COMPARING MECHANICAL PROPERTIES OF SECONDARY WALL AND CELL CORNER MIDDLE LAMELLA IN SPRUCE WOOD - Brill
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IAWA Journal, Vol. 18 (1),1997: 77-88
COMPARING MECHANICAL PROPERTIES OF SECONDARY WALL
AND CELL CORNER MIDDLE LAMELLA IN SPRUCE WOOD
by
Rupert Wimmer 1 & Barry N. Lucas 2
SUMMARY
Mechanical characterizations of the S 2 layers and the cell comer middle
lamella in the axial direction were investigated in spruce wood, A me-
chanical properties microprobe capable of measuring hardness and
Young's modulus on a spatially resolved basis similar to that of an elec-
tron beam microprobe was used. Hardness of the cell comer middle lamel-
la was found to be almost as high as that of the secondary wall, but the
Young's modulus of the cell comer middle lamella was 50% less than
that of the S2' The S2 showed constant hardness over its range of Young's
modulus, but the cell comer middle lamella exhibited a strong correlation
(R2 = 0.55) between hardness and the Young's modulus. Further inves-
tigations are needed to directly combine chemical and micromechanical
properties and also to investigate the mechanical effects of the high varia-
bility of cell comer middle lamella chemistry.
Key words: Spruce, Picea, hardness, Young's modulus, secondary wall,
middle lamella, mechanical property.
INTRODUCTION
High resolution mechanical property measurements of cell wall material have been
done previously. Such studies have included micro-tensile tests of microtomed (usu-
ally 80 11m thick) wood sections (e.g., Wellwood et al. 1965; Kennedy & Ifju 1962;
Grozdits & Ifju 1969), tensile tests on individual fibers obtained through chemical
maceration (e.g., Leopold & Macintosh 1961; Schniewind et al. 1965), and a micro-
compression test done perpendicular to the grain (Wilson 1964). Because the S 2 layer
is the thickest cell wall layer it is been suggested that it controls the strength of the
entire fiber (Wardrop & Dadswell 1951; Abe et al. 1991). An analogy can be drawn
between the structure of the S2 layer and that of a unidirectional fiber-reinforced com-
posite material, in which the cellulose fibrils represent the fiber reinforcement, and the
mostly amorphous hemicellulose and lignin represent the composite matrix. However,
the mechanical behavior of tissues cannot be explained solely by the material proper-
ties of secondary cell walls and other factors need to be considered, one being the
physical properties of the middle lamella.
1) Universitat fUr Bodenkultur Wien, Gregor Mendel-Strasse 33, 1180 Vienna, Austria.
2) Nano Instruments, Inc., P.O. Box 14211, Knoxville, Tennessee 37014, U.S.A.
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In unlignified woody tissues, the primary walls of adjoining cells are separated by a
pectic layer, called the middle lamella, that essentially acts as a cementing agent. Lignin
first is deposited in the primary wall at the cell comer and then in the cell comer middle
lamella, tangential compound middle lamella, radial compound middle lamella, and
secondary wall (Donaldson 1991). The impregnation of lignin in the cell wall and the
middle lamella can alter the material properties of these structures. When lignification
occurs, the distinctions between the primary cell walls of adjoining cells and the in-
tervening middle lamella are lost, and the three layers are then referred to as the com-
pound middle lamella. The volume fraction of the compound middle lamella in conif-
erous wood is c. 10-12% of the woody tissue volume (Fergus et al. 1969).
To measure the mechanical properties of the S 2 and the compound middle lamella,
depth-sensing indentation was applied as a testing method in this study. According to
Brinell, the impression of a steel-ball on a smooth metallic surface, not too close to the
edges, delivers clear, reproducible results. At the beginning of the century, Janka (1906)
proposed and developed a modified Brinell-hardness test for wood. The static force
required to completely embed a steel hemisphere 0.444 inch (11.5 mm) in diameter,
corresponding to a projected hemispherical surface of 1 cm 2, into the wood was meas-
ured. Because the steel hemisphere covers a wide range of cellular structures, the meas-
ured hardness is approximately proportional to the density of the wood or cell mass per
unit volume.
The distinguishing feature of the indentations used for this investigation is their
very small size. Using a mechanical properties microprobe allows one to evaluate the
mechanical response of a sample with submicron spatial resolution (Oliver 1986; Oliver
& Pharr 1992; Page et al. 1992) and this method has recently been used on woody
tissues (Wimmer et al. 1997). The objectives of the current study were to 1) simultane-
ously quantify the patterns of hardness and Young's modulus of the cell comer middle
lamella (CCML) and the secondary wall (S2), and 2) look for differences and trends in
these properties across tree rings of spruce.
MATERIAL AND METHODS
From a 80-year-old red spruce tree (Picea rubens Sarg.) as mm increment core was
taken and a sequence of five tree rings prepared so that cross-sectional areas of about
1 mm 2 were obtained. Samples were prepared as usually done for a transmission elec-
tron microscopy study. To minimize leaching effects as well as possible changes in
chemical composition, specimens were air dried at room temperature for several days
and then oven dried at 70 DC for about 20 minutes before embedding in resin according
to the method of Spurr (1969). The compact structure of the woody cell wall allows no
penetration of the embedding medium and thus a phase boundary exists between the
cell wall and the polymeride (Jayme & Fenge11961; FengelI967). Under vacuum, the
resin filled the lumina and then the resin was cured in the usual way. A surface on the
embedded specimen was sectioned using a Reichert-Ultramicrotome with a diamond
knife. Mechanical tests were done of pure cell wall material and of the cell comer
middle lamella. The microtomed samples provided sufficient surface quality for the
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A
1000
500
o
1000
1000
1000
1000
1000 1000 1000 1000
1000 1000
1000
o
1000 1000
1000
1000
1000
1000
1000
1000
1000
1000 1000
1000
1000
1000 o 0
1000
Fig. 1. Three-dimensional reconstructed atomic-foree-microscope (AFM) images: a) remaining
plastic deformation of a single S2 indentation (indentation depth = 80 nm); b) remaining plastic
deformation of a cell comer middle lamella indentation close to the tracheid cell wall.
indentation tests. The specimens were glued on aluminum stubs using a laser tech-
nique for perpendicular alignment. Mounted specimens were fixed in the stage of the
Nano Indenter II® (Nano Instruments, Inc., Knoxville, TN).
The operation principles of the computer-controlled N ano Indenter II® are described
in detail elsewhere (Oliver & Pharr 1992; Willems et al. 1993; Wimmer et al. 1997).
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The Nano Indenter II® was enclosed in a heavy wooden cabinet, the major purpose of
which is to ensure thermal stability during an experiment. The indenter itself was sus-
pended on a pneumatic antivibration table to isolate it from building vibrations. The
apparatus was located in a room stabilized at 21 DC and relative humidity was around
60% throughout the experiment. With the Nano Indenter II®, loads on the order of a
few f.!N can be applied to a pyramidal diamond indenter, with a resolution better than
50 nN, and the resulting depth displacement can be measured to 0.16 nm. A load-
controlled instrument mode is used in examining hardness at shallow depths. The sys-
tem controls the load as well as the loading rate continuously during the loading and
unloading segments of the indentation procedure. With know ledge of the indenter geo-
metry, the contact area can be determined and the nanohardness value for the applied
load can be calculated (Oliver 1986; Wimmer et al. 1997). Unlike conventional hard-
ness testers, it is not necessary to optically determine the area of an indentation in
order to calculate the hardness (Fig. la).
Because the radial and tangential oriented compound middle lamella were too nar-
row « 0.5 Ilm), the indentation tests were done solely on cell comer compound middle
lamella (CCML) which provided surfaces of approximately 2 x 2 Ilm (Fig. Ib). Two
sets of measurements were made. In the first set 5 tree ring samples were prepared and
subsequently measured with the Nano Indenter II®. Each indentation was checked
carefully using reflected light microscopy and for more detailed analysis an atomic
force microscope in contact mode (Park Scientific Instruments®) was used. In total
267 single measurements were obtained. All these measurements were done solely in
latewood because their tracheid walls were wide enough for locating the indentations.
The second set of measurements characterized properties along a radial file of tracheids.
In total, properties of 12 consecutive tracheids, and the adjacent CCML, were meas-
ured over a distance of 250 Ilm. Measurements started with the last latewood tracheid
and proceeded toward the earlywood.
RESULTS
Dataset 1
Figures 2a and b are box-and-whisker plots showing Young's modulus and hard-
ness of radial and tangential Srwalls, CCML and also Spurr's resin as a control. In
this data set (267 measurements), the average Young's modulus of tangentially and
radially oriented tracheid walls was 16 GPa and the hardness was 0.30 GPa. Differ-
ences between radial and tangential cell walls were not significant. Both hardness and
Young's modulus of the CCML were lower than the S2, but to a different degree. The
S 2 layer displayed an elastic modulus nearly twice that of CCML but the S 2 was only
10% harder than CCML.
Dataset 2
The second data set comprised measurements made across a 250 Ilm transect along
a radial file of tracheids in a single tree ring (Table 1). The Young's modulus of S2
walls in this second dataset was nearly three times as high as the CCML, and the hard-
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a
Srtang
Srrad
CCML
Resin
0 5 10 15 20 25 30
Youngs' modulus (GPa)
b
Srtang
Srrad
CCML1
Resin
0.0 0.1 0.2 0.3 0.4 0.5
Hardness (GPa)
Fig. 2. Box-and-Whisker Plot of Young's modulus (a) and hardness (b) for the anatomical com-
ponents. Number of measurements: STtang (85), Srrad (62), CCML (97), Resin (23).
ness of the S2 walls was approximately 20% more than the CCML. Coefficients of
variation were considerably higher for CCML than for S2 walls. Figures 3a and b
depict the mechanical trends for CCML and S 2 layers for the measured distance from
latewood towards earlywood.
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30
a S 2 layer
S 2 layer
25
~en
20
~ 15
~
en
'0Jj
t:: 10
~
~
5
latewood earlywood
o
o 50 100 150 200 250 300
Distance (filll)
0.50
b S 2 layer
I
0.45 CCML
0.40
~ 0.35
~ 0.30
i::r: 0.25
0.20
0.15
latewood earlywood
0.10 i
o 50 100 150 200 250 300
Distance (J.lm)
Fig. 3. Young's modulus (a) and hardness (b) of radial S2 layer and cell comer middle lamella
along a 250 J.lm distance in a tree ring.
Three out of 76 CCML hardness values were above those of the S2 walls, which
could be due to scatter. The hardness of the very last latewood tracheids in the growth
ring is low, but hardness increases significantly in the subsequent 1-2 tracheids. In
contrast, CCML values do not show this difference. The lowest values for Young's
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Table l. Comparison of longitudinal hardness and Young's modulus (OPa) of secondary
wall layers (S2Iayer) and cell comer middle lamellas (CCML).
Hardness Young's modulus
S2 layer CCML S2 layer CCML
mean 0.335 0.267 19.70 6.89
SD 0.030 0.058 3.14 2.26
CV% 9 21 15 21
mm 0.226 0.125 10.34 4.14
max 0.383 0.475 27.29 1l.69
counts 50 76 50 76
modulus and hardness of both the S z and the CCML occurred at the distance of 220
11m. Figure 4 shows a scatter plot presenting hardness values plotted against their cor-
responding Young's modulus for CCML and Sz. Different clusters are observed for
the Sz and CCML. Hardness and Young's modulus of CCML are highly correlated to
each another (RZ = 0.55, p < 0.001), but no such correlations occur for the Sz (RZ =
0.09, n.s.).
0.50
Sz layer
015 CCML
015
~ 0.35
8
~ 0.30
.§
t;;
::c: 0.25
0.20
015
0.10
0 5 10 15 20 25 30
Young's modulus (GPa)
Fig. 4. Hardness-Young's modulus scatterplot showing relationships for cell comer middle la-
mella and S2'
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DISCUSSION
Because the mechanical properties microprobe allowed measurements on a spatially
resolved basis that was smaller than the width of single cell walls, the effects of cell
wall proportions and wood density on mechanical properties were not being directly
measured as in other wood property studies (e.g. Wellwood et al. 1965). In this study,
the mechanical measurements of S2 walls and the CCML reflect the ultrastructural
and chemical variability present in wood. The S2 layer and CCML differ chemically
and structurally, thus knowledge of the factors controlling the biochemical and bio-
mechanical processes occurring during cell wall formation are important.
Cell corners of sprucewood are highly lignified (e. g., Downes et al.199l; Wimmer &
McLaughlin 1996), as is the CCML (Fergus et al. 1969). Because no data were found
for direct mechanical CCML measurements, results that refer to the mechanical behavior
of isolated lignin are discussed. Cousins et al. (1975) obtained Young's modulus of di-
0xane lignin from a continuous indentation test. The lignin was chemically dissolved
using dioxane and precipitated to a thick syrup in a thin stream and dried. The dry lig-
nin powder resulted in a rod and appeared to be hard and brittle. The Young's modulus
of these lignin samples was 3.3 GPa with a coefficient of variation of 12%. Young's
modulus for CCML was twice the dioxane lignin elasticity. A few points might be
worthy of discussion. First, isolating lignin using dioxane introduces the problem that
all methods of isolation have, namely, these methods either fundamentally or at least
partially change the native lignin structure (Fengel & Wegener 1989). Further, during the
lignin isolation process extractives and other chemical components are removed or, at
least, have changed their molecular structure. Cousins (1976) has compared different
lignins and found periodate lignin to be the one that undergoes the fewest changes in
either physical or chemical structure during isolation. This type of lignin had the high-
est moduli but was also most sensitive to moisture changes. The present work was done
at the equilibrium moisture of 10.9% which gave mechanical values 10% under those
expected at 0% moisture (see Wimmer et al. 1997 for further discussion). The periodate
lignin (Cousins 1976) would give 4 GPa at 10.9% equilibrium moisture content.
Meier (1961) analyzed spruce and pine fibers at different stages of maturation and
found that the middle lamella/primary wall of cambial cells was rich in pectic acids,
arabinose and galactose. Therefore, it is likely that a change of these chemical compo-
nents could alter the mechanical properties of the CCML. Pectin plays a important role
during the lignification of wood cells, involving the cation Ca 2+. Studies have demon-
strated that the cambium has an especially high Ca 2+ concentration (Wardrop 1976).
Pectin is a good chelator of Ca 2+, building strong 'egg in the box' structures that can
be formed between polygalacturonic acids and Ca 2+ (Grant et al. 1973). Westermark
et al. (1986) found Ca 2+ to be high in unlignified woody tissues at high lignin concen-
trations indicating that pectin may act as a selective binder for Ca 2+ in the cambial
cells. Pectin degrades or will be removed prior to the lignification apparently releasing
Ca 2+ ions that are used in lignification. The released and positively charged Ca 2+ ions
bind to negatively charged groups of lignin and this could affect the mechanical prop-
erties of the CCML.
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From cell wall fracture investigations it is known that fractures occur predominant-
ly between the middle lamella and S 1 layers or between the S 1 and S 2 layers (Donaldson
1995). The rapid change from lignin-rich to lignin-poor lamellae as observed within
the S 1 layer (Maurer & Fengel 1991) favors cell wall fracture. Mark (1967) demon-
strated that wood fracture is rarely initiated within the compound middle lamella. It is
more likely that the S 1 layer is the first to undergo mechanical failure, usually because
of shearing. Mark (1967) showed that the shear stresses in the S 1 and S 2 layer are
typically opposite in direction, as might be anticipated from the difference in their
microfibrillar orientation angles. This agrees with finding high CCML hardness which
is an indicator for high mechanical strength.
The mechanical measurements along a 250 /Jm radial transect showed the CCML to
have a higher variability than the S2' The high variability of the CCML could be ex-
plained by the high variability of its chemistry. In a recent study, Tirumalai et al.
(1996) used Raman microprobe spectroscopy to study the concentration of ligno-
cellulosics in the CCML of black spruce. They found that the relative concentration of
lignin and cellulose varies considerably, partially because of lignin deficient regions.
Similar results were found in birch (Daniel et al. 1991).
S2 layers have a significantly higher Young's modulus than the CCML, but are not
particularly harder than the CCML. Maximum Young's modulus did not exceed 27
GPa and the mean is less than theoretically calculated values for S2, which vary be-
tween 28 GPa for earlywood and 35 GPa for latewood (Cave 1968). The final 1 or 2
latewood tracheids in a tree ring are usually very narrow radially. The mechanical
properties, particularly hardness, of these final tracheids were significantly lower than
those of other tracheids. A change in cell wall chemistry likely explains this finding.
Fukazawa and Imagawa (1981) showed in a UV-microscopic study with Abies that
cells in the terminal zone of the latewood have a distinctive high lignin content. Wilson
and Wellwood (1965) and also Wu and Wilson (1967) found that in conifers earlywood
lignin content was consistently 2 to 3% higher than latewood lignin content. Con-
versely, the per cent cellulose content is higher in latewood (De Zeeuw 1965; Fengel
1969) and there are longer cellulose chains, better packing and higher crystallinity (Lee
1961). These characteristics explain the mechanically stronger latewood (Wimmer
et al. 1997) with the exception of the very final (1-2) latewood tracheids which have a
different chemistry.
A discussion of the trends in Figure 4 leads to the question of positional dependen-
cies of the indentations within CCML and S 2. Measurements in CCML, if done very
close to Sz, could be affected by the S2 mechanical properties and vice versa. How-
ever, if this were the case, the Young's modulus should be affected first due to the
longerrange of the elastic strain fields during an indentation. Therefore, the S2 Young's
modulus should be affected much more by position within S 2 than is hardness, the
latter remaining nearly constant. As far as the strong correlation between hardness and
Young's modulus of CCML is concerned, it is possible that there is not a positional
effect within the CCML, but rather that the high chemical variability due to lignin
deficient regions is the main cause for this phenomenon. Due to the variability in chem-
istry and, consequently, packing density, hardness and Young's modulus are affected
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in the same way resulting in a strong correlation. On the other hand, in the S2 the
amount of cellulose is more or less constant over the measured distance of 250 /lm and
therefore packing density has not affected hardness which stays equal along a large
range of Young's moduli. But the orientation of the cellulose in the S2 (microfibrillar
angle) might have been altered to some extent influencing the Young's modulus con-
siderably as found by Cowdrey and Preston (1966).
CONCLUSIONS
For a better understanding of the mechanical behavior of wood it is essential to have
basic information on the property values of single wood components such as the S 2
and the CCML. The mechanical properties microprobe used in this investigation was
capable of measuring hardness and Young's modulus on a spatially resolved basis
similar to that of an electron beam microprobe. The CCML was found to be mechani-
cally strong with hardness values almost as high as the secondary walls. As a hypoth-
esis, in the lignin enriched middle lamella the positively charged Ca 2 + ions could bind
to the negatively charged groups at the lignin, providing additional strength to the
CCML.
Although we have measured mechanical properties from S2 and CCML solely, it
has to be reemphasized that in solid wood the individual cells are not isolated and they
rarely operate in isolation from neighboring cell walls. Due to the high variability of
the chemistry in the CCML as well as cellulose orientation in the S2' further investiga-
tions are needed that link chemical and micromechanical properties at a high spatial
resolution.
ACKNOWLEDGMENTS
The authors gratefully acknowledge the assistance of Drs. W.e. Oliver and T.Y. Tsui for their
collaborative role. Helpful ideas and suggestions were provided by Drs. B. Gardner and B. Hinter-
stoisser. Thanks are due to Prof. E. A. Wheeler for intensive editing, and to two anonymous reviewers.
Research was conducted while the senior author was a Visiting Scientist at the Oak Ridge National
Laboratory, Oak Ridge, TN 37831 partly with joint funding by the Austrian Science Foundation
(Schrtidinger Scholarship J799-BIO) and by the U.S. Environmental Protection Agency under
Interagency agreement with the U.S. Department of Energy.
REFERENCES
Abe, H., I. Ohtani & K. Fukazawa. 1991. FE-SEM observations on the microfibrillar orientation
in the secondary wall oftracheids. IAWA Bull. n.s. 12: 431-438.
Cave, I.D. 1968. The anisotropic elasticity of the plant cell wall. Wood Sci. Techno!. 2: 268-
278.
Cousins, W. J. 1976. Elastic modulus oflignin as related to moisture content. Wood Sci. Techno!.
10:9-17.
Cousins, w.I., R.w. Armstrong & W.H. Robinson. 1975. Young's modulus of lignin from a
continuous indentation test. I. Mat. Sci. 10: 1655-1658.
Downloaded from Brill.com09/28/2021 07:59:32PM
via free accessWimmer & Lucas - Mechanical properties of cell wall and middle lamella 87
Cowdrey, D.R. & F.RS. Preston. 1966. Elasticity and microfibrillar angle in the wood of
Sitka spruce. Proc. Roy. Soc. B 166: 245-272.
Daniel, G., T. Nilsson & B. Pettersson. 1991. Poorly and non-lignified regions in the middle
lamella cell comers of birch (Betula verrucosa) and other wood species. IAWA Bull. n. s. 12:
70-83.
De Zeeuw, e. 1965. Variability in wood. In: W.A Cote Jr. (ed.), Cellular ultrastructure of woody
plants: 457-471. Syracuse University Press.
Donaldson, L.A 1991. Seasonal changes in lignin distribution during tracheid development in
Pinus radiata D. Don. Wood Sci. Technol. 25: 15-24.
Donaldson, L.A 1995. Cell wall fraction properties in relation to lignin distribution and cell
dimensions among three genetic groups of radiata pine. Wood Sci. Technol. 29: 51-63.
Downes, G.M., J.Y. Ward & N.D. Turvey. 1991. Lignin distribution across tracheid cell walls of
poorly lignified wood from deformed copper deficient Pinus radiata (D. Don). Wood Sci.
Technol. 25: 7-14.
Fengel, D. 1967. Ultramicrotomy, its application in wood research. Wood Sci. Technol. 1: 191-
204.
Fengel, D. 1969. The ultrastructure of cellulose from wood. Part I: Wood as the basic material
for the isolation of cellulose. Wood Sci. Technol. 3: 203-217.
Fengel, D. & G. Wegener. 1989. Wood - chemistry, ultrastructure, reactions. Walter De Gruyter,
Berlin, New York
Fergus, B.J., AR. Procter, J.AN. Scott & D.A. Goring. 1969. The distribution of lignin in
sprucewood as determined by ultraviolet microscopy. Wood Sci. Technol. 3: 117-138.
Fukazawa, K. & H. Imagawa. 1981. Quantitative analysis of lignin using an UV microscopic
image analyzer. Variation within one growth increment. Wood Sci. Technol. 15: 45-55.
Grant, G.T., E.R Morris, D.A Rees, P.J.e. Smith & D. Thorn. 1973. Biological interactions
between polysaccharides and divalent cations. The egg-box model. FEBS Letters 21: 195-
198.
Grozdits, G.A & G. Ifju. 1969. Development of tensile strength and related properties in
differentiating coniferous xylem. Wood Sci. 1: 137-147.
Janka, G. 1906. Die Harte der HOlzer. Cbl. Ges. Forstwes. 32: 193-202.
Jayme, G. & D. Fengel. 1961. Beobachtungen an Ultradiinnschnitten von Fichtenholz. Holz
Roh- u. Werkstoff 19: 50-55.
Kennedy, R.W. & G. Ifju. 1962. Applications of microtensile testing to thin wood sections.
Tappi 45: 725-733.
Lee, e.L. 1961. Crystallinity of wood cellulose fibers studied by X-ray methods. For. Prod. J.
11: 108-112.
Leopold, B. & D.C. McIntosh. 1961. Chemical composition and physical properties of wood
fibres. III. Tensile strength of individual fibers from alkali extracted loblolly pine holocellu-
lose. Tappi 44: 235-240.
Mark, RE. 1967. Cell wall mechanics oftracheids. Yale University Press, New Haven, London.
Maurer, A & D. Fengel. 1991. Electron microscope representation of structural details in soft-
wood cell walls by very thin ultramicrotome sections. Holz Roh- u. Werkstoff 49: 53-
56.
Meier, H. 1961. The distribution of polysaccharides in wood fibers. J. Polym. Sci. 51: 11-18.
Oliver, w.e. 1986. Progress in the development of a mechanical properties microprobe. MRS
Bull. 11 (5): 15-19.
Oliver, W.C. & G.M. Pharr. 1992. An improved technique for determining hardness and elastic
modulus using load and displacement sensing indentation experiments. J. Mater. Res. 7:
1564-1583.
Downloaded from Brill.com09/28/2021 07:59:32PM
via free access88 IAWA Journal, Vol. 18 (1), 1997
Page, T.P., W.e. Oliver & e.J. McHargue. 1992. The deformation behavior of ceramic crystals
subjected to very low load (nano) indentations. J. Mater. Res. 7: 450-473.
Schniewind, A.P., G. Ifju & D.L. Brink. 1965. Effect of drying on the flexural rigidity of single
fibers. In: Consolidation of the paper web. Cambridge Symposium 1965. Technical section
of the British paper and board makers association: 538-543. London.
Spurr, A.R 1969 A low-viscosity epoxy resin embedding medium for electron microscope.
l Ultrastruct. Res. 26: 31-43.
Tirumalai, V.C., U.P. Agarwal & lR. Obst. 1996. Heterogeneity oflignin concentration in cell
comer middle lamella of white birch and black spruce. Wood Sci. Technol. 30: 99-104.
Wardrop, A.B. 1976. Lignification of the plant cell wall. Appl. Polym. Symp. 28: 1041-1063.
Wardrop, A.B. & H.E. Dadswell. 1951. Helical thickenings and micellar orientation in the sec-
ondary wall of conifer tracheids. Nature 168: 610.
Wellwood, RW., G. Ifju & J.w. Wilson. 1965. Intra-increment physical properties of certain
western Canadian coniferous species. In: W.A. Cote Jr (ed.), Cellular ultrastructure of woody
plants: 539-549. Syracuse University Press.
Westermark, U., H.-L. Hardell & T. Iversen. 1986. The content of protein and pectin in the lig-
nified middle lamella/primary wall from spruce fibers. Holzforschung 40: 65-68.
Willems, G., J.P. Celis, P. Lambrechts, M. Braem & G. Vanherle. 1993. Hardness and Young's
modulus determined by nanoindentation technique of filler particles of dental restorative
materials compared with human enamel. J. Biomed. Mat. Res. 27: 747-755.
Wilson, J.w. 1964. Wood characteristics. III. Intra-increment physical and chemical properties.
Pulp Paper Res. Inst. Can. Res. Note No. 45.
Wilson, lW. & R.W. Wellwood. 1965. Intra-increment chemical properties of certain Western
Canadian coniferous species. In: W. A. Cote Jr (ed.), Cellular ultrastructure of woody plants:
551-559. Syracuse University Press.
Wimmer, R, B.N. Lucas, T.Y. Tsui & W.C. Oliver. 1997. Longitudinal hardness and Young's
modulus of spruce tracheid secondary walls using nanoindentation technique. Wood Sci.
Technol. 31 (in press).
Wimmer, R & S. B. McLaughlin. 1996. Possible relationships between chemistry and mechanical
properties in the microstructure of red spruce xylem. In: J.S. Dean, D.M. Meko & T.W.
Swetnam (eds.), Tree rings, environment and humanity. Proceedings of the International
Conference, Tucson, Arizona, 17-21 May 1994: 659-668. Radiocarbon.
Wu, Y. & J.w. Wilson. 1967. Lignification within coniferous growth zones. Pulp Pap. Mag. Can.
68: T159-T171.
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