1 softwood microanatomy softwoods general information overview of structural cells wood appearance...
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Softwood Microanatomy • Softwoods
• General information• Overview of structural cells• Wood appearance• Importance of pitting
• Component cells and tissues• Longitudinal tracheids• Resin canals• Rays• Longitudinal Parenchyma
• Crossfield pitting
FW1035Lecture 4Bowyer et al – Chapter 4
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Softwoods
• Tree Appearance• Needle or scale-like
foliage• Trees have ‘excurrent’
form (dominant trunk with lateral branches)
• Distribution• Worldwide, less common
than hardwoods• North America has a
large standing inventory
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Softwoods - Why are we Interested in them?
• Softwoods are the foundation of the forest products industry in North America• structural construction
lumber (light and straight grained)
• pulp and paper - long fiber cells
• plywood industry
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Cell Types in Conifer Wood
• Longitudinal Tracheids• Dominant cell (90-95% of wood volume)• Elongated 75-100 times their diameter
(aspect ratio)• Occur in radial rows (or “ranks”)• Denser latewood from smaller radial
diameter (flattening of the cell) and thicker wall
• Rays - generally uniseriate, except when a ray contains a resin canal
• Resin canals - Found in 4 genera• Pinus (Pines)• Picea (Spruces)• Larix (Larches)• Pseudotsuga (Douglas-fir)
Longitudinal Tracheids• Compose the bulk of xylem
(90-95%)• Shape and proportions are
similar amongst softwood species
• Radial diameter varies with position in growth ring
• Tangential diameter remains constant and leads to judgements of texture
• Conductive and support functions
Spiral Thickening in Longitudinal Tracheids
• Also called “helical thickening”
• Usually smooth inner cell walls
• Some species have spiral thickening on inner walls• Fine, helical ridges
• Found in Douglas-fir and western yew, plus some other minor species
Pitting in Softwoods
• Fluids transported between cells through pits - no vessels like hardwoods
• 3 types of pit pairs• Bordered pits are the most
common pit type in softwoods- bulk of xylem is composed of longitudinal tracheids
• Appearance of half-bordered pit pairs between ray parenchyma and longitudinal tracheids is important for microscopic ID - “crossfield pitting”
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Bordered Pit Pair Aspiration • Response to injury,
drying, or conversion of sapwood to heartwood
• Fibers of the margo are flexible enough to distort in response to pressure
• Torus moves and contacts the inside surfaces of the pit aperture and seals the pit pair
• Earlywood pit pairs aspirate more easily than latewood pits
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• Also called strand parenchyma
• Appear as chains of short, vertically oriented cells
• Formed by fusiform cambial initials
Longitudinal Parenchyma
Longitudinal Parenchyma
• Appear as thin walled cells on the cross section
• May have dark-colored contents
• Compose as much as 1-2% of the wood volume in:• Redwood (Sequoia spp.)
• Cedars (Thuja spp.)
• Pines (Pinuss spp.)
• Typically, only visible with hand lens in redwood and bald cypress.
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Resin Canals
• Conduct resin secreted by specialized parenchyma cells called epithelial cells lining the canal opening
• Seals wounds from insects or mechanical damage
• Occur oriented in the longitudinal direction and in the radial direction (within fusiform rays)
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More on Resin Canals
• Occur in:• Pinus spp.
• Larix spp.
• Picea spp.
• Pseudotsuga menziesii
• Useful for wood ID
• May need hand lens to see
• Appearance varies with presence/absence of resin
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More on Resin Canals
• Occurrence:• Pines - large and numerous
(all growth rings and even distribution)
• Spruces, larches, Douglas-fir - more sporadic distribution
• may not be in all rings• commonly in tangential
groups
• Pines have thin walled epithelial cells (easily damaged in cutting)
Douglas-fir
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Traumatic Resin Canals
• May occur in species that do not normally have them
• Response to injury• Appear as single,
continuous line• Different appearance
from normal resin canals
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Conifer Rays
• Almost always uniseriate (except fusiform rays)
• Non-fusiform rays are composed of ray tracheids and/or ray parenchyma cells• homogeneous• heterogeneous
• tracheids usually at top and bottom of a heterogeneous ray
Multiseriate Rays in Redwood
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Fusiform Rays
• Horizontal resin canals• Occur in all species that have
longitudinal resin canals• Resin canal within a ray• May contain three cell types
• Ray parenchyma• Ray tracheids• Epithelial cells
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Dentations
• All ray tracheids are conductive cells, form a bordered pit
• Dentations occur in ray tracheids of hard pines.
• They are localized wall thickenings that appear tooth-like as a consequence of slide preparation.
• When the cell is considered as a whole, dentations are actually just irregular ringlike structures that are similar to spiral thickenings in longitudinal tracheids.
What is the “cross-field” in cross-field pitting?
• Intersection of a longitudinal tracheid and a ray parenchyma cell produces a half-bordered pit
• In radial view, in conjuction with ray tracheids, we see distinctive pitting patterns
Cross-field