6 



ECONOMIC WOODS OF THE UNITED STATES 



colored portion, the heartwood, and a nearly colorless outer border, 

 the sapwood. In fresh-cut green sections the sapwood is further 

 differentiated by its greater moisture content. 



Indigenous arborescent plants are readily separable into two 



FIG. 1. Cross section of stem of Quercus prinus (chestnut oak); &, bark 

 showing outer and inner portions; s. w., sapwood; the darker inner portion is 

 heartwood; a. r., annual or growth ring; p. r., (pith.) ray, a large number of which 

 can be seen crossing the growth rings at right angles. Note season checks. Natural 

 size. (From Bui. 102, U. S. Forest Service.) 



great natural classes: I, Gymnosperms, and II, Angiosperms. 

 Class I is further divided into two unequal groups: Coniferce (13 

 genera), and Taxacece (2 genera). Class II embraces (according to 



