TWINING PLANTS 39 



different individuals, or different shoots on the same individual, twine in 

 opposite directions. In Loasa aurantiaca, Scyphantus elegans, Blumenbachia 

 lateritia, Tropaeolum tricolorum^ Ipomoea jucunda, and Hibbertia dentata 

 the twining may even be reversed on the same shoot 1 . 



Unless the growth is considerably diminished during the reversal 

 of circumnutation the youngest coils may untwine, and if the reversal 

 occurred frequently no permanent twining would be possible, as is the case 

 when the plant is rotated horizontally on a klinostat. If, however, only 

 a portion of the coils are untwined permanent coiling may continue, but 

 more slowly than usual. Homodromous twining may even take place, if 

 the reversal of circumnutation is only temporary and ceases before any 

 permanent coils have been formed, This was actually observed by Darwin 

 to occur in Hibbertia dentata 2 . Indeed in the normal progress of circum- 

 nutation the later coils may be partially unwound. 



There is no definite relationship between the number of coils and 

 the number of circumnutations, the latter being performed more frequently 

 than the former are produced. Darwin 3 observed that Ceropegia circum- 

 nutated once in six hours, but only formed a coil in nine and a quarter 

 hours. The same was the case with Aristolochia gigas, except that a cir- 

 cumnutation was completed in five hours instead of six. Naturally twining 

 is only produced by definitely regulated circumnutation, and it can be 

 artificially induced by causing a growing apex to slowly follow a tropic 

 stimulus around a support. 



If the support is of appropriate thickness the coils are closely applied 

 to it, but around thread, fine wire, or string, loose coils are often formed. 

 These are often subsequently closely pressed to the thinnest supports by 

 the elongation of the stem, for the same reason that the diameter of a spiral 

 spring decreases when the spring is considerably stretched 4 . This elonga- 

 tion is in part autotropic, but is also due to the increased geotropism 

 of the stem inducing the younger coils to straighten more or less 5 . This 

 tendency may cause the straightening of the younger coils formed around 

 a thick support if this is removed, whereas the older coils are permanent, 

 owing to the fact that the power of growth rapidly disappears after coiling. 

 Owing to the same tendency a considerable pressure may be exercised 

 upon the support, sufficient to crush in a hollow paper cylinder 6 , to 

 partially strangulate a soft fleshy stem, to compress a leaf or petal, or 



1 For further details see Darwin, 1. c. ; Schenck, 1. c., p. 123, and the literature there quoted. 



2 L. c., p. 47. 3 L. c., p. 13, 



* De Vries, 1. c., p. 326; Baranetzsky, 1. c., p. 58 ; Schwendener, 1. c., 1881, p. 419; Ambronn, 

 1. c., I, p. 5 ; n, p. 35- 



5 Baranetzsky, 1. c. 



6 Mohl (Ranken- u. Schlingpflanzen, 1827, p. 118) deduced the existence of this pressure from 

 the curvatures produced in a string round which coiling occurs. De Vries (1. c., p. 327) found that 

 the coils at once narrow when the support is removed. 



