60 THE BOOK OF FORESTRY 
haps the most common defects in wood They repre- 
sent the portion of a branch which is imbedded in the 
tree and are especially common toward the heart. When 
trees have grown in a dense stand the lower branches 
soon die, are broken off by wind or ice and the later 
growth rings cover the knots up. That 1s why a forest 
of old veterans produces clear lumber on the outside 
while the core of the trunk contains many knots. 
Another common defect in lumber is what is known as 
a “shake,” which is the separation of the fibers in the 
trunk. This may occur as a split running from the 
center out along the radius. If several clefts are 
present, the condition is known as “star shake.” Another 
type of this defect is the “cup” or “ring shake.” This 
is caused by the separation of one or more growth rings 
from the adjacent rings. When a log with cup or ring 
shake is sawed up the shaky portion separates from the 
remainder and a bad case may completely spoil the log 
for lumber. Such defects are common in nearly all 
overmature trees, and while usually they occur only 
in the butt log, they may affect the whole trunk and 
some of the larger branches. 
Possibility of Substitution—No other nation uses 
wood as freely as ours and the remark that the civili- 
zation of North America is founded upon wood is as 
true today as formerly. Today we are using two hun- 
dred and sixty cubic feet of wood per year for each 
citizen of our country, an amount twice as much as 
was used fifty years ago and six times as much as is 
used per capita in the German Empire. With the rapid 
growth in the steel and concrete business which has 
taken place during the past decade the prediction has 
been frecly made that these materials would rapidly 
replace wood. While it is true that bridges, houses, 
