Autor: F. Reichenbach ex Britton & Rose

Genus honouring Hermann Auguste Jacques Gruson (1821-1895), German collector of cacti of French origin (see portrait above, public domain).
Monospecific genus of plants forming impenetrable dense clumps, with segmented stems, with tuberculate low ribs, areoles with white felt. Glochids absent on adult plants. Spines without sheath, aciculate to slightly flattened. Leaves tiny, linear, soon deciduous.
Flowers diurnal, self-sterile, subapical, yellow, pollinated by insects. Fruits ellipsoid, strongly umbilicate, with unpleasant taste. Seeds (about 50 per fruit) discoid strongly compressed, pale yellow.
The monotypic genus Grusonia grows endemically in very large colonies between 1200 m and 2000 m in altitude, in the valleys of the Sierra de la Paila (Coahuila, Mexico), in a basin (depression of Mapimi) and in the state of Durango, among bushes and grasses, on limestone rocks and some basaltic volcanic areas, together with other cacti (Astrophytum, Ariocarpus, Leuchtenbergia), Agave lechuguilla, and forms sometimes an impenetrable barrier. Soils are poor in organic material. In winter, the temperature can drop down to -8°C.
Mexico (Coahuila, Durango).

Currently only one recognised species:
– Grusonia bradtiana* (J.M.Coult.) Britton & Rose 1919

References: "TAXONOMY of the CACTACEAE" -  ISBN 978-84-617-3723-9 (Vol. 1)