Fig. 4

Typical Salix Tora TW G-fibres and presence of G-layer (arrows). a Safranin/astra blue (double staining) and b astra blue only. The G-layer is shown as a thick blue staining layer lining the cell lumina and has been pulled away (i.e., sectioning artefact) in most fibres from the outer fibre secondary cell wall. c After treatment with Wiesner (i.e., for distribution of cinnamaldehyde groups), and d Mäule (for syringyl-lignin units) reagents, the gelatinous layer reacted negatively and remained almost transparent (arrows). e, f Mature control (non-TW) and TW, respectively. Red staining indicative of cinnamaldehyde groups is concentrated to the middle lamellae and vessel walls. g, h Staining of control- (non-TW) and TW with Mäule reagent showing strong staining of fibre secondary walls, a weak reaction with vessel secondary cell wall layers (S1, S2) and negative for the G-layer