Plant secondary cell walls have received considerable attention in recent years as they account for the bulk of plant biomass and are thus a renewable source of carbon for the production of biofuels and other added-value chemicals. Heteroxylans are the predominant hemicellulose in biomass from woody plants and grasses. These xylans have a backbone composed of 1,4-linked -D-xylopyranosyl (Xylp) residues that vary in the type, location and number of substitutions to the backbone. Eudicots synthesize glucuronoxylans that have -D-glucosyluronic acid (-D-GlcpA) and/or a 4-O-methyl -D-glucosyluronic acid (4-O-Me-GlcpA) linked to O-2 of the backbone residues. By contrast, grass xylans contain a-L-arabinofuranosyl residues linked to O-3 of the backbone. Dicot glucuronoxylans have a unique glycosyl sequence (4--D-Xylp-(1,4)--D-Xylp-(1,3)--L-Rhap-(1,2)--D-GalpA-(1,4)-D-Xylp) at their reducing end, which has been shown using Arabidopsis mutants to be required for normal xylan synthesis and for normal plant growth and development. In my dissertation, I describe the structural characterization of heteroxylans isolated from the walls of members of the different monocot orders and seedless land plants and discuss how these changes are correlated with the evolution of these plants. I also show that members of the Poales and Asparagales have heteroxylans that lack the glycosyl sequence 4--D-Xylp-(1,4)--D-Xylp-(1,3)--L-Rhap-(1,2)--D-GalpA-(1,4)-D-Xylp) at their reducing end. The results of my studies provide new insight into the structure of heteroxylans in the monocots and the nature of the sequence of glycosyl residues at the reducing ends of these xylans. In addition my research provides evidence that the sequence 4--D-Xylp-(1,4)--D-Xylp-(1,3)--L-Rhap-(1,2)--D-GalpA-(1,4)-D-Xylp was not eliminated when monocots and dicots diverged but was lost during the evolution of some monocot orders. Such knowledge provides additional insight into the structural diversity of land plant cell wall polysaccharides. My studies also provide a foundation for the application of genetic and molecular approaches to modify xylan structure and its interaction with other wall components to improve the economic value of plant biomass.