386 CHAP. XXXIV. THE MECHANISM OF THE TWINING STEM 



in the plate. The problem to be solved was therefore not 

 one of magnification but of reduction. This was secured 

 by the employment of a device which produced a suitable 

 reduction by a system of aluminium toothed wheels T 

 and T' (fig. 218, R). The necessary condition for perfect 

 working of the arrangement is that the centre of curvature 

 of the toothed wheel T' should be at the fulcrum of the 

 recording lever. 



Having explained the method of observation and of 

 record of torsional movement, I describe in this chapter the 

 results of the investigations of the following subjects : 



1. Modification of torsional movement by variation in 



the rate of ascent of sap. 



2. Torsional movement under variation of temperature. 



3. The critical fatal temperature. 



4. Effect of chemical agents on autonomous torsion. 



Effect of Variation in the Rate of Ascent of Sap 



I have shown ^ that the rate of ascent is increased, within 

 limits, by a rise of temperature of the water applied at the cut 

 end of the stem. Diminution of the rate of ascent is pro- 

 duced under drought or by withdrawal of water. Artificial 

 drought may also be caused by the application of a plasmo- 

 lytic solution of KNO3 at the cut end of the stem. Increase 

 in the rate of ascent causes an increase, while diminution in 

 the rate gives rise to diminution of turgor in the stem. The 

 experiments show that an increase of turgor, however produced, 

 causes an increase in the rate of normal torsion. Diminution 

 of turgor, on the other hand, causes an arrest or even a reversal 

 of normal torsion. 



Experiment 205. — The normal rate of the anti-clockwise 

 torsion of the stem of Phaseolus was 10 mm. per 5 minutes. 

 The vessel which supplied water at the cut end of the stem 

 was removed ; the effect of the resulting drought was a 

 continuous diminution in the rate of the anti-clockwise 



^ Physiology of the Ascent of Sap, p. 62. 



