Figure 4
Transverse sections of tracheids in the compression wood and opposite wood of Pinus densiflora. (a,b) Broad argon ion beam (BIB)-milled cross-section of compression wood. The tracheids have a round shape with a thick cell wall. Many intercellular spaces can be observed among tracheids. No cutting damage, such as lightning bolt cracks or detachment between the S1 and S2 layers of the secondary wall, as observed in microtome cross-sections, are present in the BIB-milled section. (c,d) BIB-milled cross-section of opposite wood (non-reaction wood). Tracheids have rectangular or hexagonal shapes with relatively thin cell walls. Arrows indicate bordered pits. (e,f) Microtome cross-section of compression wood. The structures of cell walls and helical cavities are extended in the cutting direction. (g,h) Razor blade-cut cross-section of compression wood. Several cracks are generated in the S2 layer of the secondary wall (e,h). The S2 layers are delaminated from the S1 layers due to cutting stress. Scale bars = 50 µm (a, c, e, g) or 10 µm (b, d, f, h).
