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 Seyphanthus 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 Veitcht. which on prolonged 
stimulation develop little adhesive discs, that are closely 
adpressed to roughnesses in the surface of the support and, 
becommg 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 
