46 



MANUAL OF BOTANY 



thorns are really modified branches is proved not only by their 

 structure, which is exactly the same as the stem or branch upon 

 which they are placed, but also by their position in the axil of 



Fie. 6S. 



Fiij. 68. Tubers of the common Potato {Solanum tuberosmn), 



leaves ; by their sometimes bearing leaves, as in the Sloe (fig. 

 64) and Spiny Eest-harrow ; and by their being frequently 

 changed into ordinary leaf-bearing branches by cultivation, as 

 „ in the Apple and Pear. 



Thorns are sometimes con- 

 founded with prickles, but 

 they are readily distin- 

 guished from these by their 

 structure and connection 

 with the internal parts of the 

 stem ; the prickles being 

 merely formed of hardened 

 parenchyma, arising imme- 

 diately from, and in connec- 

 tion only with, the epidermal 

 tissue and cortical cells be- 

 neath. Thorns must not be 

 confused with spines, which 

 are prickly bodies arising generally upon the margins or apices 

 of leaves, as in the Holly. Sometimes a whole leaf is metamor- 

 phosed into such a spine. 



fig. 09. Tubers o£ the Jerusalem Arti 

 choke (Ilelianthus luberosus). 



