PART I 



STRUCTURAL AND PHYSICAL PROPERTIES 



OF WOOD 



GENERAL 



Wood of a timber-producing tree may be considered under 

 three general heads, viz., root, stem, and branch. The relative 

 proportion of the three classes of wood in a tree depends on the 

 species, the age, and the environmental conditions of growth. 

 The woody portion of stem and branch has, within certain limits, 

 the same structure. Branches are of less technical value because 

 of their irregular shape and small dimensions. The latter is due 

 to the fact that the number and thickness of the layers of growth 

 are less and the wood elements smaller than in the bole. 



Wood of roots always differs somewhat from that of the stem 

 in form, structure, and distribution of the elements; the growth 

 rings are narrower, the elements have wider lumina, and the 

 wood is as a rule lighter, softer, and more porous. Roots, with 

 occasional exceptions, are a very subordinate source of wood in 

 America. 



Stem wood, on account of its more desirable dimensions and 

 shape and its greater uniformity, is of the greatest utility and 

 value. The form and character of the stem are of greater impor- 

 tance than the relative volume; with few exceptions the more 

 nearly straight and cylindrical and the freer from limbs, knots, 

 and defects, the greater are its technical properties and value. 

 These properties are largely determined by the age of the tree 

 and the inherent characteristics of the species, though affected 

 by environment. Straightness and clearness are materially 

 influenced by density of stand. 



A woody stem, branch, or root is composed of three unlike 

 parts (Fig. 1) . Through the central portion runs a narrow cylinder 

 of soft tissue, the pith. On the outside is bark. Between these 

 two and making up the bulk of the structure is the wood or xylem. 

 The wood, particularly in old sections, usually shows a central 



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