Fig. 6. Comparative schemes of secondary cell walls in grass, hardwood, and softwood.

The figure shows the spatial arrangement of lignin (yellow), cellulose (white fibrils), two-fold xylan (red flat ribbons), three-fold xylan (blue twisted ribbons), mixed forms of xylan (magenta), and mannan (GGM; green) in secondary plant cell walls. Numbered spheres highlight the structural features of (1) lignin–xylan interaction in all plants, (2) cellulose bundles in woody plants, (3–6) two-fold and three-fold xylan with different sidechains, and (7) galactoglucomannan. The molecular fraction of polysaccharides is considered in the depiction, but the illustration may not be strictly to scale. The model of grass cell wall is generated using the data recently reported²⁴, for comparison with the models of woody plants. Lignin and carbohydrates are much better mixed in woody plants than in grass, resulting in the binding of lignin to both three-fold and two-fold xylan as well as to cellulose microfibrils. The structural assembly of woody cell walls is thus different from the domain-separation scheme of grass cell walls.