88 MORPHOLOGY OF LEAVES. 



work, the separate parts or ramifications of which form what are 

 variously called the ribs, a sufficiently proper term, nerves or 

 veins. The latter names ma}- suggest false analogies ; but they 

 are of the commonest use in descriptive botan}*. That of veins, 

 and of its diminutive, veinlets, for the smaller ramifications, is 

 not amiss ; for the fibrous framework not only gives firmness 

 and support to the softer cellular apparatus, i. e. forms ribs, 

 but serves in the leaf, as it does in the stem, for the more rapid 

 conveyance and distribution of the sap. The subdivisions con- 

 tinue bej'ond the limits of unassisted vision, until the fibro-vas- 

 cular bundles are reduced to attenuated fibres ramified through 

 the parenchyma. In leafstalks, the woody bundles are parallel, 

 not ramified, and arranged in various ways ; in Exogens usually 

 so as to form in cross-section an arc or an incomplete or com- 

 plete ring. In leaves serving as foliage or organs of assimilation, 

 the blade is the important part, and this only is here regarded. 



168. The characteristic contents of these cells of parenchyma 

 are grains of chlorophyll (142 3 ), literally leaf-green, to which 

 the green color of foliage is wholly owing, and which may be 

 regarded as the most important of all vegetable products ; 

 because it is in them (or in this green matter, whatever its form, 

 189) that all ordinary assimilation takes place. As it acts only 

 under the influence of light, the expanded leaf-blade may be 

 viewed as an arrangement for exposing the largest practicable 

 amount of this green matter the essential element of vegeta- 

 tion to the light and air. 



169. The Parenchyma-cells, constituting the green pulp, are 



themselves arranged in accordance 

 with this adaptation. The upper stra- 

 tum is mostly of oblong cells, compactly 

 arranged in one or more layers, their 

 longer diameter perpendicular to the 

 surface. The stratum next the lower 

 surface of the leaf consists of loosely 

 arranged cells, with longer diameter 

 usually parallel to the plane of the leaf, 

 133 often irregular in form, and so disposed 



as to leave intervening sinuous air-spaces freely permeating all 



FIG. 136. A magnified section through the thickness of a leaf of lllidum Flori- 

 danum, showing (ho irregular spaces or passages between the cells, which are small in 

 the upper layer of the green pulp, the cells of which (placed vertically) are well com- 

 pacted, so as to leave only minute vacuities at their rounded ends; but the spaces are 

 large and copious in the rest of the leaf, where the cells are very loosely arranged : 

 also the epidermis or skin of the upper (a) and of the lower surface of the leaf (b\ 

 composed of irfectly combined and thick-walled empty cells. 



