36 FIELD AND WOODLAND PLANTS 



to the increasing size of a living stem round which they have grown. 

 Tlie spirals of the Honeysuckle, however, do not increase in this 

 way ; and consequently, when they surround the trunk or branch 

 of a young tree, the latter is constricted, often to such an extent 

 that it is strangled and becomes stunted in its growth. 



Another class of cUmbing plants cling to their surroundings by 

 means of tendrils, which are modifications of leaves or shoots 

 that grow spirally like the stems we have been considering. 



Whatever be the origin of a tendril, it generally grows straight 

 until it has reached some favourable support, and in order to 

 obtain such supjjort it performs circular movements similar to those 

 of the tips of twining stems. Like these stems, too, the tendril is 

 always sensitive, and forms a close spiral round the object it 

 touches. 



Some tendi'ils will grow spirally without ever touching a support, 

 but these often become stunted and wither, while those which 

 reach and embrace a stem or other structure are apparently incited 

 to a luxuriant growth by the stimulating effect of the pressure 

 produced. 



When the tip of a tendril is successful in gripping a stem firmly, 

 the portion behind it often takes part in the spiral movement, 

 thus becoming shorter, and pulling the support towards its own 

 j)lant in such a manner as to bring it within the reach of additional 

 tendrils. 



Of course the tendrilled plants have a much better chance of 

 securing a suitable support than the twiners, for the latter have to 

 depend on the searching and clinging powers of but one structure, 

 while the tendrils are usually very numerous on the same plant, 

 and throw themselves out in all directions in search of the required 

 aid. The production of tendrils as a means of support is also much 

 more economical than the method of clinging by a twining stem, 

 for the former are usually very slender, while the latter must 

 necessarily be sufficiently thick to convey the nutritive requirements 

 of the whole plant ; and thus the process of clinging by tendrils 

 is more in accordance with the usual economy of Nature. 



We have observed that twining stems can, as a rule, twine 

 round only those supports which are erect or nearly so. This is 

 not the case with tendrils, which are better adapted for twisting 

 round horizontal stems and leafstalks. Often, too, they pass 

 from one branch or leaf to another, and so secure the plant to which 

 they belong by fastenings both above and below. Further, while 



