The primary walls of the grasses and most other commelinid monocotyledons except the palms (Arecaceae) are rich in glucuronoarabinoxylans (GAXs) (3) (Table 4.1). Examination
of the primary cell walls of the grass maize by transmission electron microscopy after prepar­ing them by fast freeze, deep etch, and rotary shadowing showed similar bridges between the cellulose microfibrils to those described for pectin-rich primary walls [Section 4.4.1.1, (151)]. Again, two co-extensive, but independent, polymer networks have been proposed as models of the architecture of these walls (152). The bridging molecules are considered to be GAXs carrying few side chains. GAX with low degrees of substitution may interact reversibly through hydrogen bonding with surfaces of cellulosic microfibrils and may do so in the twofold helical conformation (153). With increasing degrees of substitution the affinity for microfibril surfaces decreases, presumably due to the increase in the threefold conformation which would be unfavorable for association with cellulose and additionallyto increased steric hindrance to interaction imposed by the Araf substituents themselves. GAXs with low degrees of substitution with arabinosyl units, as well as (1^3,1^4)-p-D-glucans, have been assumed to coat the cellulose microfibrils (152). However, it is possible that, as in the primary walls ofmungbean (Vigna radiata) (150), non-cellulosic polysaccharides are ad­sorbed onto only a small proportion of the microfibril surfaces. Highly substituted GAXs and small proportions of pectic polysaccharides are considered to comprise the second network. The primary cell walls of all other commelinid monocotyledons (excluding the Arecaceae), including the other families that contain (1^3,1^4)-p-D-glucans in their cell walls (17), presumably have similar wall architectures. The high molecular weight (1^3,1^4)-p-D — glucans show high viscosities in solution and at high concentrations can be induced to form gels. Associations between (1^3,1^4)-p-D-glucan chains are proposed to be due to junction zone formation between pairs of consecutive cellotriosyl units (154) and puta­tively there are also non-covalent associations between the (1^3, 1^4)-p-D-glucan and heteroxylan components in the wall matrix (155).