26 STRUCTURAL AND PHYSIOLOGICAL BOTANY. 



208. The principal vein of a leaf is a continuation of the 

 petiole, and runs in a direct line from the base to the apex of 

 the lamina; this vein is called the midrib. It usually pro- 

 duces other veins from its base or sides, or from both : such 

 veins are called ribs, if very strong, and proceeding from the 

 base to the apex; under other circumstances, they are fre- 

 quently named nervures. 



209. There are two strata of veins, the one belonging to the 

 upper, and the other to the under surface. 



210. The upper stratum conveys the juices from the stem 

 into the lamina, for the purpose of being aerated and elabo- 

 rated ; the under returns them into the bark. 



211. The veins are interposed among cellular substance, 

 called diackyma, diploe^ or mesophyllum ; which is often strati- 

 fied differently below the two surfaces of the leaf; the upper 

 stratum being more compact than the lower, and having its 

 cells perpendicular to the plane of the leaf : in such cases, the 

 cells of the lower stratum are commonly more or less parallel 

 with the under surface. 



212. The lamina is variously divided and formed; it is 

 usually thin and membranous, with a distinct upper and under 

 surface ; but sometimes becomes succulent, when the surfaces 

 are often not distinguishable. 



213. The upper surface is presented to the sky, the lower 

 to the earth ; this position is rarely departed from in nature, 

 and cannot be altered artificially, except by violence. 



214. A leaf is simple when its lamina is undivided, or when, 

 if it is separated into several divisions, those divisions do not 

 reach the midrib ; Ex. Lime-tree, Oak. 



215. The form of the simple leaf is extremely variable, and 

 the terms employed to denote the variations are numerous in 

 proportion. 



216. Some leaves have the margin so continuous, that the 

 outline is scarcely interrupted, except by small toothings. 

 Of such leaves the following are among the more common 

 forms : 



Orbicular 120 ; ovate 121 ; lanceolate 122 ; oval 118 ; oblong 117 ; roundish oblong 113 ; 

 peltate 124 ; cordate" 9 ; cordate ovate 114 ; cordate acuminate 119 ; renifonn 123 ; 

 oblique" ; auriculate 139 . 



