1917] 
ZELLER—DURABILITY OF YELLOW PINE 99 
In answer to an article by von Schrenk (716) on the grading 
of yellow pine, Hoxie (716) produces a plate showing the 
cross-sections of three planks of yellow pine heart-wood upon 
which Merulius lacrymans had grown for three years under 
identical, favorable conditions of moisture and temperature. 
The decay was greatest in that plank having the lowest spe- 
cific gravity and at the same time the lowest resin content; 
and the plank having the highest specific gravity and the 
highest resin content was most resistant to decay. This is 
another example to substantiate the conclusions of Mayr 
(294), cited above. Although Hoxie attributes the resistance 
of the heaviest plank to its resin content, it is impossible for 
the writer to draw the same conclusion, for this resistant 
plank, besides having the highest resin content of the three, 
had also a higher specific gravity and narrower growth rings 
than the other two planks. 
Since the specific gravity of wood is to a more or less extent 
a function of the percentage of the summer wood (Johnson, 
93, р. 27) contained, and since the density of wood and Ше 
breadth of the growth rings have been so closely related in 
the grading of coniferous timber, the limited literature deal- 
ing with these several properties of wood will be taken up as 
a whole. 
Density has long been held as an index of the durability of 
wood. Ав early as 1818 McWilliams (1818, рр. 182-183) said 
that ‘‘from the experience of those most deserving of notice 
it appears that the durability of timber is in proportion to its 
solidity." He later defines ‘‘solidity’’ in the following man- 
ner: ‘‘When different sorts of timber are equally dry, the 
respective depths to which they will sink in water is a very 
good criterion of their proportionate solidity.’’ 
Mayr (’86), in a discussion of various species of pine, con- 
cludes that the wood which is heavier, although less resinous, 
is more valuable and durable. Later (’94) he has shown that 
the resin content does not markedly influence the specific 
weight of the wood, and he states that the more heavily ligni- 
fied cell walls of the summer wood offer a mechanical resist- 
