503 335w5 01 wood structure 2

Wood s structure and variability 2 The microscopic structure of wood Content • Macroscopic structure of wood • Cell types and function • Comparative wood structure www.csaw.utas.edu

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Wood s structure and variability 2

The microscopic structure of wood

Content

•  Macroscopic structure of wood

•  Cell types and function

•  Comparative wood structure

www.csaw.utas.edu.au

KDA503/335 Timber, its origin and characteristics

Wood s structure and variability

Macroscopic wood structure

•  Anisotropic structure

–  Longitudinal, radial, tangential planes

radial

tangential

•  Various parts of the stem

–  Cambium, bark, wood

•  Variable wood types

–  Sapwood, heartwood, juvenile and

reaction woods

•   Various cell types

–   Arranged longitudinally and radially

(rays)

Cell type and function

Cambium and various wood cells in softwoods and

hardwoods

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Cambium

•  The cambium is a thin delicate

layer of tissue between the bark

and the wood

•  During dormancy, the cambium

consists of a single cell layer

•  As the growing season begins, this

single cell layer subdivides radially

to form the cambial zone eight to

ten cells wide

•  During the growing season, these

cells continue to subdivide to

produce cells that:

–  remain in the cambial zone,

–  develop into bark

–  develop into wood

Cell differentiation

•  About three weeks after formation, wood cells differentiate

•  The cells tend to change in shape and a secondary wall is formed

•  The cell dies and becomes ready to serve for:

– conduction, – support or – storage

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Major cell structure in the wood

wood ra

ys

lumen

tangential

Cell structure: Wall and pits

•  Cell walls are made up of a primary layer and often a secondary layer enclosing a lumen

•  The secondary layer often has interruptions called pits

•  Pits occurring in matched pairs in adjacent cells allow

fluids and gases to move between adjoining cells

Cell wall in section, showing simple pits (left) and bordered pits (right)

Source:

http://www.tutorvista.com/content/biology/biology-iii/cell-organization/cell-wall.php

SEM photo of bordered pits in softwood

Source: http://www.scielo.cl/scielo.php?

pid=s0718-221x2005000100001&script=sci_arttext

Cell structure: Longitudinal and rays cells

•  Rays are bands of tissue

running radially from the

pith towards the

cambium, perpendicular

to the longitudinal axis

•  They mainly store food

and are often coupled

with resin or gum canals

•  Rays in different species

have different ray

heights, widths and

composition and are

often used for

identification

Source: http://www.museum-albersdorf.de/BERNSTEN/holzblok.gif!

Rays viewed from major wood planes

Major cell types

Table 1: Functions and wall thickness of the various types of cells found in

softwoods and hardwoods (Source: Desch & Dinwoodie 1996)

Cells Softwood Hardwood Function Wall thickness Parenchyma √ √ Storage

Tracheids √ √ Support

Conduction

•  Parenchyma and tracheid cells can operate longitudinally or radially (as ray cells)

Parenchyma cells

•  Parenchyma cells are generally short and

thin walled cells responsible for storage in

softwoods and hardwoods

•  They make up:

–  Less than 10% of the total wood volume in

softwoods

–  On average, 20% of hardwoods but it can be as

high as 30%

•  They can be aligned longitudinally and radially

•  The shape of parenchyma cells vary but ray

parenchyma cells are generally brick-shaped

Tracheids

•  Tracheids cells are generally long, slender, hollow needle-shaped cells, with closed and pointed ends

–  They are 2.5 -5.0mm (or up 10mm) long,

100 times greater than their diameters

•  They are rounded or rectangular in shape radially and have a hollow centre: the lumen or cavity

•  Earlywood tracheids are thin-walled with relatively large diameters compared to thick-walled smaller diameters in latewood

•  They conduct fluids through the tree and provide support

•  They can be aligned longitudinally and radially

–  In softwoods, 95% of the cells are vertically aligned, whereas in hardwoods, 80%

•  They make up about 90-95% of the volume of softwood stem,

Conduction cells with pits lining the cell walls:

A longitudinal file of tracheids (left) and a single tracheid

http://www.tutornext.com/system/files/u73/Chapter%203.3-8_1.jpg

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Fibers

•  Fibers or fiber tracheids in hardwoods are

usually long, tapered and thick walled

–  approx 1 to 2 mm in length and 20-30 micrometers

in diameters

•  They are:

–  shorter than the longitudinal tracheids in softwoods

(1-2 mm compared to 3-4mm),

–  are rounded in shape (compared to the rectangular

in softwoods),

–  have bordered pits like longitudinal tracheids in

softwoods but with less developed borders

Fibers in Australian hardwoods

ESEM images of E obliqua & E pilularis at 700x magnification The images have been cropped to show fiber cells only

Courtesy of Adam Redman, DEEDI, Qld

Vessels

•  Vessels are longitudinal cells

found in hardwoods that link end to end and conduct sap along the stem

•  They differentiate hardwoods from softwoods

•  Compared to fibers and tracheids, they are shorter, with larger diameter cells and thinner walls This makes

them appear like pores

•  Their walls are heavily pitted

These providing lateral movement of water and air between adjacent cells

Vessels and fibers in birch

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

Vessels

•  Vessels are connected end to

end through end walls with holes

called perforated plates

•  Perforations develop near the end

of the cell maturation process

•  Vessels are connected side to

side through numerous bordered

pit pairs

•  Just like perforated plates, the shape and arrangement of the vessel-to-vessel pitting is often consistent within a species

•  Other cell types typically form pits where they contact vessel elements, although tracheids and vessels are sometimes not linked

by pitting

Types of vessel perforations: simple, scalariform, foraminate Source http://t2.gstatic.com/images?q=tbn:ANd9GcQHG1569Bj9ignj-QrWBwNw85f_Mpw8KFiSUez3mZPio91YxEqxrA&t=1

Vessels

•  Depending on their vessel

arrangement , hardwoods are

either ring porous or diffuse

porous

•  Ring porous hardwoods have

large earlywood vessels, and a

distinct latewood zone with small

vessels

•  Diffuse porous hardwoods have

vessels with a relatively uniform

size, distributed fairly evenly

across the growth ring Most

hardwood are diffuse porous

•  When hardwoods are milled into

boards, the cut vessels produce

distinctive patterns on the radial

and tangential surfaces

Ring porous sassafras Sassafras albidum Source:http://forest.mtu.edu/classes/fw1035/2011/Lab lecture 1-Ring porous hardwoods.pdf!

Comparative wood structure

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softwood and hardwood cells

•  Softwoods are basically made up of two cell types,

–  Tracheids constitute about 90-95% of softwoods, the rest are parenchyma

•  Hardwoods are made up of at least four cell types: tracheid, vessel

element, parenchyma, and fibers

–  Each hardwood cell type constitutes at least 15% of the total volume of wood

•  Softwood cells are aligned in parallel straight radial rows, with straight spoke-like rays

•  Hardwood elements and rays are seldom aligned in radial rows Distortion from the radial orientation occurs in the vicinity of large vessel elements

Early/spring wood

Late/summer wood

•  Softwood tracheids are generally rectangular whereas

hardwood vessels are more rounded

•  The longitudinal tracheids in softwoods perform both

conduction and support

•  In hardwoods, vessels are largely for conduction, while

fiber tracheids are largely for support

Softwood rays

•  The cells in softwood rays can be either ray parenchyma or ray

tracheids

•  The ray parenchyma cells are brick-shaped, about 20 x 25 micro-millimeters

•  Ray tracheids, similar to longitudinal tracheids, are thick walled and have bordered pits

•  When a ray contains a resin canal, it

is called fusiform ray Fusiform rays

usually contain both tracheids and

parenchyma

•  Resin canals, consistently found in pines, are intercellular spaces

surrounded by parenchyma cells that secrete resin into the canal

•  They can be orientated in the longitudinal or radial direction

Resin canal

by Dongyang Sun Sample cutting by Stefan Lehneke 3D by Dan Ridley-Ellis, Edinburgh Napier University, August 2010!

http://farm5.static.flickr.com/4080/4911220541_c19a716cb9.jpg!

softwood and hardwood rays

•  Softwood rays are only a

few cells wide

–  They usually constitute

around 5-7% of the total

volume,

•  In hardwoods, rays are

wider up to 30 cells

wide

–  They constitute on

average, 17% but up to

30% of the total volume

•  In hardwoods, ray cells

are very large and these

add to the wood's

aesthetic appeal

storied ray cells

Source http://delta-intkey.com/wood/images/brachy-t2.jpg!

Summary

•  During the growing season, cambium cells subdivide to produce cells that remain in the cambial zone, develop into bark or develop into wood

•  About three weeks after formation, wood cells differentiate to serve for conduction, support or storage

•  Cell walls are made up of a primary layer and often a secondary layer enclosing a lumen Liquids and gases can move between them through pits

•  Softwoods are basically made up of two cell types –  Tracheids constitute about 90-95% of softwoods, the rest are parenchyma

–  These are aligned in parallel straight radial rows, with straight spoke-like rays

•  Hardwoods are made up of at least four cell types: tracheid, vessel element, parenchyma, and fibers

–  Each hardwood cell type constitutes at least 15% of the total volume of wood

–  These are seldom aligned in radial rows Distortion from the radial orientation occurs in the vicinity of large vessel elements

•  Rays in different species have different ray heights, widths and composition and are often used for identification

•  Resin canals, consistently found in pines, are intercellular spaces surrounded by parenchyma cells that secrete resin into the canal

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