94 
usually of only 2—3 layers, but in L. palustre 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 
