LESSON 8.] 



THKIK VF.NATION. 



55 



l3o. AVitliout here entering uj)on tlie subject of the anatomy of 

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

 rial, viz.: 1. the green pidp, or parenchyma ; and 2. the Jibrous 

 framework, or skeleton, whieh extends througliout the soft grecf. 

 pulp and suj)ports it, giving the leaf a strength and firmness which 

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

 ered witli a transparent skin, culled the 

 epidermis, like that which covers the 

 surface of the shoots, iScc. 



136. The framework consists of 

 wood, — a fibrous and tough material 

 which runs from the stem tliTough the 

 leaf-stalk, when there is one, in the 

 form of parallel threads or bundles of b\ 

 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 ; 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 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 witlj the general shape of the leaf, and with the kind of division 

 when the bhule is divided or lobed, that the reatliest way to study 

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



131). Various jls it appears in difierent leaves, the veining is all 

 reducible to two princijjal kinds; namely, \\\g parallel-veined and the 

 netted-veined. 



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

 branch off from the main ril) or ribs, divide into liner and liner 



riG. Kk Laaf ofUiF Qiiinco; b, blade ; p, [wiwlo ; <(, Hti|iiil<M. 



