ECONOMIC WOODS OF THE UNITED STATES 77 



b 4 Woods varying from moderately to extremely 

 heavy and hard. Southern Pines.* 9 



a 5 Woods usually very dense and resinous, with 



large proportion of sharply defined late wood. 



a 6 Growth rings mostly wide, variable. Sap- 

 wood thick. Sp. gr. .50-.90, usually be- 

 tween .65 and .75. Slash or Cuban Pine, 

 P. caribaea Mor., P. heterophylla (Ell.) 

 Sudw., or P. cubensis Gris. (S). 10 



b 6 Growth rings mostly narrow beyond first 2 

 or 3 inches of radius, fairly uniform. Sap- 

 wood thin. Sp. gr. .50-. 90, usually be- 

 tween .60 and .70. Longleaf Pine, Georgia 

 Pine, P. palustris Mill., or P australis 

 Michx. f. (S). 11 

 b 5 Woods usually moderately dense and resinous, 



widely variable; medium to small proportion 



of late wood not always sharply defined. 



a 6 Growth rings variable, narrow to extremely 

 broad. Sapwood thick. Sp. gr. .40-. 80, 

 usually between .45 and .55. Loblolly or 

 North Carolina Pine, P. taeda L. (S). 12 



b 6 Growth rings fairly regular, broad toward 

 pith and narrow beyond 5-7 inches. Sap- 

 wood rather thick. Sp. gr. .40-. 80, usually 

 between .45 and .55. Shortleaf Pine, P. 

 echinata Mill., or P. mitis Michx. (S). 13 



b Tracheids normally with spirals.! Resin ducts widely variable in 

 size and arranged often in short tangential groups; ducts con- 



* Specific identification of the southern pines is very uncertain. Since the 

 mechanical and physical properties of the woods are factors of the density, 

 classification for commercial purposes is made on that basis. 



f The spirals in the tracheids serve to distinguish the wood of Pseudolsuga 

 from that of all others resembling it. The tracheids of Taxus and Tumion 

 are spiralled but the woods are wholly devoid of wood parenchyma, resin 

 ducts, and ray tracheids. The sporadic occurrence of true spirals in Picea, 

 Larix and Pinus has been noted and in the rare instances where such is the 

 case the other anatomical features must be taken into account. Spirals, 

 which are thickenings upon the inside of the secondary wall, must not be con- 

 fused with striations which are slits or cracks running spirally in the thick 

 walls of the late wood and of " compression wood " (rotholz) of many conifers. 



