Fig. 4

SEM images obtained using the secondary electron signal from Norway spruce wood subjected to different pretreatments. Lignin droplets are clearly visible (red arrows) on the surface of A STEX_210°C/auto and B STEX_210°C/SO2 samples. Low-magnification SEM images obtained using the secondary electron signal of FIB cross-sections reveal the internal structure of tracheids in C STEX_210°C/auto and D STEX_210°C/SO2 samples. The collapsed cell walls of tracheids are highlighted with red arrows