774 MECHANICAL LAWS OF GROWTH. 



of this kind, not embracing any support, are frequently produced when the stem 

 rises above its support. 



The youngest coils of a twining stem are not usually in contact with its support ; 

 they are wide and flat ; w^hile the older coils are in close contact with it, and are 

 narrower and more oblique. This shows that the close clinging of climbing stems 

 to their support is a subsequent result, the coils being at first looser and wider, and 

 becoming afterwards closer and more oblique. This fact, which is of great import- 

 ance in the interpretation of the phenomena of climbing plants, w^as placed beyond 

 doubt by de Vries, who caused the summits of climbing plants to coil in this manner 

 without having any support in the middle. In this case also the coils were at first 

 wider and flatter, and became closer and more oblique with increasing age, until at 

 length the piece became quite erect, a revolution of torsion being all that represented 

 each spiral revolution. It is not improbable that geotropism is the cause of the 

 coils — at first flatter and sometimes almost horizontal — becoming afterwards more 

 oblique. It is clear that the stronger the force with which the coils become closer 

 and more oblique, the more closely must they cling to their support. If there is a 

 support in the axis of the coils, the younger parts of the summit will be constantly 

 prevented by it from performing their normal revolution of nutation, and the apex 

 will therefore continue to grow in a spiral, and will climb continually further up the 

 support, the older coils always becoming more oblique 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 recom- 

 mence the reyolution 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 

 had 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 contains almost all that we at present know on the mechan- 

 ical laws of the twining of climbing stems. A few remarks, borrow-ed 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 : — 



Scyphanthiu elegans i hour 17 min. 

 Akebia quinata I 30 



Cori'vol'vulus sepium i 42 



Phaseolus -vulgaris 1 55 



Ad ha tod a 48 



* I may take this opportunity of rema iking that, according to Dutrochet, the genetic spiral of 

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



