Macrophyte-flow interactions in aquatic plants.

Linear-bladed ('Vallisneria americana'), whorled (' Elodea canadensis'), and dissected ('Ceratophyllum demersum ') leaf morphologies occur convergently in aquatic plants. It is likely that fluid dynamics have had an effect on plant morphology, by acting on exchange processes at low flows and drag forces at higher flows. These species were examined using flow visualization and digital imagery at five velocities (1-11 cms-1) in a flume. Results indicated that in terms of fluid retention, 'Elodea' and 'Vallisneria ' acted like a cylinder, whereas 'Ceratophyllum' acted like a mesh. Transitional flows occurred at lower velocities than the cylinder, indicating an increase in local mixing. However, this was reversed when the internodal spacing was increased experimentally for 'Elodea' and 'Ceratophyllum'. This reduction in plant-flow interaction would allow these plants to remain erect in a relatively low energy environment. These results suggest that the flow environment has influenced the plasticity of leaf types and the evolution of macrophyte form in general.