Fig. 3

The sample A cutting diagram for SEM and FE-SEM observations. a Radial view of sample A. A white circle indicates the area observed by SEM from the front and the underside. A black arrow indicates the direction of gravity. Scale bar, 5 mm. b Sample A was cut into 4 pieces, A-1, A-2, A-3, and A-4. Samples A-1 and A-3 were used for lignin analysis. Sample A-2 was used for FE-SEM observation