Hatiora

Hatiora is a small genus of epiphytic cacti which belongs to the tribe Rhipsalideae within the subfamily Cactoideae of the Cactaceae. Recent taxonomic studies have led to the three species formerly placed in subgenus "Rhipsalidopsis" being removed from the genus, including the well known and widely cultivated ornamental plants known as Easter cactus or Whitsun cactus (cultivars or hybrids of the former "Hatiora gaertneri").
Description.
All "Hatiora" species are found as epiphytes growing on trees or (rarely) lithophytes growing on rocks. They are found in the tropical rainforests of the Mata Atlântica in eastern Brazil. The plants are weakly succulent, growing more or less upright and becoming woody at the base when older. Spines are usually missing. The insect-pollinated flowers are borne terminally. They are small, with a diameter of about , actinomorphic (radially symmetrical), bell-shaped and always coloured (yellow, yellow-orange or pink). The fruit is a berry. By contrast with species of the genus "Schlumbergera", most of which have flattened stems, "Hatiora" species have stems with a circular cross-section.
Taxonomy.
Cacti belonging to the tribe Rhipsalideae are quite distinct in appearance and habit from other cacti, as they grow on trees or rocks as epiphytes or lithophytes. However, for a long time there has been confusion as to how the rhipsalid species should be divided into genera. In 1819, Haworth described the first discovered species of the modern genus "Hatiora" under the name "Rhipsalis salicornioides". In 1834, A.P. de Candolle recognized the distinctness of this species and transferred it to a new genus "Hariota", named after Thomas Hariot, a 16th-century botanist. Later a second species, "H. gaertneri", was initially named as "Epiphyllum russellianum" var. "gaertneri" ("Epiphyllum russellianum" is now "Schlumbergera russelliana") and then in 1889 as "Epiphyllum gaertneri". A third species, "H. rosea", was described in 1912 as "Rhipsalis rosea".
By 1923, many nomenclatural uncertainties and confusion had arisen over the name "Hariota". Nathaniel Britton and Joseph Rose created a new name "Hatiora" as a taxonomic anagram of "Hariota". Of the species known at the time, they placed "Hariota salicornioides" in "Hatiora" along with "H. cylindrica"; they had already placed "H. gaertneri" in "Schlumbergera" in 1913 and left it there; and they erected a new genus, "Rhipsalidopsis", for "H. rosea". Two further species which have been assigned to "Hatiora" were placed in various genera, including the original "Hariota" and "Rhipsalis". According to Anderson, the confusion among the Rhipsalideae was not clarified until work by Wilhelm Barthlott and Nigel Taylor in 1995, which placed six species in "Hatiora", divided between two subgenera.
Phylogenetic studies using DNA have led to a modification of the Barthlott and Taylor classification and the three species of "Hatiora" they placed in subgenus "Rhipsalidopsis" have been transferred out of the genus. There is agreement that "Hatiora epiphylloides" should be placed in "Schlumbergera" (as "Schlumbergera lutea"). There is disagreement over the other two species. Some sources also include them in a broadly defined "Schlumbergera", others place them as the only two species in the genus "Rhipsalidopsis". "Hatiora" and the most broadly circumscribed "Schlumbergera" both branch from the tip and have short segments (less than 7 cm long). "Hatiora" has stems that are round in cross-section and radially symmetrical (actinomorphic) flowers, whereas "Schlumbergera" has flattened or otherwise angular stems and its flowers may be radially symmetrical or radially unsymmetrical (zygomorphic).
Subgeneric classification and species.
In the taxonomic treatments of the genus by Barthlott & Taylor (1995) and Hunt (2006), "Hatiora" was divided into two subgenera with six accepted species, plus a hybrid created in cultivation. Subgenus "Rhipsalidopsis" has subsequently been removed from "Hatiora".