TWINING OF TENDRILS. 



867 



These processes must however be considered as abnormal, the tendrils having 

 failed of performing their purpose of coming into contact, by means of their revolving 

 nutation, with a support during the period that they, are sensitive and still in a grow- 

 ing state. If this contact takes place on the sensitive side, a curvature arises at the 

 spot, and the tendril clings to the support; fresh sensitive spots are thus constantly 

 brought into contact with it, and the free apex twines firmly round the support 

 in a larger or smaller number of coils (Fig. 487). The nearer the spot where 

 contact first takes place to the base of the 

 tendril the larger are the number of revolu- 

 tions round the support, and the stronger the 

 attachment; though even a small number of 

 coils is sufficient to attach it with con- 

 siderable force. The portion of the tendril 

 between its base and the point of attachment 

 is obviously unable to twine round the support 

 like the free apex ; and therefore the irritation 

 caused by the contact extending to the portion 

 that is not in contact produces a different form 

 of curvature, consisting in a rolling up of this 

 portion into the form of a corkscrew, as shown 

 in Fig. 487 «, w, w'. This coiling is similar to 

 that already mentioned as taking place of its 

 own accord in many tendrils which do not take 

 hold of a support, especially in the circum- 

 stance that the under or dorsal side of the 

 tendril is always the concave one ; but it differs 

 from a spontaneous coiling in being always the 

 result of irritation, occurring invariably when 

 tendrils take hold of a support, and also in 

 taking place some time (half a day to a day) 

 after the attachment, at a time when the tendril 

 is still perfectly sensitive and growing rapidly 

 in length; while the spontaneous coiling occurs 

 only with the cessation of growth and of irri- 

 tability. The coiling which is the result of the 

 irritation caused by contact also takes place 

 much more rapidly than that which is sponta- 

 neous ; both can be readily observed by noticing 

 older tendrils which are still straight and have 

 not attached themselves, and younger ones on the same shoot that are attached and 

 already coiled up. The coiling of tendrils attached to supports is therefore a result 

 of stimulation in the same sense as the twining of the free portion round a support ; 

 and it is only the physical impossibility of also twining round the support that forces 

 the portion of the tendril between its base and the support to coil up like a cork- 

 screw. The coiling of this intermediate portion, like the curvature of a longer piece 

 of a tendril in consequence of the contact of a single point, is a proof that the local 



3 K 2 



Fig. 487.— Coiling of a tendril of Bryom'ii dioica. B a 

 portion of the branch from which the tendril springs by 

 the side of the petiole b and the axillary bud k; the 

 lower part of the tendril u is straight ; the upper part x 

 has coiled round a twig A; the long intermediate 

 part between the rigid basal portion « and the point of 

 attachment x has coiled spirally, and thus raised the 

 branch ^; w iv' the two spots where the direction of the 

 coil is reversed. 



