LECT. XI.] CAULINAR AND FOLIAR APPENDAGES. 647 



piratory organs of plants ; and, independent of 

 other reasons that might be advanced against the 

 opinion that they are absorbents, we have only 

 to notice the fact, that the succulent plants of arid 

 soils, which live almost entirely by cuticular ab- 

 sorption, are often nearly devoid of hairs: besides, 

 they are observed in the interior of plants, as in 

 the vacuities within the stems of aquatics, where 

 their absorbent function, did they possess it, is 

 not required. Whether their presence can modify 

 the action of light, air, and temperature upon 

 plants, has not yet been determined ; and we must 

 confess that we are still ignorant of the use of these 

 minute organs in the vegetable economy. 



iii. THORNS, Spince. These are rigid, sharp- 

 pointed processes firmly connected with the tex- 

 ture of the parts on which they appear*. They 

 are either simple or compound. The simple spine 

 (spina simplex) is a slender tapering body ter- 

 minating in a sharp point, and covered with a 

 bark and cuticle the same as those of the stem or 

 the branch: it is, also, generally solitary, as in the 

 common Hawthorn (see cut 1, p. 268) and the 

 Cockspur-thorn, Mespilus oxycantha, and Crus 

 galli; Box-leaved Staff Tree, Celastrus buxifolius, 

 &c. The compound spine (spina composita) com- 

 prehends several kinds, which are named accord- 



* Linnaeus defines the thorn thus : " Spina est mucro plantae 

 " e Hgno plantae protrusus." Phil. Bot. 84. 3. 



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