The genus Echinodorus comprises 27 species found throughout North, Central, and South America. The most recently described species, Echinodorus floridanus (Haynes and Burkhalter), 1997, is diagnosable from other North American Echinodorus species by its large size, leaf shape, hardy rhizome, and stellate pubescence, and is endemic to the Florida panhandle. We have hypothesized that Echinodorus floridanus is closely related to South American taxa based on shared morphological traits. From a cladistic analysis of 127 morphological characters taken from 31 Echinodorus species and subspecies we have hypothesized that E. floridanus is closely related to South American taxa. Preliminary data using the 5S ribosomal DNA non-transcribed spacer (5S-NTS), a rapidly evolving nuclear region also supports Echinodorus floridanus as being sister to South American taxa, specifically to E. grandiflorus. We discuss relationships within the genus, focusing on two major groupings, and among putative outgroup taxa, providing characters uniting the various groupings therein. The first grouping consists of the 4 species of Echinodorus native to the United States while the second group of interest, E. floridanus, E. grandiflorus, and E. bracteatus, are of mixed geographic origin but appear to be closely related to one another.

Key words: Alismataceae, Aquatic, Echinodorus, Endangered