400 CHAP. XXXV. TORSION UNDER DIFFUSE STIMULATION 



the electric potential. I have found that a sudden variation 

 of temperature also acts as a stimulus. 



Experiment 217. — I have already explained that a 

 steady rise of temperature up to an optimum enhances the 

 rate of torsion. In the present experiment a specimen of 

 Clitoria was mounted within a double cylinder through 

 which a flow of water at the constant temperature of 25° C. 

 was maintained. The rate of normal anti-clockwise move- 

 ment was 12. Warmer water at 30° C. was now made to 

 flow through the cylinder, thus subjecting the plant to a 

 sudden variation of temperature. The result was an abrupt 

 reversal in the torsional movement from 12 to — 75- This 

 persisted for 2 minutes. As the temperature became steady, 

 the movement was gradually converted from negative to 

 positive. After half an hour it had become 14, slightly 

 greater than at the beginning. This is due to the fact that 

 the steady temperature inside the cylinder was now about 

 28° C. 



Effect of Mechanical Stimulation 



Experiment 218. Feeble stimulation. — The normal rate 

 of Thunbergia was 20, which was increased to 40 under the 

 application of a feeble mechanical stimulus. The recovery 

 to the normal rate of 20 was attained in 5 minutes. 



Experiment 219. Effect of stronger stimulation. — 

 Stimulus of friction was applied by rubbing the specimen 

 lengthwise with a piece of fine emery paper for about 

 3 minutes. The normal rate in Porana was 35 ; after the 

 frictional stimulation the rate was diminished to 3. The 

 specimen exhibited a recovery of the normal rate in 

 the course of 20 minutes. 



It has been shown (Experiment 50) that frictional 

 stimulation induces a general retardation of growth. In a 

 twisting stem the retardation is the greater in the more 

 actively growing half, which is also the more excitable. This 

 differential effect causes a diminution of the rate, or even 

 a reversal of the direction, of normal torsion. 



