54 MOVEMENTS OF CURVATURE 



supports of less than 3 to 5 mm. diameter, and the hooks of Strychnos and 

 Roitcheria are unable to coil around supports more than 7 to 10 mm. 

 diameter 1 . The less actively curving and less irritable tendrils are unable 

 to apply themselves closely to the sides of a flattened support, whereas 

 a thin sensitive tendril may come into close contact with both sides of 

 a thin strip of sheet zinc 2 . If the tendril undergoes secondary growth on 

 attachment, the coils of thick tendrils often become extremely closely 

 applied even to irregular supports 3 . 



Similarly, if a tendril strives to tighten its coils it may exert pressure 

 upon the support, and either roll up a leaf around which coiling has 

 occurred or diminish the diameter of a paper cylinder slit along one side 4 . 

 Hence, on withdrawing a solid support, the coils usually tend to narrow, 

 and de Vries found that a tendril of the cucumber which had formed 

 five and a half coils around a support 6 mm. thick showed eight narrower 

 coils when the support was removed. 



When a tendril is in contact at one point only, the main curvature is 

 produced here, but the stimulus is perceptibly propagated in both directions 

 to a distance of 5 to 10 millimetres 5 . Similarly, the secondary thickening 

 which tropical tendrils such as those of Bauhinia and Strychnos undergo 

 takes place mainly at the point of contact, the effect of the stimulus ceasing 

 to be perceptible at a distance of i to 3 cms. 6 The continued curvature of 

 the tendril usually brings fresh acropetal surfaces in contact with the 

 support until the whole terminal portion has coiled. The same tendency to 

 coil takes place basipetally, but is prevented by the tension existing in the 

 free portion between the plant and the support. If the tendril is allowed 

 to coil around a light paper shell a few coils may be formed basipetally 

 from the original point of contact, and this causes the shell to be drawn 

 towards the plant. 



Tendrils may not only coil around horizontal supports or loose objects, 

 but may coil in different directions, so that either left- or right-hand coiling 

 may be shown by the tendrils of the same plant 7 . The coils are usually 

 somewhat inclined, and though near together are not superimposed. 

 Tendrils are unable to coil around thick supports, since, if the stimulated 

 part cannot form a sufficient curvature, it is drawn away by the old-age 

 coiling 8 . By the aid of this coiling long tendrils may sometimes succeed 



1 Ewart, 1. c., pp. 189, 214. 



2 Mohl, Ranken- u. Schlingpflanzen, 1827, p. 82. 



3 Ewart, 1. c. 



* Mohl, 1. c., p. 63 ; de Vries, I.e., p. 307. Cf. Macdougal, Ber. d. hot. Ges., 1896, p. i^. 



5 De Vries, Arb. d. bot. Inst. in Wiirzburg, 1873, Bd. I, p. 304; Pfeffer, Unters. a. d. bot. Inst. 

 zu Tubingen, 1885, Bd. I, p. 509; Fitting, I.e. On petiole-climbers cf. Derschau, Einfluss von 

 Contact u. Zug auf rankende Blattstiele, 1893, p. 13. 



6 Ewart, I.e., pp. 208, 223. 



7 Cf. de Vries, 1. c., p. 307. 8 Mohl, 1. c., pp. 80, 142. 



