PARENCHYMA OF THE LEAF-BLADE. 



159 



446. The centric arrangement has two modifications : (1) that 

 in which the whole pulp is composed of chlorophyll parenchyma, 

 but towards its mid- 

 dle plane has larger 

 cells with less chlo- 

 rophyll, and some- 

 times has conspicu- 

 ous lacunae (many 

 grasses, Yucca fila- 

 mentosa, Crassula, 

 etc.) ; (2) that in 

 which it is composed 

 of layers which are 

 uniformly distrib- 

 uted above and be- 

 low a middle layer 

 of colorless cells free 



from chlorophj-ll, but, in succulents, very rich in sap (Aloe, 



Mesembryanthemum, 

 etc.). In both the 

 foregoing modifica- 

 tions the upper layer 

 of the parenchyma 

 ma}' be composed of 

 somewhat longer cells 

 than those below, and 

 to them can be applied 

 the term more gener- 

 ally given to those in 

 the next type, namely, 

 palisade-cells. 



447. The bifacial 

 arrangement has the 

 denser tissue in that 

 part of the leaf which 

 is exposed to the 

 light. This usually consists of several layers of palisade paren- 



PIG. 122. Leaf of Pinus Laricio. Cross-section of a part of the leaf, showing the 

 stomata, hypoderma, and parenchyma. The folded walls of the parenchyma-cells (see 

 208) are plainly shown in the cells below the resin-passage (ffC), where they have been 

 emptied of their contents. ( Kny. ) 



FIG. 123. Transverse section of a leaf of Ilex Aquifolium, showing arrangement of 

 the parenchyma: pp, palisade parenchyma; pc, spongy parenchyma: /i, hypoderma; 

 la, fibro-vascular bundle. Stomata are found only upon the lower surface of the leaf. 

 (Areschoug. ) 



