386 VEGETABLE PHYSIOLOGY 
will ultimately straighten itself and move as before, till it 
touches something else. If, on the other hand, the body 
first touched is one round which the tendril can twine, it 
coils itself round it; the stimulus thus persists and the 
resulting curvature increases it, bringing more and more of 
the sensitive side into contact with the support, till the 
latter is encircled many times by the sensitive twiner. The 
coiling is seldom confined to the part of the tendril in 
contact with the support, but the free part between the 
latter and the axis of the plant also twists itself into a 
kind of helix. If the two are not very close together this 
helix usually shows two parts, the coils of which are in 
opposite directions. This is, however, only because the 
filamentous body is attached at both ends. 
A tendril, though thus sensitive to contact, does not 
coil, according to Darwin, if its sensitive surface is struck 
by drops of rain, nor, in the case of the Passiflora already 
alluded to, if contact takes place between two tendrils. 
The sensitive region varies in different tendrils, but it 
cannot be so strictly localised as in the case of a growing 
root. They are usually irritable on one side only, which 
is slightly concave, though in some cases the sensitive- 
ness extends all round them. The lower part of a tendril 
is, as a rule, only sensitive to prolonged contact. Their 
susceptibility further varies with their age, being greatest 
when they are about three parts grown. The part which 
first responds to the stimulus is usually the part touched, 
but, as we have seen, the coiling also takes place nearer 
their bases, so that we have an evident transmission of the 
stimulus backwards, as in other cases noted. The method 
of response is usually an increase of turgidity upon the 
convex side, followed by greater growth. In many instances 
careful measurements have shown that both the concave 
and convex parts grow during the coiling, but in a few 
cases the concave side either does not grow or becomes 
actually shorter than before. 
This sensitiveness to contact which is so markedly 
