Gymnocalycium denudatum aka Spider Cactus

Gymnocalycium denudatum is a small cactus. It belongs to the Cactaceae family. Its common name is spider cactus. The name refers to its curved spines. These lie against the stem and look like small spiders. Kew places its native range from southern Brazil into Argentina. In Argentina, this includes the provinces of Misiones and Corrientes. LLIFLE also lists Uruguay, but is less sure about Paraguay. Kew describes its climate as subtropical. It calls the plant a succulent subshrub.

The plant tends to grow as one stem without branches. Its shape ranges from a low, flat ball to a fuller globe. The stem is glossy and grey-green to dark green. LLIFLE gives its usual height as 0.07–0.10 feet [0.02–0.03 metres]. Its usual width is 0.20–0.26 feet [0.06–0.08 metres]. Plants grown in gardens can reach 0.33 feet [0.10 metres] tall. They can reach 0.49 feet [0.15 metres] across, too. These sizes exceed the source's usual range. The species varies a great deal in form.

Most plants have five ribs, though some have eight. The ribs look round but have flat surfaces. Their tubercles are broad, low bumps, not large raised points. These give the plant a chin-like outline. This shape is less marked than in some of its relatives. A few round areoles bear a thin coat of wool. They sit about 0.03–0.07 feet [0.01–0.02 metres] apart. All spines are radial and lie close against the body. They measure about 0.03–0.05 feet [0.01–0.015 metres] long. They range from white to pale pink. Each thin spine narrows to a point. Spines may twist, bend to the side, or curve upward.

The large flowers contrast with the small, low stem. Their glossy petals are cream-white. Some have a faint pink tint. Flowers reach 0.16 feet [0.05 metres] long. They reach 0.23 feet [0.07 metres] wide. The perianth segments are oblong, with sharp tips. The supplied species account ends before it gives full flower details. It gives no usable facts on fruit or bloom time. The evidence for this species leaves its growing needs unclear. Its frost limits are unclear, too. It does not establish edible uses or toxicity. Nor does it establish which pests cause problems.