Three species and their names
In the Plants of the World Online treatment adopted here, the genus comprises Aztekium hintonii, A. ritteri and A. valdezii. The last remains a separate species, although some earlier works included it in A. ritteri. Checking the name therefore matters when comparing material from different years. [2-WWW]
A surface divided into small areas
The most thoroughly studied surface, that of A. ritteri, consists of ribs, transverse grooves and still smaller depressions. This arrangement divides the epidermis into exposed and sheltered zones. Fold crests have a thicker covering and reinforced hypodermal cells. Most stomatal apparatuses lie deeper, where neighbouring stem portions shelter them from direct radiation. The spatial arrangement of these structures explains more than the plant’s appearance from above alone. [Porembski 1996]
Water loss reduces turgor, the pressure exerted by cell contents on the wall. In A. ritteri, this is accompanied by the groove walls moving closer together. The anatomical description of this species explains how stem contraction changes air access to stomatal apparatuses. The extent and importance of a similar mechanism in the other species require separate studies of their surfaces. [Porembski 1996]
Soft tissues and wood
In the comparative analysis from 2017, both A. hintonii and A. ritteri had wood without fibres and a parenchymatous cortex and pith. Here, wood is secondary vascular tissue. Alongside vessels, it contains wide tracheids with annular or helical wall thickenings. Wall portions between these thickenings remain able to change dimensions. This organisation is compatible with deformation of the highly hydrated stem. [Vázquez-Sánchez et al. 2017]
The distribution of supporting tissues can differ even among close relatives. Geohintonia mexicana produces dimorphic wood, whose later portion contains fibres. Comparison of these plants reveals several ways of maintaining the shape of a small stem. Height, ribs, hypodermis and wood type form a shared mechanical system whose components have changed repeatedly during Cacteae evolution. [Vázquez-Sánchez et al. 2017]
Relationships and identification
In 2002, Butterworth et al. united Aztekium and Geohintonia in a strongly supported clade that was sister to the remaining Cacteae in their tree. The analysis included 62 members of the tribe and was based on the plastid intron rpl16. The result describes relationships reconstructed from this marker. Strombocactus, formerly compared with Aztekium, occurred elsewhere in the tree. [Butterworth et al. 2002]
Detailed surface characters, distribution and documented propagation methods are presented in the profiles of A. ritteri, A. hintonii and A. valdezii. Tissue structure is compared in the article “How a cactus stem maintains its shape during drought”. [Vázquez-Sánchez et al. 2017]