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.