect stimulation, established in 

 previous chapters (pp. li>6, 216). The resulting curvature 

 is thus brought about by the joint effects of direct sti- 

 mulation of the proximal, and indirect stimulation of 

 the distal side. We may now recapitulate some of the 

 important facts relating to tropic curvatures : 



Indirect stimulation gives rise lo dual impulses, positive 

 and negative ; of these the positive impulse is practically 

 independent of the conducting power of the tissue ; but 

 the transmission of the excitatory negative impulse is 

 tlependent on the conducting power. No tissue is a perfect 

 conductor, nor is any a perfect non-conductor of excitation, 

 the difference is a question of degree. In a petiole or a 

 stem the conducting power along the direction of length is 

 considerable, but very feeble in a transverse direction. In 

 a semi-conducting tissue, a feeble stimulus will transmit 



