Rapicactus zaragozae

Species and names from earlier treatments

POWO accepts Rapicactus zaragozae as a separate species native to Nuevo León and Tamaulipas. Davide Donati published the combination in this genus in 2003. The literature also contains Turbinicarpus zaragozae, Neolloydia zaragozae and treatments at subspecies or variety rank within R. subterraneus, Turbinicarpus subterraneus and T. mandragora. In the current account, these names connect to one accepted species. [2-WWW]

In the 2019 analysis, R. zaragozae was placed in the strongly supported Rapicactus lineage. The authors compared it with R. beguinii, R. booleanus, R. mandragora and R. subterraneus. The distinctness of the genus was supported by a combination of structural characters, including seeds with tuberculate microsculpture and fruits that open longitudinally. [Vázquez-Sánchez et al. 2019, tab. 1, p. 409, 412, 416–417]

A thin layer beneath the epidermis

The hypodermis, the tissue immediately beneath the epidermis, consisted of one cell layer in the material studied. It contained concentric druses, crystal aggregates organised around a centre. Druses were also recorded in the pith. The single hypodermal layer distinguishes this stem in the anatomical sample, while the crystal type in this layer was shared with other Rapicactus examined. [De la Rosa-Tilapa et al. 2019, wersja online, p. 2]

The main wood was non-fibrous. Vessels were accompanied by tracheids with broad annular or helical wall thickenings. The conducting cells formed a system within the fleshy stem. The histological comparison documents their structure, while the degree of dehydration and rate of water transport require separate physiological measurements. [De la Rosa-Tilapa et al. 2019, online version, pp. 2–4]

An exception in cork structure

A periderm, a secondary protective tissue, developed in older sections of Rapicactus stems. In most members of the genus compared, cork cells formed alternating thin-walled and thick-walled layers. R. zaragozae was an exception to this arrangement. The phellogen, the layer of dividing cells that produces cork, originated in the epidermis. [De la Rosa-Tilapa et al. 2019, wersja online, p. 3–4]

Both the stem layer and its age matter in the microscopic image. The hypodermis belongs to the primary structure of a younger section, while the periderm develops later. In R. zaragozae, comparing these zones provides two independent identifying details. This variation was described in identified material from the 2019 study. [De la Rosa-Tilapa et al. 2019, online version, methods, results and discussion]

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