Gross features
In viewing end-grain surfaces (Fig. 2a, b), individual wood cells usually
cannot be seen without magnification. (In certain hardwood species, the
largest cells, vessel elements, may be evident as visible pores on cleanly cut
surfaces.) However, the familiar pattern of circular growth rings is appar-
ent, concentrically arranged around the central pith. Within each ring,
depending on the species, afirst-formed earlywood layer may be distinct
from an outer latewood layer. The visual pattern, or figure,on longitudinal
board surfaces is most commonly the result of this earlywood-latewood
variation. Distinct earlywood-latewood contrast usually indicates variation
in cell characteristics, with latewood having greater density than early-
wood. In some woods, however, there may be no significant difference in
properties within growth rings.
Individual wood cells usually have an elongated shape, although
they vary in proportions, from short and barrel shaped to long and needle-
like. Most cells are longitudinal; that is, they are elongated vertically in the
standing tree, parallel to the stem axis. On an end-grain surface we there-
fore see these cells in cross section. Scattered through the longitudinal
wood cells are horizontally oriented ray cells, grouped to form flattened
bands oftissue called rays. These ribbonlike rays (with their flattened sides
oriented vertically) radiate horizontally outward from the pith, crossing
perpendicularly through the growth rings. Individual ray cells are always
too small to be seen without magnification, and therefore narrow rays are
not apparent. However, some hardwood species have rays of up to tens of
cells in width, which are therefore visible as distinct radial lines on cross
sections. Collectively, the ray cells in most species account for less than
10% of the volume of the wood. It is important to understand that rays
are present in every species and, whether visible or not, have an important
role in many properties of wood.
The cylindrical form and arrangement of the growth rings in the
tree stem, along with the vertical and horizontal arrangement of cells,C P P W 5Figure 2a, b
Cross-sectional (end-grain) surfaces of
(a)an uneven-grained softwood, Scots pine,
and (b) a ring-porous hardwood, white oak,
showing features visible with a hand lens
(10 3 magnification).
a b