396 VEGETABLE PHYSIOLOGY 



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 thicknesses 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 particularly 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 opposite. The 

 direction of the twining is not, however, always constant. 

 Darwin noticed that it is not so always even in a single 

 individual. In Scyphanihus elegans it is reversed in succes- 

 sive internodes of the same stem. Many of our ordinary 

 climbers can twine up a support having only the thick- 

 ness of a piece of string ; other plants, particularly 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 out- 

 growth or hypertrophy of the part affected. This is seen 

 in the tendrils of Ampelopsis Veitchi, which on prolonged 

 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 TJiesium 

 show a similar property. When they come into contact 



