THE LIGNEOUS SYSTEM. 



139 



vascular, abounding in spiral vessels, and strengthened with 

 liber. 



432. The parenchyma exists in two strata, more or less dis- 

 tinct. In all those leaves which are ordinarily horizontal in 

 position, one surface being upward and the other downward, 

 these two layers are dissimilar; but in leaves with a vertical 

 lamina (Iris), and in Phyllodia (321), the two layers are similar. 



517 



518 



517, Section of a stem at the origin of a leaf p, cellular, or pith; a, vascular, the medullary shentli 

 sending off a bundle into the leaf-stalk; <7, the swelling (pulvinus) just below the articulation of the leaf- 

 stalk (/); /;, the axillary bud. 518, Minute portion of a leaf of Viola tricolor, viewed in perspective, 

 showing, a, cells of epidermis above; b, compact parenchyma of the upper portion of the leaf; c, loose 

 parenchyma; d, epidermal-cells of the lower surface with stomata, one cut and opening into the inter- 

 cellular passages. (Magnified 100 diameters.) 



433. Our cut (518) displays a superficial layer of the empty 

 tabular cells (a) of the epidermis. Next beneath, in the surface 

 on which the sun shines, are one or two layers of oblong cells 

 (//) placed perpendicularly to that surface, and more compact 

 than the cells beneath them (c), which are pervaded by intercel- 

 lular passages and by the veins. 



434. The stomata as a rule belong to the shaded side of the 

 leaf, avoiding the sun's direct rays. On the sunny side there 

 are few, comparatively, or none. In the submerged leaves of 

 water-plants, the epidermal layer is hardly distinguishable, and 

 is wholly destitute of stomata. In such leaves as float upon water 

 (Water Lilies) stomata are found in the upper surface alone. 



435. Within all the vesicles of the parenchyma are seen ad- 

 hering to the walls the green globules of chlorophyl, which 

 give color to the leaf dark green above, where it is more com- 

 pact ; paler beneath, where the cells are more loose and separate. 



