STRUCTURE OF LEAVES. 



67 



have found by experiment, that these excretions are not injurious, 

 and it is now shown, that the necessity for rotation depends on the 

 want of certain nutritive matters in the soil.* In very rich and fertile 

 land, the same crop may be grown successively for many years. 



LEAVES AND THEIK APPENDAGES. 



Structure of Leaves. 



133. 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. These 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. 



134. Aerial Leaves. These leaves consist of vascular tissue in the 

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

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



135. The Vascular 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 external become inferior. Thus, in 

 the upper part of the leaf, which may re- 

 present the woody layers, there are spiral 



vessels (fig. 126 t), annular reticulated or 

 porous vessels, v, and woody fibres, f; 

 whilst in the lower side, which may repre- 

 sent the bark, there are laticiferous ves- 

 sels and fibres, resembling those of liber, I. 

 There are usually two layers of fibro- 

 vascular 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 removed, and 

 the fibro-vascular left. The vascular 

 system of the leaf is distributed through 

 the cellular tissue in the form of simple 

 or branching veins. 



* This subject is considered when the sources whence plants derive their nourishment are 

 treated of. 



Fig. 126. Bundle of fibro-vascular tissue, passing from a branch, 6, into a petiole, p. The 

 vessels are first vertical, then nearly horizontal, but they continue to retain their relative posi- 

 tion. Changes take place in the size of the cells at the articulation a. 1 1, Tracheae, in which 

 the fibre can be unrolled, v v, Annular vessels. //, Woody fibres. 1 1, Cortical fibres, or fibres 

 of liber. 



