times their diameters. Average tracheid lengths range from 2 mm to 6 mm
among coniferous species, with a corresponding diameter range of approx-
imately 20–60 μm. The relative diameter of tracheids is a basis for classify-
ing texture among conifers. Tracheid size is important to the porosity and
tothe performance of coatings applied to wood.
Across a softwood growth ring, latewood is distinguished from
earlywood by decreased radial diameter and increased cell-wall thickness.
The transition may be gradual in some woods, abrupt in others. The
earlywood-latewood contrast may be slight in some woods (“even-
grained” woods) or may be pronounced in others (“uneven-grained”
woods). In uneven-grained woods such as hard pines or larches, there
may be as much as a threefold difference in specific gravity (0.3–0.9) from
earlywood to latewood.
Some coniferous species have resin canals, tubular passageways
lined with epithelial cells that exude resin, or pitch, into the canals. Resin
canals are a constant feature of some genera in the family Pinaceae (the
pine family), including Pinus(pine), Picea (spruce), and Larix(larch). Resin
canals are largest and most numerous in the pines—they are usually dis-
tinct 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 offinished surfaces. The rays in softwood are
narrow, usually one cell wide (except occasional rays with horizontal resin
canals in some species), and therefore cannot be seen without magnifica-
tion. With a hand lens they are barely visible—appearing as light streaks
across radial surfaces.Anatomical characteristics: Hardwoods
In comparing the anatomy of the hardwoods with that of the softwoods,
several general differences are apparent. There are many more cell types
present in hardwoods, and there is more variation in their arrangement.
Rays in hardwoods vary widely in size, from invisibly small to conspicuous
to the eye. Temperate hardwoods lack normal resin canals.
Hardwoods have evolved specialized conductive cells called vessel
elements, which are distinct in having relatively large diameters and thin
cell walls.They occur in the wood in end-to-end series, and their end wallsC P P W 7abc
Figure 4a–c
Transverse sections of (a) a typical softwood,
spruce; (b) a ring-porous hardwood, oak; and
(c) a diffuse-porous hardwood, maple.