Claytonia (Portulacaceae) comprises 27 species, including 14 subspecies, of fibrous-rooted annuals as well as tuberous, stoloniferous, rhizomatous, and caudicose perennials, which are indigenous to eastern Asia and North America.
The biogeography and fossil history of Claytonia are reviewed and a preliminary phylogenetic re-construction is presented. Characters of leaf anatomy and morphology, underground parts, flowers, fruits, chromosomes, and biochemistry were included in the study.
Examination of 50,000 herbarium collections and numerous wild populations, as well as chromosomal, phytochemical, and greenhouse studies revealed the extent of interspecific and infraspecific variation. Evaluation of prior generic and infrageneric taxonomic treatments showed that the group is better treated as the single genus Claytonia. An extensive literature review and study of herbarium holdings permitted clarification of nomenclatural matters and typifications. Most species are fully illustrated and all are mapped. One new species (C. ozarkensis) is described, and two new combinations (C. multiscapa subsp. czukczorum, C. multiscapa subsp. pacifica) are proposed.