SPIRAL TWINERS. 13 



"When a stick was placed so as to arrest the lower and rigid 

 internodes of the Ceropegia at the distance at first of 15 and 

 then of 21 inches from the centre of revolution, the shoot slowly 

 and gradually slid up the stick, so as to become more and more 

 highly inclined; and then, after an interval sufficient to have al- 

 lowed of a semirevolution, it suddenly bounded from the stick 

 and fell over to the opposite side, to its ordinary slight inclina- 

 tion. It now recommenced revolving in its usual course, so that 

 after a semirevolution it again came into contact Avith the stick, 

 again slid up it, and again bounded from it. This movement 

 of the shoot had a very odd appearance, as if it were disgusted 

 with its failure but resolved to try again. "We shall, I think, 

 understand this movement by considering the former illustration 

 of the sapling, in which the contracting surface was supposed to 

 creep from the southern, by the eastern, to the northern, and 

 thence back again by the western side to the southern face, suc- 

 cessively bowing the sapling in all directions. Now with the 

 Ceropegia, the stick being placed a very little to the east of due 

 south of the plant, the eastern contraction could produce no 

 effect beyond pressing the rigid internodc against the stick ; but 

 as soon as the contraction on the northern face began, it would 

 slowly drag the shoot up the stick; and then, as soon as the 

 western contraction had well begun, the shoot would be drawn 

 from the stick, and its weight, coinciding with the north-western 

 contraction, would cause it suddenly to fall to the opposite side 

 with its proper slightly inclined positions ; and the ordinary 

 revolving movement would go on. I have described this case 

 because it first made me understand the order in which the con- 

 tracting or turgescent cells of revolving shoots must act. 



The view just given further explains, as I believe, a fact ob- 

 served by Yon Mohl (S. 135), namely, that a revolving shoot, 

 though it will twine round an object as thin as a thread, cannot 

 do so round a thick support. I placed some long revolving shoots 

 of a Wistaria close to a post between 5 and G inches in diameter, 

 but they could not, though aided by me in many ways, wind 

 round it. This apparently is owing to the flexure of the shoot, 

 when winding round an object so gently curved as this post, not 

 being sufficient to hold the shoot to its place when the con- 

 tracting force creeps round to the opposite surface of the shoot ; 

 so that it is at each revolution withdrawn from its support. 



When a shoot has grown far beyond its support, it sinks down- 

 wards from its weight, as already explained in the case of the 



