62 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, 
Fic. 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 isan 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 
