wall. The incidence of degradation of the lamellae is in 
the following order: 
(1) Central layer of the secondary wall 
(2) Innermost layer of the secondary wall 
(8) Outermost layer of the secondary wall 
(4) Primary wall’ 
Progressive stages in the deterioration of the cell wall 
are shown in Plates I-IV. Three outstanding features 
may be noted in these thin sections of degraded wood: 
(1) areduction of the major portion of the secondary wall 
to a granular, virtually amorphous residue; (2) the reten- 
tion of a structurally intact cell wall ‘‘layer’’ or ‘‘mem- 
brane’’ which corresponds in position to the primary 
wall; and (8) the presence of isolated or contiguous 
groups of cells whose secondary walls are relatively un- 
altered. 
That the conspicuous granular remnants of degraded 
wall layers are essentially lignin residues may be demon- 
strated by their total extraction during delignification 
(compare Plate I, Figs. 1, 2 and 8 with Plate III, Figs. 
1, 2 and 38). The visibly intact, persistent layers, or oc- 
casionally entire cell walls, are truly cellulosic, as may be 
shown by their brilliant birefringence in the microscope 
when viewed between crossed Nichols. Birefringence in 
the thinner cellulosic residual layers is greatly accentu- 
ated by delignification. 
Because of their extremely tenuous character, certain 
structural residues, such as those in Plate I, Figs. 1, 3 and 
4 and Plate III, Figs. 1, 2 and 8, might be assumed to 
consist solely of primary walls. The composition of these 
tenuous ‘‘membranes,’’ however, can be shown to com- 
‘The term primary wall refers here specifically to the wall layers 
of individual contiguous cells exclusive of the true intercellular sub- 
stance which is often erroneously, and confusingly, included as a part 
of the ““primary wall’’ (Kerr and Bailey, 1934). 
[7] 
