In the aqueous gel matrix of the primary wall polysaccharide chains interact non­covalently to form a continuous three-dimensional network. The polysaccharide chains in the gel have two types of domains: open, hydrated, unassociated regions and regions where the complementary conformations of two or more chains permit association over restricted segments (junction zones) (78). The forces stabilizing these junction zones are intermolecular hydrogen bonds or ionic forces. The capacity of matrix polysaccharides to form junction zones is variable and depends on the stereoregularity of the chain, determined by the monosaccharide and linkage sequence, the presence of bulky side chains and the proximity of mutually repulsive charged residues. The physical characteristics of the gel matrix will depend on the lengths and numbers of junction zones. The non-cellulosic wall polysaccharides generally have features that make them potential gel-forming polymers. They have linear backbones, are more or less soluble in water, and in contrast to the cellulosic fibrillar phase, show conformational irregularities. The non-covalent interactions of the non-cellulosic polysaccharides with one another and the surfaces of cellulosic microfibrils are important in determining the cohesivity of cell walls as is discussed in Section 4.4 — Molecular architecture of plant cell walls.

In addition to the functional strengthening of walls by non-covalent interactions be­tween individual matrix polymers and matrix polymers and the cellulosic microfibrillar phase, these interactions are reinforced by direct covalent associations between polysac­charides, polysaccharides and lignin, polysaccharides and protein, and between proteins, as well as indirect associations between polysaccharides and polysaccharides and lignin through covalently-linked bridging molecules. The chemistry of this cross-linking is described in the following sections.