34 WESTERN YELLOW PINE IN ARIZONA AND NEW MEXICO. | 
| 
On a smooth transverse section of the wood the annual rings of © {] 
erowth are distinctly visible, though more so in wood having wide | 
rings (fig. 6, @) than in wood with very narrow rings (fig. 6, c). 
The width of the annual rings varies greatly in different trees (fig. | 
1, a, b, and c), which in great measure accounts for the difference in 
the character and quality of the wood. Very old trees, as well as 
young ones growing on poor soil, invariably develop narrow annual 
rings. Such wood has a more uniform structure than that with 
wider rings, and superficially often resembles white pine. Thrifty, 
rapidly grown trees have wood with wide annual rings, in each of 
which there is a conspicuous contrast between the soft early wood and | 
the hard late wood. di 


a NEG E EINE NAL ATT TE 
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Fic. 6.—Various types of western yellow pine wood as shown in smooth transverse sec- 
tions photographed natural size; e. w., early wood; J. w., late wood; r. d., resin ducts. 
The contrast between the wood formed in early spring (fig. 6 a, 
e. w.) and that formed later (fig. 7 a, 7. w.) is due partly to the 
| general reduction in the radial diameter (fig. 7, f. ¢.) of the cell ele- 
ments, as growth advances from the early to the late wood, and 
partly to the greatly increased thickness of the cell walls (fig. 7, 
1. w.) of the late-wood portion of the ring. The cell cavities in the 
wood formed in the early spring are usually much larger, and the 
cell walls thinner than in the wood formed later. (Fig. 7 0, e. w.). 
These two parts of the annual ring show a gradual transition from 
one to the other (fig. 7 a, a. 7.) which is characteristic and often 
serves to distinguish western yellow pine wood from that of other — 

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