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 2.1.3.3 Tangential. The tangential surface is exposed when a board is cut parallel
to the bark and tangent to the log diameter. The lumber is called flat or plain sawn. This is the most
common way of producing lumber today, and it results in a characteristic U or V-shaped grain pat-
tern (Figure 2-2) in the softwoods with distinct earlywood and latewood bands, such as southern
pine and Douglas-fir, and the coarse grained hardwoods such as oak.
2.1.3.4 Interlocked. Interlocked grain occurs when the longitudinal wood cells
spiral normally in one direction for one to several years, and then reverse direction. Interlocked
grain is characteristic of gums. Woods with interlocked grain are difficult to split, may shrink some-
what more longitudinally than normal upon drying and often warp excessively. Figure 2-5 shows a
characteristic checking pattern in a gum railroad tie due to interlocked grain.
2.1.4 Sapwood And Heartwood. The wood formed immediately inside the bark of a tree
is called sapwood. Sapwood is light in color and contains living cells that transport water from the
roots to the branches and leaves at the top of the tree. Heartwood is formed in the central part of the
tree stem, as those water conducting cells die.
The relative thickness of sapwood and heartwood in stems of various tree species is an important
characteristic. Heartwood of some tree species is difficult to treat with preservatives, but sapwood
of all species accepts preservatives. Heartwood may compose most of the stem in mature Douglas-
fir and oak. Southern yellow pine poles and lumber readily accept preservatives because these trees
and lumber are mostly sapwood (Figure 2-1). By contrast, poles and lumber cut from mature
Douglas-fir trees contain a high proportion of impermeable or refractory heartwood that is difficult
to treat. The heartwood of red oak is permeable whereas that of white oak is some of the most im-
permeable wood. It is this type of species, and variable treatability, that shapes the grouping of
wood products by species in standards for wood preservative treatments.
Very few, if any, differences exist in the mechanical properties of sapwood and heartwood.
Heartwood of some species may have a slightly higher weight per unit volume than sapwood due to
the presence of significant amounts of extractives. Due to an accumulation of various materials, ex-
tractives, pitch, oil, and other extraneous substances, the heartwood is often darker than sapwood.
2.2 MICROSCOPIC STRUCTURE. Wood is composed of many individual cells held together
much like a hand full of soda straws. The cells that contribute most to wood strength are oriented in
a longitudinal direction, essentially parallel to or with a very slight deviation from the long axis of
the tree stem. Other cells that store food or produce extractives are of different shapes and structure.
The presence or absence of certain types of cells, their size, and the manner in which these cells are
arranged determines the properties of any one particular wood species, and are critical keys in the
accurate identification of wood species. The microscopic structure of softwoods and hardwoods is
summarized below.
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