649 Polygonum hydropiper L.

This polymorphic species s. lat. has a very wide distribution. It belongs to the circumpolar plants (Hultén, Circumpol. Il, p. 218 and map 209). In N. America both native and naturalized populations are considered as occurring. Further, this species has been introduced into many parts of the world. Cf. also Meusel et al. 1965, map p. 131.