Homalocephala polycephala

Name and branching stem

Homalocephala polycephala (in older literature Echinocactus polycephalus, also Emorycactus polycephalus) is a species accepted in POWO. It occurs in the southwestern United States and northwestern Mexico. The subspecies occupy different parts of this area, so identifying a specimen’s geographical provenance complements assessment of its structure. [2-WWW]

A young plant may have one stem, while an older plant produces further stems at the base and gradually builds a dense clump. Britton and Rose described clusters of 20–30 heads. The current flora of North America gives an even broader range of branch numbers. A many-headed habit is therefore a developmental feature that a single seedling does not show for a long time. [Britton and Rose 1922] [Zimmerman and Parfitt 2003a]

The stems have sharp, tall ribs and large areoles. Dense spines partly conceal the green surface. In Britton and Rose’s description of the nominotypical subspecies, young spines were covered with fine hairs and became more exposed with age. Their red tint, flattening and transverse rings contribute to the plant’s appearance. The strongest central spines arise among shorter spines arranged around them. [Britton and Rose 1922]

Flowers, dry fruits and seed release

The inner tepals are yellow throughout their length. This character helps distinguish the species from H. parryi, whose flower has a reddish base. The developing flower may be mechanically constrained by dense spines and then opens less widely. Scales and numerous hairs occur at the flower base and remain on the developing fruit. [Vargas-Luna et al. 2018] [Zimmerman and Parfitt 2003a]

The fruit dries into a thin covering, usually before releasing its seeds. It opens at the base, and hairs cover its surface for a long time. Britton and Rose described dark seeds with distinctly tuberculate sculpturing in the nominotypical form. Fruit structure differs from the bright red, juicy fruit of H. texensis, although both species belong to the same closely related group. [Britton and Rose 1922] [Vargas-Luna et al. 2018]

Two subspecies visible under magnification

POWO recognises subsp. polycephala and subsp. xeranthemoides. In the key from 2018, the nominotypical subspecies has hairy spines and scales surrounding the lower flower portion, and convex outer walls of seed-coat cells. In subsp. xeranthemoides, the spines and scales are smooth, while seed-coat cell surfaces remain much flatter. These characters can be assessed under magnification and are more informative than the general colour of the clump alone. [2-WWW] [Vargas-Luna et al. 2018]

Britton and Rose treated Echinocactus xeranthemoides as a separate species, basing the division partly on its smooth, shiny seeds. In the molecular analysis from 2018, the two taxa were very similar in the DNA regions studied. The authors retained subspecies rank, combining small sequence differences with discernible morphological differences. This example shows why an old label bearing the name E. xeranthemoides should be retained with specimen documentation. [Britton and Rose 1922] [Vargas-Luna et al. 2018]

Comparison with other Homalocephala species

The clearest set of species characters consists of the clump-forming habit of adult specimens, uniformly yellow perianth and dry fruit. H. parryi branches much less often and has a reddish flower base. H. texensis usually retains a solitary, low stem, and its pink flowers precede fleshy fruit development. Detailed identification of the subspecies of H. polycephala additionally requires examination of spines, scales and seeds. [Vargas-Luna et al. 2018]