and Sherard, 1937; Virtanen, Koistinen and Kiuru, 
1988). Additional evidence confirming the ‘‘protective’’ 
effect of lignin has been obtained by the author in a series 
of experiments utilizing a species of the cellulolytic bac- 
terium Cytophaga. In a series of liquid cultures each 
containing mineral salts, filter paper and thin sections of 
various woods (Pinus, Sequoia and the hardwood T'rocho- 
dendron), it was found that the filter paper was com- 
pletely broken down within a period of a week, whereas 
the various wood sections were unaffected after six weeks 
incubation. 
In addition to lignin, other normal constituents of the 
cell wall may serve to inhibit biological degradation of 
cellulose, viz., resins, terpenes and tanniniferous sub- 
stances. Differential resistance of cell wall layers might 
logically be ascribed, therefore, solely to the effect of 
these encrusting ‘‘protective’’ substances of which lignin 
is undoubtedly the most significant in anaerobic degra- 
dation of woody tissues. In further support of this ex- 
planation of the selective breakdown of the cell wall, it 
should be emphasized that the most resistant layer of the 
secondary wall, viz., the outermost or first formed layer, 
is often far more heavily lignified than the broad central 
layer (Bailey and Kerr, 1985). Moreover, the primary 
wall, the most resistant of all the lamellae of the cell 
wall, is intensely lignified (Ritter, 1925; Kerr and 
Bailey, 19384). 
The effect of degrees of lignification in different lamel- 
lae of the cell wall often produces striking morphological 
effects on the mode of attack of wood by various fungi. 
Fungi belonging to diverse genera of the Pyrenomycetes 
and Fungi Imperfecti commonly degrade wood by dis- 
solving the central and often the innermost layers of the 
secondary wall, but leaving the outermost layer and the 
primary wall visibly unaltered (Bailey and Vestal, 1937; 
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