pretation is strongly supported by the fact that primary 
wall cellulose often persists after degradation of the cel- 
lulose of heavily lignified layers of the secondary wall. 
For this and other reasons it seems difficult to interpret 
selective degradation of cellulose in different parts of the 
plant cell wall except in terms of chemical differences and 
resistance to hydrolysis in successively formed lamellae. 
The persistence of the primary and occasionally the 
outermost layers of the secondary cell wall is strikingly 
shown in many silicified woods. Not uncommonly, sili- 
cified wood, regardless of geologic age, contains unusual 
amounts of organic residues, occasionally present to such 
an extent that the demineralized wood may be embed- 
ded, sectioned and stained much as living tissue (Arnold, 
1981; 1941). In such material of diverse groups of 
plants which may range in age from Devonian to Ter- 
tiary, there is no cellulose remaining; it seems likely that 
silicification took place at a time when degradation of 
the cell wall had not yet passed the stage of the partial 
retention of a cellulosic structural residue. In other 
words, the cellulosic framework of the tissues was re- 
tained for a sufficient length of time to allow preservation 
of structure before silicification began. After or during 
the silicification process the remaining cellulose was lost 
from the tissue, leaving a modified but coherent lignin 
residue. ‘This modified and silicified residue simulates 
the original cellulosic residue of anaerobically degraded 
wood (compare Plate I, Figs. 1 and 8 and Plate II, Figs. 
2 and 8 with Plate V, Figs. 1, 2 and 5). 
Whether the greater resistance to degradation of the 
cellulosic lamellae of the primary wall and the outermost 
secondary wall of plant cells is due to intrinsic chemical 
factors or primarily to physical protection by extraneous 
non-cellulosic substances cannot be determined without 
further codrdinated histological and chemical study. 
[15 ] 
