676 



LECTURE XXXVIII. 



and which sometimes in the case of the Hop and of the common Bindweed extend 

 close up to the bud, have nothing to do with the problem of twining proper, 

 although they are probably of use for fixing the shoots to the support eventually. 

 On the other hand a torsion is necessarily connected with the actual twining itself, 

 even in the case of twining plants in which the above-mentioned torsions never 

 occur, simply because the laws of mechanics demand it. Since this unavoidable 

 torsion is usually not visible at all, it will be well for the student to convince 

 himself of its necessary existence. Suppose a long caoutchouc tube laid on the 

 table in the form of a helix : hold the outer end of the tube fast with one hand, 

 and seize the inner end with the other, and raise the arm till the whole tube stands 

 vertical on the table : supposing a black or white straight line to have been pre- 

 viously drawn on the tube so that, when the tube lies on the table the line runs, 

 for instance, on the convex side of the turns of the helix, it is then noticed that, after 

 the tube has been stretched in the manner described, that the line runs round the 

 tube in the form of a spiral, and this so that one turn of this torsion line comes on 

 each turn of the original helix, but in the opposite direction to the original coils of 

 the helix. 



Finally, the additional remark that the numbers of twining plants known are 

 greater than those of tendril-plants, and, like the latter, they occur in all parts of the 

 world, but especially in America. Mohl, even in 1827, gave the numbers of twining 

 plants known to him as 866, and it is certain that more than a thousand could now 

 be enumerated. 



The preceding considerations have always reference to the young shoot-apex 

 only, and its movement on or without a support. Further considerations might 

 now have to be extended to the other biological relations of these plants, and in this 

 connection those twining plants, especially tropical ones, &c., in which the shoot-axis 

 after having wound round a support then developes wood and secondary cortex by 

 subsequent growth in thickness, would be of peculiar interest. However, we must 

 here leave these matters and the conclusions to be drawn from them. 



