6 



ECONOMIC WOODS OF THE UNITED STATES 



colored portion, the heartwood, and a nearly colorless outer l)order, 

 the f<aptrood. In fresh-cut green sections the sapwood is further 

 ditferentiated by its greater moisture content. 



Indigenous arborescent plants are readily separable in o two 



Fia. 1. — Cross section of stem of Quercus prinus (chestnut oak); 5, bark 

 showing outer and inner portions; s. w., sapwood; thje 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 growtli rings at rjgjit 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: Conifercv (13 

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



