Wood xylem fiber cell consists of two main parts: a thick secondary wall and thin compound middle lamella (CML). A middle layer (S2) is the thickest layer of the secondary wall and is reinforced with fibrous polysaccharide crystals oriented more or less parallel to the fiber cell axis. The orientation angle (microfibril angle; MFA) determines the mechanical properties of the wood xylem fibers, including the longitudinal Young's modulus, anisotropic drying shrinkage, surface growth stress, longitudinal tensile creep deformation, and so forth. Consequently, the S2 layer takes an important role in determining the macroscopic properties of clear wood specimens, with the mechanical interaction between the reinforcing polysaccharides and the encrusting matrix substance ultimately controlling the material properties of the wood.
