Studies of the cellular structure of different softwood species have shown that there are large similarities between them, which are likely to have arisen early in the evolution of woody plants and are clearly so important to their survival that they have changed little since (Barnett and Bonham, 2004). Within the stem, wood cells exist to carry out two primary functions; provide a passage for water and nutrient transportation from roots to crown, and to support the large static and dynamic forces induced by the biomass above (Booker and Sell, 1998). In order to fulfil these functions, softwoods are made up of tracheids and rays. Tracheids are aligned vertically and account for approximately 90 % of the cells present, they allow the vertical transportation of materials up the stem and carry the imposed loads (Dinwoodie, 2000). Ray tracheids are found in the horizontal plane and allow for the movement of water and minerals radially, with ray parenchyma cells acting as a material store (Romberger et al., 1993, Walker, 2006). The flow of nutrients between tracheids occurs through openings between them known as bordered pits, as described by Walker (2006).
Table 2-1: Documented values for wood chemical constituent mechanical properties Experimental Modelled/Estimated Cellulose Ex (N/mm2) 1370001, 1380002, 120000 - 1350003 168000 4, 2460005 Ey (N/mm2) 177004, 180006, 277007 Gxy (N/mm2) 30008, 45007, 51004 νxy 0.17 Hemicellulose Ex (N/mm2) 80008 20009 Ey (N/mm2) 8009, 34006 Gxy (N/mm2) 10009, 180010 νxy 0.29 Lignin Ex (N/mm2) 310011 200010 Ey (N/mm2) 1000 - 200010 Gxy (N/mm2) 120011 6009,8007 νxy 0.3312
1) (Sakurada et al., 1962), 2) (Nishino et al., 1995), 3) (Matsuo et al., 1990), 4) (Tashiro and Kobayashi, 1991), 5) (Mark, 1980), 6) (Cave, 1978), 7) (Mark, 1967), 8) (Cousins, 1978), 9) (Salmén, 2004), 10) (Bergander and Salmén, 2002), 11) (Cousins, 1976), 12) (Bodig and Jayne, 1982)
where: Ex longitudinal modulus of elasticity
Ey transverse modulus of elasticity
Gxy shear modulus
ν Poisson’s ratio
The now commonly accepted softwood cellular structure is that shown in Figure 2-2, in which the tracheid is divided into an amorphous middle lamella, a thin primary wall and a three-ply secondary wall comprising the S1, S2 and S3 layers. This model was first proposed by Bailey and Kerr (1935), who used polarization microscopy to show the presence of a three-ply structure in which the layers were characterised by different predominating orientations of the cellulose. This was in disagreement to earlier proposal by Preston (1934) that the angle of cellulose fibrils was constant throughout the thickness of the cell wall. Bailey and Kerr’s model was found to be correct and understanding was deepened with the early application of
transmission electron microscopy (Wardrop, 1954, 1957, 1958, Harada et al., 1958), and more recently field emission scanning electron microscopy (Abe and Funada, 2005). Within the secondary wall layers the cellulose microfibrils are highly aligned, as discussed below. The deviation of the helical winding of the microfibrils from the tracheid cell longitudinal axis is termed the microfibril angle, as depicted in Figure 2-2. A summary of the functions believed to be carried out by each cell wall layer and their characteristics is given in the sub-sections that follow.
Figure 2-2: Softwood cell wall structure from Dinwoodie (2000)
2.3.3.1 Middle lamella and primary wall
The middle lamella is situated between the cells, acting as a binding material (Walker, 2006). The primary cell wall contains a loose network of microfibrils that were proposed in the model of Harada and Côté Jr (1985) to be orientated approximately longitudinally on the outer face and transversely on the inner, a fact that was confirmed by Abe et al. (1995) with the use of scanning electron microscopy. The transverse orientation on the inner face is thought to restrain lateral cell expansion at the end of the first tracheid developmental phase (Taiz, 1984). It is often difficult to distinguish the middle lamella from the primary cell wall, resulting in the two commonly referred to as the compound middle lamella (Jyske, 2008). The thickness of the compound middle lamella is typically within the range of 0.25 - 0.35 μm, with no significant differences between early and latewood. Typical values of the proportions of chemical constituents at 12 % moisture content are 5 %, 40 % and 55 % for cellulose, hemicellulose and lignin respectively (Persson, 2000, Bergander and Salmén, 2002, Qing and Mishnaevsky Jr, 2009).
Microfibril angle
2.3.3.2 S1 layer
In early studies investigating the arrangement of microfibrils within the S1 layer of the cell wall,
such as those conducted with the use of transmission electron microscopy by Wardrop (1954, 1957, 1958) and Harada and Côté Jr (1985), it was thought that the microfibrils were organised in an alternating structure of crossed S and Z helices1 within the cell wall. It has however been
shown more recently with the use of field emission scanning electron microscopy (Abe et al., 1991) and a combination of polarised light and transmission electron microscopy (Donaldson and Xu, 2005), that the arrangement of microfibrils is not crossed, but instead gradually changes from an S helix on the outer face to a Z helix on the inner surface nearest the lumen, with microfibril angles varying from approximately 45° to 70° in the respective orientations, with no systematic variation with age evident. No documented results were found regarding variations in the S1 layer microfibril angle in Douglas-fir. It was postulated by Donaldson
(2008) that due to the thin nature of the S1 layer, the outer surface of the S2 layer may have also
been observed in early studies, leading to the conclusion of a crossed arrangement. It is thought that this microfibril arrangement aids in limiting excessive radial expansion of the S2
layer under compressive loads by forming a protective sheath, while also preventing the transformation of intra-wall fracture along the boundary between the compound middle lamella and S1 layer into transwall fracture across the entire cell wall (Booker and Sell, 1998).
The thickness of the S1 layer has previously been reported as increasing from 0.2 μm within the earlywood to 0.3 μm in the latewood, with typical values of the chemical constituents at 12 % moisture content of 30 %, 30 % and 40 % for cellulose, hemicellulose and lignin respectively (Fengel and Stoll, 1973, Bergander and Salmén, 2002, Qing and Mishnaevsky Jr, 2009).
2.3.3.3 S2 layer
The microfibrils within the S2 layer are arranged in a highly ordered Z helix (Barnett and
Bonham, 2004). Variation in the microfibril angle is large both within and between trees, typically ranging from 40° to 5° (Donaldson, 2008). The trends of, and factors responsible for these variations are discussed further in Section 2.5.2. The thickness of the S2 layer has
previously been reported as increasing from 1.4 μm within the earlywood to 4 μm in the latewood, with typical values of the chemical constituents at 12 % moisture content of 50 %, 30 % and 20 % for cellulose, hemicellulose and lignin respectively (Fengel and Stoll, 1973, Bergander and Salmén, 2002, Qing and Mishnaevsky Jr, 2009). Given the large thickness
1 The helix, when observed from the outer face of the cells, is designated an S-helix when it is a left handed
relative to other cell wall components, it is the S2 layer that is predominately responsible for
carrying loads imposed by the mass of the tree, as well as tension and compression forces generated by imposed loads which may cause the stem to flex, such as wind and passing animals in younger stems (Booker and Sell, 1998).
2.3.3.4 S3 layer
Data regarding the orientation of microfibrils within the S3 layer is comparatively limited compared with the other secondary wall layers. This is largely due to difficulties associated with its measurement, with incorrect measurement of the inner S2 layer possible (Donaldson
and Xu, 2005). The results presented by Wardrop (1964), Tang (1973) and Donaldson and Xu (2005) are in general agreement, and show that the microfibril orientations range from an S to Z helix on moving towards the lumen, with microfibril angles typically varying by ± 50° or less from the tracheid longitudinal axis. The differences between the fibril angles reported in the above studies is likely due to a combination of factors, including the limited number of tracheids assessed, differences between trees and species and also variations in measurement techniques. No documented data was found regarding the orientation of microfibrils within the S3 layer of Douglas-fir trees. This fibril arrangement within the S3 layer is thought to help
greatly in the stiffening of the tracheids against the hydrostatic collapse forces imposed on to the faces of the lumen, generated by the conduction of materials up the stem. Booker (1993) demonstrated that the stiffness of the cell walls to sideways deflection is increased by a factor of approximately 2.5 by the presence of the S3 layers acting as cross-banding on the inner
lumen face. Having the lowest proportion within the cell wall, the thickness of the S3 layer has
previously been reported as increasing from 0.03 μm within the earlywood to 0.04 μm in the latewood, with typical values of the chemical constituents at 12 % moisture content of 48 %, 36 % and 16 % for cellulose, hemicellulose and lignin respectively (Fengel and Stoll, 1973, Bergander and Salmén, 2002, Qing and Mishnaevsky Jr, 2009).
2.3.3.5 Helical thickening
A further cell wall feature specific to Douglas-fir and a limited number of other softwoods are helical or spiral thickenings, ridges of cell wall material, deposited on the lumen side of the tracheid. Helical thickenings are usually found in an S helix (Wardrop, 1964) and have been observed to be an extension of the S3 layer (Jute and Levy, 1973). The helix angle of the
thickenings is thought to be related to tracheid size, with longer and thinner cells having a helix angled nearer to perpendicular to the longitudinal axis of the tracheids (Meylan and Butterfield, 1972). The role that helical thickenings play within the cell is still not fully
understood, it is however thought that they increase cell wall strength, preventing collapse arising due to hydrostatic forces (Yang, 2006), similar to the role of the S3 layer microfibrils. Images of helical thickenings in the tracheids of Douglas-fir are shown in Figure 2-3.
(a) (b)
Figure 2-3: Helical thickenings in Douglas-fir tracheids (a) 1750x magnification (b) 10000x magnification from Meylan and Butterfield (1972)