STIMULATION AND ITS KESULTS 395 



body, they begin to curve round it. If the contact is not 

 prolonged the tendril will curve for some time, but 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. 



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 sensitiveness 

 extends all round them. The lower part of a tendril is, as 

 a rule, only sensitive to prolonged contact. Their sus- 

 ceptibility 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 shown 

 by tendrils is possessed also, though to a much smaller 

 extent, by most climbing stems. These organs show the 



