STIMULATION AND ITS RESULTS 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 
cireumnutation 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 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 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 
