Xylem fibers and tracheids are the main cell types in hardwoods and softwoods, respec­tively, and have thick, lignified secondary walls that have been extensively studied. In these secondary walls, the cellulose microfibrils are more densely packed and highly ordered than in primary walls, and usually occur in three layers (Si, S2, and S3) (156). In the thinner outer (S1) and inner (S3) layers, the microfibrils have a cross-helical organization, whereas in the thicker middle layer (S2), the helical organization is uniform and steeper (Figure 4.10) (157). In the S2 wall layer, there is evidence from microscopy that aggregates of cellulose microfibrils and matrix material form alternating, concentric lamellae (158-160). In the walls of the softwood spruce (P. abies), the galactoglucomannans are associated more with the cellulose than the lignin, whereas with the heteroxylans the reverse is true (161, 162). These results led to a model (Figure 4.11) in which much of the galactoglucomannan is asso­ciated with the aggregates of cellulose microfibrils, and the matrix material contains lignin with associated heteroxylans and the remaining galactoglucomannans (10). This is consis­tent with the observations that heteromannans (glucomannans and galactoglucomannans) form oriented associations with surfaces of cellulose microfibrils since the conformation of their backbone chains is similar to cellulose (163) and are dissociated from the surfaces of cellulosic microfibrils only in strongly alkaline borate solutions [17.5% NaOH-4% borate (6 M NaOH-0.81 M H3BO3)] (164).

In the xylem fibers of hardwoods, immunogold labeling indicates that heteroxylans (4- O — methylglucuronoxylans) with low degrees of substitution are associated with the aggregates of cellulose microfibrils, whereas heteroxylans with higher degrees of substitution are in the matrix (165). Alow degree of acetylation of polysaccharides enhances their water solubility (23) and may prevent their association with one another and other polysaccharides in walls.

Although the cellulose microfibrils in the secondary walls of sclerenchyma fibers of some species of grasses are organized in a similar way to those in the xylem fibers and tracheids of hardwoods and softwoods (166), the walls of bamboo fibers have many layers and are described as polylamellate (167). As in the walls of hardwood fibers, in the lignified secondary walls of the grasses there maybe populations of heteroxylans with low degrees of substitution

associated with the aggregates of cellulose microfibrils and heteroxylans with higher degrees of substitution in the matrix (8, 168).

Studies of lignified secondary walls of hardwoods, softwoods, and grasses by transmission electron microscopy after preparation by fast freeze, deep etch, and rotary shadowing re­vealed bridges between cellulose microfibrils (147, 148, 169). Unlike the bridges in primary walls, these occur across slit-shaped pores, 8-40 nm wide, in the cell walls (148). The bridges were visible only in walls before lignification had occurred, or in lignified walls that had been delignified with acid sodium chlorite solution. Although the model proposed for the secondary wall of spruce tracheids, showed no bridges between the aggregates of cellulose microfibrils (10), similar principles used in modeling primary walls can be applied to mod­eling lignified secondary walls (8,168). Thus, polysaccharides associated with the aggregates of cellulose microfibrils may also form the bridges between the aggregates.