TENDRILS AND THORNS. 



57 



remarkable phenomena of irritability of the tendrils, and shall only mention here 

 that these climbing organs may arise by metamorphosis of very different parts of 

 the shoot ; and this either at the expense of the development of leaves, or without 

 the latter being essentially affected. 



In a certain sense the so-called thorns stand in contrast to the tendrils ; they 

 agree with them, however, in that they arise from various parts of the shoot, 

 generally with suppression of the leaf-formation. As thorns we distinguish certain 

 hard pointed, simple or branched bodies, which serve the plants concerned either 

 as a protection against the attacks 

 of larger animals, or as climbing- 

 organs. 



Frequently, thorns arise from 

 the metamorphosis of foliage 

 leaves, as in the Barberry [Ber- 

 beris); or they are remnants of 

 fallen foliage leaves, as in Isoeies 

 hystrix, Astragalus tragacantha ; 

 or the end of a foliage shoot be- 

 comes transformed into a thorn, as 

 in Ramnus cathartica; or an entire 

 branched shoot, furnished with 

 small leaflets, at first delicate and 

 soft, eventually hardens to a system 



Fig. <,i.—Trofccolum minus. The long 

 petiole (a, a, a) of the leaf (/) is sensitive to 

 continued contact, and has so wound itself 

 round a support and its own stem (rt), that the 

 latter becomes firmly fixed to the support ; z 

 ti>e axillary shoot of the leaf. 



1-lG. ^z.—IiryoHta dioica. B a portion c.f the stem 

 from which the tendril arises, together with the leaf-stalk 

 (b) and the bud (k) ; the lower part of the tendril («) is stiff 

 (not tendril-like) ; the upper portion (x) has coiled round 

 a branch. The long intermediate part of the tendril, 

 between the rigid basal portion (») and the point of attach- 

 ment (jf), has become spirally coiled, and thus raised the 

 stem B; luiu' the points whore the direction of the coil is 

 reversed. 



of thorns, as in GlediiscJiiaferox, Iriacajilhus, Sec. Thorns of this kind are shoot-axes, 

 on which the development of leaves ceases, while the tissue, and even that of the 

 growing point, is changed into hard, woody, permanent tissue. The older nomen- 

 clature has hitherto carefully avoided classing together with thorns the prickles, 

 which agree with them physiologically. These are hard, pointed, short or long bodies, 



