146 THE LEAVES. 



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 very delicate fibres, ramified throughout the 

 green pulp. 



261. The cellular portion of the leaf consists of thin-walled cells 

 of loose parenchyma, containing grains of chlorophyll, to which 

 the green color of fohage is entirely owing. The 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 

 different 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 

 upper 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 or more compact layers of 

 oblong cells, placed endwise, or with their long diameter perpen- 

 dicular to the surface ; while that of the lower stratum is very loosely 

 arranged, leaving numerous vacant spaces between the cells ; and 

 when the cells are oblong, their longer diameter is parallel with the 

 epidermis. Tliis is shown in Fig. 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 



parenchyma consists of only a 

 single series of perpendicular 

 cells. Also in Fig. 220, which 

 represents a similar view of a 

 thin slice of a leaf of the Gar- 

 den Balsam. Fig. 221 repre- 

 sents a piece cut out of a leaf 

 of the White Lily ; where the 

 upper stratum is composed of 

 only one compact layer of ver- 

 tical cells. The parenchyma is alone represented ; the woody por- 

 tion, or veins, being left out. The more compact structure of the 



FIG 220. Magnified flection 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. (After Brongniart.) 



