Wood anatomy, Parts of wood structure

1. CHEM-E2105
Wood and Wood Products
Wood structure and anatomy
Mark Hughes
19th January 2016

2. 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

3. 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

4. Gross structure of wood
• Bark, pith, heartwood, sapwood
• Growth rings
• Growth features (defects, usually in wood products):
e.g. knots, grain angle

5. 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)

6. 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.

7. Growth rings

8. 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)

9. Earlywood and latewood
• Sometimes referred to a ‘springwood’ and
‘summerwood’
• Earlywood lighter in colour as it is less dense than
the darker latewood

10. 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

11. •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

12. Knots

13. Knots
(Source: Wilson & White, 1986)
Live (or tight) knot Dead (or lose) knot

14. Grain angle (spiral grain)
(http://commons.wikimedia.org/wiki/File:Lodgepole_pine_spiral_g
rain.jpg)
(http://saki.iwarp.com/061228-30.html)

15. Grain angle (spiral grain)
(http://commons.wikimedia.org/wiki/File:Lodgepole_pine_spiral_g
rain.jpg)
(http://saki.iwarp.com/061228-30.html)

16. Grain angle (spiral grain)
(http://commons.wikimedia.org/wiki/File:Lodgepole_pine_spiral_g
rain.jpg)
(http://saki.iwarp.com/061228-30.html)

17. Grain angle (spiral grain)
(http://commons.wikimedia.org/wiki/File:Lodgepole_pine_spiral_g
rain.jpg)
(http://saki.iwarp.com/061228-30.html)

18. Balsa: density ~ 160 kg/m3
Uses: modeling to core materials in
high performance composites
Technical importance of wood anatomy

19. Greenheart: density ~1055 kg/m3
Uses: piers, jetties boatbuiling

20. 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

21. Structure
(Source: Wilson & White, 1986)
Trunk is
pseudocylindrical
(tapered)
Tangential surface
Radial surface
Transverse surface

22. (Source: Society of Wood Science and Technology)
Wood structure

23. Microscopic structure of wood
Cellular structure
Wood fibres

24. 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

25. 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

26. 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

27. 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)

28. 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)

29. 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)

30. 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.)

31. 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

32. Transverse section

33. Longitudinal sections
Tangential RadialRays

34. Pine (transverse section)
(x150 magnification)

35. Hardwoods

36. 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.

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

38. Arrangement of vessels

39. 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/)

40. Semi-ring porous
• Persimmon, White
Ebony (Diospyros
virginiana)
• Intermediate between
ring porous and diffuse
porous; vague
definition
(http://www.wood-database.com/lumber-
identification/hardwoods/persimmon/)

41. Diffuse porous
Liquidambar 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)

42. Birch
(x150 magnification)

43. Greenheart
(Richter and Dallwitz, 2000)

44. Greenheart (transverse)
(Richter and Dallwitz, 2000)

45. Greenheart (tangential)
(Richter and Dallwitz, 2000)

46. Greenheart (radial)
(Richter and Dallwitz, 2000)

47. 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)

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

49. 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

50. (Desch & Dinwoodie 1981)

51. 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

52. 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

53. (Desch & Dinwoodie 1981)
Terminal parenchyma Diffuse parenchyma

54. (Desch & Dinwoodie 1981)
Banded parenchyma

55. (Desch & Dinwoodie 1981)
Banded parenchyma Vasicentric parenchyma

56. (Desch & Dinwoodie 1981)
Confluent parenchyma Aliform-confluent parenchyma

57. 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 wood
is formed in parts under
tension

58. 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

59. 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

60. 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
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)