# Atlas of Micromorphological Degradation of Archaeological Birch Bark
## 3 Results
### Reference Material: Contemporary Intact Birch Bark  
The bark morphology of the reference sample was examined with light microscopy (Figure 2). The radial thin sections revealed a compactly arranged and regular cell structure consisting of an equal number of thin-walled and thick-walled cell bands, with the former being more hydrophilic than the latter (Figure 8). The thin-walled cells were heavily compressed in the outer portion of the bark but originally had a square shape, visible in the centre of the radial thin section. The shape of the lumen varied and was generally lenticular. The compound middle lamella and tertiary walls had a deeper blue colour than the secondary walls, an indication of their greater affinity for water [12]. The barely coloured thick-walled cell walls, on the other hand, showed strong birefringence. Suberin had a crystalline structure showing birefringence [72]. The TEM investigation of the reference sample shows the arrangement of cells and the layered structure of cell walls (Figure 9). In the thick-walled cell bands, the radial  
Figure 8: Reference sample. LM images of radial thin sections (**left**, outer surface; **right**, inner side) reveal thick-walled and thin-walled cell bands (arrowheads, **top left**). Tips of arrows indicate boundaries between thin-walled and thick-walled cell bands. Compressed thin-walled cells are marked with “co” (**top right**). Water affinity of thin-walled cell walls, compound middle lamella and tertiary walls can be appreciated from the intensity of blue staining. Thick-walled cell walls are hydrophobic (light blue) and show strong birefringence (**bottom**, **left** and **right**). Sample shows overall strong birefringence of thick-walled cells. While thin-walled cells usually have only very weak birefringence, the birefringence of thin-walled cell layer at boundary with thick-walled cells is just as strong as that of thick-walled cells. compound middle lamella was about 0.24 \(\pm\) 0.03 \(\upmu\)m thick with a net-like structure in the centre and dark rims. The tangential compound middle lamella was 0.08 \(\pm\) 0.01 \(\upmu\)m thick and electron dense, an indication of higher lignification [73; 74]. Interruptions in the tangential compound middle lamella were present. At the transition between the thick- and thin-walled cell bands, the radial compound middle lamella built a thick corner with ends susceptible to fracture. The thick-walled cells were filled with electron-dense compounds with an irregular shape. In the thin-walled cell bands, it was not possible to distinguish between radial and tangential compound middle lamella, as the cells were compressed. The compound middle lamella had a homogeneous structure and was about \(0.08\pm 0.02\)\(\upmu\)m thick. The secondary wall is suberized, and therefore had a lamellar structure with very fine dark lines, arranged around dense globular particles. The tertiary wall is the boundary layer at the lumen and was very dense. The thin-walled cells were partly filled with a very dark multiform compound. Close to the lenticel, the reference sample was delaminated (Figure 2, left). LM images show that separation of the layers occurred at the boundary between thin- and thick-walled cell bands. Fracture occurred in the radial thin-walled cell walls (Figure 10, left). The consequence was a tangential separation (delamination) of the layers and the unfolding of thin-walled cells. The dark blue and purple balls and bubbles and the pinkish cloudy structures visible on the outermost layers of the toluidine-stained radial thin sections indicate the presence of hyphae. TEM images of the same area show fungal structures in the lumen of the cells and a hypha penetrating from one cell to another. Figure 9: Reference sample. TEM images of thick-walled and thin-walled cells. The compound middle lamella (CML) is thicker in radial direction (ra) than tangential direction (ta) in thick-walled cell bands (**top left**). In tangential direction, interruptions of composite middle lamella are present (in, **bottom left**).