Introduction
1 All the figures are for conifers and on dry wieght basis.
Major constituents of wood are cellulose, lignin and hemicelluloses. Small amounts of pectic material, starch and other extraneous substances are also present. The ash content of wood seldom
exceeds 0,5%..) (2) o Normal hardwoods and softwoods usually contain
42?2 percent of cellulose. The lignin content of hardwoods varies
from IS to 25%, while in softwoods the range is 25 to 35%. A partly
acetylated5 acidic xylan accounts for 20 to 35% of the wood of hardwoods, with a second hemicellulose, a glucomannan occuring in small
amounts. A partly acetylated galacto-gluco-mannan makes up almost
20% of coniferous wood, but the xylan, .in this case, comprises only
10% of the total wood substance(2).
S3
In a typical wood cell, the primary wall forms an envelope
for the secondary wall, which in turn, encloses the lumen. The primary wall, as well as the intercellular substance, are highly lignified (71 per cent lignin and 14 per cent pentosan in Douglas Fir) ,
whereas the secondary wall is largely cellulose ( over 50% in 8^
and slightly less in layer) , though lignified ( 13 to 25% in
$2 and 11 to 13/j in layer). The hemicellulose content is about
20% in the primary wall, 25 to 30% in the So, and 30 to 35% in the
layer ( 37)—. Regions of. approximately parallel change alternate
with less ordered regions in the secondary wall cellulose. The
molecules in the crystalline regions and to a certain extent, in
the amorphous regions, are held primarily together by a large
number of hydrogen bonds while Van der Waal’s forces probably make
some contribution to the cohesion.