ECONOMIC WOODS OF THE UNITED STATES 45 



P. heterophylla, 50; P. tceda, 55; P. strobus, 30; Tilia americana, 

 65; Juniperus virginiana, 25; Liriodendron tulipij 'era, 20; Quercus 

 alba, 20; Robinia pseudacacia, 12. 



In the same species there generally exists a constant relation 

 between the crown space and the cross-sectional area of the 

 sapwood in the stem. Rapidly growing trees and trees in the open 

 have a larger proportion of sapwood than those of the same species 

 growing in less open stands. In the latter case the number of 

 rings in the sapwood is almost always greater. 



Heartwood in general is of a darker color than sapwood, due 

 to the presence of gums, resins, and other substances. In some 

 genera, however, there is little difference in appearance between 

 these two portions, for example, in Nyssa, Ilex, Celtis, Populus, 

 Salix, Picea, Abies, and Tsuga. 



Change from sapwood to heartwood is never accompanied by 

 increased lignification. Deposition of large amounts of gum or 

 resin materially increases the weight of the wood, and on that 

 account in certain tropical species the heartwood averages fully 

 one-third heavier than the sapwood. 



While physiologically heartwood is that portion of the woody 

 cylinder which does not contain living elements, yet technically 

 only discolored parts are so called, though it of course is without 

 living elements. Branches form heartwood as soon as they cease 

 to grow vigorously, no matter in what part of the crown they are 

 located. In a whorl one branch may be practically all heartwood 

 while none of the others shows any. 



Usually heartwood is commercially more valuable than sap- 

 wood, partly on account of its color, but more especially because 

 of its greater durability under exposure. In grading lumber 

 sapwood is often considered a defect. Important exceptions are 

 found in the use of paper birch for spools, hickory and ash for 

 handles, spokes, etc., woods for manufacture of pulp, and timber 

 to be impregnated with preservatives, where heartwood is con- 

 sidered undesirable. 



The average thickness of the sapwood and the character of the 

 demarcation between heartwood and sapwood are features fre- 

 quently made use of in classification. 



References 



Roth, F.: Timber, Bui. 10, U. S. Div. of Forestry, p. 13. 

 Boulger, G. S.: Wood, p. 17. 



