PARENCHYMA OF THE LEAF-BLADE. 



159 



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

 in whicli the whole pulp is composed of chlorophyll parenchyma, 

 but towards its mid- 

 dle plane has larger 

 cells with less chlo- 

 rophyll, aud some- 

 times has conspicu- 

 ous lacunte (many 

 grasses. Yucca flla- 

 uieutosa, Crassula, 

 etc.) ; (2) that in 

 which it is composed 

 of la3-ers which are 

 uniformly distrib- 

 uted above and be- 

 low a middle laj'er 

 of colorless cells free 



from chloroplyll, but, in succulents, ver}' rich in sap (Aloe, 



IMesembryanthemum, 

 etc.). In both the 

 foregoing modifica- 

 tions the upper layer 

 of the parenchyma 

 may 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 

 (jlenser tissue in that 

 part of tlio leaf which 

 is exposed to the 

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



FlG. ]22. Leaf of Piiius Laricio. Cross-section of a part of the ]eaf, Blmwing the 

 stoniata, hypnderraa, ami parenchyma. The fnldeil walls of the parenchvma-oells (see 

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

 emptied of their contents. (Kny.) 



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

 the parenchyma: 2^p^ palisade parenchyma; pr, spongy parenchyma; /i, hypoderma; 

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

 (Areschoug. ) 



