514 LECTURE XIX. 



It has been incidentally mentioned that twining can only 

 take place when the support is of appropriate thickness. 

 In illustration we may cite Darwin's general observation, 

 that the English twiners, excepting the Honeysuckle (Lonicera 

 Periclymenum], never twine round trees, whereas tropical 

 twiners can ascend thick trees. In one particular case, that 

 of Phaseolus, he found that the stem could twine round a 

 support four inches in diameter but failed with a support 

 nine inches in diameter. There appears to be no limit of 

 thinness, for, as we have seen, stems will twine round 

 thin strings or wires, but when the support, assumed to be 

 vertical, is very thin, the spiral is long drawn out, so that 

 the stem is nearly straight, clearly because, under these 

 circumstances, negative geotropism can most fully assert 

 itself. The thicker the support, the more do the turns of 

 the stem round the support approach the horizontal. When 

 the support is of such a thickness that the stem, in order to 

 embrace it, would have to twine horizontally, twining no 

 longer takes place, for it is prevented by negative geotropism. 

 But the limit of thickness is not the same in all cases ; one 

 stem can twine, whereas another cannot, round a support of 

 a given thickness. This appears, according to Schwendener, 

 to depend on the length of the still growing portion of the 

 stem ; the longer it is, the less need it approach the hori- 

 zontal in growing round the support, but this view is ren- 

 dered improbable by Darwin's observation that the long 

 circumnutating shoots of Ceropegia Gardnerii failed to twine 

 round a support six inches in diameter. 



Allusion has been incidentally made to the torsion ex- 

 hibited by twining stems, and as this torsion has been re- 

 garded by some as an important factor in the process of 

 twining, we may now devote a short time to a consideration 

 of it. Von Mohl, for instance, considered that the circum- 

 nutation of twining stems, and therefore indirectly their 

 twining also, was due to torsion. 



We must first clearly understand what torsion means ; 

 it means a twisting of the organ about its own axis. This 

 is always exhibited sooner or later by twining stems, but it 



