Hydrodynamics and Biomechanics of the Submerged Water Moss Fontinalis antipyretica ‐ a Comparison of Specimens from Habitats with Different Flow Velocities

Flow velocity has an influence on the hydrodynamic and biomechanical properties, as well as on the morphology and the anatomy of the submerged water moss Fontinalis antipyretica Hedw. Cross‐sections of the plant stems show two main types of tissues. The strengthening tissue in the outer part is characterized by thick‐walled cells with a small lumen, the parenchyma in the centre by thin‐walled cells with a large lumen. The specimens from habitats of different flow velocities differ in the proportions of the strengthening tissue and the branching angle of the leaves. A flow tank with a special sensitive two‐component balance inserted into the bottom of the flume was used to measure the hydrodynamic drag, which acts on the plant stems at different flow velocities. The drag forces increase with the length of the plant. Mechanical properties such as elasticity and ultimate strength of the plant stems were tested in tension. Relating the data to the relative proportions of the strengthening tissue results in different estimates of Young's moduli for the strengthening tissue of plants from the different sites. The critical strains to which the stems can be extended are remarkably high. Loading and unloading cycles reveal viscoelastic behaviour of the stem tissues. In the first cycle plastic deformation is also observed, but only to a lesser degree in subsequent cycles.