Wood structure II: Anatomy and properties

Transcription

1 CHEM-E0120: An Introduction to Wood Properties and Wood Products Wood structure II: Anatomy and properties Mark Hughes 21 st September 2017

2 Today The relationship between the technical properties of wood and its anatomy Cell types and characteristics Softwood microstructure Hardwood microstructure Reaction wood

3 Relationship between mechanical properties and density (or specific gravity) (Source: Dinwoodie 2001)

4 Technical importance of wood anatomy Balsa: density ~ 160 kg/m 3 Uses: from model aeroplanes to core materials in high performance composites!

5 Greenheart: density ~1055 kg/m 3 Uses: piers, jetties boatbuilding

6 Orientation Trunk is pseudo-cylindrical (slightly tapered) Three directions: Longitudinal Radial Tangential Properties differ significantly in different directions wood can be regarded as an orthotropic material

7 Spiral grain ( (

8 Spiral grain ( (

9 Spiral grain ( (

10 Spiral grain ( (

11 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

12 Cell types Softwood: Hardwood: Tracheids (support and conduction) Aspect ratio ~100:1 Parenchyma (storage mainly in the rays) Tracheids Parenchyma Fibres (thick walled cells) whose main function in mechanical support Vessels (or pores), specialised conductive tissue

13 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

14 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 (

15 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 to form the long tube-like vessels ( webb/bot410/xylem/xylem-1.htm)

16 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 ) (

17 Ray cells (Populus tremula L) Ray 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 (Pirus malus L.) uniseriate ray biseriate ray (

18 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

19 Transverse section

20 Longitudinal sections Tangential Rays Radial

21 Pine (transverse section) (x150 magnification)

22 Hardwoods

23 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.

24 Hardwoods (

25 Arrangement of vessels

26 Ring porous Red oak (Quercus rubra) ( nsverse-section-of-northern-red-oak-a-ring-poroushardwood) (

27 Semi-ring porous Persimmon, White Ebony (Diospyros virginiana) Intermediate between ring porous and diffuse porous; vague definition (

28 Diffuse porous Liquidambar styraciflua L. (Red gum, sweet gum) Swietenia macrophylla King (Echtes Mahagoni, true mahogany, caoba) (

29 Birch (x150 magnification)

30 Greenheart (Richter and Dallwitz, 2000)

31 Greenheart (transverse) (Richter and Dallwitz, 2000)

32 Greenheart (tangential) (Richter and Dallwitz, 2000)

33 Greenheart (radial) (Richter and Dallwitz, 2000)

34 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)

35 (

36 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

37 (Desch & Dinwoodie 1981)

38 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

39 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

40 Terminal parenchyma Diffuse parenchyma (Desch & Dinwoodie 1981)

41 Banded parenchyma (Desch & Dinwoodie 1981)

42 Banded parenchyma Vasicentric parenchyma (Desch & Dinwoodie 1981)

43 Confluent parenchyma Aliform-confluent parenchyma (Desch & Dinwoodie 1981)

44 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 Compression wood Tension wood

45 Literature and further reading Society of Wood Science and Technology: Desch, H.E. and Dinwoodie, J.M. (1981): Timber: its structure, properties and utilisation. 6 th 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 onwards. Commercial timbers: descriptions, illustrations, identification, and information retrieval. In English, French, German, Portuguese, and Spanish. Version: 25th June Databases: Wood Anatomy ( The wood database ( Inside wood (