STIMULATION AND ITS EESULTS 387 



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 increased 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 movement of circumnutation very conspicuously, 

 the portion which takes part in the formation of the spiral 



c c 2 



