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FOR 4362

Wood Structure andForest Products

Lecture 3 Topics:

- Softwood Structure

Softwoods: the mainstay of thewood products industry Straight-grained

Light in weight

Fasteners attach easily

Relatively homogeneous

Long-fibers strong paper

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Longitudinal Tracheids

Makes up 90-95% of softwood

Characteristics

About 100x greater in length than diameter

3-4 mm in length

25-45 m (microns) in length (25-45

millionths of a meter)

Rectangular in cross section

Hollow centers (lumen)

Closed at the end Pits are normally bordered

Earlywood to latewoodtransition in softwoods

Abrupt transition

Douglas-fir (below)

Hard pines

Larch

Gradual transition

True firs

Hemlock

Eastern white pine

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Pits in tracheids

Bordered pits are tracheid totracheid passages

Remember that in each pit is athickened central torus supportedby a microfibral network known asthe margo.

If the membrane shifts and thetorus blocks the pit, the pit is said tobe aspirated.

Aspirated wood is resistant topenetration by wood preservatives

Aspiration can be caused by thedrying process

Aspiration is more common in:

Sapwood to heartwood transition

Earlywood

Longitudinal parenchyma

1-2% of softwood volume

Same general shape as longitudinal

tracheids, though subdivided by

secondary cell walls along their length.

Mature parenchyma a series of short cells

end-to-end

Thin walled

Simple pits

Tangential view showing ray

parenchyma

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Epithelium

Epithelial cells line resin canals

Epithelial cells secret resin

Longitudinal resin canals form whencells that fail to develop secondarycell crosswalls and remain thinwalled these are epithelial cellsand are visible in cross-section ofwood

Horizontal resin canals accompanylongitudinal canals

Horizontal canals are present insome of the rays

Traumatic resin canals form inresponse to injury

Larger and occur in tangentialbands at start of growth ring.

Rays

Uniform narrow rays are the rule in

softwoods except when horizontal

resin canals are present

Uniserate rays are one cell wide and

many cells in height (tangential

view)

Redwood rays are biserate (2 cellswide)

Rays may be composed of ray

parenchyma or ray tracheids

Fusiform ray contains a resin canal

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For test. Study Figure 4.12 and accompanying

text, pages 75-77 in Chapter 4 of Shmulsky!

Silviculture and softwoods

Faster growth means tracheids with larger

diameter (more earlywood formation)

larger growth rings

Lower specific gravity less strength

Pruning means fewer and smaller knots

more clear, straight-grained wood and thus

more strength and uniformity

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