8 BULLETIN 139, U. S. DEPARTMENT OF AGRICULTURE. 
little contrast between early and late growth. Lumber cut from 
slow-growing trees, on dry, sandy soils, is redder in color, more 
resinous, and somewhat harder and more durable than the other. 
There is also a marked difference between the weight and quality 
of lumber cut from young stands and from mature timber, due to 
the percentage of sapwood in the former. Sargent gives the specific 
gravity of dry wood (unquestionably cut from mature trees) as 
0.485. The sapwood of immature trees has a specific gravity of 
0.9, and the heartwood of 0.6. It is this difference between sapwood 
and heartwood which perhaps gives rise to the term "pigiron," 
since second-growth Norway pine with, a wide sapwood would not 
float. In the course of experiments by the Forest Service, under 
the direction, of H. D. Tiemann. small blocks cut from "pigiron" 
floated from 2 to 9 days, while heartwood floated from 3 months 
to 1 year. Thus the floating ability of timber cut from young 
stands can be determined by computing the volume per cent of sap- 
wood and heartwood in the logs. In mature trees the sapwood is 
narrow, rarely exceeding 3 inches. 
In the softer grades of Norway pine the late wood of the annual 
rings does not contrast very sharply with the early wood, as is the 
case with the hard grades, or in the yellow or hard pines, of which 
longleaf pine is typical. Moreover, the late wood usually forms much 
less than one-half of the width of the annual ring. This and the 
slight contrast between the inner and outer part of the annual ring 
gives the wood a rather uniform structure and density, rendering 
it equal to white pine for many purposes. 
Microscopic characters which distinguish Norway pine from other 
pine woods with which it is likely to be confused are the conspicuous 
dentate projections on the inner walls of pith-ray tracheids and the 
large simple pits (from 1 to 2) to each longitudinal tracheid and 
the radial walls of the ray-parenchyma cells. The following ana- 
lytical key will be of assistance to technical students in the identifi- 
cation of red, white, and jack pine: 
Inner walls of pith-ray tracheids without dentate projections. 
One to 2 large simple pits to each longitudinal tracheid on the radial walls of the 
ray-parenchyma cells. Late wood narrow, inconspicuous; wood sparingly 
resinuous White pine (Pinus strobus) 
Inner walls of pith-ray tracheids with conspicuous dentate projections. ' 
One to 2 large simple pits to each longitudinal tracheid on the radial walls of the 
ray-parenchyma cells. Dentate projections regular, short. Late wood con- 
spicuous, not sharply defined from the lighter early wood of the same annual 
ring. Wood usually very resinous Norway pine (Pinus resinosa) 
One to 6 (usually 3 to 6) oval, simple pits to each longitudinal tracheid on the 
radial walls. Dentate projections irregular, long, often branched and con- 
necting across the cells. Late wood very conspicuous, sharply defined from 
the early wood. Wood moderately resinous Jack pine (Pinus divaricata) 
