One of the most widespread, and confusing, genera in the tribe Trichocereeae is Harrisia. Even its placement in that tribe is open to question, for Alberto Areces-Mallea (pers. comm.) has found that DNA sequences suggest it may belong in the tribe Hylocereeae.
Harrisia (type, Cereus gracilis = H. gracilis) was described by Nathaniel Britton in 1908, the name honoring William Harris of Jamaica, who contributed much to the knowledge of the flora of that island. Britton included 17 species in Harrisia, not all are now accepted. Subsequent research has shown that most of the species of the former Eriocereus belong in Harrisia, as well as those included by Curt Backeberg in his Roseocereus. This treatment of Harrisia is based primarily on the research of Areces-Mallea, recognizing 20 species.
Harrisia is characterized as treelike or shrubby, sometimes erect, but also sprawling or even prostrate. The stems are slender, not segmented, and ribbed. Flowers are large, white, open at night in summer, and funnelform. Some of the cacti make attractive plantings, though they may clamber widely.
Harrisia Britton 1908
Cereus subg. Eriocereus A. Berger 1905, Eriocereus (A. Berger) Ricco-
bono 1909 Roseocereus Backeberg 1938
Subfamily Cactoideae.tribeTrichocereeae. Plants treelike to shrubby, sometimes climbing or totally prostrate, with upright or arching branches, to 7 m (23 ft) high, often with slendertrunks. Roots sometimes tuberous. Stems cylindrical, ribbed, notsegmented, without aerial roots. Ribs 4-12. Tubercles low or absent. Areoles borne on rib prominences ortubercles. Spines few to many, variable. Flowers funnelform, open at night, white, 12-22 cm (4.7-8.7 in) long, 8-12 cm (3.1-4.7 in) in diameter; pericarpels and floral tubes with hairlike spines orwool; floral tubes slender, thin walled. Fruits fleshy, yellow to orange, sometimes dehiscent, with areoles bearingscalesorspines. Seeds broadly oval, black, warty, with deeply impressed hilum areas forming chambers. Distribution: Florida, throughout the Caribbean, south into South America in Brazil, Paraguay, Bolivia, and Argentina.
Two subgenera of Harrisia are often recognized. Subgenus Harrisia has indehiscent fruits and tends to be treelike; it is restricted to the United States and the Caribbean. Subgenus Eriocereus has dehis-centfruits, is usually climbing or shrubby, and occurs in South America. Although the two subgenera are disjunct, theirseeds are identical.
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