A Conspectus of Myriophyllum (Haloragaceae) in North America

Thirteen species of the genus Myriophyllum are found in North America; ten are native, three have been introduced. A key to the species is provided and previous conflicting taxonomic treatments are assessed. The possibility of a polyploid series involving four species is considered and dismissed. Eight of the common species are found to exhibit sufficient phenotypic plasticity to account for most if not all the taxonomic problems encountered.