22 ECONOMIC WOODS OF THE UNITED STATES 



Plate IV, Figs. 1,2), (c) terminal, i.e., comprising the outer limit 

 of the growth ring (Plate III, Fig. 6; Plate V, Fig. 2; Plate VI, 

 Fig. 2), (d) surrounding pores (Plate III, Figs. 3, 5), (e) ar- 

 ranged in radial rows. These features are quite important in 

 classifying woods. For example, in Fraxinus americana the pores 

 in the late wood are usually joined tangentially by narrow bands 

 of wood parenchyma, while in F. nigra (Plate V, Fig. 2) the pores 

 are rarely so united. In Hicoria (Plate IV, Fig. 1) wood paren- 

 chyma is in numerous, fine, concentric lines as distinct as the 

 rays, while in Diospyros (Plate IV, Fig. 2) the lines are finer than 

 the rays and very indistinct. In Tilia wood parenchyma is in 

 tangential lines, but is not so disposed in Liriodendron, Magnolia, 

 and dEsculus. In Liriodendron (Plate VI, Fig. 2) and Magnolia 

 the outer limit of the growth ring consists of 24 rows of tan- 

 gentially flattened wood-parenchyma cells with very thick, copi- 

 ously pitted radial walls. 



Wood parenchyma is present in the wood of all Gymnosperms 

 except the Taxacece. The cells are invariably associated with 

 resin formation and are usually referred to as resin cells or epithelial 

 cells, according as they are more or less scattered or surrounding 

 resin ducts. 



Resin cells are usually cylindrical or prismatic, thin-walled, 

 with transverse terminations more or less strongly marked with 

 simple pits. The pits in the side walls are often few and invariably 

 simple. Resin cells can usually be distinguished on cross sections 

 under the microscope by their thin walls, simple pits, or better 

 by the deep color of their contents. If the section passes near 

 enough to an end wall the simple pits therein give the appearance 

 of a sieve plate (Fig. 10). While in most cases resin cells are 

 invisible without the microscope, and often not readily found with 

 it, yet in Juniperus, Taxodium, and Sequoia they are usually 

 conspicuous, not infrequently giving rise in the first two species 

 to wavy tangential lines in the growth ring, visible to the unaided 

 eye. 



The distribution of the resin cells is variable. In some cases 

 (e.g., Thuya) they are scattering; in others (e.g., Taxodium [Plate 

 II, Fig. 1], Juniperus [Plate II, Figs. 3, 4], Libocedrus) they are 

 disposed in well-defined zones concentric with the growth ring, 

 being most abundant as a rule in the transition zone between 

 early and late wood. In still other cases (e.g., Tsuga) there is 

 often a tendency of some of the resin cells to aggregation, and in 



