STIMULATION BY LIGHT 409 



twisting organ. The modification of torsion produced was 

 due to the differential excitability of the two longitudinal 

 halves of the organ. In the case of the stimulus of gravity, 

 stimulation is also diffuse in the erect and in the inverted 

 positions. But a new class of phenomena makes its 

 appearance when the two flanks of the anisotropic organ 

 are successively excited by lateral stimulation. The plane 

 of junction of the two halves of the twisting stem, it is 

 to be remembered, is always undergoing slow revolution 

 {see p. 376). The results are of high significance, though 

 at first sight they appear to be contradictory. I will first 

 describe the effects of unilateral application of light to the 

 different sides of the organ. 



Method of Procedure 



The normal rate of torsion is first taken in the dark ; 

 the effect is then observed of subjecting the different 

 sides of the organ to a narrow line of light produced by a 

 cylindrical lens interposed in the path of a parallel beam 

 of light from an arc-lamp. The different sides of the organ 

 are successively exposed to light by rotating the small glass 

 bottle in which the specimen is mounted ; the angle through 

 which the plant is rotated can be read on the horizontal 

 scale (cf. fig. 215). 



Experiment 226. — I give detailed results obtained with 

 Clitoria which may be taken as typical. The rate of torsion, 

 when one of the sides was exposed to light, was ascer- 

 tained ; readings of the rate of torsion were next taken 

 when light acted on another side. In the first position the 

 natural rate of torsion in the dark was 30 mm. per 5 minutes. 

 Exposure to light increased it to 35. When the plant 

 was rotated through 22-5°, the rate increased to 45. This 

 increase continued till a maximum rate of 61 was attained. 

 Further rotation of the plant in the same direction now 

 induced an increasing retardation which culminated in 



