# Atlas of Micromorphological Degradation of Archaeological Birch Bark
## 3 Results
### Sample 0: Contemporary Birch Bark Degraded in Nature  
This sample presents all identified degradation features except for decay by bacteria and the accumulation of foreign substances, which was present only in sample 13, retrieved from a cave environment. In the LM pictures, it is possible to identify degraded areas characterized by unfolding of the thin-walled compressed cells and an irregular arrangement of the cell structure. Birefringence is strongly present in the well-preserved areas, whereas it is completely missing in the degraded areas. All layers of thick-walled cells are visible in the well-preserved areas (Figure 13). The toluidine staining is stronger on the suberized secondary cell walls and weaker on the compound middle lamella. The phenolic compounds in the cell lumen are absent. The penetration of the secondary wall by hyphae caused circular voids located at the cell corners. Further, swelling and loosening of the wall is visible, leading to complete secondary wall disintegration. Finally, only a dark blue unstructured substance remains. Hyphae are visible in the lumen of the cells, leading to decay in parts of the tertiary walls (Figure 14, bottom left and top right). Figure 12: Reference sample. TEM images of thick-walled cell of lenticel with plasmodesmata (pla) in tangential wall and intercellular spaces (in-sp) in radial cell corners. Tubular plasmodesmata are filled and occasionally have a deposit (de) towards the cell lumen. Figure 14: Sample 0. LM overview of sample with clear boundary between well-preserved area and fungal degraded area (arrowheads). Secondary walls of thick-walled cells are penetrated mainly in the corners by hyphae (vo-fu), and hyphae are present in thin-walled cell lumen (hy). These adhere to the tertiary cell wall and sometimes cause it to detach (det-fu) (top right). At bottom left, complete dissolution of cell structure is visible (me-fu). At bottom right, fungal structures can be seen (fu-bo). These cause a decrease in lignin-containing middle lamella (rem-fu) and phenolic lumen filling of thick-walled cells. At the same time, darker staining (in-st-fu) and swelling of secondary cell wall occur (sw-fu). Fractures due to breaking of radial thin-walled cells (fra) are also visible. Figure 13: Sample 0. LM overview under transmitted light (**left**) and polarised light (**right**). Arrows on the left mark transitions between differently preserved areas. Degraded areas are characterized by unfolding of thin-walled cells (un), increased irregularity of cell arrangement (ir) and reduction of cell wall substance, especially suberized secondary wall of thick-walled cells (re-th) up to complete dissolution of cell structure (me-fu). Fracture of thin radial walls and layer separation are also visible (fra). In degraded area, thick-walled cells do not show birefringence (no-bi). The TEM pictures show that the secondary cell walls, which are infested with hyphae in the corners, causing oval voids, are swollen, their structure is loosening and their shape is irregular (Figure 15). Advanced degradation is indicated by disintegration of the wall; in some cases the cell wall seems to be completely digested, while in others an electron-dense granular residue is left. Similar compounds have been identified in scleroid of beech bark in the area around the hyphae, and interpreted as remnants of not completely degraded lignin [75]. Connected to the infestation with hyphae, the phenolic compounds in the lumen, compound middle lamella and secondary walls were removed (Figure 16). This led to separation of the cells along the compound middle lamella. Figure 15: Sample 0. TEM images illustrating degradation feature called circular voids. At top right, empty cavities not filled with hyphae are visible (vo-fu). In the same picture, swelling and loosening of secondary cell wall can be seen (sw-fu). At bottom right, cells with corners penetrated by hyphae (hy) are visible. These have almost completely metabolised the supervised wall, leaving only a little granular dark residue (re).