Transcript Softwood Structure

FW1035
Lecture 4
Bowyer et al – Chapter 4
Softwood Microanatomy
• Softwoods
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General information
Overview of structural cells
Wood appearance
Importance of pitting
• Component cells and tissues
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Longitudinal tracheids
Resin canals
Rays
Longitudinal Parenchyma
• Crossfield pitting
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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
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Pinus (Pines)
Picea (Spruces)
Larix (Larches)
Pseudotsuga (Douglas-fir)
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E = Epithelial cells
Diagram of Pine Wood Microanatomy
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Softwood
Structure
Can you recognize
and name all the
indicated features?
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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
Earlywood / Latewood Transition
Abrupt Transition
Gradual Transition
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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 softwoodsbulk 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”
Simple Pit Pairs
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Bordered and Half-Bordered Pit Pairs
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Bordered Pit Pair Structure - Conifers
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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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Aspirated Earlywood/Latewood Bordered Pits
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Longitudinal Parenchyma
• Also called strand
parenchyma
• Appear as chains of
short, vertically oriented
cells
• Formed by fusiform
cambial initials
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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.
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:
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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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(Fusiform Ray)
How can you tell
if a wood sample
has resin canals?
Longitudinal resin canal
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
Douglas-fir
• Pines have thin walled
epithelial cells (easily
damaged in cutting)
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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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Heterocellular Rays
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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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Dentate Ray Tracheids - Hard Pines
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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.
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What is the “cross-field” in crossfield pitting?
Cross-field
• 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
Types of
Cross-Field
Pitting
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