Xyloglucan is a polysaccharide found in the primary cell walls of all higher plants examined. Its cellulose-like backbone, which is about 0.15 to 1.5 μm long, consists of 300 to 3 000 β-(1→4)-linked D-glucopyranose residues. About 60–75% (or, in grasses, about 30–40%) of the glucose residues have side-chains attached to position 6. The major side-chains are: D-xylopyranosyl-α-1 -→, D-galactopyranosyl-β-(1 →2)-D-xylopyranosyl-α-I→ , L-arabinofuranosyl-(1 -→2)-D-xylopyranosyl-α-1 -→, and (except in grasses) L.-fucopyranosyl-α-(1 -→2)-D-galactopyranosyl-β-(1-→2)-D-xylopyranosyl-α-1 -→. There is some regularity in the distribution of these side-chains along the backbone. Xyloglucan plays two very different rôles in the control of cell growth: (a) as a major building material of the wall [concentration of xyloglucan in the wall in vivo ≈ 10% (w/v)] it probably directly dictates wall extensibility and, therefore, the rate of cell expansion and (b) it can be broken down to a fucose-containing oligosaccharide which [at a concentration of ≈ 0.0000001% (w/v)] exerts a hormone-like anti-auxin effect on growth. In addition, xyloglucan lacking fucose is used by certain dicotyledonous seeds as a food reserve which is mobilized after germination. Xyloglucan is, therefore, the subject of considerable current interest in several apparently disparate areas of botany.