Class of plant cell-wall polysaccharide (chains of simple sugar) that cannot be extracted from the wall by hot water or chelating agents, but can be extracted by aqueous alkali. Includes xylan, glucuronoxylan, arabinoxylan, arabinogalactan, glucomannan, xyloglucan, and galactomannan. Part of the cell-wall matrix and as storage of carbohydrates in some seeds.