ECONOMIC WOODS OF THE UNITED STATES 103 



flecks rare. Yellow Birch. B. lutea 

 Michx. f. (N, C). 

 b' Wood light and soft, not strnij g. Sap-^ 

 vfnn(} thick, whit e. Pith flecks usually- 

 abundant. Paper Birch. B. papyrifera 

 Marsh. (N, Rn, Pn).^" Gray Birch. 

 populifolia Marsh. (N). 

 b* Wood cross-graine d, tough to split, difiicult to 

 work. Growth rings usually indistin ct. Ves- 

 sels sparsely to densely pitted with moderately 

 large bordered pits sometimes scalariform. 

 Rays heterogeneous; 1-5 cells wide, few to 40 

 cells high. Wood parenchyma mostly around 

 vessels, not in tangential lines. Color brown 

 to nearly white. 



a^ Rays mostly 1-2-seriate (sometimes wider). 

 Pores numerous, usually evenly distributed, 

 a^ Pores of medium size. Wood fibres with 

 rather thin walls and large lumina. Wood 

 light (sp. gr., .54), soft, not strong. 

 Tupelo. Nyssa aquatica L. (S, C).^' 

 b^ Pores small. Wood fibres with thick 

 walls and small lumina. Wood com - 

 paratively heavy (sp. gr., .64), hard and 

 strong. Black Gum. N. sylvatica Marsh. 

 (C, N, S). 

 b^ Rays mostly 3-4-seriate. Pores compar- 

 atively large, not very numerous, unevenly 

 distributed. Wood fibres with thin waUs 

 and large lumina. Wood light (sp. gr., .53), 

 soft, not strong. 



Sour Tupelo. N. ogeche Marsh. (S). 



d^ Rays very fine, indistinct, uniseriate. 



a' Growth rings limited by 1-2 rows of thin wood- 

 parenchyma fibres. Pores very numerous. Ves- 

 sels densely pitted; perforations simple. Wood 

 light and soft. 



a* Rays distinct under lens, mostly 10-12 cells 

 high. Pores minute, invisible to unaided eye. 



