THE TWINING OF STEMS 37 



conditions by many plants, such as Akebia quinata, Menispermum canadense, and 

 Humuhis Lupulus, which have no support or have grown beyond it. The 

 fixation of the shoot a little below the apex often causes free coiling, although 

 this may be but slight *. It is possible that this physiological tendency to coiling 

 may be directly or indirectly awakened by contact with a support, and may aid in 

 twining. The fact that the free coils are often straightened again shows nothing, 

 for the same happens when a stick is withdrawn from the coils just made around it. 



The factors which determine the permanent homodromous curvature of the asym- 

 metrically nutating free apex are uncertain. It is possible that the epinastic growth 

 of the side becoming convex during winding is more pronounced as the internode 

 approaches the adult condition, hence causing the permanent winding of the stem. 

 Although the apex is curved, the stem remains physiologically radial, and the 

 curvature passes in turn from side to side of the stem as the latter circumnutates. 

 In the same way the contact line of an adult twiner follows usually a more or less 

 spiral path around its stem 2 . 



The homodromous curvature of the apex is probably partly due to autonomic 

 and aitionomic variations of tone in which the external world and the progress of 

 twining act as directive stimuli. Baranetzsky and Noll 3 , on insufficient grounds, 

 assume the existence of a diageotropic irritability in the apex inducing paranasty. 

 Ambronn ascribes the homodromous curvature to the conjoint action of circum- 

 nutation and negative geotropism, a conclusion which Schwendener disputes 4 . The 

 latter erroneously regards circumnutation and geotropism as factors of constant mag- 

 nitude, and forgets that the circumnutation and the klinotropic position of the shoot 

 caused by it are themselves the result of regulated geotropic reactions. De Vries 

 supposed the curvature to be due to the torsion produced by the weight of the free 

 portion of the apex, but this has been shown to be untrue by various investigators 8 . 



The causes of twining are therefore unknown, but the very fact that regular coils 

 sloping at a definite angle are produced in each individual case suffices to show that 

 the position, thickness, and resistance of the support act as directive agencies upon 

 coiling or regulate the internal tendencies responsible for twining. It is quite 

 possible that the coiling is in one case produced by grasping movements, but in 

 another by an active curvature of the internodes. The stems of Cuscuta and Lopho- 

 spermum are able to twine partly as the result of their irritability to contact, which 

 in tendrils is the main factor in producing coiling. Although most twiners seem to 



1 De Vries, 1. c., pp. 324, 339 ; Baranetzsky, 1. c., p. 42 ; Sachs, 1. c., p. 707. Pfefier has observed 

 in a culture of Phycomyces nitens that most of the sporangiophores were spirally twisted, and the 

 shoots of some varieties, as for instance Juncus effusus, var. spiralis, always show a spiral coiling. 



3 De Vries, 1. c., p. 329. Circumnutation is the result of a changeable or labile induction of 

 a physiological dorsiventrality. 



3 Baranetzsky, 1. c., p. 38 ; Noll, Sitzungsb. d. Niederrhein. Ges. f. Natur- u. Heilkunde, 4. Febr. 

 1895 ; Strasburger, Lehrb. d. Botanik, 1898, 3. Aufl., p. 225 ; Noll, Sitzungsb. d. Niederrhein. Ges., 

 8. Juli 1901. That centrifugal force should act similarly to gravity is not surprising. 



* Ambronn, Zur Mechanik d. Windens, 1885, 2. Thl., pp. 19, 47 (Repr. from Sitzungsb. d. sachs. 

 Ges. d. Wiss.) ; Ber. d. bot. Ges., 1887, p. 105 ; Schwendener (1886), Gesammelte bot. Mitth., Bd. 

 I, p. 452. 



5 De Vries, I.e., p. 337; Baranetzsky, I.e., p. 69; Schwendener, 1881, I.e., pp. 403, 416; 

 Ambronn, 1. c., 1885, p. 25. 



