152 APPENDAGES OF THE STEM. 



the petioles. The sap vessels of the bark are con- 

 tinued into the leaves ; but their office as ducts ceases 

 with the fall of the latter. 



In structure leaves are furnished with a beautifully 

 branched petiole, curiously divided and articulated like 

 arteries and veins, or ranged in nearly parallel lines ; 

 the interstices being- filled up with a pulpy, paren- 

 chymous, green substance, covered on both sides by 

 a cuticle, which is said to be porous to allow the 

 transmission of fluids. That the superior and inferior 

 surfaces have different functions, or are differently 

 acted on by the air and light, is manifest from the 

 circumstance, that if their natural position be reversed 

 by force, they will very soon regain it if at liberty. 

 There is a remarkable exception to this, however, 

 exemplified in the genus Alstrcemcria, the leaves of 

 which having tortuous petioles present their inferior 

 disc to the sky. 



Leaves differ much in substantiality ; some are 

 thin and transparent, others thick and fleshy. Those 

 of some of the Aloes and Crassulacea, as has been 

 before observed, cannot with propriety be called 

 leaves, because, if slipped off and placed in a favour- 

 able situation, they become perfect plants ; thus ex- 

 hibiting a property of a stem rather than of simple 

 leaves. Such appendages are in fact foliaceous stems. 

 There are several plants wiiose leaves, if planted in 

 moist heat, will exsert root fibres, and keep the leaves 

 alive for a considerable time ; but being destitute of 

 the organisation of a stem, never can produce any 



