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Mixed‐linkage (1→3),(1→4)‐β‐d‐glucan (MLG) is widely considered to be a defining feature of the cell walls of plants in the Poales order. However, we conducted an extensive survey of cell‐wall composition in diverse land plants and discovered that MLG is also abundant in the walls of the horsetail Equisetum arvense. MALDI‐TOF MS and monosaccharide linkage analysis revealed that MLG in E. arvense is an unbranched homopolymer that consists of short blocks of contiguous 1,4‐β‐linked glucose residues joined by 1,3‐β linkages. However, in contrast to Poaceae species, MLG in E. arvense consists mostly of cellotetraose rather than cellotetriose, and lacks long 1,4‐β‐linked glucan blocks. Monosaccharide linkage analyses and immunochemical profiling indicated that, in E. arvense, MLG is a component of cell walls that have a novel architecture that differs significantly from that of the generally recognized …