Series editors' preface - Wood Tools

may be gradual in some woods, abrupt in others.
The early-wood–late-wood contrast may
be slight in some woods (even-grained woods)
or may be pronounced (uneven-grained
woods). In uneven-grained woods such as
hard pines or larches, there may be as much as
a threefold difference in density (relative density
0.3–0.9) from early-wood to late-wood.
Some coniferous species have resin canals,
tubular passageways lined with epithelial cells,
which exude resin or pitch into the canals.
Resin canals are a constant feature of genera in
the family Pinaceae (the pine family), including
Pinus (pine), Picea (spruce), Larix (larch) and
Pseudotsuga (Douglas fir), and are therefore an
important feature for initial identification
screening of unknown wood samples. Resin
canals are largest and most numerous in the
pines, usually distinct to the naked eye. In
other species, magnification may be required
to locate them. The resin from canals may
bleed through paint films and result in yellowish
speckling of finished surfaces.
Some softwoods also have a longitudinal cell
type called longitudinal parenchyma (parenchyma
is the collective name for cells specialized
for the storage of food materials). If
present, these cells represent a very small percentage
of the wood’s volume and are of
importance mainly as an identification feature.
The rays in softwoods are narrow, usually
one cell wide (except for occasional rays with
horizontal resin canals in some species) and
therefore cannot be seen without magnification.
With a hand lens they are barely visible
as light streaks across radial surfaces.
Microscopic examination of softwood rays provides
valuable information on how many cells
high or wide a ray is, the type(s) of cell(s) present
and the type of pitting and other features
present on the cell walls. This information is
very important for identification of softwoods.
Most narrow softwood rays contain two
types of cells, ray tracheids and ray parenchyma.
A softwood ray with both types of cells
is termed heterocellular or heterogeneous. A
ray composed of either ray tracheids or ray
parenchyma alone is termed homocellular or
homogeneous (it should be noted that these
terms have different meanings in hardwoods).
The pitting that occurs between ray
parenchyma cells and early-wood longitudinal
tracheids is called cross-field pitting. Cross-field
Wood and wooden structures 57
Windowlike
pits
Smooth ray
tracheids
Figure 2.5 Radial microscopic views of eastern white
pine, Pinus strobus (a) and Scots pine Pinus sylvestris
(b) showing cross-field pitting. The term ‘cross field’
refers to the area where a ray parenchyma cell (seen
here where the cells run horizontally) crosses an
earlywood longitudinal tracheid (seen here where the
cells run vertically). The two cell types share a cell wall
and the shape inside the cell wall (here the pitting is
described as ‘windowlike’) can be used as a diagnostic
feature in softwoods. An additional diagnostic feature
can be seen in the ray tracheids above and below the
cross fields. In the case of eastern white pine (a) the
ray tracheids are smooth, whilst in Scots pine (b) the
ray tracheids are jagged in appearance and are
described as ‘dentate’
pitting may be window-like (Figure 2.5),
pinoid, piceoid, cupressoid, or taxodoid. For
further information see Grosser (1977),
Hoadley (1990), Phillips (1960), Schweingruber
(1990) and the anatomical features list produced
by the International Association of
Wood Anatomists (IAWA).
2.2.3 Cell structure: hardwoods
In comparing the anatomy of the hardwoods
with that of the softwoods several general differences
are apparent. There are many more
(a)
Dentate ray
tracheids
Windowlike
pits
(b)