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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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