STKUCTURE OF LEAVES. 



79 



Leaves and theie Appendages. 

 Structure of Leaves. 



Leaves are expansions of the bark, developed in a symmetrical 

 manner, as lateral appendages of the stem, and having a connection 

 with the internal part of the ascending axis. They appear at first as 

 small projections of cellular tissue, continuous with the bark, and 

 closely applied to each other. The points from which they arise are 

 called nodes. In the early stages of their development they are 

 undivided. The cellular papiUse, from which they originate, gradually 

 expand in various ways, acquire vascular tissue, and ultimately assume 

 their permanent form and position on the axis. They may be divided 

 into aerial and submerged leaves, the former being produced in the air, 

 and the latter under water. 



Aerial Leaves. — These leaves consist of vascular tissue iu the 

 form of veins, ribs, or nerves, of cellular tissue or parenchyma filling up 

 the interstices between the veins, and of an epidermal covering. 



The Vasctjlae System of the leaf is continuous with that of the 

 stem, those vessels which occupy the internal part of the stem becoming 

 superior in the leaf, while the more ex- 

 ternal, become inferior. Thus, in the 

 upper part of the leaf, which may re- 

 present the woody layers, there are spiral 

 vessels (fig. 141 t), annular, reticulated, 

 and pitted vessels, ■;;, and ligneous fibres, 

 /; whilst in the lower side, which may re- 

 present the bark, there are laticiferous 

 vessels and fibres, resembling those of 

 liber, I. There are usually two layers 

 of fibro-vasoular tissue in the leaf, which 

 may be separated by maceration. They 

 may be seen in what are called skeleton 

 leaves, in which the cellular part is re- 

 moved, and the fibro-vascular tissue is 

 left. The vascular system of the leaf 

 is distributed through the cellular tissue 

 in the form of simple or branching veins. 



The Epidekmis (fig. 142 e s, e i), composed of cells more or less 

 compressed, has usually a difi'erent structure and aspect on the two 



Kg. 141. Bundle of fibro-vaseular tissue, passing from a 'branch, t, into a petiole, p. 

 The vessels are first vertical, then nearly horizontal, but they continue to retain their 

 relative position. Changes take place In the size of the cells at the articulation a. 1 1, 

 Tracheae or spiral vessels in which the fibre can be unroUed. v v, Annular vessels. //, 

 Fibres of wood. 1 1, Cortical fibres, or fibres of liber, or the inner bark. 



Fig. 141. 



