PRICKLES AND BRISTLES 



105 



156. Leaf-spine of 

 barberry. 



softer growth, so that the side buds do not have a chance 

 to start. The thorns of osage orange and honey locust 

 are also branches. Those of the honey locust usually 

 arise from supernumerary buds which are 

 borne somewhat above the axils. 



216. Prickles, bristles, and tveaJc spines, 

 which have a definite arrangement on the 

 stem, are tisually modified leaves or parts 

 of leaves. The spines of 

 thistles are hardened 

 points of leaf-lobes. The spines of the 

 barberry are reduced leaves; in their axils 

 are borne short branches or leaf -tufts 

 (Pig. 156) ; in spring on young shoots 

 may be found almost complete gradations 

 from spiny leaves to spines. The prickly 

 ash has prickles that simulate stipules and 

 stipels, but the irregularity of position in- 

 dicates that they are not homologous with 

 stipules. The prickles of the common locust 

 (robinia) are usually interpreted as stipules. 

 217. Prickles, bristles, and hairs, which 

 are scattered or 

 have no dofintie ar- 

 rangement, are usu- 

 ally mere out-grow^ths of the epi- 

 dermis. They usually are re- 

 moved with the bark. Of such are 

 the prickles of squashes, briars 

 (Fig. 158), and roses. 



218. The reason for the exis- 

 tence of spines is difficult to de- 

 termine. In many or most cases 

 they seem to have no distinct use or function. In some 

 way they are associated with the evolution of the plant, 



57. Small prickles of 

 the prickly ash. 



158. Prickles of dewberry. 



