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Gymnocalycium denudatum

Gymnocalycium denudatum (an old specimen)
This species has nearly armless spines flattened against body ,
looking like a bunch of white spiders crawling on a cactus. 
 

Morphology (Identifying Characteristics): Small globular usually unbranched cactus with glossy dark green body and rounded ribs (The 'chin' effect is not as visible as other Gymnocalycium)  Grows up to 15 cm in diameter and 10cm tall.
Spines:  Nearly harmless spines flattened against body in shape of small, white spiders
Flowers: Large cream white, that are up to 7 cm wide.
Blooming time:  In spring and summer.
 

Cultivation: Very easy to grow, this plants offers no cultivation difficulties. Summer grower needs moderate to copious waterings in summer. Quite frost resistant if kept dry in winter  -4° C.  Feed with a high potassium fertilizer  in summer. Sun Exposure: Full sun to light shade.

Propagation: Seeds (seldom produces offsets)

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• Echinocactus denudatus  Link & Otto
• Cereus denudatus
• Gymnocalicium  paraguayense (Sch.) Schuetz

In wildlife G. denudatum with pin/red flower is not found. Probably this red flowered form is a cultivar (may be Czech cultivar 'JAN SUBA' )

Photo and © copyright by Konstantin V. Korotkov.
 (Sergiev Posad, Moscow reg., Russia)

I want to add detail as regards Gymnocalycium denudatum.
In austrian journal "Gymnocalycium"  [18 (2) 2005] an article of Hans Till was published, which dedicates to G. megalothelon (the old species epithet “megalotelos” had grammatically invalid ending). This species is very closed to G. denudatum.
Some specialists (G. Frank) even referred G. megalothelon to denudatum-forms group. In given article the author has described a new variety with light-red flower (see photo in this journal on page 622) - G. megalothelon var. susannae. However the collection material that was used by H. Till at description, imported previously from Uruguay by W. Rausch, but not furnished with field number. In due course the information about discovery in wild nature G. denudatum with red flower comed from A. Fric, Mueller-Melchers, L. Horst and other researchers. However the cases in point are single instances. The K. Prestle's controversial variety G. denudatum var. roseiflorum has not at all rose flower, but white with rose tips of petals. H. Till offers to identify this variety as G. megalothelon.  
In general, nor one of officially registered populations of red-flowering Gymnocalycium from Schuetz’s subgenus Macrosemineum (G. denudatum and G. megalothelon, in particular, pertain to this "large-seed" subgenus) until wasn't known.
Present-day classification for these taxa sensu H. Till looks somewhat strangely:

G. denudatum
subgenus:  Microsemineum
section:  Microsemineum
subsection:  Macrosemineum series:  Macrosemineum

G. megalothelon
subgenus:  Microsemineum
section:  Microsemineum
subsection:  Macrosemineum series:  Multiflora

P.S. On this subject see also an article about south-Brazilian gimnos in “Kakteen und andere Succulenten” 2002, 53 (2): 29-39 written by well-known German specialists – N. Gerloff and D. Metzing. This article was published in Russian “Kaktus-Klub” 2002, 2:35-44 too.
 

◄ Here a photo of  G. denudatum with half-disclosed flower (max diameter of fully disclosed flower - 7 cm)
 

Respectfully , Konstantin.

All the information and photos in cactus art file are now available also in the new the Enciclopedia of Cacti. We hope you find this new site informative and useful.