864 MECHANICS OF GROWTH. 



and clinging to the support. If the support is removed soon after a few loose coils 

 have been formed round it, the shoot will retain its spiral form for a time, but will 

 then straighten itself and recommence the revolution at its apex. 



A revolution of torsion of the twining internodes must, on purely mechanical 

 grounds, accompany every revolution of twining; but torsions of the parts which 

 have already coiled also occur, especially with round rough irregular supports ; their 

 direction is sometimes to the right, sometimes to the left. 



During the course of the twining the leaves must sometimes stand on the out- 

 side, sometimes on the inside of the coils ^ ; in the latter case the leaf-stalk will be 

 pressed against the support on which it slips laterally under the pressure of the 

 contracting coil, dragging the internode sideways with it, and thus causing a local 

 torsion. 



What has now been said includes almost all that we at present know on the 

 mechanism of the twining of climbing stems. A few remarks, borrowed from Darwin, 

 may be added. 



The revolution of the free overhanging apex is often strikingly uniform in the same 

 plant under the same external conditions (as e. g. in the Hop, Micania, Phaseolus, &c,). 



The following table of Darwin's gives some idea of the time required, under favour- 

 able conditions, for a revolution : — 



The direction of the twining is usually constant in the same species ; but it does 

 sometimes happen, as in Solanum Dulcamara and Loasa aurantiaca, that different indi- 

 viduals twine in opposite directions. Darwin found, in these two species and in Scyphan- 

 thus elegans and Hibbertia dentata, that the same stem will sometimes twine first in one 

 and then in the other direction. 



The positive heliotropism of twining internodes is generally feeble ; a powerful 

 heliotropism would obviously be only a hindrance to the twining and especially to the 

 revolution, by which an effort, so to speak, is made to reach the support. Heliotropism 

 is however shown by the fact that when the light falls from one side only, revolution 

 takes place more quickly towards the source of light than away from it ; as e.g. in 

 Ipomcea jucunda, Lonicera brachypoda, Phaseolus, and Humulus. 



It may be concluded from what has been said on the mechanism of twining that 

 there is for every species a certain maximum of thickness of the support at which the 

 twining is possible. The support must not be much thicker than the diameter of the 

 coils which the shoot can make without a support ; if the support is too thick, the apex 

 of the shoot attempts to make coils by its side, and these eventually become effaced. 

 Darwin (/. c. p. 22) acknowledges his ignorance of the cause why the climbing plant 

 cannot twine round supports which are too thick ; de Vries's experiments however seem 

 to give a sufficient explanation. 



The movements of twining internodes are more energetic the more favourable the 

 external conditions of growth, and the more rapid the growth itself; they are therefore 

 vigorous when food is abundant, temperature high, and the plants contain abundance of 



' I may take this opportunity of remarking that, according to Dutrochet, the genetic spiral of 

 the phyllotaxis takes the same direction in climbing plants which have their leaves arranged spirally 

 as the twining ; and therefore also the same as the spontaneous torsion and the revolving nutation 

 of the same plants. 



