ECONOMIC WOODS OF THE UNITED STATES 7 



Sargent's " Manual of the Trees of North America"), Mono- 

 cotyledons (2 families and 8 genera), and Dicotyledons (62 fam- 

 ilies and 162 genera) . The Monocotyledons are of comparatively 

 slight importance as sources of wood, and for that reason, as well 

 as on account of their peculiar structure,* are omitted from the 

 general discussion of wood and from the key. 



The woods of the Gymnosperms are commonly referred to as 

 ''coniferous woods," "softwoods," and " needle-leaved woods." 

 These terms are inexact since (1) the Taxacece do not bear cones; 

 (2) many of the so-called "softwoods" (e.g., Pinus palustris, 

 Pseudotsuga, Taxus) are harder than many of the so-called "hard 

 woods" {e.g., Populus, Salix, JEsculus, Tilia); and (3) the con- 

 trast in the leaves is by no means always as great as the terms 

 "needle" and "broad" would indicate. Common usage, how- 

 ever, has given these names sufficient definiteness for ordinary 

 purposes, though they should be avoided where scientific exact- 

 ness is desired. 



PITH 



The central portion of the young shoot, branch, and root is 

 composed of loosely aggregated, mostly thin-walled, isodiametric, 

 parenchymatous cells — the pith. It is usually of small diameter, 

 does not increase in size after the first year, in fact, may even in 

 some instances be compressed, and appears to be of only temporary 

 utility to the tree. In some cases, according to Gris (loc. tit.), 

 the cells remain active for several years, and alternately store 

 and give up products of assimilation, especially starch and tannin, 

 according to the periods of vegetation. In such instances the 

 walls of the active cells are thickened and densely pitted. 



The pith in woody stems of Gymnosperms is fairly uniform in 

 shape, size, color, and structure; in Dicotyledons there is great 

 variation. As to outline in cross section: it is star-shaped in 

 Quercus, triangular in Fagus, Betula, and Alnus; ovoid in Tilia, 

 Fraxinus, and Acer; circular in Juglans, Ulmus, and Cornus. 

 In Juglans the color is black; in Gymnocladus it is red; in many 

 others it is brown or gray. In Rhus, Sambucus, and Ailanthus the 



* In adult steins of Monocotyledons the fibro-vascular bundles are scat- 

 tered throughout the central cylinder instead of being disposed in a circle, 

 as in the Dicotyledons. The bundles are closed and the tracheary tissue 

 surrounds the phloem. 



