223. The lamina is generally of a rounded oval outline, longer 

 than wide, with equal sides but unequal ends. It is, how-ever, 

 subject to variety almost infinite in this respect. The end of 

 the blade next the stem is the base, and that most remote, the 

 apex. 



224. A leaf is simjjle when its blade consists of a single piece, 

 however cut, cleft, or divided ; and compound when it consists 

 of several distinct blades, supported by as many branches of a 

 compound petiole. 



225. The frame-work, or skeleton, of the lamina above men- 

 tioned, consists of the ramifying vessels of the petiole, while the 

 lamina itself is, of course, parenchyma (29, 221). 



226. The manner in wliich the veins are divided and distrib- 

 uted, is termed venation. The organs of venation are, as they 

 are called, the midrib, nerves, and veins; distinctions which musr. 

 be regarded as purely arbitraiy, since there is no diiFerence in 

 their functions, but only in their size. 



227. The midrib, or casta, is the principal prolongation of the 

 petiole, running directly through the midst of the leaf to the 

 apex. If several such ribs of nearly equal size radiate from the 

 base, they are called nerves, and the leaf is said to be three- 

 nerved, five-nerved, &c. 



228. The primary divisions sent off from the midrib, or nerves, 

 are properly the veins. (In descriptive botany, however, the 

 terms nerves and veins are too often used indiscriminately.) 

 The secondary divisions, or the branches of the veins, are called 

 veinlets. 



229. There are three principal modes of venation which are, in general, char- 

 acteristic of the three grand divisions of the vegetable kingdom. 



1st. Reticulate or net veined, as in Exogens. The petiole is 

 prolonged into the leaf in the form of the midrib, or several pri- 

 mary branches, dividing and subdividing into branchlets, which 

 unite again, and by their frequent inosculations fonn a kind of 

 network. Ex. maple, bean. 



2nd. Parallel-veined, as in Endogens. In this kind of vena- 

 tion the veins are all parallel, whether proceeding from the base 

 of the leaf to the apex, or sent off laterally from the midrib, and 



