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Evaluation of Transgenic Wood for Pr oductivity & Quality     355


                                         (I) Nanoindentation
                   40                    (II) Model for radiata pine
                               (II)      (III) Model for spruce
                   35      (III)         (IV) Model for radiata pine (horizontal shifted)
                  Longitudinal modulus (GPa)  25  (VI)  (I)
                                         (V) Model for spruce (horizontal shifted)
                   30
                                         (VI) Vertical shifting of curve (V)

                   20
                         (IV)
                   15
                   10

                    5    (V)
                    0
                      5    10    15   20    25   30    35   40    45
                                     Microfibril angle (°)

               FIGURE 12.3  The S-2 cell wall longitudinal stiffness as a function of
               microfi bril angles: experimental and modeled values (Tze et al. 2007).


               observed, and there is a preliminary indication that the dependence
               could be affected by cell wall extractives. In cell wall modification
               studies, it is desirable to minimize any unintended variations by using
               samples that are from the same growth ring, so that any treatment-
               induced changes in cell wall hardness can be identified.


               12.3.3 Chemical Composition
               Wood tissue is composed of a group of polymeric components, which
               make up the bulk of wood cell walls. All polymeric components are
               mixed in the wall and determine the physical and mechanical proper-
               ties of wood. A typical wood cell includes a primary wall, a second-
               ary wall (S1, S2, S3), and a lumen.
                   There are two chemical components in a wood cell wall. The
               primary components, which establish the chemical and physical
               nature of the cell wall and constitute the bulk of materials in the
               wood cell wall, include cellulose (40 to 50 percent), hemicellulose (20 to
               35 percent), and lignin (15 to 35 percent). Cellulose is the most impor-
               tant single component in the cell wall and has a primary relationship
               to the physical and mechanical behavior of the wood. Hemicellulose
               and lignin are also important in the wall because they act as matrixes
               in the natural polymer composites. The secondary components in the
               wall include ash (<1 percent) and extractives, such as tannins, volatile
               oils and resins, gums, latex, alkaloids, and other compounds. The sec-
               ondary components may not significantly affect the structure of the
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