94 



usually of only 2 — 3 layers, but in L. illustre frequently of 

 from 5 to 6 layers of palisade-cells of medium height. 



The spongy parenchyma, which comprises about one-half 

 of the thickness of the leaf, consists in the older leaves of 

 cells which are either non-branching or branch only slightly, 

 and which unite so as to form intercellular spaces at fairly 



Fig. 11. Ledum palustre. 

 Stem in transverse section; slightly mag. (Greenland). (Phot.) 



regular intervals. In these spaces, in older leaves, traces of a 

 previous tissue may be found in the form of empty cells or 

 portions of cell-walls. In the young leaves these spaces are 

 entirely filled with a closely-packed tissue consisting of trans- 

 parent cells which are doubtless filled with cell-sap and which 

 perhaps serve as reservoirs of water and are of importance as 



