Cleistocactus
Cleistocactus is a genus of flowering plants in the cactus family Cactaceae, native to mountainous areas - to  - of South America (Peru, Uruguay, Bolivia and Argentina). The name comes from the Greek kleistos meaning closed because the flowers hardly open. Description
The plants of the genus are slender stem succulents that are tall, mostly slender and often many-branched up to about 3 m high. They usually form basally branching shrubs, rarely they branch higher and form small trees. The shoots stand upright and then often hang over as they get older; they usually lie down with their ends rising up; more rarely they grow hanging. They usually have many ribs, closely set areoles and spines. The areoles on the ribs usually have many fine, hair-like spines with a few firmer spines in between; the spines are rarely longer and coarse. In most species, the flowers appear in large numbers individually from the areoles. The flowers are tubular and the tips hardly open with only the style and stamens usually protruding. In some species (from the earlier genera Borzicereus and Cephalocleistocactus) they appear from conspicuous zones of heavy bristle and hair formation. In adaptation to the pollinators (hummingbirds), the flowers are long, tubular with upright bracts, which are sometimes not or only slightly folded outwards at the tips and thus appear almost closed. They are often slightly zygomorphic due to an upward bend near the base and/or an oblique flower border (longer or straighter at the top, shorter or more folded at the bottom). The flower colors range from green to white, yellow, orange and red to violet, with shades of red predominating. The densely scaled fruits that emerge after fertilization are relatively small, but usually contain numerous seeds. Species
Species of the genus Cleistocactus according to Plants of the World Online :

Synonyms
The following genera have been brought into synonymy with this genus:
Akersia Buining
Bolivicereus Cárdenas
Borzicactella H.Johnson ex F.Ritter
Borzicereus Fric & Kreuz. (orth. var.) Cephalocleistocactus F.Ritter
Cleistocereus Fric & Kreuz. (orth. var.) Clistanthocereus Backeb. Demnosa Fric
Gymnanthocereus Backeb. Hildewintera F.Ritter
Maritimocereus Akers
Pseudoechinocereus Buining (nom. inval.) Seticereus Backeb. Seticleistocactus Backeb. Winteria F.Ritter
Winterocereus Backeb. References

 Innes C, Wall B (1995). Cacti, Succulents and Bromaliads. Cassell & The Royal Horticultural Society. The species list is referenced from http://www.cactiguide.com/ which is in turn referenced from several books which are listed on that site. The principal book listed here is The Cactus Family by Edward F. Anderson. External links 

   photos on www.AIAPS.org
   photos on www.cactiguide.com

 
Cacti of South America
Flora of Argentina
Flora of Bolivia
Flora of Peru
Flora of Uruguay
Cactoideae genera