Morphological variation of two taxonomically distant plant species along a natural flow velocity gradient.

•  The similarity of morphological plastic response to physical stress has been poorly tested among distant plant taxons as yet. •  The effect of flow stress was compared for two species -Berula erecta and Mentha aquatica- through 14 morphological traits measured for five sets of 30 individuals in five patches organised along a natural velocity gradient. Size-dependent and size-corrected traits were analysed by single correlations and multivariate analyses. •  B. erecta exhibited the expected pattern, that is a sharp decrease of all but one size-dependent trait as velocity increased. Five and four size-corrected traits were correlated with velocity, for B. erecta and M. aquatica, respectively, but three of them showed an opposite trend for the two species. Within-patch trait variability, as hypothesized, tended to decrease with velocity for B. erecta. •  The two species exhibited partly opposite responses despite the involvement of common traits. Small size allowed B. erecta to escape flow stress, whereas M. aquatica acquired a more streamlined morphology. The adaptive value of these morphological adjustments should be assessed through drag measurements.