XIII] A NOTE UPON TORSION 625 



ascribed, such as contact (thigmotaxis), exposure to light 

 (heliotropism), and so forth, need not be discussed here*. 



A simple stem growing upright in the dark, or in uniformly 

 diffused light, would be in a position of equilibrium to a iield of 

 force radially symmetrical about its vertical axis. But this 

 complete radial symmetry will not often occur; and the radial 

 anomalies may be such as arise intrinsically from structural 

 peculiarities in the stem itself, or externally to it by reason of 

 unequal illumination or through various other localised forces. 

 The essential fact, so far as we are concerned, is that in twining 

 plants we have a very marked tendency to inequalities in longi- 

 tudinal growth on different aspects of the stem — a tendency which 

 is but an exaggerated manifestation of one which is more or less 

 present, under certain conditions, in all plants whatsoever. Just 

 as in the case of the ruminants' horns so we find here, that this 

 inequality may be, so to speak, positive or negative, the maximum 

 lying to the one side or the other of the twining stem ; and so it 

 comes to pass that some climbers twine to the one side and some 

 to the other: the hop and the honeysuckle following the sun, 

 and the field-convolvulus twining in the reverse direction ; there 

 are also some, like the woody nightshade {Solanum Dulcamara) 

 which twine indifferently either way. 



Together with this circumnutatory movement, there is very 

 generally to be seen an actual torsion of the twining stem — a 

 twist, that is to say, about its own axis; and Mohl made the 

 curious observation, confirmed by Darwin, that when a stem 

 twines around a smooth cylindrical stick the torsion does not take 

 place, save "only in that degree which follows as a mechanical 

 necessity from the spiral winding": but that stems Avhich had 

 climbed around a rough stick were all more or less, and generally 

 much, twisted. Here Darwin did not refrain from introducing 

 that teleological argument which pervades his whole train of 

 reasoning: "The stem," he says, "probably gains rigidity by 

 being twisted (on the same principle that a much twisted rope 



* Cf. (e.g.) Lepeschkin, Zur Kenntnis des Mechanisinus der Variationsbewe- 

 gungen, Ber. d. d. Bot. Gesellsch. xxvi A, pp. 724-735, 1908 ; also A. Trondle, Der 

 Einfluss des Lichtes auf die Permeabilitat des Plasmahaut, Jahrb. iviss. Bot. 

 XLvni, pp 171-282, 1910. 



T. G. 40 



