Differences in Stachys palustris populations

Stachys palustris has a wide distribution in North America and Europe with groups of different ploidy levels. Europe is apparently mostly hexaploid, while North America has tetraploid and diploid populations.

Here are two seedlings of S. palustris. European (and presumedly hexaploid) on the left and North American (and probably tetraploid) on the right. The differences are most apparent at the seedling stage.