ORGANS OF NDTEITION. 



109 



dulous ; in other cases this weeping appearance arises from the 

 weakness and flexibility of the branches, as in the Weeping 

 Willow and Weeping Birch. The relative length also of the 

 upper and lower branches will give rise to corresponding diifer- 

 ences in the general appearance of trees. Thus, if the lower 

 branches are the longest and become shorter as they approach 

 the top, the whole will be shaped like a cone or pyramid, as in 

 the Spruce Fir ; if the middle branches are longer than those of 

 the base and apex, the general appearance will be rounded or 

 oval, as in the Horsechestnut ; if those of the top are the most 

 developed, the form will be umbrella-like, as in the Italian Pine 

 (Pinus Pinea). The above are the extreme forms only, between 

 which, as may be readily supposed, there are a number of inter- 

 mediate ones. 



Besides the forms of stems and branches already described, 

 there are some others which have received special names. 

 These we now proceed to notice. 



Spines or Thorns. — It sometimes happens that a leaf-bud in- 

 stead of developing as usual, and forming an ordinary branch 

 with leaves on its surface, becomes arrested in its growth, 

 and forms a hardened projection terminating in a more or 

 less acute point, as in Thorns {fig. 207), Gleditschia {fig. 206), 



Fig. 206. 



Fig. 20: 



Fig. 206. Branching spine of the Honey Locust {_Gleditschia). 

 Fig. 207. Spine of a species of Thom. 



&c. Such an irregularly-developed branch is called a spine or 

 thorn. That the spines are really modified branches is proved 

 not only by their structure, containing as they do all the ele- 

 ments of the stem or branch upon which they are placed ; but 

 also by their position in the axils of leaves ; by their some- 



