TWINIK& OP TENDBILS 497 



curvature ; in the case of the tendrils of Passiflora gracilis, 

 indeed, Darwin found that the contact of a small loop of thread 

 weighing not more than -^ grain induced it, while a mere touch 

 caused one to assume the form of a helix in about two minutes. 

 This is perhaps the most sensitive tendrU known ; with others a 

 stronger stimulus is needed and the time taken for the response 

 is longer, the irritability varying greatly. The behaviour of 

 tendrils in twining is- somewhat pecuhar. When young they 

 are generally circumnutating, and if in their movement they 

 come into contact with any foreign body they begin to curve 

 towards it. If the contact be not prolonged, the tendril will 

 curve for some time, but will ultimately become straight again 

 and move as before, tiU it touches something else. If, on the 

 other hand, the body first touched is one round which the 

 tendril can twine, it coils itself roimd it ; the stimulus thus 

 persists and the resulting curvature increases it, till the support 

 is encircled many times by the sensitive twiner. The coiliug 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 spiral coil. If the two be not 

 very close together, this spiral 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 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 the 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 the 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, though in cer- 

 tain cases careful measurement has shown that the concave side 

 becomes perceptibly shorter. 



This sensitiveness to contact, which is so markedly shown 



VOL. II. K K 



