192 College of Forestry 
the accumulation of carbon at the expense of oxygen and 
hydrogen. The cell contents and certain other substances, 
particularly the hemicellulose xylan (wood gum), that origi- 
nally were combined with the cellulose to constitute the cell- 
wall, furnish the formative material which coagulates to 
thick drops and gives rise to the humic by -products “of decom- 
position which ultimately infiltrate certain portions of the 
wood, causing them to appear as blackish zones. 
By the action of a strong oxidizing reagent on fresh 
sap-wood a brown humic product can be prepared artificially, 
which is essentially like that occurring naturally in wounded 
areas of living trees, dead wood, or as the result of their decay 
by Polyporus pargamenus and other wood-destroying fungi. 
The properties of the extracts obtained respectively from 
sound and decayed hickory wood indicate that in the course 
of the decomposition a portion of the carbohydrate substance, 
particularly the hemicellulose xylan, becomes converted into 
humic substances of two prince ipal groups, namely humie acid 
and humin. It is the accumulation of these humic substances 
which give rise to the dark brown decomposition products 
which are of such common occurrence in decaying wood. 
The chemically resistant by-products often forming black- 
ish zones in wood decayed by Polyporus pargamenus have 
proven to be a group of substances analogous to the humic 
products which arise under certain circumstances in wounded 
parts of living trees or in fallen woody parts that may be 
entirely free from fungous attack and which have been known 
under the name “ wound gum.” In wood free from fungous 
attack the formation of the brown decomposition product is 
mainly dependent upon the concurrence of three factors 
(a) The presence of dead cells; (b) an optimum supply of 
moisture; and (¢c) a supply of oxygen sufficient to promote 
oxidation. The formation of the brow n product 1 in this case 
is likewise a sign of the humification which sets in gradually 
when their walls and contents undergo decomposition. 
In general the manner and extent of the decay varies 
greatly, not only with the species of tree, but also with the 
development of the heartwood and its chemical and physical 
