2 BULLETIN" 606, U. S. DEPARTMENT OF AGRICULTURE. 



on penetration, but were purposely eliminated from this study in 

 order that the variables introduced by the structure of the wood 

 could be studied independently. This point should be kept in mind 

 when applying the results to the treatment of wood in commercial 

 plants. 



The experiments were made at the Forest Products Laboratory, 

 maintained by the Forest Service, United States Department of 

 Agriculture, in cooperation with the University of Wisconsin. The 

 results should assist in establishing a basis for the grouping of 

 species to secure the best treatment and the most efficient use of the 

 preservative. 



This bulletin is a study of the resistance to injection with creosote 

 of some of the more important hardwood species native to this 

 countn r . A similar study on coniferous woods was published in 

 Bulletin 101 of the Department of Agriculture. 



STRUCTURE OF THE HARDWOODS. 



The term ' ; hardwoods " as here used means those trees which have 

 comparatively broad leaves and do not bear cones. The hardwoods 

 (Angiosperms) differ in structure from the softwoods or conifers x 

 (Gymnosperms) chiefly in the possession of pores or vessels 

 (tracheae). For this reason the conifers are frequently called non- 

 porous woods and the hardwoods either diffuse-porous or ring- 

 porous, according to the arrangement of the pores or vessels in the 

 annual ring. The different varieties of specialized cells and fibers 

 and the arrangement of the cells in less uniform rows make the struc- 

 ture of the hardwoods much more complex than that of the conifers. 



GROSS STRUCTURE. 



Sapwood and heartwood. — The lighter-colored layer of wood on 

 the outer circumference of the tree contains the living cells, and is 

 known as the sapwood. In some species, such as aspen, the difference 

 in color between sapwood and heartwood is hardly notieeable. As 

 trees grow older the cell walls tend to become more and more in- 

 filtrated with various substances, which are often responsible for the 

 color of the heartwood. During the change from sapwood to heart- 

 wood alterations sometimes occur in the pores, which, as a result, have 

 a tendency in most species to become clogged or closed up. 



Annual rings. — Each year the tree adds a fresh la} r er of wood sub- 

 stance under the bark around its circumference. These layers are 

 called annual rings. The part which is formed early in the year is 

 called springwood; that which grows later in the season, summer - 



1 For a description of the wood of the conifers see Bulletin 101 of the Department of 

 Agriculture. 



