Humphrey: Durability of American Woods 
81 
balance in plant life, for where trees develop luxuriantly they 
must also rot rapidly. 
The manner and rate at which different species of fungi act on 
given kinds of wood is often highly variable, more or less a spe- 
cific property of the fungus. Under natural conditions certain 
species seem exclusively or predominantly to attack conifers, while 
others confine their activities to hardwood (broadleaf) timber. 
Still others may attack almost exclusively hardwood trees in one 
region and conifers in another. In some cases this may be an 
apparent rather than a real condition, however, since our judg- 
ment as to the identity of a fungus rests upon the morphological 
characters of the fruit-bodies which it produces and it is an oft- 
observed fact that many decaying timbers fail to develop the fruit- 
ing stage of the organism at all. 
Laboratory Tests on Durability 
Comparatively few laboratory tests on the durability of timber 
are recorded either in European or American literature. The 
European continent, however, has few timber species compared 
with the great variety on the American side. 
Through the activities of the United States Forest Service 
strength values and other physical properties of our commercial 
woods have been worked out, but this paper is the first of a series 
on the relative resistance which the different timbers offer toward 
different fungi. It is proposed to carry on the work from year to 
year, using various wood-rotting fungi and adding new kinds of 
timber as they are made available. 
The work was begun about three years ago at the Forest Prod- 
ucts Laboratory on material collected from all portions of the 
United States primarily for timber test purposes. Both heart- 
wood and sapwood were secured whenever possible, but since the 
available material comprised in many cases only corners and slabs 
remaining after sawing out the specimens for strength tests it was 
not always possible to get everything desired. In the case of hem- 
locks, firs and spruces heartwood and sapwood are poorly differ- 
entiated as far as color changes go, the differences in physiological 
functions in the living trees, however, are just as marked as in 
