Rapicactus subterraneus

Name and range

Rapicactus subterraneus is an accepted species from Nuevo León, Tamaulipas and San Luis Potosí in Mexico. It was initially described as Echinocactus subterraneus. In scientific literature, it also appears as Turbinicarpus subterraneus and T. mandragora subsp. subterraneus. All these names lead to the current species in POWO. [2-WWW]

The 2019 publication discusses sites in the Mier y Noriega region, within the Sierra Madre Oriental. It indicates an association of local populations with a substrate containing gypsum outcrops. This describes the populations discussed in the study, while the current database account covers the full accepted species range. [Vázquez-Sánchez et al. 2019, p. 415]

Thickened root and narrow neck

The plant has a thickened main root and a narrow section joining it to the stem portion. This feature was among the arguments used to distinguish Rapicactus historically. Comparison of seed, fruit and tissue structure supplemented the description of underground organs, and molecular analysis confirmed the lineage's distinctness. [Vázquez-Sánchez et al. 2019, p. 407, 409, 412]

In the stem studied, the hypodermis contained concentric druses characteristic of the analysed Rapicactus group. A druse forms a crystal cluster with concentric organisation. In Turbinicarpus and Kadenicarpus, the same layer contained prismatic crystals. This comparison concerns a specific location in the stem. [De la Rosa-Tilapa et al. 2019, wersja online, p. 2]

Body position and comparison with related species

Hinton compared R. subterraneus (called Turbinicarpus subterraneus in the article) with the newly described R. booleanus. The longer, thinner neck of the first species raises part of its body above the substrate surface. Radial spines were fewer and shorter than in R. booleanus, and central spines retained a glassy white colour along their full length. Both underground-part proportions and spine appearance are therefore useful for identification. [Hinton 1996, p. 64]

In this 1996 comparison, R. subterraneus sites were described as limestone hills with agaves, dasylirions and yuccas. In contrast, the 2019 study discusses populations on gypsum outcrops in the Mier y Noriega region. These observations document habitats from different accounts. Substrate type refers to a specific site, and broad range boundaries alone do not determine the composition of all soils occupied by the species. [Hinton 1996, Hinton, p. 64. Vázquez-Sánchez et al., p. 415] [Vázquez-Sánchez et al. 2019, Hinton, p. 64. Vázquez-Sánchez et al., p. 415]

Two zones of vascular tissues

Amphicribral bundles were found in the cortex, with central xylem surrounded by phloem. The main xylem contained few fibres, distinguishing R. subterraneus from the other taxa compared in the study, whose wood was fibre-free. Vessels were accompanied by wide-band tracheids. Fibres in the main cylinder are a different feature from fibres in small cortical bundles, which distinguished Kadenicarpus in this study. [De la Rosa-Tilapa et al. 2019, wersja online, p. 2–4, ryc. 4C]

The structure of xylem and parenchyma forms an arrangement in which water conduction lies beside succulent stem tissues. The limited proportion of fibres is a feature observed in sections. The authors did not compare forces acting on the neck or the mechanical resistance of the underground connection in this experiment. [De la Rosa-Tilapa et al. 2019, online version, methods, results and discussion]

In vitro propagation under a former name

The species was one of eight taxa studied in micropropagation in 2005 as Turbinicarpus subterraneus. Seedling fragments provided material for areole stimulation and shoot production. The abstract then presents rooting and acclimatisation. The combined results include several taxa, so the pooled mean survival in this study is not a separate result for R. subterraneus. [Dávila-Figueroa et al. 2005, abstract]

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