is invalid. And so is the first description as Caltha palustris by Carl Linnaeus in his Genera Plantarum of 1737. But Linnaeus re-describes the species under the same name in Species Plantarum of 1 May 1753, thus providing the correct name. Caltha palustris is a highly variable species. When the growing season is shorter, plants are generally much smaller and may root at the nodes of the stems after flowering. Through history, many proposals have been made to split it into different (often numerous) taxa. Popular characters to distinguish between taxa concern the follicle. Most of the differences between populations are probably phenotypic adaptations to particular circumstances without a genetic basis. Variability within populations is also considerable. Varieties that are widely recognised are C. palustris var. palustris, C. palustris var. radicans (small plants with decumbent stems rooting at the nodes), C. palustris var. araneosa (big plants with erect stems forming young plants at the nodes), C. palustris var. alba (with white flowers) and C. palustris var. purpurea (with magenta flowers). Caltha leptosepala also is highly variable. There may be mostly one or mostly two flowers per stem, many lanceolate sepals or fewer ovate sepals, smaller hart-shaped or larger kidney-shaped leaves, and pollen may be of two different types. Populations on the US westcoast and the US Rocky Mountains consistently differ from each other by fixed combinations of these character states and two subspecies are distinguished: ssp. leptosepala and ssp. howellii. Curiously, these fixed combinations cannot be found in the Canadian Rocky Mountains and in Alaska. For this reason the subspecies status is generally preferred over distinguishing a separate species (Caltha biflora). Caltha sagittata has a rather large distribution. Usually the leaves have so called appendages, which are lobes at the base that are at a sharp angle with the top lobe. In some northern forms these appendages are in the same plane as the remainder of the leafblade, and these plants are sometimes recognised as C. alata. Some character states gradually change over its distribution area, and the angle of the basal lobes does not seem to be special in this respect. The remaining species vary less and have not been divided into subtaxa. Modern classification Historically, the genus Caltha has been divided over two sections: Populago (now Caltha) that included all Northern Hemisphere species, and Psychrophila that contained all Southern Hemisphere species. The latter is sometimes regarded as a