LESSON 8.] 



THEIK 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 pvlp, or parenchyma ; and 2. the fihrous 

 frarrtfiworlc, or skeleton, which extends throughout the soft green 

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

 U 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 stem through the 

 leaf-stalk, when there is one, in the 

 form of parallel threads or bundles of »/ 

 fibres ; and in the blade these spread 

 out in a horizontal direction, to form 

 the rihs and veins of the leaf. The 

 stout main branches of the framework 

 (like those in Fig. 50) are called the 

 rihs. 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 ; and their 

 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 hoUow ; 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 



FIG. 8ft Leaf of the Quince.* &, blade ; 7), petiole j «t, stipules. 



