Wood structure and anatomy - MyCourses

CHEM-E2105

Wood and Wood Products

Wood structure and anatomy

Mark Hughes

11th January 2019

Today

• The gross structure of wood (macrostructure)• The relationship between the technical properties of wood

and its anatomy• Cell types and characteristics• Softwood microstructure• Hardwood microstructure• Reaction wood

Structural levels

• Gross structure of wood:– Visible to the naked eye– Heartwood/sapwood, growth rings, grain, knots

• Microstructure of wood– Visible under a light microscope– Different cell types, morphology of cells

THE FIBRE (CELL)• The cell wall

– Visible by electron microscopy (some features by optical microscopy)

• Chemical composition– Spectroscopic & chemical techniques

• Providing background to:– Appearance– Properties– Behaviour

Macro- & micro-structure

Ultra-structure & chemistry

Gross structure of wood

• Bark, pith, heartwood, sapwood

• Growth rings

• Growth features (defects, usually in wood products): e.g. knots, grain angle

Heartwood and sapwood

• Heartwood usually darker in colour (extractives), generally more durable

– Also gums and resins

• Sapwood – lighter in colour, often perishable

(Source: Society of Wood Science and Technology)

Extractives in heartwood

• Darkening of timber heartwood is caused by extractives. Different compounds (that are extractable by organic solvents). They include:– Lipids

– Terpenoids

– Phenolic compounds

• Extractives have an effect on:– Colour

– Durability (e.g. pine heartwood much more durable than sapwood)

– Can also affect gluing etc.

Growth rings

Growth rings

• Width varies according to ring age and external conditions

• The width of a growth ring in Finland is an average of 1.5 to 2 mm, however:

• Variation is great:– Pine 0.1...10 mm

– Spruce 0.5...12 mm

– Birch 0.5...10 mm

• Composed of “earlywood” and “latewood” (springwood/summerwood)

Earlywood and latewood

• Sometimes referred to a ‘springwood’ and ‘summerwood’

• Earlywood lighter in colour as it is less dense than the darker latewood

Pine 25% (variation 15...50%)

Spruce 15% (variation 10...40%)

latewood

Earlywood/latewood proportions

• Wood stronger the more latewood it contains– (Strong relationship between density and strength of

wood)

• Strength etc. qualities can be determined according to the relative share of latewood

• Share of latewood depends on a) ecological factors & b) species

• As growth decelerates latewood percentage grows

• A warm autumn increases the latewood percentage

• A drought in the autumn results in a lower proportion of latewood

• The relative and absolute share of latewood is greatest at the base of the tree

Differences between earlywood and latewood

Volume weight (pine): Earlywood 300...370 kg/m3

Latewood 810...920 kg/m3

Knots

Knots

(Source: Wilson & White, 1986)

Live (or tight) knot Dead (or lose) knot

Grain angle (spiral grain)

(http://commons.wikimedia.org/wiki/File:Lodgepole_pine_spiral_g

rain.jpg)(http://saki.iwarp.com/061228-30.html)










Balsa: density ~ 160 kg/m3

Uses: modeling to core materials inhigh performance composites

Technical importance of wood anatomy

Greenheart: density ~1055 kg/m3

Uses: piers, jetties boatbuiling

Microstructure

• Wood composed of cellular tissue that has different functions

• Cells aligned either parallel (mainly “grain direction” ~90%) or perpendicular (“rays”) to the axis of the tree

Structure

(Source: Wilson & White, 1986)

Trunk is

pseudocylindrical

(tapered)

Tangential surface

Radial surface

Transverse surface

(Source: Society of Wood Science and Technology)

Wood structure

Microscopic structure of wood

Cellular structure

Wood fibres

The cell

• Features:

– Tube like structure

– Wall thickness depends on function

– Void space in the centre is called the lumen

– Structures known as pits connect cells

– Formed by cell division

Cell types

Softwood:

• Tracheids (support and conduction)– Aspect ratio ~100:1

• Parenchyma (storage –mainly in the rays)

Hardwood:

• Tracheids

• Parenchyma

• Fibres (thick walled cells) whose main function in mechanical support

• Vessels (or pores), specialised conductive tissue

Cell types

• Fibres: elongated cells, dead and empty when functional. The cell wall surrounds the lumen. Their function is to transport fluids, and/or for strengthening

• Parenchyma: these are ‘brick-like’ cells. Unlike tracheids, wood parenchyma normally live for many years. Wood with living parenchyma is known as sapwood. When the cells die the wood becomes known as heartwood and this occurs towards the centre of the tree. When the cells die the cell contents are converted to waste products that are known as extractives. Parenchyma can be in the rays (ray parenchyma) where the cell’s long-axis is horizontal or in the wood (wood parenchyma) where the long axis is vertical

Cell types

• Tracheids: are fibres whose function is both conduction and strengthening

– Earlywood conduction

– Latewood support

• Note: the pits on the radial surface of the lumen

• Note also the cracking in the cell structure

(http://sciencewise.anu.edu.au/articles/timbers)

Cell types

• Vessels: are vertical tubes that are formed from a stack of cells that have lost or partially lost their end walls. Their function is for the rapid transport of fluids

• Vessel elements are stacked one on top of the other for form the long tube-like vessels

(http://www.biologie.uni-hamburg.de/b-online/library/

webb/BOT410/Xylem/Xylem-1.htm)

Cell features

• Cells are connected by structures known as pits that are to be found on the radial walls of the cells

• Where the vertical tissue interconnects with the rays, the pits are known as cross-field pits

• During drying the pits can become irreversibly closed a condition known as pit aspiration. This can be problematic if trying to infiltrate the structure with fluids (e.g. for pressure treatment, modification, pulping )

(http://www.sbs.utexas.edu/mauseth/weblab/webchap15wood/15

.2-5.htm)

Ray cells

• Rays cells form “bands” or “flecks” on the tangential surface that are clearly visible in some species (e.g. beech) and can also be seen in other species like oak)

• They can be “uniseriate”, i.e. they are only one cell wide, or multiseriate (or bi-, tri- seriate)

• This is a useful aid in identification

(http://www.woodanatomy.ch/mic_tang.html#c)

uniseriate ray

biseriate ray

(Populus tremula L)

(Pirus malus L.)

Softwoods

• Relatively simple structure (compared with hardwoods)

• Composed of earlywood and latewood tracheids and wood and ray parenchyma. Wood parenchyma is rather scarce

• Tracheids are mainly oriented vertically, but in some species are also found in the rays.

• They also contain resin canals that are channels in the wood (not cells), lined with an epithelium of parenchyma cells that secrete resins into the canal. Canals can be both vertical in the wood and radial in the rays where they are called fusiform rays

Transverse section

Longitudinal sections

Tangential RadialRays

Pine (transverse section)

(x150 magnification)

Hardwoods

Hardwoods

• More complex structure than softwoods

• In addition to tracheids and parenchyma, hardwoods contain vessels and fibres, known as libriform fibres whose function is that of providing mechanical strength

• The arrangement of the vessels can be used to help in identifying the species

• Likewise the arrangement of the wood parenchyma can also be used to help in identification. There is more wood parenchyma in hardwoods than found in softwoods.

Hardwoods

(http://steurh.home.xs4all.nl/engloof/eloofht.html)

Arrangement of vessels

Ring porous

(http://www.britannica.com/EBchecked/media/56305/

Transverse-section-of-northern-red-oak-a-ring-

porous-hardwood)

Red oak (Quercus rubra)

(http://www.wood-database.com/lumber-

identification/hardwoods/red-oak/)

Semi-ring porous

• Persimmon, White Ebony (Diospyrosvirginiana)

• Intermediate between ring porous and diffuse porous; vague definition

(http://www.wood-database.com/lumber-

identification/hardwoods/persimmon/)

Diffuse porousLiquidambar styraciflua L. (Red

gum, sweet gum)

(http://www.biologie.uni-hamburg.de/b-

online/wood/english/melswmac.htm)

Swietenia macrophylla King (Echtes

Mahagoni, true mahogany, caoba)

Birch

(x150 magnification)

Greenheart

(Richter and Dallwitz, 2000)

Greenheart (transverse)


Greenheart (tangential)


Greenheart (radial)


Tyloses

• Tyloses form in vessel when conduction ceases (i.e. when the wood becomes heartwood) and the pressure in the vessel drops

• The cell walls of the parenchyma expand though the pits in to the vessel like a balloon, blocking the vessel

• This makes it difficult to impregnate the heartwood of some hardwood species

(Desch & Dinwoodie 1981)

(http://sciencewise.anu.edu.au/articles/timbers)

Hardwood parenchyma

• Useful aid in identifying wood

• Two types of wood parenchyma can be identified in hardwoods

• Apotracheal parenchyma, which is independent of the vessels and paratracheal parenchyma, which is associated with the vessels

• Apotracheal parenchyma can be further subdivided into – Terminal

– Diffuse

– Banded

• Paratracheal parenchyma can be subdivided into– Vasicentric

– Aliform

– Confluent

• Further subdivisions are possible


Apotracheal parenchyma

(Independent of vessels)

• Terminal parenchyma: narrow band of parenchyma found at the close of the growing season

• Diffuse parenchyma: single strands distributed irregularly among the fibres

• Banded parenchyma: In tangential layers independent of the vessels

Paratracheal parenchyma

(Associated with vessels)

• Vasicentric parenchyma: forms complete sheaths or borders around the vessels

• Aliform parenchyma: tangential “wing like” arrangements appearing in cross section as diamond shapes areas

• Confluent parenchyma: tangential projection of parenchyma masses join up to form confluent parenchyma


Terminal parenchyma Diffuse parenchyma


Banded parenchyma


Banded parenchyma Vasicentric parenchyma


Confluent parenchyma Aliform-confluent parenchyma

Reaction wood

• Reaction wood forms when the tree tries to restore a displaced stem or branch

• In softwoods, compression wood is formed in parts that are under compression

• In hardwoods, tension woodis formed in parts under tension

Compression wood

• Forms in softwoods and is concentrated on the underside of the stem or branch

• Heavier, harder and more dense than normal wood

• Tracheids are short and thick-walled

• Compared to normal wood, S1 is thicker, the fibril angle in S2 is greater, and S3 is completely lacking

• Cellulose content is low, lignin content high

• Cross-section of fiber circular, intercellular spaces between fibers

• Layer with high lignin content in S1

Normal latewood Opposite wood Compression wood

Compression

wood

Tension wood

• Forms on the upper side of inclined stems and branches

• Contains more fibres than normal wood• The fibres are longer and their diameter

is lower• Cell walls are thick• In hardwoods, the vessels are less

frequent and smaller• Often contain a gelatinous layer (G)

consisting of almost pure and crystalline cellulose

• High content of cellulose, low content of lignin

Literature and further reading• Society of Wood Science and Technology:

http://www.swst.org/teach/set2/struct1.html

• Desch, H.E. and Dinwoodie, J.M. (1981): Timber: its structure, properties and utilisation. 6th

Edition, Macmillan London

• Dinwoodie, J.M. (2001): Timber: Its Nature and Behaviour

• Wilson, K. and White, D.J.B. (1986): The Anatomy of Wood: Its Diversity and Variability

• Richter, H.G., and Dallwitz, M.J. 2000 onwards. Commercial timbers: descriptions, illustrations, identification, and information retrieval. In English, French, German, Portuguese, and Spanish. Version: 25th June 2009. http://delta-intkey.com

• IAWA List of Microcopie Features for Hardwood Identification. Link at: https://www.researchgate.net/publication/294088872_IAWA_List_of_Microcopie_Features_for_Hardwood_Identification

Databases:

• Wood Anatomy (http://www.woodanatomy.ch/ident_key.html)

• The wood database (http://www.wood-database.com/wood-identification/)

• Inside wood (http://insidewood.lib.ncsu.edu/search?11)

http://www.swst.org/teach/set2/struct1.html

http://delta-intkey.com/

https://www.researchgate.net/publication/294088872_IAWA_List_of_Microcopie_Features_for_Hardwood_Identification

http://www.woodanatomy.ch/ident_key.html

http://www.wood-database.com/wood-identification/

http://insidewood.lib.ncsu.edu/search?11

Wood structure and anatomy - MyCourses

Documents