ANATOMY OF LEAVES. 293 



shall now examine the modifications depending on 

 two principal states of dicotyledonous leaves : name- 

 ly, 1st. When the leaf is thin or membranaceous : and, 

 2d. When it is thick and fleshy. 



1st. In the thin leaves of this class of plants, the 

 vessels of the costae proceed from the principal faci- 

 culus of the midrib, and run between the laminae of 

 cuticle, imbedded in the cellular matter, in cords 

 which form visible elevations on the back of the leaf, 

 and corresponding furrows on its face. Each fascicle 

 consists of spiral and proper vessels throughout all its 

 ramifications ; and, in whatever manner these vessels 

 are arranged in the fasciculi, the spiral and proper 

 vessels are always associated, and, in general, tan- 

 gent. This arrangement is common both to sessile 

 and to petiolated, to simple and to compound leaves, 

 as far, at least, as respects the expansion. In sessile 

 leaves, however, all the fasciculi do not proceed from 

 the midrib, but some are given off directly from the 

 stem or the branch, and enter the expansion of the 

 leaf at its base, on each side of the midrib. In petio- 

 lated leaves, also, the petiole is generally dilated at 

 its point of union with the branch, and at this point the 

 vessels enter the petiole in distinct bundles ; the re- 

 mains of which are visible in the scar produced by 

 the falling of the leaves in autumn. 



Thus in the Apple, the Pear, the Peach, and many 

 other trees, the leaf is attached to the wood by three 

 fasciculi, one of which enters the middle of the peti- 

 ole, and the others, on each side of it. In the Lau- 

 rustine the whole of the vessels pass from the wood 

 into the petiole in one fascicle, the transverse section 

 of which is nearly a complete semicircle. In com- 

 pound leaves, the number of fasciculi passing into the 

 petiole from the wood, is in some instances regulated 

 25* 



