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| The genera of Cactaceae |
Including Hamatocactus Britton & Rose, Thelomastus Fric (nom. inval.)
The plants cerioid, or condensed-cactoid; low and very compacted in their entirety, or not ‘low and very compacted’. The stems spiny (sometimes fiercely so); discoid (sometimes depressed), or globose, or shortly cylindric to elongate cylindric; 2–20 cm in diameter; apically depressed, or not apically depressed; pseudocephaliate. The plants unbranched, or offsetting; erect; solitary, or clustering; to 0.03–0.25 m high. The stems columnar, or not columnar. The stems not segmented; ribbed and grooved to not ribbed and grooved (i.e., the ribs low, often weakly defined). The ribs 7–20; borne spirally, or longitudinal. The grooves deep and narrow (usually), or wide. The plants conspicuously tuberculate (usually), or not conspicuously tuberculate. The tubercles when present, rounded-terete to conical gibbous or nipple-like, usually shortly grooved ventrally from the terminal areoles; longitudinally grooved adaxially from the areole towards their ‘axils’. The tubercular groove short, extending less than halfway to the ‘axil’. The tubercles more or less connected by the ribs, or not connected by ribs; spirally disposed, or borne in longitudinal series. The areoles associated with tubercles (usually), or not tubercle-associated (T. setispinus only); distant; spirally disposed, or borne in longitudinal series; simple (round to elongate, nearly aways borne at the tips of tubercles); associated with extra-floral nectaries, or not associated with extra-floral nectaries; with spines (nearly always), or without spines. The spines when present, solitary, or clustered; 1–25; 0.3–6 cm long; with radials and centrals differentiated (the radials straight, 3–36 mm long, radiating or sometimes pectinately arranged, the centrals stronger, 5–60 mm long, straight, curved or hooked, rarely flattened and papery, porrect or ascending, rarely absent). Central spines 0, or 1–5. Radial spines 1–25. The spines stiff, or flexible, or stiff and flexible; usually including some hooked members, or never hooked; straight, or curved, or hooked (in one species only); flattened, or terete, or flattened and terete. The mature stems leafless.
Flowering during the day. The flowers solitary; terminal (borne on young areoles); one per areole; funnelform, or campanulate; sessile; medium-sized to large; 2.5–6.2 cm long (and 2–8 cm in diameter); regular. The receptacle conspicuously produced beyond the ovary into a tubular hypanthium. The pericarpel scaly. The hypanthial tube rather stout; not naked; at least above with scales (these broad, thin, scarious-margined). The axils of the scales of the hypanthial tube more or less naked, or not naked (then sparsely hairy). The hypanthial tube spineless. The perianth sequentially intergrading from sepals to petals, or petaline, or of ‘tepals’ (?); usually brightly coloured, white, or yellow, or red, or pink (or in combinations). The perianth segments relatively short, broad; blunt, or pointed. Stamens numerous, inserted in the tube and clustered around the style; not exserted. The stigma lobes yellow.
The mature fruit 0.6–3 cm long; globose, or ovoid, or ellipsoidal; green, or red; not naked (scaly); without spines; with persistent floral remains; non-fleshy when mature; dehiscent (via a basal pore), or indehiscent (T. setispinus again the exception); when dehiscent, dehiscing via basal pores. The seeds 1–2.3 mm long (and 0.5–1.7 in diameter); pyriform, warty or with tesselate ridges, black; pyriform; not encased in bony arils. The testa with tesselate ridges or verrucose. Cotyledons reduced or vestigial.
Natural Distribution. Texas, northern and central Mexico.
Classification. About 12 species. Subfamily Cactoideae. Tribe Cacteae.
Cf. Hunt (1967).
Images. • Thelocactus hexaedrophorus: © Zoya Akulova (2016). • Thelocactus setispinus (as Hamatoactus): Britton & Rose (1922). • Thelocactus macdowellii (as Echinomastus), withEchinomastus intertextus (as dasyacanthus): Britton & Rose (1922).
We advise against extracting comparative information from the descriptions. This is much more easily achieved using the DELTA data files or the interactive key, which allows access to the character list, illustrations, full and partial descriptions, diagnostic descriptions, differences and similarities between taxa, lists of taxa exhibiting or lacking specified attributes, and distributions of character states within any set of taxa. See also Guidelines for using data taken from Web publications.
Cite this publication as: ‘Watson, L., and Dallwitz, M.J. 2018 onwards. The genera of Cactaceae: descriptions, illustrations, identification, and information retrieval. Version: 14th November 2021. delta-intkey.com’.
