42 



ECONOMIC WOODS OF THE UNITED STATES 



In ring-porous woods of good growth it is usually the middle 

 portion of the ring in which the thick-walled, strength-giving 

 fibres are most abundant. As the breadth of the ring diminishes, 

 this middle portion is reduced so that very slow growth (fine 

 grain) produces comparatively light, porous wood composed 



ar 



PIG. 13. 



FIG. 13. Quercus macrocarpa (bur oak) : cross section through three entire 

 growth rings showing very large pores in early wood and general absence of dense- 

 walled wood fibres. Such wood is light, soft, and not strong. Magnified 20 

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



FIG. 14. Quercus macrocarpa (bur oak) : cross section through one entire 

 growth ring and parts of two others, showing comparatively small pores (t>) in 

 early wood (e. w.), and presence of abundant thick-walled wood fibres in the late 

 wood (Z. ;.). Such wood is heavy, hard, and strong. Magnified 20 diameters. 

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



mostly of thin-walled vessels and wood parenchyma (Figs. 13, 14). 

 This explains why " second-growth " (i.e., rapidly grown) hickory, 

 ash, and chestnut are stronger than the slowly grown "virgin" 

 stock of the same species. Moreover, in trees of this type there 

 is less early wood formed at the base of a stem than farther up, 



