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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)

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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?

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

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Abrupt Transition Gradual Transition

Earlywood / Latewood Transition

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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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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• 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

How can you tell if a wood sample has resin canals?

(Fusiform Ray)

Longitudinal resin canal

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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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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.

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

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Types of Cross-Field Pitting

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