THE ADAPTATION OF ORGANISMS 97 



being plants which grow on, but not at the expense 

 of, other plants. 



Then, again, we have adaptation of different Adapta- 

 types of organ to subserve special purposes or organs to 

 functions. For example, protection from destruc- special 



. * fiinnt ir\ 



tion by animals is well exemplified in such 

 ABC 



> 



functions. 



FIG. 46.. A, prickles ; B, leaf thorns ; C, branch thorns. 



common plants as the rose, the hawthorn, and the 

 holly. In each case the protection is afforded by 

 sharp spines, but a little knowledge of morphological 

 botany teaches us that the spines are of very different 

 origin in each case (Fig. 46). In the hawthorn they 

 are modified branches, in the holly they are extensions 

 of the veins of leaves, in the rose they are merely 

 hardened and sharpened emergencies from the 

 surface layers of the stem or leaf-stalk, and have no 

 connection with the internal vascular system. Once 



