Thelocactus bicolor

Name and range

The current combination Thelocactus bicolor was published in 1922. Older works use Echinocactus bicolor and also Ferocactus bicolor. POWO treats the species with five subspecies, including the typical subspecies and T. bicolor subsp. heterochromus (formerly T. heterochromus). The natural range covers southwestern Texas and northeastern Mexico, in the desert and dry scrub biome. [2-WWW]

Growth habit and flowering

Flora of North America gives stems 3–38 cm tall and most often 4–10 cm wide, with 8–13 ribs. In young specimens, tubercles may remain so clearly separated that the rib arrangement is difficult to see. Plants at this stage may already be capable of flowering. Stem size and armature change with age and among populations. [Zimmerman and Parfitt 2003d]

Flowers are funnel-shaped, pink to purple. RHS gives a diameter up to about 8 cm. Spines can combine reddish and pale portions, with older ones becoming whitish. In the typical subspecies description, Flora of North America usually distinguishes 9–18 radial spines and up to four central spines. [1-WWW] [Zimmerman and Parfitt 2003d]

Variation described in the genus revision

Mosco and Zanovello compared 34 taxonomic units using 65 morphological characters. In the T. bicolor group, they highlighted the red flower throat and polycolpate pollen. The subspecies heterochromus combines a flattened habit with strong spines and an absence of areolar glands. The study supported its treatment within the variable T. bicolor. This was an analysis of morphological similarity supplemented by field observations. [Mosco and Zanovello 2000]

Cultivation and propagation

For cultivation under shelter, RHS recommends full light, low air humidity and a freely draining cactus substrate. Water moderately in summer and keep dry in winter. Liquid fertiliser can be applied two or three times during the growing season. Plants are propagated from seed, and those producing offsets can also be propagated vegetatively. The possible occurrence of mealybugs should be considered in cultivation. [1-WWW]

A broad niche in the species comparison

In a model comparison of nine species, Mosco used 213 records of Thelocactus bicolor. Levins’ niche breadth index was 0,282, the highest in this set. The most important variables by permutation importance were isothermality (39%) and precipitation of the wettest quarter (33,1%). These results describe climatic variation among occupied sites in the data set used. Isothermality is the ratio of the mean daily temperature range to the annual range, multiplied by 100. [Mosco 2019, p. 3–6, tabele 1–2]

Widespread occurrence and broad modelled niche breadth can coexist with local population specialisation. Mosco highlighted the diversity of environments occupied by T. bicolor. Data at roughly kilometre scale cover climate and topography, while details of substrate and microsites require field observations. [Mosco 2019, p. 6, 10–12]

Chemical composition in the 1968 study

Domínguez et al. studied six kilograms of whole plants of T. bicolor from Mina in Nuevo León. The material was sun-dried, ground and extracted successively with petroleum ether, ethanol and water. Itesmol, identified as a steroid, and eisacol, identified as a pentacyclic triterpenoid, were isolated from the unsaponifiable fraction of the ether extract. [Domínguez et al. 1968, p. 458]

The study from 1968 presents the isolation, spectroscopic characterisation and reactions of derivatives of these compounds. The authors left further structural determination to continued analysis. The results concern the material’s chemical composition and its separation. The publication did not study effects of the extract on the human body or medicinal properties. [Domínguez et al. 1968, p. 458–461]

Related articles

Materials from the author's archive

The species also appears in Konrad Szychowski's private material archive. [Kaktologia 2009b, file p. 16] [Neciu b.d., file p. 1]