Autor: Britton & Rose

Genus honouring Manuel Fraile (1850-?), born in Spain, responsible for the cactus collection of the Department of Agriculture of the United States in the late nineteenth century. Portrait is unknown.
Genus of dwarf plants, usually solitary, also forming clumps of small globose to shortly elongated low stems, with ribs and tubercles little developed but visible. Spines very small, not aggressive.
Flowers diurnal, self-fertile, always pale yellow, appearing at the apex of stems, cleistogamous (self-pollinating) or opened for a crossed pollination made by insects, usually bees (Schlindwein & Wittmann 1995, in Emerson 2007), with a floral tube densely woolly and setose. Fruits with floral remains persistent, thin-walled, with irregular dehiscence or indehiscent, full of seeds. Seeds shiny, helmet-shaped or ovals, brown to blackish brown, finely papillose, of various size according to the species, floating, dispersed by rainwater (hydrochory).
The genus Frailea has an extensive geographic range, and usually grows in the shade of shrubs, among grasses, but also in sunny places, at ground level, in plains, on rocky or sandy soils, gravels, rocky outcrops, in pockets of fine sand, between mosses and lichens, sometimes together with fungi (F. cataphracta), from 20 m up to 900 m in altitude, often with other cacti (Notocactus, Echinopsis, Opuntia). Plants tend to bury themselves and disappear almost completely into the soil during the dry season.
Argentina (Corrientes, Entre Rios, Misiones, Santa Fe), Bolivia (Santa Cruz, Tarija), Brazil (Mato Grosso do Sul, Rio Grande do Sul), Colombia (Dagua)? (never found again), Paraguay (Alto Paraguay, Asuncion D.C., Boqueron, Concepcion, Cordillera, Guaira, Itapua, Misiones, Paraguari), Uruguay (Artigas, Cerro Largo, Colonia, Durazno, Flores, Florida, Lavalleja, Maldonado, Paysandu, Rivera, Rocha, Salto, Soriano, Tacuarembo, Treinta y Tres).

Currently 17 recognised species + 6 subspecies:
– Frailea alexandri Metzing 2006
– Frailea buenekeri Abraham 1990
– Frailea buenekeri subsp. densispina Hofacker & Herm 1997
– Frailea castanea Backeb. 1936
– Frailea cataphracta* (Dams) Britton & Rose 1922
– Frailea chiquitana Cardenas 1951
– Frailea colombiana (Werderm.) Backeb. 1935 (not found, doubtful taxon)
– Frailea curvispina Burning & Brederoo 1972
– Frailea friedrichii Burning & G.Moser 1971
– Frailea fulviseta Burning & Brederoo 1973
– Frailea gracillima* (Monv. ex Lem.) Britton & Rose 1922
– Frailea gracillima subsp. horstii (F.Ritter) P.J.Braun & Esteves 1995
– Frailea mammifera Burning & Brederoo 1972
– Frailea mammifera subsp. angelesiae Kiesling & Metzing 2006
– Frailea perumbilicata Ritter 1970
– Frailea phaeodisca* (Speg.) Y.ito 1957
– Frailea pumila* (Lem.) Britton & Rose 1922
– Frailea pullispina Backeberg 1963
– Frailea pygmaea (Spegazzini) Britton & Rose 1922
– Frailea pygmaea subsp. albicolumnaris (F.Ritter) Hofacker 1998
– Frailea pygmaea subsp. tuyensis (Burning & Moser) Metz 2006
– Frailea schilinzkyana (F.Haage) Britton & Rose 1922
– Frailea schilinzkyana subsp. concepcionensis (Burning & Moser) Braun & Esteves 1995

References: "TAXONOMY of the CACTACEAE" -  ISBN 978-84-617-3723-9 (Vol. 1)