Geohintonia mexicana

Name and environment

Geohintonia mexicana is the only accepted species in the genus. POWO lists Nuevo León and Tamaulipas. Earlier combinations Aztekium mexicanum and Echinocactus mexicanus refer to the same plant. [2-WWW]

The protologue describes a usually solitary stem about 10 cm tall and 10,5 cm in diameter, with 18–20 ribs. Short-lived, papery spines persist near the apex. Dark pink to purple flowers open in the afternoon. Type material came from a gypsum site in Galeana municipality. In these habitats, the species occurred alongside Aztekium hintonii. [Glass and Fitz Maurice 1992]

The three areolar spines are 9–12 mm long and have characteristic, spoon-shaped broadened bases. Flowers reach up to 4 cm in diameter and close after dark. The fruit remains hidden in the apical wool. It has a thin wall and is about 9 mm long and 4–5 mm in diameter. The black, shiny seeds measure about 1,2 mm long, 0,7 mm wide and 0,5 mm thick. The seed-coat surface bears small, rounded elevations visible under magnification. [Glass and Fitz Maurice 1992]

Wood changing during development

Dimorphic wood has been demonstrated in Geohintonia mexicana. This term denotes two different structural types produced successively during the plant’s life. The early wood contains vessels embedded in tissue of wide tracheids. Later, the cambium, the cell layer producing secondary vascular tissues, also begins to produce wood richer in fibres. A section through an older stem therefore records different stages of activity of the same meristem. [Vázquez-Sánchez et al. 2017]

Wide-band tracheids have walls thickened into rings or helices. Thinner sections between the bands are more susceptible to deformation. This structure favours tissue shortening as the stem dehydrates. Wood fibres, by contrast, have more strongly lignified walls and increase stiffness. A change in the kinds of cells produced shifts the tissue’s mechanical properties towards greater stem support. [Vázquez-Sánchez et al. 2017]

In dimorphic wood, fibres appear in all strands of the vascular cylinder. At the same time, vessel element wall structure changes from a simpler helical pattern towards more developed secondary walls with pits. Pits are areas allowing water flow between neighbouring conducting elements. Increased lignification limits the wood’s capacity for vertical compression. The surrounding storage tissues can still change size, however, and the rib arrangement facilitates the spatial distribution of these changes. [Vázquez-Sánchez et al. 2017]

Why this species has a distinctive structure

In the study from 2017, dimorphic wood often accompanied larger, taller plants. Geohintonia mexicana was among the small species that were exceptions to this relationship. Its globular habit shows that height alone does not explain the transition from tracheids to fibres. The authors pointed to the need to understand regulation of secondary wall development. Different Cacteae lineages may activate a similar anatomical programme at different stem sizes. [Vázquez-Sánchez et al. 2017]

The picture of the wood is complemented by other tissues. In the character matrix, G. mexicana had a thick-walled hypodermis and a parenchymatous cortex and pith. A stiffer layer beneath the epidermis therefore surrounds an extensive interior more capable of changing volume. The analysis linked these components with shape maintenance during drought. It was based on anatomy and character distribution on the phylogenetic tree, without a direct test of mechanical strength in Geohintonia. [Vázquez-Sánchez et al. 2017]

Relationship with Aztekium

The analysis of the plastid intron rpl16 from 2002 placed Geohintonia together with Aztekium. The possible hybrid origin of Geohintonia discussed at that time remained a hypothesis. A single plastid marker identifies one inheritance history and cannot by itself establish the contribution of two parental lineages. [Butterworth et al. 2002]

Anatomical comparison does identify a specific difference between close relatives. In the A. hintonii and A. ritteri studied, the wood remained without fibres. G. mexicana produced a later wood phase with fibres. Shared ancestry is therefore accompanied by variation in vascular tissue development. From a physiological perspective, the timing of stiffer tissue formation and its share of the whole stem become relevant. [Vázquez-Sánchez et al. 2017]

Propagation and transfer to greenhouse cultivation

Quail included Geohintonia among the cacti whose seedlings he kept through their first winter in a moist, covered environment with artificial light. After moving them to the greenhouse, he initially kept them under a cover that diffused light and limited rapid drying. The account concerns practice in his collection. It distinguishes the requirements of a small seedling from later seasonal cultivation under natural light. [Quail 2002]

Related articles

Materials from the author's archive

The species also appears in Konrad Szychowski's private material archive. [Rodríguez Ortega and Franco 2001, file p. 6]