STIMULATION AND ITS EESULTS 387 



shown by tendrils is possessed also, though to a much 

 smaller extent, by most climbing stems. These organs 

 show the movement of circumnutation very conspicuously, 

 the portion which takes part in the formation of the spiral 

 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 is not so always even 

 in a single individual. In Scyphanthus elegans it is 

 reversed in successive inter nodes 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 Veitchi, 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 



