CHAPTER XLVII 



TORSIONAL RESPONSE TO HELIOTROPIC AND GEOTROPIC 

 STIMULUS: AUTONOMOUS TORSION AND ITS VARIATIONS 



Torsional effect — Method of recording torsional response — Torsional response 

 under the lateral action of light — Torsional response to other forms of lateral 

 stimulation — Torsional response of compound strip of ebonite and stretched 

 india-rubber — Modification of torsional response by artificial variation of the 

 relative excitabilities of the two halves — Laws of torsional response — Demon- 

 stration of differential geotropic excitation in a dorsi ventral organ — Torsional 

 response to lateral geotropic stimulation — Modification of torsional geotropic 

 response by artificial variation of differential excitability — Autonomous torsion : 

 effect of temperature — Effect of light— Effect of electrical current — Effect of 

 gravity — The twining of stems. 



It has been shown in the course of the last chapter that the 

 movement of the leaf in response to light constitutes a 

 simple instance of that general reaction of plant tissues to 

 stimulus with which we have now become familiar, and that 

 no specific sensibility requires to be postulated in order to 

 account for it. 



Torsional effect. — There is one effect, however, which 

 has hitherto appeared to be inexplicable, except on the 

 supposition that some such specific sensibility had actually 

 been acquired by the leaf for the definite purpose of sub- 

 serving the advantage of the plant by placing the upper 

 surface of the lamina at right angles to incident light. A 

 leaf when struck laterally by light undergoes a torsion, which 

 carries the upper surface of the lamina into such a position 

 that it faces the light. No result could seem at first sight 

 more conclusively to support the theory of dia-heliotropic 

 sensibility. Before going further into this question, I shall 

 give records of the actual effect observed. In fig. 265 is 

 shown a curve which exhibits the increasing torsion induced 



