Copiapoa humilis subsp. tenuissima (Ritter ex D. Hunt) D. Hunt, Cact. Syst. Init. 16: 6 (2003)
Basionym : C. hypogaea subsp. tenuissima Ritter ex D. Hunt, Cact. Syst. Init. 13: 14 (2002)
Synonym: C. tenuissima Ritter, Taxon 12 (1963) Invalidly published

South of Antofagasta, the coastal hills are not accessible due to the absence of roads in the region. Only some distance south of the city does a road head from the Pan-American highway towards the coast to the now abandoned mine of El Cobre.

Figure 8 Large clumps of Copiapoa solaris
at the habitat of Copiapoa humilis subsp. tenuissima

It is in this region that Ritter discovered C. humilis subsp. tenuissima which he described (although invalidly) as Copiapoa tenuissima back in 1960. It made a brief appearance as a subspecies of C. hypogaea before David Hunt placed it under C. humilis, where I believe it belongs. The plants grow on barren hills at an altitude of 600m in association with C. solaris. This appears to be a rare taxon not growing in any great quantity and I have seen only a few plants in habitat. The small heads grow flush with the ground, underneath which is a very large tuberous root. This is the smallest growing taxon of the Copiapoa humilis group with the heads reaching only 3 to 4cm in diameter in habitat. Cultivated specimens can grow much larger but it is still slow growing and does not readily offset. The epidermis is very dark, almost black and the tubercles are much smaller than the type at only 2mm across and 4mm deep and more angular in shape.

Figure 9 Copiapoa humilis subsp. tenuissima PH251.02 (El  Cobre 600m)

The dark colour of the body is retained on seed raised plants in cultivation, demonstrating that it is not only due to exposure to intense sunlight. The spination is only weakly developed with no central spines. Radial spines grow flush with the areole and are approximately 4mm long and are 8 to 10 in number. Spine colour is brown to golden yellow. Nigel Taylor suggested that this plant exhibits neoteny (retaining the juvenile state even at maturity) and I agree with him. All other subspecies of C. humilis initially have similar juvenile spination as young plants but then go on to develop stronger adult spination. This taxon has lost the ability to produce adult spination and can flower in the juvenile state. The flowers are approximately 25mm across growing from the woolly apex and are yellow with a red tint on the outside of the petals.

Figure 10 Copiapoa humilis subsp. tenuissima Lau 876
ex habitat plant in cultivation for many years, growing much larger than in habitat
 but still retaining the weak juvenile spination