THE LEAF IN RELATION TO ITS ENVIRONMENT 



301 



I 



P/fo/o &/] 



FIG. 367. SPANGLE-GALLS ON OAK. 



for they spring 

 from the epider- 

 mis or cortex of 

 a plant member, 

 and contain no 

 fi br o -vascular 

 bundles ; while 

 spines, it will be 

 remembered, are 

 traversed by 

 those bundles 

 which connect 

 them with the 

 vascular system 

 of the stem arid 

 root. Good ex- 

 amples of prickles 

 are offered by the 

 Dog-rose (Rosa 

 canina) and the 



Holly (Ilex), to name no other plants. In the former they occur on the 

 petioles and branches ; in the latter they spring from the margins of the 

 leaves. 



Plants of a prickly nature seldom develop those structures on leaves and 

 branches which are out of reach of grazing animals. The Common Holly 

 (Ilex aquifolium), 

 the bristly den- 

 tate leaves of 

 which form a 

 characteri s t i c 

 feature of 

 shrubby speci- 

 mens of the 

 plant, produces 

 only unarmed 

 leaves, with en- 

 tire margins, 

 on its upper 

 branches, when it 

 attains to t h e 

 dignity and di- 

 mensions of a 

 tree. Indeed, the 

 gradations from FIG. 368. CHERRY-GALL ON OAK. 



Photo by} 



[/. Ilolmns. 



