establishes a three-dimensional orientation to the wood tissue (Fig. 3). A
plane perpendicular to the stem axis is termed the transverse plane,or cross-
sectional plane,also appropriately called the end-grain surface,as represented
bythe end ofa log or board. The tree cross section is analogous to a circle,
and a longitudinal plane passing through the pith of the stem (as would a
radius of the circle) is a radial planeor surface. A plane parallel to the pith
but not passing through it forms a tangent to the circular growth-ring
structure at some point and is termed a tangential planeor surface, at least
at that point. Relative to the anatomical structure of the wood, the tan-
gential “plane” would take on the curvature ofthe growth ring. However,
any slabbed log surface or “flat-sawn” board is accepted as a tangential cut,
ev en ifthe board surface is truly tangential only in a limited central area.
In a small cube of wood, as used for anatomical study, the curvature of
the rings is insignificant, allowing the cube to be oriented to contain quite
accurate transverse, radial, and tangential faces (Fig. 3).
Thin slices or sections of wood tissue, as commonly removed
from the surfaces for study, are termed transverse, radial, and tangential
sections. These tissue sections, as well as the planes they represent, are
often designated simply by the letters X, R,and T,respectively.
In a further exploration of the anatomical nature of wood, gener-
alities must give way to more specific detail according to the type ofwood
considered. A systematic approach is to follow the standard botanical
classification of wood.
Within the plant kingdom, timber-producing trees are found in
the division spermatophytes, the seed plants. Within this division are two
classes, the gymnosperms and the angiosperms. Trees belonging to the
gymnosperms (principally in the order Coniferales) are called softwoods.
In the angiosperms, a subclass known as dicots (dicotyledonous plants)
includes hardwoods.Anatomical characteristics: Softwoods
The cell structure of softwoods is relatively simple compared to that of
the hardwoods (Fig. 4a–c). Most of the cells found in coniferous woods
are tracheids, which account for 90–95% of the volume of the wood.
Tracheids are fiberlike cells with lengths of approximately one hundred6 Hoadley
Figure 3
Block of coniferous wood, Douglas-fir, cut
into a cube along the principal structural
planes: transverse or cross-sectional (X),
radial (R), and tangential (T).