RESPONSIVE CURVATURE OF ANISOTROPIC ORGAN 89 



by the unilateral stimulus of light, we shall next proceed to 

 consider the similar effects induced by unilateral mechanical 

 stimulation. We shall find, if we touch a tendril unilaterally, 

 that it responds to this one-sided stimulus by the concavity 

 of the excited side, and we obtain a curvature. After a 

 more or less prolonged contact, this curvature becomes fairly 

 permanent. Thus, by means of unilateral excitation, the 

 originally radial tendril — like the unilaterally excited plagio- 

 tropic organ — has been made to become bilateral and 

 anisotropic, and the excited concave surface should now be 

 relatively less excitable than the convex. 



A tendril of Passiflora was taken, in which a curvature 

 of half a spiral had been induced by stimulus of unilateral 

 contact. The straight lower end of the tendril was now fixed 

 in a clamp, the hooked end being attached to the Optic 

 Lever. A spiral of tinsel made one of the electrical contacts 

 at the hooked end, the other being made at the clamp. An 

 electrical shock of moderate intensity was now passed through 

 the length of the tendril. From what has been said already, 

 we should expect that diffuse excitation would now produce 

 concavity of the more excitable, or convex, side of the hooked 

 tendril. We should expect, in other words, that the electric 

 stimulation would have the effect of undoing the existing 

 curvature, or straightening out the curved tendril. Such a 

 responsive uncurling would, if it occurred, relax the tension 

 on the Lever, and cause a concomitant movement upwards 

 of the spot of light. On the cessation of stimulus, again 

 provided this have been not too strong, there should be a 

 restoration of the tendril to its original curvature. Greater 

 intensity of stimulus, or stimulus of longer duration, should, 

 other things being equal, produce greater responsive move- 

 ment. And the recovery from such stronger stimulation 

 would require a relatively longer time. 



All these theoretical considerations are found fully verified 

 in the record given below (fig. 42). It will there be seen that 

 a stimulus of moderate electric shock, lasting fifteen seconds, 

 produced a correspondingly moderate response of fourteen 



