388 VEGETABLE PHYSIOLOGY 



being frequently of considerable length. This is of course 

 a great advantage in enabling the stem to find a support. 

 The continuation of the circumnutating movement after 

 contact with such support, has given rise to the view that 

 circumnutation alone will enable climbing to take place. 

 Consideration of the behaviour of various twining stems 

 with supports of various thickness has shown however 

 that this is supplemented by changes resulting from the 

 contact effected by circumnutation, and therefore from the 

 possession of the sensitiveness under consideration. 



Twining stems show individual peculiarities in the 

 direction of their twisting, and in the nature and particu- 

 larly the thickness of the support they need. The stem 

 of the Hop twists in the direction taken by the hands of a 

 watch ; that of the Convolvulus in one diametrically oppo- 

 site. The direction of the twining is not however always 

 constant ; Darwin noticed that it was not so always even 

 in a single individual. In Scyphanthus elegans it was 

 reversed in successive internodes of the same stem. Many 

 of our ordinary climbers can twine up a support having 

 only the thickness of a piece of string ; other plants, par- 

 ticularly the climbers of tropical forests, need supports of 

 some inches in diameter. 



The twining of stems is often accompanied by a torsion 

 of the stem, or a twisting round its own axis. This is not 

 however of universal occurrence. 



The stimulus of contact is sometimes followed by an 

 outgrowth or hypertrophy of the part affected. This is 

 seen in the tendrils of Ampelopsis Veitclii, which on pro- 

 longed stimulation develop little adhesive discs, that are 

 closely adpressed to roughnesses in the surface of the 

 support and, becoming mechanically attached to them, 

 enable the plant to maintain a very strong hold upon the 

 wall or other support to which it is clinging. The roots of 

 Thesium show a similar property. When they come into 

 contact with other roots growing near them they develop 

 a swelling at the point of contact, from which certain cells 



