ABSTRACTTo investigate the possible link between cell wall alginic acid composition and tissue mechanics, juveniles ofEgregia menziesii(Turn.) Aresch. were grown under controlled conditions in an outdoor flowing seawater system under three different force regimes. After 6–10 weeks of growth, tissue strength, breaking strain, modulus, toughness, work of fracture, and the percentage of polymannuronate, polyguluronate, and alternating sequences in the cell wall alginic acid were examined. The force regime had significant effects on all mechanical indices except toughness. Juveniles grown under high energy conditions (water velocity = 1.2 m · s−1) were about two times stronger, two times stiffer, and had a 1.5 times greater work of fracture than those from low energy conditions (<1 cm ·−1). Treatment effects on thallus strength and modulus were predicted from alginic acid composition data to test for the importance of this cell wall material in whole plant adaptation to hydrodynamic stress. However, the prediction that differences in alginic acid composition were responsible for differences in tissue mechanical properties was inconsistent with observations. Therefore, the hypothesis that alginates play a central role in structural adaptation could not be a