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

The mechanical adaptations of the stems of four species of Ranunculusto 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 stiffenedby 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 withstandtrampling. The aquatic R. peltatus and R. fluitans, which live in stilland fast-flowing water, respectively, are both more flexibleand have higher breaking strains, of 0.1–0.15, than terrestrialplants, which may allow them to withstand sudden tugs due toflow. R. peltatus maintains the central lumen, places structuralelements away from the centre, and has a higher rigidity thanR. fluitans, which may allow it to avoid self-shading, and supportitself when the water level falls. The stem of R. fluitans shows adaptations for withstanding dragfrom fast-flowing water. The stem has a low rigidity which allowsit to minimize inertial drag forces by aligning itself parallelto the direction of the local flow. However, the rigidity (andthe 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 ‘mechanicalfuse’ which protects the root system by allowing seasonalgrowth to be lost. Key words: Ranunculus, mechanics, flow, anatomy