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 



£ur. 



Fig. 1. — Cross section of stem of Quercus prinus (chestnut oak); b. bark 

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

 heartwood; a. r., annual or growth ring; p. r. t (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 



