DIURNAL VARIATION OF TORSION 407 



increasing intensity of light is retardation of the rate of 

 torsional movement, which may culminate in reversal. The 

 retarding effect ceases under diminished intensity of light. 



Effect of gravitational stimulus. — Gravity also has a 

 pronounced effect on torsional movement, which is depen- 

 dent on the angle of inclination. 



A continuous fluctuation of torsion occurs as the resultant 

 of these different factors ; some of them are concordant 

 in their effect, thus increasing the normal movement, while 

 others diminish or even reverse it. The record of torsional 

 movement taken for a long period thus appears to be highly 

 erratic, a sudden increase being followed by an equally 

 abrupt reversal. 



Diurnal Variation of Torsion 



I give an automatic record (fig. 223) taken continuously 

 for 24 hours, the movement being reduced to keep the trac- 

 ings within the limits of the smoked glass plate. The experi- 

 ment was carried out under relatively simple conditions. 

 A young internode of a stem of Thunbergia was clamped 

 at the node below, and held erect in the manner already 

 described (p. 385). The shoot was introduced, through a 

 hole in a table, into a glass chamber which protected the 

 recording apparatus from disturbance caused by air-currents. 

 Ground glass was employed for the cover, to give a more 

 uniform illumination during the day. The record was com- 

 menced at 4 P.M. and continued for 24 hours. Light was at 

 that time disappearing, the retarding effect of light being 

 thus removed ; the temperature, very high at thermal noon, 

 had fallen to the optimum at about 4 p.m. Owing to these 

 two concordant factors, the direction of torsion was positive, 

 that is, anti-clockwise. After 9 p.m. the fall of temperature 

 was rapid and the torsional movement underwent a slight 

 reversal up to early morning. From 6 a.m. onward, the 

 temperature rose above 20° C. and light increased. The 

 effect of rise of temperature was predominant, so that 



IluILIBRARY^^ 



