Thelocactus

Thelocactus
File:Thelocactus bicolor.jpg
Kingdom	Plantae
Family	Cactaceae
Subfamily	Cactoideae
Tribe	Cacteae
SubTribe
Genus	Thelocactus
Species
Notes	Taxonomy follows Joël Lodé (2015) and Mosco & Zanovello (2000).

Etymology

The name means "nipple cactus", referring to the characteristic tuberculate (nipple-like) structure of the stems.

Description

Thelocactus is a genus of small to medium-sized cacti, solitary or forming clumps.

• Habit: Globose, conical to cylindrical plants, usually with depressed apex; up to 40 cm tall.
• Ribs: Often indistinct or fragmented.
• Tubercles: Well developed, rounded or conical, sometimes elongated and grooved.
• Areoles: Situated at the apex of tubercles; nectariferous glands may be present or absent.
• Spines: Usually straight, variable in size and colour; central spines stronger and longer than radials.
• Flowers: Diurnal, funnel-shaped; white, yellow, pink to magenta, rarely orange or red; arising at the apex; pollinated by insects.
• Fruits: Green to reddish, scaly; dehiscing basally through a pore; floral remnants persistent.
• Seeds: Black, pear-shaped; testa cells smooth to warty; dispersal by ants (myrmecochory) and possibly water.

Habitat

The genus occupies a wide range of environments:

• limestone, gypsum, sandy and alluvial soils
• rocky slopes and plains
• desert scrub (matorral) and grasslands
• occasionally mountainous areas and pine forests

Altitude range: approximately 50–2700 m.

Distribution

• Mexico: Coahuila, Durango, Hidalgo, Nuevo León, Querétaro, San Luis Potosí, Tamaulipas, Zacatecas
• USA: New Mexico, Texas

Taxonomy

Originally described by K. Schumann (1898) as a subgenus of Echinocactus, Thelocactus was elevated to genus rank by Britton & Rose (1922).

The circumscription of the genus has historically been unstable:

• Gymnocactus was separated and later merged into Turbinicarpus or reassigned
• Hamatocactus setispinus was temporarily included but later excluded due to clear morphological differences

Molecular studies:

• Butterworth et al. (2002): placed Thelocactus within a poorly resolved "Ferocactus clade"
• Bárcenas et al. (2011): showed that Thelocactus s.l. is not strictly monophyletic
• Hernández-Hernández et al. (2011): confirmed instability of the Ferocactus-related clade
• Vázquez-Sánchez et al. (2013): supported a broader clade including Thelocactus, Leuchtenbergia and related taxa

Pending further revision, the genus is treated here in its traditional circumscription.

Species

Following Mosco & Zanovello (2000) and Lodé (2015).

• Thelocactus bicolor
  • Thelocactus bicolor subsp. bolaensis
  • Thelocactus bicolor subsp. flavidispinus
  • Thelocactus bicolor subsp. heterochromus
  • Thelocactus bicolor subsp. schwarzii
• Thelocactus buekii
  • Thelocactus buekii subsp. matudae
• Thelocactus conothelos
  • Thelocactus conothelos subsp. argenteus
  • Thelocactus conothelos subsp. aurantiacus
  • Thelocactus conothelos subsp. flavus
  • Thelocactus conothelos subsp. garciae
• Thelocactus hastifer
• Thelocactus hexaedrophorus
  • Thelocactus hexaedrophorus subsp. lloydii
• Thelocactus lausseri
• Thelocactus leucacanthus
  • Thelocactus leucacanthus subsp. schmollii
• Thelocactus macdowellii
• Thelocactus multicephalus
• Thelocactus rinconensis
  • Thelocactus rinconensis subsp. freudenbergeri
  • Thelocactus rinconensis subsp. hintonii
  • Thelocactus rinconensis subsp. nidulans
• Thelocactus tulensis

Notes

• The genus remains taxonomically complex and partially unresolved.
• Some taxa historically included (e.g. Hamatocactus setispinus) are now excluded.
• Molecular data suggest relationships within a broader Ferocactus-related lineage.