PRICKLES AND SPINES 



109 



buds that are 



which have a 



168. Leaf-spine of 

 barberry. 



locust usually arise from supernumerary 



borne somewhat above the axils. 



228. Prickles, bristles and weak spines, 



definite arrangement on the stem, are usually 



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 (Fig. 168); in 

 spring on young shoots may 

 be found almost complete gradations from 

 spiny leaves to spines. The prickly ash has 

 prickles (Fig. 169) that simulate stipules 

 and stipels, but the irregularity of position 

 indicates that they are not homologous with 

 stipules. The prickles of the common locust 

 (robinia) are usually interpreted as stipules. 



229. Prickles, bristles and hairs that are 

 scattered or have no definite arrangement, are 

 usually mere outgrowths of the epidermis. 

 They commonly are removed with the bark. 

 Of such are the prickles of squashes, briars 

 (Fig. 170), and the 



roses. 



230. The reason 



for the existence of spines is difficult 

 to determine. 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, and one cannot deter- 

 mine why they came without know- 

 ing much of the genealogy of the 

 plant. In some cases they seem 



169. Small prickles of 

 the prickly ash. 



170. Prickles of dewberry. 



to be the result of the 



contraction of the plant-body, as in the cacti and other 



