HELIOTROPIC TORSIONAL RESPONSE 657 



by lateral application of sunlight in the terminal leaflet of 

 Desmodium. A light index was attached transversely to 

 the lamina, by means of which, and with the help of a pro- 

 tractor, the gradually increasing torsion was measured at 

 definite intervals of time. It will be seen from the curve 



Fig. 265. Torsional Response of Terminal Leaflet of Desnwdiitm under 

 the Action of Lateral Sunlight 



Abscissa represents lime in minutes, and ordinate angular movement 

 in degrees. 



thus obtained, of which the abscissa represents time, and 

 the ordinate the induced angular torsion, that in this 

 case, within twenty-five minutes, a torsion of 14° had been 

 induced. 



Method of recording- torsional response. — For the 

 purpose of certain investigations, presently to be described, 

 in connection with this torsion, it became necessary to devise 

 special experimental arrangements by which a continuous 

 record of the rate of torsion and its variations could be 

 obtained. The mode in which this was accomplished will 

 be understood from the illustration in fig. 266. A mirror, 

 carried on a light spring-clip made of aluminium, is slipped 

 over and attached to the petiole, at a short distance from the 

 pulvinus or pulvinoid, which is the seat of the torsional 

 movement. As the object is to record only the torsion, the 



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