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 : — 
Scyphanthus elegans i hour 17 min. 
Akebia quinata i „ 30 „ 
Con'vol'vulus sepium i „ 42 ,, 
Phaseolus vulgaris i „ 55 „ 
Adhatoda [cydoncßfolid) 48 ,, „ 
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 H'tbbertia 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 (A 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. 
