510 LECTURE XIX. 



or in other words, is the part which comes into contact with 

 the support, the turns of the spiral which it describes will 

 encircle the support provided it be of appropriate thickness. 

 The spiral growth is promoted by torsion due to internal 

 causes (p. 353), and may be either promoted or hindered by 

 torsion due to external causes of which the weight of the 

 terminal bud, of the leaves, and friction against the support 

 are the most important. 



In his paper on the subject, which affords much valuable 

 information as to the mechanism of the process, Schwendener 

 attributes twining to circumnutation and to an antidromous 

 torsion, that is, a torsion in an opposite direction to that of 

 circumnutation, which causes the nutating stem to present 

 constantly its concavity to the support. 



Again, Baranetzki attributes twining to what he terms 

 the asymmetrical circumnutation of the apex of the stem, 

 that is, to circumnutation round an axis which is inclined to 

 the vertical. His mode of regarding the process would seem 

 to approach that of de Vries. His views concerning the 

 influence of geotropism are worthy of note. He finds, like 

 Schwendener, that a stem will not twine whilst it is being 

 rotated on a clinostat ; he considers that this is due to an 

 arrest of its circumnutation, for he finds that under these cir- 

 cumstances the nutation is of the kind which is termed 

 undulating (p. 368). It is on this ground that he concludes, as 

 mentioned in a previous lecture (p. 361), that circumnutation 

 is not spontaneous, but is induced by gravity. 



It will be seen that in nearly all these suggested explana- 

 tions twining is referred to negative geotropism, to circum- 

 nutation, and to torsion, in various combinations. But we 

 have seen that stems which do not twine exhibit negative 

 geotropism, circumnutate, and may undergo torsion. The 

 question of the ultimate difference between stems which do 

 and those which do not twine, still seems to remain unan- 

 swered. If twining simply depends upon these factors, why 

 cannot we induce any flexible stem, for instance excessively 

 elongated etiolated stems, to twine ? 



Of all these various attempts at explanations of twining 



