ECONOMIC WOODS OF THE UNITED STATES 105 



ments; pits into ray cells simple or half-bordered; 

 no " resin plates." Rays 1-2 cells wide, few to 

 30 cells high; containing dark gum. Red or 

 Sweet Gum, Gum wood, " Hazel," Li- 

 quidambar styraciflua L. (S, C). (Plate VI, 

 Fig. I.) 58 



b 6 Color brown to nearly white, fairly uniform. 

 Lustre high. Pores variable in size and 

 abundance in different species; tyloses ab- 

 sent. Gum ducts absent. Light-colored trans- 

 verse "resin plates" usually present in vessels and 

 fibres of heart wood.* Vessels wholly without spirals. 



a 7 Wood usually rather dense, tough and 

 strong; sp. gr. .5G-.75, average .64. 

 Pores minute, not crowded. Black or 

 Sour Gum, Pepperidge, Nyssa sylvatica 

 Marsh. (C, N, S). 



b 7 Wood rather light and soft; tough but 

 not strong; sp. gr. .40-.56, average about 

 .50. Pores small, crowded. Tupelo, Bay 

 Poplar, N. aquatica L. (S, C); N. biflora 

 Walt. (S). 59 



b 1 Woods mostly light and soft. Rays fine.f 

 a 2 Rays distinct to unaided eye. 



a 3 Growth rings terminated by distinct light-colored 

 line of parenchyma; no parenchyma lines within 

 growth ring. No "ripple marks" on wood. 

 Vessels as below; tyloses few, thin-walled, incon- 

 spicuous. Woods soft but firm, occasional speci- 

 mens rather hard; straight-grained, as a rule. 

 Rays fairly uniform for each species; heterogeneous 

 (marginal cells square or upright); cell walls very thick, 

 abundantly and irregularly pitted; pits into vessels often in 

 groups with common border. Terminal parenchyma in 2-3 

 rows; cells thick-walled and very irregularly pitted (tangen- 

 tial section). 



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

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



t A partial exception occurs in Alnus where aggregate rays are often found. 



