OEGAKS OF NrTKlTION. 129 



6f the leaf is usually that part also which is most developed, 

 which performs the more important functions of the leaf, and 

 which is also in ordinary language known under the name of leaf. 

 It is the part, therefore, which will come more particularly 

 under our notice. Before proceeding however to describe it and 

 the other parts of the leaf separately, it will be necessary to 

 treat of the internal structure of leaves, and of their insertion 

 and arrangement. 



2. THE IKTERNAL STRUCTURE OF LEAVES. 



In describing leaves with reference to their structure we 

 divide them into two kinds, namely, aerial and submerged; by 

 the former, we understand those that are produced and live 

 entirely or partially in the air; by the latter, those that are 

 formed and dwell wholly immersed in water. 



1. Aerial Leaves. — In the lowest leaf-bearing plants, such as 

 Mosses, as already noticed, the leaves consist simply of paren- 

 chymatous tissue, formed by the growing outwards of the paren- 

 chyma of the circumference of the stem or branch ; while in the 

 majority of the higher plants they contain, in addition to this 

 parenchyma, a framework or skeleton formed of wood-cells or 

 liber-cells, or of both these tissues, and vessels of diflferent 

 kinds, which are in direct connexion with corresponding parts of 

 the fibro-vascular system of the stem. We distinguish there- 

 fore, in such leaves, as in the stem— both a parenchymatous and 

 a fibro-vascular system — the former constituting the soft parts 

 or the -parenchyma of the leaf ; the latter the hard parts, which 

 by their ramification form what are called veins or nerves. 



The whole of the leaf is clothed by the epidermis, which is 

 commonly furnished with stomata in the manner already de- 

 scribed. The stomata are, however, almost confined to that 

 portion of the epidermis which corresponds to the parenchyma 

 of the leaf. The epidermis is also furnished with various ap- 

 pendages, as Hairs, Grlands, and their several modifications. 

 These, together with the epidermis, have been already fully 

 described under their respective heads : we have now therefore 

 only to allude to the fibro-vascular system and parenchyma 

 which are situated between the epidermis of the upper and 

 lower surfaces of the leaf. 



a. Fibro-vascular System. — This is in direct connexion with 

 that of the stem in the three great classes of plants respectively. 

 We shall direct our attention more especially to that of the 

 leaves of Dicotyledonous Plants. 



The fibro-vascular system is in by far the majoi'ity of cases 

 double, that is, it consists of an iipper layer which is in con- 

 nexion with the woody system of the stem {^fig. 250, t, x\ f) ; 

 and a lower which is continuous with the liber (l). The upper 



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