XIII] A NOTE UPON TORSION 889 



evolved together in the course of ages, to bring about something 

 more and more fitted for survival in the struggle for existence. 

 They were, in short, of the nature of acquired habits, rather than 

 physical phenomena. But there was no gainsaying the fact that the 

 immediate cause of curvature was inequahty of growth on opposite 

 sides*. 



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

 light, would be in a position of equilibrium to a field of force radially 

 symmetrical about its vertical axis. But this complete radial sym- 

 metry will not often occur; and the radial anomalies may be such 

 as arise intrinsically from structural pecuHarities in "the stem itself, 

 or externally to it by reason of unequal illumination or through 

 various other locahsed forces. The essential fact, so far as we are 

 concerned, is that in twining plants we have a very marked tendency 

 to inequahties in longitudinal 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 coiftes 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 (/So/ai<Mm Dulcamara), which 

 twine indifferently either way. 



Together with this circumnutatory movement, there is very 

 generally to be seen an actual torsioyi 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 follow^ as a mechanical necessity from 

 the spiral winding": but that stems which had cHmbed around 

 a rough stick were all more or less; and generally much, twisted. 

 Here Darwin did not refrain from introducing that teleological argu- 

 ment which pervades his whole train of reasoning: "The stem," 

 he says, "probably gains rigidity by being twisted (on the same 



* On the whole controversy, see F. P. Blackman's obituary notice of Francis 

 Darwin in Proc. R.8. (B), ex, 1932. 



