Collinsonia L. (Lamiaceae) sensu stricto is a genus of mints endemic to eastern North America. At present, four species and two subgenera are commonly recognized. C. canadensis L., C. serotina Walter and C. tuberosa Michaux form subgenus Collinsonia and are morphologically quite similar to one another. Shinners (1962) published the only taxonomic treatment of the genus, dealing primarily with nomenclature and considering only morphological variation, as represented on herbarium specimens. He performed no quantitative analyses. A considerable amount of taxonomic confusion continues to surround Collinsonia; therefore, a modern systematic study of the genus was performed. Field studies of natural populations were conducted, and leaf material for molecular work was collected. Species boundaries within subgenus Collinsonia were examined by subjecting macromorphological variation to principal components analysis (PCA) and canonical variates analysis (CVA). Results of the ordination suggest that subgenus Collinsonia is composed of two distinct taxa and possible hybrids. C. tuberosa is indistinguishable from C. canadensis, showing considerable overlap in the phenetic analysis. Therefore, the two taxa are best merged under C. canadensis. C. serotina (as delimited by Shinners) is a combination of true C. serotina (those specimens with four stamens), unusual C. canadensis and hybrids between the two. In addition to the phenetic analyses, molecular investigations have been undertaken. The ITS sequence analysis focuses on the phylogeny of Collinsonia and of tribe Elsholtzieae and provides additional evidence concerning species boundaries.

Key words: Collinsonia, Lamiaceae, ordination, phenetic analysis, sequence analysis