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Wood ChemistryPSE 406

Tree & Wood Structure

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Agenda Tree components

» Stem, crown, roots» Hardwood versus softwood

Macro wood structure» Reaction wood

Micro wood structure» Cell types» Cell wall layers

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Why Wood Structure? Chemical distribution is dependent

upon macro and microscopic structure.» Tree species dependent» Dependent upon position in tree» Cell type dependent» Dependent upon position in the cell

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Tree Structure I It has been my

experience that the majority of students taking wood chemistry can’t tell the difference between a hardwood and a softwood. In the next few slides I am going to present you with the layman's view of what is a tree.

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Tree Structure II In general, trees contain these

structural components:» Stump/Roots» Stem (wood & bark)» Crown: live/dead branches,

foliage (leaves/needles), flowers and fruits.

There are major chemical components which are found in all of these components. We will focus on the components found in the stem.

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Tree Structure III

Hardwoods» Trees containing leafs» Typically lose foliage in

winter» Maple, alder, oak,

Softwoods» Trees containing needles» Typically retains needles

over winter.» Pines, firs, cedars, spruce

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Tree Species Differences

Softwoods Hardwoods

Nomenclature Conifers/ Gymnosperms Deciduous/ Angiosperms

# of Species 520 30,000

Cellulose No Difference* No Difference*

Hemicellulose Majority 6 carbon Sugars Majority 5 carbon Sugars

Lignin 23-33% of tree

16-25% of tree

Extractives Big Differences Big Differences

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Tree Composition Composition of Trees1 Species Branches Foliage Bark Wood Roots2

Softwood 13 8.5 10.5 66 20

Hardwood

12 5 15 68 19

1. Values for branches, foliage, bark, and wood = % of tree above ground2. Values for roots is a separate measurement = % of total tree

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Macroscopic StructureOuter Bark (dead, protection, high extractives)

Cambium(growth, inward wood,outward bark)

Xylem=wood

Phloem(inner bark)(transportation of waterand nutrients)

Heartwood(support, dead, dark)

Sapwood(younger, light color, living cells, transportation)

Pith

Annual Rings

Earlywood

Latewood

Knot

Definitions in notes section

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Macroscopic Structure (2)

Earlywood

Latewood

Heartwood

Sapwood

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Reaction Wood

Tension Wood (Hardwoods)

Compression Wood (Softwoods)

Tension orCompressionWood

This is a very poor representation of a very bent tree

Notes

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Wood Microscopic Structure

Imagine that wood is made up of millions and millions of toilet paper rolls glued together. These rolls are the fibers that will make paper

Most often the ends of these tubes are sealed. There are small holes in the sides of the tubes to allow water to pass through

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Microscopic Structure

Microscopic structure of wood (Textbook of Wood Technology, Panshin, A. J., page 118

Tracheid (support, water transport, softwoods),in hardwoods we have libriform fibers)

Resin canals (epithelium parenchyma secretes resin epithelium parenchyma secretes resin)

Rays (transportation of water)

Pits (wholes, transport between fibers, different typs)

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Hardwood & Softwood Fibers

Softwood Cells Source: Wood Chemistry, Fundamentals

and Applications. Sjostrom page 7

Hardwood Cells Source: Wood Chemistry, Fundamentals

and Applications. Sjostrom page 10

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Microscopic Structure

Structure of woody cell by Cote, 1967. This figure is used by almost every wood chemistry text. It can be found in Wood Chemistry, Fundamentals and Applications by Sjostrom on page 14.Notes

ML-space between cells, 70-80% lignin, glue

P-primary wall, very thin, random microfibrils,

S (S1+S2+S3)-secondary wall, the thickest, microfibrils - opposite direction

W-warty layer, thin, storage of metabolites

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Cell Cross SectionPrimary Secondary 1

Secondary 2

Secondary 3Warty Layer

Middle lamella

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T/F Earlywood: wide, thin walled cells for

water transport (T/F)? Phloem: this is where growth takes

place in the stem (T/F)? Heartwood: outer (younger) portion of

the woody tissue (T/F)? Cambium: this is the dead protective

layer (T/F)?

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T/F Vessels: short, wide, thin-walled cells

found in hardwoods (T/F)? In hardwoods, epithelium parenchyma

cells secrete resin (T/F)? …………. are holes in the fibers which

allow water to flow between fibers. Primary Layer: this is the thickest

layer of the cell (T/F)?