74 ECONOMIC WOODS OF THE UNITED STATES 



A Resin ducts, both vertical and iiorizontal (fusiform rays), 

 present; scattered *; the vertical appearing on longitudinal 

 surface as fine lines or scratches, light or dark in color. Rays 

 with tracheids. (For B, see p. 80.) 



a Resin ducts plainly visible without lens; numerous to 

 moderately so, and fairly well distributed. (For b, see p. 

 78.) 



a* Tracheids normally without spirals. Resin ducts not constricted 

 but often closed with tylosal outgrowths of the epithelial cells; 

 the latter are thin-walled and normally flattened. Wood paren- 

 chyma only in association with resin ducts, not isolated or zonate. 

 Ray tracheids comparatively large and numerous, in one to sev- 

 eral marginal rows and frequently interspersed in high rays and 

 often entirely composing low rays. Woods with character- 

 istic but not always pronounced resinous odor. Color 

 contrast between heartwood and sapwood usually sharp 

 and distinct. (For b\ see p. 77.) Pine.^ 



a^ Moderate contrast in color and density between 

 seasonal growths; transition between the two por- 

 tions of growth ring gradual; texture uniform. 

 Woods soft to medium, comparatively non-resinous. 

 Color pale straw to reddish-brown. Ray tracheids 

 with upper and lower walls uniformly thickened or smooth 

 (Figs. 4, 5, pp. 25-26). Pits present in the tangential walls 



of the late wood. Soft Pine Group. 



a^ Woods soft, straight-grained. Ray parenchyma cells 

 in early wood with 1 or 2 large simple pits in each cross- 

 field, f (Fig. 4, p. 25.) 



White or Five-leaved Pine Group-J- 



* As a result of injury, compact peripheral rows of ducts may arise as in 

 the case of certain woods in which resin ducts do not occur normally. 



t By " cross-field " is meant the area of intersection of a ray cell and a wood 

 tracheid. The typical condition of pitting is found only in the early wood as 

 the pits may be semi-bordered in the summer wood. 



X The other members of this group (a') are P. flexilis James (P), P. albi- 

 caulis Eng. (R, P), and P. strobiformis Eng. (R). Their woods are of no com- 

 mercial importance. That of P. albicauHs is characterized by resinous tra- 

 cheids. See author's "Significance of Resinous Tracheids," Botanical Gazette, 

 66:1:61-67 (July, 1918). 



