30 FIELD AND WOODLAND PLANTS 



to tlie increasino; size of a living stem round wliieh they have grown. 

 The 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 climbing plants chng 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 

 oljtain such sup2:)ort 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 tendrils will grow spkally 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 tij) of a tendril is successful in gripping a stem firmly, 

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

 thus becoming shorter, and pulling the support towards its own 

 plant in such a manner as to bring it within the reach of additional 

 tendrils. 



Of coixrse the tendrilled plants have a much better chance of 

 securing a suital)lc support than the twiners, for the latter have to 

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

 while the tench-ils are usually very numerous on the same plant, 

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

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

 more economical than the method of cUnging 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, wliile 



