prise five layers viz., the outermost secondary wall layer 
and the primary wall of the two contiguous cells plus the 
true intercellular substance between the adjacent primary 
walls. The five-layered structure of these tenuous ‘‘mem- 
branes”’ is revealed by their relation to the pit border of 
bordered pit-pairs. This relationship is most easily ob- 
served in the late wood tracheids of coniferous woods, as 
shown in Plate III, Figs. 1, 2 and 3 (also compare Plate 
ILI, Fig. 8 with Fig. 5). The pit border is solely a struc- 
tural feature of the secondary wall. It is apparent, there- 
fore, that the tenuous innermost layer of the pit border, 
shown in Plate III, Figs. 2 and 8, is the resistant outer- 
most or first formed layer of the secondary wall. Hence, 
it may be deduced that the persistent fraction of the cell 
wall in these tissues consists in large part of the remaining 
outermost layers of the secondary wall of two adjacent 
cells, including of course the retained and still more ten- 
uous primary walls and true intercellular substance. That 
the persisting or visibly intact cell wall is actually a very 
minor part of the original unmodified secondary wall is 
shown in thin sections in which cells possessing intact 
secondary walls are intermingled with cells exhibiting 
extensively degraded secondary walls (Plate I, Figs. 1 
and 2 and Plate III, Fig. 1). The reason or reasons for 
the retention of the entire secondary wall in certain cells 
which are in direct contact with others in which these 
wall layers are extensively degraded remain obscure. 
It should be emphasized in connection with its greater 
resistance to degradation that the outermost or first 
formed layer(s) of the secondary wall possesses quite dif- 
ferent staining reactions, optical properties and physical 
behavior from those of the broad central layer. The first 
formed layer is more heavily lignified than the central 
layer and shows far less tendency to swell in strong min- 
eral acids, such as 72 per cent sulfuric acid or 40 per cent 
[ 8] 
