Discocactus heptacanthus

Discocactus heptacanthus is a small, solitary cactus characterized by its discoid to nearly globular shape. Its stems, reaching 3 to 9 cm (1.2 to 3.5 in) in height and 8 to 16 cm (3.1 to 6.3 in) in diameter, possess a pale green to olive epidermis and branched roots. The plant features 9 to 16 prominent, vertically or slightly spirally arranged ribs, which are grooved between the areoles, creating flattened, nipple-shaped tubercles. Each rib bears 2 to 6 oval, sunken areoles, measuring 3 to 8 mm (0.12 to 0.31 in) long and 2 to 7 mm (0.079 to 0.276 in) wide. The spines vary in color from pale yellow to gray or brown. They are straight or slightly curved and typically consist of a single central spine, 1.5 to 2.4 cm (0.59 to 0.94 in) long, though this may be absent. Additionally, 3 to 9 radial spines, measuring 0.3 to 3.5 cm (0.12 to 1.38 in) long, are present. Mature plants develop a woolly cephalium at their apex, a structure 1 to 2.7 cm (0.39 to 1.06 in) high and 2.5 to 6 cm (0.98 to 2.36 in) in diameter. This cephalium, composed of white to dull pastel wool and occasional brown or black bristles up to 5 cm (2.0 in) long, protects the plant's sensitive tip from cold and intense ultraviolet radiation. It is also thought to attract pollinators and is conspicuous even before flowering. The fragrant, white flowers are tubular to funnel-shaped, emerging from the cephalium's edge. …

This species is native to central-western Brazil, extending from Goiás, Mato Grosso, Mato Grosso do Sul, Pará, Rondônia, to Tocantins, Eastrern Bolivia, and Paraguay. It primarily grows at low elevations within the seasonally dry tropical biome, preferring exposed, iron-rich gravelly soils. Occasionally, it can also be found on sandy or quartz gravel, as well as on granite and sandstone outcrops.