12 TREES OF THE NORTHERN UNITED STATES 
terial very durable and fit for furniture. A stem of this 
kind is called endogenous (inside-growing). Fig. 1 repre- 
sents a longitudinal and a cross section of an exogenous 
stem, and Fig. 2 of 
an endogenous one. 
Since all the stems 
with which we have 
to deal are exogens, a 
particular description 
of that class will here 
H] be given. Fig. 1 shows 
‘WW the appearance of a 
section of an Ash 
stem six years old. 
The central portion, 
which is about as thick as wrapping-twine, is the pith; 
from this outward toward the bark can be seen the six 
annual layers of the wood ; and then comes the bark, con- 
sisting of two portions. First there is an inside layer of 
greenish material, the fresh-growing portion, and lastly 
the outer or dead matter. This outer portion“must crack 
open, peel off, or in some way give a chance for the con- 
stant growth of the trunk. The different kinds of trees 
are readily known by the appearance of the bark of the 
trunk, due to the many varieties of surface caused by the 
allowance for growth. None of the characteristics of 
trees afford a better opportunity for careful observation 
and study than the outer bark. 
The Birches have bark that peels off in thin horizontal 
layers—the color, thinness, and toughness differing in the 
different species; the Ashes have bark which opens in 
many irregular, netted cracks moderately near each other; 
the bark of the Chestnut opens in large longitudinal 
cracks quite distant from one another. The color of the 
bark and the character of the scales are quite different 
in the White and the Black Oaks. 
In the woody portion radiating lines may be seen; 
