RESPONSIVE CURVATURE OF ANISOTROPIC ORGAN 85 



example, in those of Cucurbita and Convolvulus. In order 

 to demonstrate the greater contraction of the shaded side — 

 which is seen as responsive curvature — while eliminating the 

 effect of gravity, I have employed two different modes of 

 experiment. In the first, stems are held with their tips 

 vertically downwards, and electric shocks are passed through 

 them ; a curvature is then produced by the greater contrac- 

 tion of the shaded side, in consequence of which the free end 

 of the stem is lifted up against the force of gravity. The 

 second method consists in supporting the stem horizontally 

 in such a way that the plane which divides the previously 

 shaded and unshaded sides is vertical : on strong stimulation, 

 the stem now moves in the horizontal plane, in a definite 

 direction which is determined by the induced concavity of 

 the shaded and more excitable side. Here we see the plagio- 

 tropic stem behaving like the pulvinus of Mimosa, the more 

 excitable side becoming concave under diffuse stimulation in 

 both. 



Response of plagiotropic stems. — In order to obtain a 

 series of responses, made from plagiotropic stems, demon- 

 strating their similarity to those obtained from the pulvini of 

 sensitive plants like Mimosa, we may use specimens of 

 Cucurbita or Convolvulus, selecting the last internode of 

 the stem as the most sensitive. The cut ends of the 

 specimens are placed in water, and the abnormal turgidity 

 thus produced may sometimes at first cause erratic responses ; 

 but after a while these become very regular. Stimulation 

 is produced by thermal shocks, the specimen being held 

 erect, within an inclosing spiral of heating wire, in the 

 manner already described. The responses are now given 

 in the form of lateral movements, which are recorded by 

 the use of a magnetically controlled horizontal recorder, fully 

 described in a subsequent chapter. The responses might 

 easily have been recorded also by the use of the Optic Lever, 

 which was employed in the case of Mimosa. But my object 

 in the present case was to eliminate as far as possible 

 the effect of gravity, and this could not have been done by 



