Three species and genus history
In the POWO treatment, Homalocephala includes three species occurring in the southwestern and central United States and northern Mexico. Britton and Rose introduced the genus name in 1922 for H. texensis. They highlighted its flattened stem, large seeds and red, juicy fruits. The Greek components of the name refer to the plant’s flat apex. [2-WWW] [Britton and Rose 1922]
The analysis from 2018 expanded this genus to include H. parryi and H. polycephala (previously Echinocactus parryi and E. polycephalus). It included representatives of all six species then assigned to Echinocactus, along with comparison taxa. A set of 42 morphological characters was combined with four chloroplast DNA regions and two nuclear genes. This broad set allowed comparison of adult plant similarity with seedling characters and molecular information. [Vargas-Luna et al. 2018]
Stem, seedling and seed structure
Sharp rib edges are visible even in young plants. The apex is relatively sparsely hairy, and flowers remain narrower at the base than the broadly bell-shaped flowers of Echinocactus. Small hairs may also cover spine surfaces and the outer sides of scales at the flower base. In H. polycephala subsp. xeranthemoides, these surfaces are smooth, so the single character of hairiness must be considered alongside other diagnostic features. [Vargas-Luna et al. 2018]
The seed contains a curved embryo. Its cotyledons have pointed tips. In the material studied, seedlings of H. parryi, H. texensis and the nominotypical subspecies of H. polycephala usually had 5–8 spines per areole, while subsp. xeranthemoides had 9–12. These data concern the first year of development, whereas spine number and proportions in adult specimens form a separate character set. On the seed surface, boundaries between seed-coat cells form sunken grooves. [Vargas-Luna et al. 2018]
Relationships recorded in structure and DNA
In the combined analyses, Homalocephala formed a shared evolutionary lineage, sister to the group containing Echinocactus and Astrophytum. The close relationship between the latter two genera was also supported by seedling characters, including the limited spine number in the first areoles. The densely armed Kroenleinia grusonii (formerly Echinocactus grusonii) lay outside this group, alongside Ferocactus. Globular or barrel-shaped habits therefore occur in several distinct cactus lineages. [Vargas-Luna et al. 2018]
The distinctness of the entire genus was clearer than the sequence of divergence among its three species. Some analyses grouped H. texensis with H. polycephala, while others left their relationships unresolved. Recognition of the genus therefore has stronger support in this study than a detailed reconstruction of its internal branching. [Vargas-Luna et al. 2018]
Flowers and fruits distinguish similar plants
H. texensis usually grows singly, with pink flowers and juicy fruits. H. parryi flowers yellow with a reddish perianth base, and its fruits dry during ripening. In H. polycephala, the flower is uniformly yellow, and the adult plant often branches from the base to form a large clump. Its fruit also matures into a dry structure that opens at the base. This comparison covers habit and reproductive organs, helping identify plants at different times of year. [Vargas-Luna et al. 2018] [Zimmerman and Parfitt 2003a]
| Species | Growth form | Flower | Mature fruit |
|---|---|---|---|
| H. parryi | Usually a solitary stem | Yellow with a reddish base | Drying out |
| H. polycephala | Often an extensive clump | Yellow throughout the perianth | Drying out |
| H. texensis | A solitary, usually low stem | Pink | Soczysty |
Propagation under controlled conditions
Propagation studies of H. parryi include greenhouse sowing and tissue cultures. In the greenhouse, seed storage, soaking and preparation at low temperature were compared. Tissue culture studies separately examined shoot production, rooting and subsequent survival after transplanting. Each stage is described in the species profile, together with experimental conditions and the scope of the results. [Vargas-Luna et al. 2026] [García-González et al. 2020]