THE LESS EXCITABLE SIDE 97 



1. The proximal side contracts because it is directly 



stimulated, the expansion of the distal side being 

 due to indirect stimulation. The curvature is 

 brought about by the joint action of contraction on 

 the one side and expansion on the opposite side. 



2. The recovery of the tendril after brief stimulation is 



hastened by the after-effect of stimulation, which 

 is an active expansion and enhancement of growth 

 {cf. Experiments 53, 54). . 



The contraction of the directly excited side, and the 

 expansion of the indirectly stimulated distal side of the 

 organ, will explain Fitting's important observation ^ that 

 in a unilaterally stimulated tendril there is (i) an accelera- 

 tion of growth on the convex side, and (2) a contraction 

 on the concave side. Fitting ascertained that the tendril 

 became straightened by the renewal of active growth on the 

 excited side. 



Response of the Less Excitable Side of Tendril 



It is generally supposed that in the tendril of Passifiora 

 the upper side is devoid of moto-excitability. My experi- 

 ments show, however, that direct stimulation does induce 

 contraction and concavity of that side, though the actual 

 movement is relatively feeble, as shown below. 



Experiment 58.— Feeble stimulus of the same intensity 

 was applied on the upper and under sides of the tendril 

 alternately. Successive stimuli were rendered more or less 

 uniform by the following device. A flat strip i cm. in 

 breadth was coated 2 cm. of its length with shellac varnish 

 mixed with fine emery powder. On drying, the surface 

 became rough ; the flat surface was then gently pressed 

 against the area of the tendril to be stimulated, and quickly 

 drawn sideways so as to rub the upper or the under side of 

 the tendril in each experiment. Stimulation, thus effected, 

 induced a responsive movement of both sides of the organ. 



1 PfefEer, ibid. voL iii. p. 57. 



n 



