Mechanical and anatomical adaptations in terrestrial and aquatic buttercups to their respective environments

The mechanical adaptations of the stems of four species of Ranunculus to their respective environments were studied by combining tensile, bending and flow-tunnel tests, with anatomical observation. Stems of the two terrestrial species, R. acris and R. repens, had high values for rigidity, EI, because they were stiffened by large quantities of peripherally placed lignified material. This trend is less evident in R. repens, which had a lower rigidity, though it with stood a higher breaking strain than R. acris. This may adapt R. repens to its creeping habit and help it withstand trampling. The aquatic R. peltatus and R. fluitans, which live in still and fast-flowing water, respectively, are both more flexible and have higher breaking strains, of 0.1–0.15, than terrestrial plants, which may allow them to withstand sudden tugs due to flow. R. peltatus maintains the central lumen, places structural elements away from the centre, and has a higher rigidity than R. fluitans, which may allow it to avoid self-shading, and support itself when the water level falls. The stem of R. fluitans shows adaptations for withstanding drag from fast-flowing water. The stem has a low rigidity which allows it to minimize inertial drag forces by aligning itself parallel to the direction of the local flow. However, the rigidity (and the second moment of area, I) does not appear to be minimized. This may allow the plant to avoid drag due to flag-like fluttering. A weak region of the stem near the base may act as a ‘mechanical fuse’ which protects the root system by allowing seasonal growth to be lost.