300 ANATOMY OF LEAVES. 



another part remains in the tubular cells, and under- 

 goes that change, which is the usual result of the agen- 

 cy oflight on the juices of all leaves exposed to its in- 

 fluence. The green color of the fluids contained in 

 these cells, marks out their limits, in a transverse sec- 

 tion of the leaf, even to the naked eye. 



The structure of the vessels in succulent dicotyle- 

 donous leaves is the same as in all other leaves. The 

 conducting vessels are spiral tubes, of the same diame- 

 ter at the apex as at the base of the leaf ; and the 

 proper or returning vessels are membranous, and ap- 

 parently perforated, although their transparency ren- 

 ders it difficult to determine their real character. The 

 ramifications are all given off at acute angles ; and ap- 

 pear to be merely separations from the caulinar or pe- 

 tiolar cluster, as Grew supposed to be the case in all 

 leaves ; at least they do not anastomose until, as I 

 have already stated, they approach the apex of the 

 leaf. 



II. THE CELLULAR SYSTEM OF THE LEAF. 



On cutting a thick, succulent leaf transversely, we 

 immediately perceive that it consists chiefly of a pulp, 

 which, when placed under the microscope, or exam- 

 ined by a good magnifying glass, is evidently compos- 

 ed of cellular tissue ; and indeed we find that this sub- 

 stance forms a large part of the structure of leaves ; 

 filling up the meshes of the net-work formed by 

 the vessels in the thin and very vascular leaves ; 

 and, in all, occupying that space which separates the 

 two cuticular layers, which constitute the upper and 

 the under disks of the leaf. 



The cellular substance of leaves differs very consid- 

 erably in density : but this diversity depends more on 

 the quantity and quality of the juices the cells contain, 

 than on any diversity of structure In the cells. To 



