TORSIONAL RESPONSE TO STIMULUS 675 



The general law of induced torsional response is that the 

 less excitable side of the organ is made to face the incident 

 stimulus. If the excitability of the lower half of the pulvinus 

 be artificially abolished by the local application of chloro- 

 form, the torsional responsive movement is reversed. The 

 leaf now executes a movement by which its lower surface is 

 made to face the light. 



On artificially increasing the natural difference, as between 

 the two halves, by altogether abolishing the excitability of 

 the upper, the normal torsional response is accelerated. 



That the upper and lower halves of a dorsi-ventral organ 

 are unequally sensitive to geotropic stimulus is shown by the 

 fact that they give torsional response, under which the 

 less excitable half is carried so as to face the incident lines 

 of gravitational force. An artificial increase of this natural 

 difference of excitability has the effect of accelerating the 

 rate of torsional response. The artificial reversal, on the 

 other hand, of these relative excitabilities has the effect 

 of reversing the direction of the natural responsive move- 

 ment. 



From these considerations it is seen that none of the 

 responsive movements caused in dorsi-ventral organs by 

 light or gravitation are due to any specific dia-heliotropic or 

 dia-geotropic sensibility, and that they are in reality due 

 to the differential excitability of the two halves of the organ. 

 The autonomous torsional movement of growth increases 

 in rate up to an optimum temperature, after which it begins 

 to decrease. There may be a second acceleration after the 

 attainment of the first minimum. 



This autonomous torsional movement is retarded or even 

 reversed in sign by unilateral stimulus of light. A constant 

 current, flowing along the length of the organ, often retards 

 or reverses the normal autonomous torsion. The stimulus 

 of gravity also has the effect of retarding or reversing normal 

 torsional movement. The effective intensity of this stimulus 

 appears to be greatest when the organ acted upon is in a 

 position at 180° from the vertically upright. 



