THEIR ANATOMY. 153 



the skeleton or framework which ramifies among and strengthens 

 the former. 



260. The woody or fibrous portion fulfils the same purposes in 

 the leaf as in the stem, not only giving firmness and support to the 

 delicate cellular apparatus, but also serving for the conveyance and 

 distribution of the sap. The subdivision of these ribs, or veins, of 

 the leaf, as they are not inappropriately called, continues beyond 

 the limits of unassisted vision, until the bundles or threads of woody 

 tissue are reduced to nearly separate fibres, ramified throughout the 

 green pulp, so as to convey to every portion the sap it consumes. 



261. The cellular portion, or parenchyma, of the leaf is not a 

 structureless, pulpy mass, such as it appears to the naked eye. 

 The chlorophyll (87), to which the green color is entirely owing, 

 and which consists of innumerable rounded globules, is all inclosed 

 in cells of lax parenchyma (51) ; and these cells are not heaped 

 promiscuously, but exhibit a regular arrangement ; upon a plan, 

 too, which varies in different parts of the leaf, according to the dif- 

 ferent conditions in which it is placed. 



262. Leaves are almost always expanded horizontally, so as to 

 present one surface to the ground and the other to the sky ; and 

 the parenchyma forms two general strata, one belonging to the up- 

 per and the other to the lower side. The microscope displays a 

 manifest difference in the parenchyma of these two strata. That 

 of the upper stratum is composed of one, two, three, or several 

 compact layers of oblong cells, 



placed endwise, or with their 



long diameter perpendicular to 



the surface ; while that of the 



lower is very loosely arranged, 



leaving numerous vacant spaces 



between the cells ; and when 



the cells are oblong, their longer 



diameter is parallel with the epi- 



dermis. This is shown in Fig. 178 



7, which represents a magnified 



section through the thickness (perpendicular to the surface) of a 



leaf of the Star-Anise of Florida ; where the upper stratum of pa- 



FIG. 173. Magnified section through the thickness of a leaf of the Garden Balsam : a, sec- 

 tion of the epidermis of the upper surface ; 6, of the upper stratum of parenchyma; c, of the 

 lower stratum ; d, of the epidermis of the lower surface. 



