Growth form and storage organs
The genus includes short cylindrical, depressed globose and decumbent stems, as well as slender stems with a more columnar growth form. Plants may remain solitary or develop clusters of offsets. In the 2018 phylogenetic study, some similar growth forms occurred in separate lineages. External resemblance is useful for identification, while its significance for relationships requires comparison with other characters. [Sánchez et al. 2018]
In some lineages the main root becomes thickened, whereas in others lateral roots act as storage organs. Section Wilcoxia includes plants with slender stems and thickened lateral roots. Several small, depressed globose species have an enlarged main root. The authors interpret these changes as different patterns of distributing water storing tissues between aboveground and underground organs. [Sánchez et al. 2018]
Flowers and different reproductive systems
Green stigmas, funnel shaped flowers and armed fruits are among the features described in the genus. Floral tube structure, nectary position, anthers and seed characters help distinguish its groups. The red flowered section Triglochidiati includes species with traits associated with hummingbird visitation. Field observations and pollinator exclusion experiments also demonstrate contributions from bees, depending on the taxon and site studied. [Sánchez et al. 2018] [Aslan 2015] [Hoffman 1992]
Most species discussed in these studies have bisexual flowers, but functional dioecy has been demonstrated in part of section Triglochidiati. Individuals then act as pollen donors or produce seeds. In the studied male plants, the pistil retained the ability to receive pollen and permit fertilisation, while seed development stopped later. Identifying the sexual system therefore required observations across successive developmental stages. [Hernández-Cruz et al. 2018]
Phylogeny and the scope of names
Sánchez and colleagues combined 44 morphological characters with six chloroplast DNA regions. The study included 59 species of the genus and ten comparative taxa. Eight main lineages were recovered and used to propose a sectional classification. The position of Echinocereus poselgeri varied between analytical methods. An additional test indicated an effect of long branch attraction, an artefact that groups strongly diverged sequences. [Sánchez et al. 2018]
POWO accepts the genus and treats Wilcoxia as its synonym. Species counts refer to particular taxonomic circumscriptions. The 2018 phylogenetic study and the subsequent treatment of the Echinocereus pulchellus complex represent successive stages in resolving its classification. Biological findings remain linked to the material and name used in the particular publication. [2-WWW] [Sánchez et al. 2018] [Sánchez et al. 2020]