Epithelantha. How age changes stem appearance

White stems on limestone

In the saddle above Anthony Gap, at the northern edge of Franklin Mountains State Park in Texas, Epithelantha micromeris grew in pinkish limestone at an elevation of approximately 1500 m. Gorelick described numerous small specimens there, usually with a stem diameter of 15–25 mm, and occasional larger plants up to approximately 40 mm. Among the greyish, rough stems were smaller, creamy-white, smooth ones. Their apices were concave, giving them a resemblance to E. bokei. [Gorelick 2026, pp. 1–3]

Identification of the small plants was also based on spine structure. They had 31–37 spines per areole, in one or two series, with adjacent spine groups overlapping. This set of characters corresponded to E. micromeris. Colour and the overall smoothness of the stem could therefore lead to confusion between two species, while areole structure provided other information. [Gorelick 2026, pp. 2–3]

Maturation involves the entire apex

The transition between forms was observed near a diameter of 15 mm. All small plants with a pale covering had concave apices and bore neither flowers nor fruits. Larger stems were mostly greyer and rough, with convex apices, longer apical spines and buds. Spines near the apex formed a conical covering and broke during the first or second season. Changes in spine length and persistence therefore affected the appearance of the entire surface. [Gorelick 2026, pp. 2–3]

A specimen with four stems approximately 12–18 mm in diameter was particularly informative. The larger stems had convex apices and buds, while the smaller ones retained the concave juvenile form. This arrangement combines different stages on a single individual. The age of the whole plant and the developmental phase of each stem can therefore provide different information. The four stems are parts of a shared plant, and their observations describe that specimen. [Gorelick 2026, pp. 2–4, Fig. 5–9]

The author also reported the subsequent fates of two collected individuals, later cultivated by him and Mike Powell. Over time, they became grey, rough and convex at the apex. Observing the direction of change strengthens the ontogenetic interpretation. The paper describes a population and a few tracked plants, however, so approximately 15 mm remains a local transition point. Developmental rates in other populations require separate comparison. [Gorelick 2026, pp. 3–4]

Why comparison with Epithelantha bokei is interesting

Flora of North America describes E. bokei with a much denser spine covering, usually 50–90 spines per areole in more than three rows, and larger flowers. The account also distinguishes floral biology. It reports self-pollination for North American E. micromeris and cross-pollination for E. bokei. Mature plants therefore differ in a set of characters. [Parfitt and Zimmerman 2003, t. 4, pp. 239–240]

Gorelick proposed that E. bokei may retain part of the form corresponding to young E. micromeris. Such a relationship is called paedomorphosis. It concerns a shift in the rate or termination of development of certain characters relative to the attainment of reproductive maturity. A plant may already flower while part of its stem structure remains similar to an earlier stage of a closely related plant. In this article, the observed evidence was a similar smooth covering and apical form. [Gorelick 2026, pp. 1–4]

Paedomorphosis is a hypothesis about developmental history and may occur between distinct species. Inferring its presence requires establishing which characters are homologous, how they change in both plants and the state of the common ancestor. The study described mainly tracked E. micromeris. A comparative developmental series for E. bokei would still need to be assembled. POWO continues to accept both species separately. [Gorelick 2026, pp. 1–4][2-WWW]

Relationships already have a molecular basis

DNA studies of the genus were available before publication of the 2026 observations. The 2021 paper by Aquino and co-authors included all ten recognised species, comparing four plastid genome fragments. E. bokei and E. micromeris belonged to one larger lineage with E. cryptica, E. ilariae, E. potosina and E. spinosior. The result provides a framework of relationships for developmental comparison, while the direction of ontogenetic changes itself requires developmental data. [Aquino et al. 2021, pp. 4522–4525, Fig. 2]

The current ten species names define the taxonomic scope of the genus in POWO. Older literature also uses variety and subspecies ranks, including E. micromeris var. bokei and subsp. bokei. Distinguishing the historical name from the paedomorphosis hypothesis preserves two kinds of information. These are how the plant was classified in a particular account and the proposed explanation of its development. [2-WWW]

Stem constrictions and the hidden part of the plant

On larger, rough stems, often above approximately 25 mm in diameter, the author saw circumferential constrictions. He called them slump rings, rings formed by stem collapse, and tentatively associated them with severe dehydration. The material shows the shape of the constrictions and their occurrence in some older stems. Linking them to a specific drought period would require measurements of the same plants before dehydration, during it and after water recovery. [Gorelick 2026, pp. 4–5]

In excavated specimens, much of the stem lay below the surface. Underground fragments still bore spines, and beneath them lay a taproot with several lateral roots. The author interpreted this arrangement as a result of a contractile root pulling the plant down, considering ordinary burial unlikely in the specimens' positions. The observations document an underground stem. Root contraction remains a proposed mechanism because neither shortening nor the force of movement was measured. [Gorelick 2026, pp. 5–7, Fig. 11–15]

Recognising a plant over time

The Anthony Gap account shows that stem phase matters in assessing Epithelantha. Spine number and arrangement, apex structure and mature flower characters provide a fuller description than the general white silhouette. Provenance records and successive observations of the same plant allow changes in appearance to be attributed to maturation. This is particularly important for branched specimens whose stems may be at different stages simultaneously. [Gorelick 2026, pp. 2–4][Parfitt and Zimmerman 2003, t. 4, pp. 239–240]