Oxalis simplicifolia (Oxalidaceae), an unusual new unifoliolate species from the Marquesas Islands (French Polynesia)

Oxalis simplicifolia Lorence & W. L. Wagner sp. nov., a new species from the Marquesas Islands (Ua Huka) is described and illustrated. It differs from the other Marquesas species, Oxalis gagneorum, in having simple, unifoliolate glabrous leaves, minutely glandular-puberulent calyx lobes, shorter corolla lobes, erect capsules, and smaller seeds. As its habitat is under serious threat from human impact, feral animals, and weeds, we conclude this new species should be added to the IUCN Red List as critically endangered (CR).