VEINS. 



85 



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

 than wide, with equal sides but unequal ends. It is, however, 

 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 simple 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). These vessels are 

 collectively called veins, from the analogy of their functions. 



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

 uted is termed venation. The organs of venation, differing from 

 each other only in size and position, may be termed the midvein, 

 veins, veinlets, and veinulets. ( The old terms midrib and nerves, 

 being anatomically absurd, are here discarded.) 



227. The midvein is the principal prolongation of the petiole, 

 running directly through the lamina to the apex ; as in the leaf 

 of the birch. If there be several similar divisions of the petiole, 

 radiating from the base of the leaf, they are appropriately 

 termed the veins; and the leaf is said to be three-veined, five- 

 veined, &c. Ex. maple. 



228. The primary branches sent off from the midvein or the 

 veins we may term the veinlets; and the secondary branches, or 

 those sent off from the veinlets, are the veinulets. 



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 midvein, or several pri- 

 mary branches, dividing and subdividing into branchlets, which 

 unite again, and by their frequent inosculations form 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 midvein, and 



^o ' V N 



UNtVER '" I 



