Rapicactus booleanus

Name and discovery

George S. Hinton described the species in January 1996 as Turbinicarpus booleanus. Davide Donati published the combination Rapicactus booleanus in 2003. POWO accepts it as the name of a species native to Nuevo León. The names Turbinicarpus mandragora subsp. booleanus and Rapicactus subterraneus subsp. booleanus refer to the same plant in treatments that assign it subspecies rank. [Hinton 1996, Hinton, pp. 62–66. POWO, as of 11 October 2026] [2-WWW]

The type material was collected on 1 March 1992 on a gypsum slope in the municipality of Galeana, at 1860 m above sea level. The epithet commemorates George Boole, the author's son, who accompanied him during fieldwork. The original publication reported two sites a few kilometres apart. This reflects the state of knowledge in 1996. [Hinton 1996, p. 62, 64, 66]

Body, neck and storage root

The body is broadly obovoid, usually solitary and rarely branched. In the original description it reached 2.5–4.5 cm in height and 2.5–5.5 cm in diameter. Usually about one third of the body projected above ground. The older base of the body was truncate, connected to the thickened root by a narrow neck below the soil surface. [Hinton 1996, p. 62–63]

In the English description, the neck is 1–5 cm long and 3–5 mm in diameter. The thickened root is pear-shaped to globose, 0.8–2.8 cm in diameter and 1.2–5.7 cm long. The neck may be curved or angled. In the author's comparison, R. subterraneus had a longer, thinner and partly above-ground neck region. The body of R. mandragora was globose and above ground. [Hinton 1996, pp. 63–64]

Tubercles, areoles and spines

The tubercles have a rhomboid cross-section, are about 4 mm high and 5 mm wide at the base. There is no groove on the dorsal surface. The green surface bears whitish spots, and the tubercles are arranged in spirals. The elliptical areoles are 1.5–2 mm long. Near the apex they are hairy, losing their hairs with age. [Hinton 1996, p. 63]

Two central spines arise from each areole, white at the base with a brown or black distal part. The upper one usually points forwards, while the lower one projects away from the stem. They are most often 12–15 and 12–18 mm long, respectively. There are usually 18–20 radial spines, with a full range of 14–28. They are white, sometimes with brown or reddish tips. The lower ones are shorter, while the upper ones may reach 17 mm, exceptionally 29 mm. [Hinton 1996, pp. 63–64]

Flower and fruit

The flower described by Hinton was about 2 cm in diameter and 2.5 cm high. The inner tepals are light to dark magenta, with a darker central stripe. The filaments and anthers are yellow. The stigma has seven to nine pale yellow lobes. Flower colour helps distinguish this plant from the white-flowered R. mandragora compared in the paper. [Hinton 1996, pp. 63–64]

The fruit reaches about 6 mm in diameter and 7 mm in height. Its surface is dark green to purple, with a slight bloom and small scales. It opens along a longitudinal split. This mode of seed release contributes to the characterisation of Rapicactus, later compared with seed-coat structure and the molecular position of the genus. [Hinton 1996, Hinton, p. 64. Vázquez-Sánchez et al., pp. 409, 412] [Vázquez-Sánchez et al. 2019, Hinton, p. 64. Vázquez-Sánchez et al., pp. 409, 412]

Exposed gypsum landscape

In the original description, the plants occupied exposed gypsum slopes. Associated species included Dasylirion berlandieri, Muhlenbergia gypsophila, Pinus greggii and Selaginella gypsophila. The mineral composition of the substrate and the substantial proportion of underground parts distinguish this site among the varied habitats of the genus. The presence of the species on this substrate is a field observation. The publication did not experimentally determine its gypsum requirements in cultivation. [Hinton 1996, pp. 64–65]

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