Anatomic structure of wood. Legend: (a) Spruce anatomic structure with scanning electron microscopy (SEM) in the RT plane. The width of the image corresponds to approximately 240 μm ([13], Fig. 14, p. 54); (b) 3D reconstruction of approximately 200 μm high section of the microsample ([13], Fig. 6, p. 54); (c) Microfibril angle (MFA) defined as the angle of the microfibril bundles with respect to the cell axis in the S2 wall layer ([14], Fig. 3, p. 183); (d) SEM micrograph of spruce tracheid. The fracture between layers S1 and layer S2 is oriented perpendicular to the tracheid axis (lower right large arrow). Surface corrugations (SC) running approximately parallel to the tracheid axis are present on the S1 surface. Cracks (upper left arrow) originate from helical striations present in the S2 layer ([15], Fig. 2, p. 419).