LESSON 8.] 



THEIR VENATION. 



55 



135. Without here entering upon the subject of the anatomy of 

 the leaf, we may remark, that leaves consist of two sorts of mate- 

 rial, viz.: 1. the green pulp, or parenchyma; and 2. the fibrous 

 framework, or skeleton, which extends throughout the soft greer: 

 pulp and supports it, giving the leaf a strength and firmness which 

 it would not otherwise possess. Besides, the whole surface is cov 

 ered with a transparent skin, called the 



epidermis, like that which covers the 

 surface of the shoots, &c. 



136. The framework consists of 

 wood, a fibrous and tough material 

 which runs from the stern through the 

 leaf-stalk, when there is one, in the 

 form of parallel threads or bundles of frj 

 fibres ; and in the blade these spread 

 out in a horizontal direction, to form 

 the ribs and veins of the leaf. The 

 stout main branches of the framework 

 (like those in Fig. 50) are called the 

 ribs. When there is only one, as in 

 Fig. 83, &c., or a middle one decid- 

 edly larger than the rest, it is called 



the midrib. The smaller divisions are termed veins ; 

 still smaller subdivisions, veinlets. 



137. The latter subdivide again and again, until they become so 

 fine that they are invisible to the naked eye. The fibres of which 

 they are composed are hollow ; forming tubes by which the sap is 

 brought into the leaves and carried to every part. The arrangement 

 of the framework in the blade is termed the 



138. Venation, or mode of veining. This corresponds so complete* 

 ly with the general shape of the leaf, and with the kind of division 

 when the blade is divided or lobed, that the readiest way to study 

 and arrange the forms of leaves is first to consider their veining. 



139. Various as it appears in different leaves, the veining is all 

 reducible to two principal kinds ; namely, the parallel-veined and the 

 netted-veined. 



140. In netted-veined (also called reticulated) leaves, the veins 

 branch off from the main rib or ribs, divide into finer and finer 



their 



8& Laf of UM Quince: b, blade ; p, petiole ; .st, stipules. 



