LECT. XI.] CAULINAR AND FOLIAR APPENDAGES. 649 



case. In some instances, however, as in the Pear 

 tree, Pyrus sativa, and in some other fruit-trees 

 which have thorns in their wild state, they dis- 

 appear by culture. The petioles of several pin- 

 nate leaves, as those of Astragalus Tragacantha, 

 which are persistent, become acuminated, and 

 change into thorns, after the leaflets fall ; the 

 peduncles, also, of some flowers, as those of 

 Pisonia, undergo a similar transformation: and 

 in the genus Mimosa and a few other tribes of 

 plants, we find that the stipules* sometimes be- 

 come ligneous, and pass into thorns. The struc- 

 ture of the foliar thorn, which appears either 

 on the margin of the leaf or on the costee, does 

 not so closely resemble that of the part on which 

 it is seated. It generally consists of a cord of 

 vessels derived from the nearest fasciculus of the 

 leaf, enclosed in a firm cellular tissue, and co- 

 vered with a horny cuticle. 



iv. PRICKLES, Aculei, may be defined rigid 

 sharp-pointed processes that do not adhere firmly, 

 but come off with the bark of the parts on which 

 they are seated (see cut 2 p. 268) -f~. They are, in 

 general, laterally compressed, and either straight 



* As these organs have not yet been described, it is ne- 

 cessary to state here, that they are small foliaceous appendages, 

 generally situated on each side of the base of many petioles. 



j " Aculeus est mucro plantae, ejusdem cortici tantum af- 

 " fixus." Phil. Got. 8-1-. 4. 



