BULLETIN 101, U. S. DEPARTMENT OF AGRICULTURE. 



GROSS STRUCTURE. 



On any cross section of a log a zone of light-colored wood next to 

 the bark may usually be distinguished from the inner portion of the 

 log. This outside zone is the sapwood; the inner darker portion is 

 the heartwood. In the sapwood many of the cells are used for con- 

 duction and storage of materials necessary in the life processes of the 

 tree ; the outer layer of cells, called the cambium, forms the growing 

 part. The darker color of the heartwood is due to the infiltration of 

 chemical substances into the cell walls, but the cavities of the cells 

 generally are not filled up, as is sometimes believed. Sapwood varies, 

 even within the same tree, in its relative width and in the number of 

 rings which it contains ; the same year's growth may be sapwood in 



Fig. 1. — Wood of spruce. 1, natural size ; 2, small part of one ring magnified 100 times. 

 The vertical tubes are wood fibers, in this case all " tracheids." m, medullary or pith 

 ray ; n, transverse tracheids of pith ray ; a, b, and c, bordered pits of the tracheids, 

 more enlarged. 



one part of the cross section and heartwood in another part. In some 

 species (those of Abies, Tsuga, and Picea) there is no sharp color 

 distinction between the sapwood and heartwood. 



The concentric rings, each of which represents one year's growth, 

 also vary in thickness in different trees and in different parts of the 

 same tree. Each ring consists of two portions; one is an inner, softer, 

 lighter-colored portion formed early in the season, called spring- 

 wood; the other is an outer, firmer, and darker-colored portion, 

 formed late in the season and called summerwood. In some species, 

 as in longleaf pine, the dark summerwood appears as a distinct, 

 sharply defined band, constituting 50 per cent or more of the cross 

 section; in other species, as in white pine, the springwood passes 

 gradually into the darker summerwood. 



