The present-day distributions ofCeratophyllum demersumandC.echinatumare useful in interpreting the Quaternary distributional history of aquatic plants. Eastndash;west North American disjunct distributions of submersed hydrophytes are probably due to incomplete interglacial migrations. Widespread species such asC.demersumhave successfully bridged gaps formed when populations migrated into isolated southern reacute;fugia to escape glaciation. Species with narrower ranges, such asC.echinatum, are still in the process of doing so. Northern disjunct distributions ofC.demersumindicate that the species also survived glaciation in northwest refugia. It is apparent that many submersed aquatic plants have endured the migrational pressures imposed during glacial periods. Their ability to do so is attributable to four factors: (i) long species durations, (ii) formerly continuous distributional ranges, (iii) high vagility, and (iv) adaptation to cold, oligotrophic conditions. Extant hydrophyte communities in glaciated North America may resemble those in existence during former interglacial periods.
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