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A TEXT-BOOK OF BOTANY 



(3) Thorns. Branches, as well as leaves ( 20), may 

 develop as thorns; an excellent illustration of a branching 

 thorn being seen in the honey locust (Fig. 60, A), and of a 

 simple thorn in hawthorn (Fig. 60, B). In dry regions, 



' 



A V. B 



FIG. 60. Thorns: A, honey locust; B, hawthorn. 



such as may be found along the Mexican border, thorns and 

 spiny branches are very common; and since in some cases 

 these spiny branches develop into ordinary branches when 

 the plant has a sufficient supply of water, it is thought that 

 such thorns and spines are results of unfavorable conditions 

 for growth. The same statement applies, of course, to those 

 cases in which thorns have replaced leaves. 



The most common modifications of the stem are those 

 which arise when it is an underground structure. Although 

 it is natural to think of all underground structures as roots, 

 this is far from being true. Since the stem is primarily a 



