ORGANS OF NUTRITION. 



103 



Spine or Thorn. — It sometimes happens that a leaf-bud in- 

 stead of developing as usual, so as to form a leaf-bearing 

 branch, becomes arrested in its growth, and forms a hardened 

 projection terminating in a more or less acute point, as in Thorns 

 (Jiff. 199) and Gleditschia {fig. 198). Such an irregularly-de- 

 veloped branch is called a spine or thorn. That the spines are 

 really modified branches is proved not only by their structure, 

 which is exactly the same as the stem or branch upon which 

 they are placed ; but also by their position in the axil of leaves ; 

 by their sometimes bearing leaves, as in the Sloe {fig. 200), and 

 Spiny Kest-harrow ; and by their being frequently changed into 

 ordinary leaf-bearing branches by cultivation, as in the Apple 

 and Pear. The spines are sometimes confound-ed by the young 

 observer with prickles already described (see p. 56), but they are 



Fig. 198. 



Fig. 200. 



Fig. 198. Branching spine of the Honey Locust (Gledifschia). Fig. 199. 



Spine of a species of Thorn, Fig.-20Q. Leafy spines of the common Sloe. 



readily distinguished from these by their structure and connexion 

 with the internal parts of the stem ; the prickles being merely 

 formed of hardened parenchyma, arising immediately from, and 

 in connexion only with, the bark. 



Tendril ot Cirrhus. — This term is applied to a thread-like 

 leafless branch, which is twisted in a spiral direction, as in the 

 Passion-flower (fig. 201). It is one of those contrivances of 

 nature by means of which weak plants are enabled to rise into 

 the air by attaching themselves to neighbouring bodies for sup- 

 port. Tendrils are also observed in (he Vine {fig. 202), where 

 they are regarded by many botanists as the terminations of 

 separate axes, or as transformed terminal buds. Both spines 

 and tendrils are sometimes produced from leaves and some other 



