2 BULLETIN 606, U. S. DEPARTMENT OF AGKICULTURE. 
oii 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 
country. A similar study on coniferous woods was published in 
Bulletin 101 of the Department of Agriculture. 
STRUCTURE OF THE HARDWOODS. 
The term u 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 1 
(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 noticeable. 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 layer 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, sunnner- 
1 For a description of the wood of the conifers see Bulletin 101 of the Department of 
Agriculture. 
