Rapicactus beguinii

Accepted name and subspecies

POWO accepts Rapicactus beguinii and two subspecies, subsp. beguinii and subsp. hintoniorum. In earlier studies the species appears as Turbinicarpus beguinii, Thelocactus beguinii or Turbinicarpus mandragora subsp. beguinii. Its synonymy also includes the name Neolloydia glassii. The main name in the current treatment can therefore be linked to literature published under several different names. [2-WWW]

In 2019, Vázquez-Sánchez and colleagues included both subspecies as separately identified tree tips. The sampled Rapicactus group formed a strongly supported lineage, closest to Acharagma in that analysis. The sample provides data on the relationships and diversity of the genus. The subspecies status used in the atlas follows the separately checked POWO list. [Vázquez-Sánchez et al. 2019, p. 407–409, tab. 1, ryc. 1] [2-WWW]

Mountain sites and microenvironment

The phylogenetic publication gives an elevational range of 1300–3120 m above sea level for R. beguinii and describes it as the species with the widest range in the genus. It lists calcareous soils and soils rich in humus. This picture extends the habitat description beyond exposed rock surfaces. Temperature, radiation and the course of drying change with elevation, while shelter and the amount of fine material between rocks also vary within a single slope. [Vázquez-Sánchez et al. 2019, p. 415]

Donati and colleagues described frequent sites in north-facing valleys, in pine undergrowth and on gypsum slopes with shrubs. Some were near streams and waterfalls. The authors proposed that cooler conditions and slower evaporation might help explain the larger size of some northern populations. This explanation of field observations supplemented the analysis of distribution and stem diameter. [Donati et al. 2017, online version, p. 8]

Stem tissues and crystals

The anatomical study of the stem found concentric druses in the hypodermis, the layer immediately beneath the epidermis. These are crystal aggregates organised around a centre. Druses also occurred in the pith of R. beguinii. The pith also had its own small vascular bundles. The location of these structures forms part of the description, since the hypodermis, cortex and pith have different roles in the organisation of the succulent stem. [De la Rosa-Tilapa et al. 2019, wersja online, p. 2]

Concentric druses and a periderm with alternating layers of thin-walled and thick-walled cork cells were illustrated for material of subsp. hintoniorum. The periderm is the secondary covering of an older stem. Its meristem, the phellogen, originated in the epidermis. Shared characters of the sampled Rapicactus are thus supplemented by variation in specific tissues of the identified subspecies. [De la Rosa-Tilapa et al. 2019, wersja online, p. 3–4, ryc. 3, 5]

Seeds and their link to the genus

The examined seeds had a tuberculate seed-coat microsculpture. The micropyle, the remnant of the opening in the ovule integuments, was separate from the hilum, the scar marking the seed's attachment. This combination helped distinguish Rapicactus from Turbinicarpus and Kadenicarpus. The ripe fruit dries and opens longitudinally. These details are particularly useful when comparing specimens with similar small, tuberculate stems. [Vázquez-Sánchez et al. 2019, pp. 409, 416–417]

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