have disappeared; thus they form continuous vessels ideal for sap conduc-
tion. When vessels are cut transversely, the exposed open ends are referred
toas pores. Pores vary in size among and within species. In certain woods
such as chestnut and oak, the largest pores are up to 300 μm in diameter
and can be easily seen without magnification, whereas in some species,
such as holly, the pores are no larger than 40 μm in diameter and are barely
perceptible even with a hand lens. Among hardwoods, pore size serves as a
measure of texture. Oak has large pores and is coarse textured; pear has
very small-diameter pores and is fine textured.
In some species (e.g., oaks, ashes, elms) the largest pores are con-
centrated in the earlywood. Such woods are said to be ring porous;they are
inherently uneven grained and therefore have distinct growth-ring-related
figure. Ring-porous structure results in uneven density and affects wood-
working behavior with characteristics such as uneven resistance to abra-
sive paper or uneven retention of pigmented stains. In certain other
woods (e.g., maple, birch, lime, poplar) pores are more uniform in size
and evenly distributed across the growth ring; these are said to be diffuse
porous. Such woods may show inconspicuous figure, or figure may be asso-
ciated with uneven pigmentation or density offiber mass in the outer late-
wood. Most diffuse-porous woods ofthe temperate regions have relatively
small-diameter pores, but among tropical woods, some diffuse-porous
woods (e.g., mahogany) have rather large pores. A third classification,
semi-ring-porous(also called semi-diffuse-porous), refers to woods in which
the first-formed pores in a growth ring are large, but the pores decrease
in size gradually to small pores in the latewood, without clear delineation
between earlywood and latewood.
Hardwoods have three other types of longitudinal cells: fibers, tra-
cheids, and parenchyma cells. All are uniformly small in diameter (mostly
in the range of15–30 μm) and therefore can be seen individually only with
microscopic magnification. Fibers are present in all woods and are charac-
teristically long and needlelike, with tapering, pointed ends and relatively
thick walls. On transverse surfaces, masses offibers appear as the darkest
areas of the tissue. Thick-walled fibers are characteristic ofhigh-density
woods such as oak and ash. Low-density hardwoods such as poplar have
thin-walled fibers. Tracheids and parenchyma cells range from absent or
sparse to fairly abundant. They are thinner-walled cells than are fibers, and
when they are present in sufficient numbers, the resulting areas of tissue
usually appear lighter in color than adjacent fiber masses.
Rays are quite variable among hardwood species. The size of rays
is expressed by cell count as viewed microscopically on tangential sections,
particularly ray width, or seriation,of the largest rays present. In woods
such as chestnut and willow, the rays are uniseriate(that is, only one cell
wide) and therefore visible only with a microscope. At the other extreme,
such as oak, the largest rays are up to 40 seriate and up to several inches in
height. Rays in oak are conspicuous to the unaided eye.
Rays influence physical and mechanical behavior as well. Rays,
especially larger ones, represent planes of weakness in the wood. Shrinkage
stresses associated with the seasoning ofwood may develop separations,
or checks, through the ray tissue. Also, the restraining effect of the rays
results in differential radial and tangential shrinkage, a common cause of
cupping in flat-sawn boards and of radial cracking in timbers.8 Hoadley