Cactus tubercles. Shape, arrangement and examples

Tubercle, areole and axil

A tubercle is a projection of the stem, rather than a spine. In Mammillaria, the spine-producing zone is at its apex, while flowers appear near the axil between tubercles. The shape of the green body should be distinguished from spine length, wool quantity and the density of neighbouring tubercles. Together, these traits create the plant's appearance, but describe different structures. [Mauseth 2006] [Fleischer and Schütz 1978, pp. 26–28]

How to compare the eight diagrams

The plates organise descriptive vocabulary rather than establish eight taxonomic groups. Conical shape, a rounded tip, flattening and close spacing can apply to the same tubercle. The examples below refer to traits recorded in sources. They do not identify the species in the illustration. The label “awl-shaped” particularly requires commentary, because it has not been confirmed as a separate category for the example indicated. [Britton and Rose 1923]

Short-conical

Example, Mammillaria mammillaris. The tubercle has a broad base and tapers towards the areole. Britton and Rose described short conical tubercles in this species, nearly rounded in cross-section. “Short” refers to the tubercle's projection beyond the stem axis, rather than spine length. [Britton and Rose 1923, p. 70] [2-WWW]

Elongated

Example, Mammillaria longimamma. Marked tubercle elongation changes the silhouette of the whole plant. In the monograph, the species appears as Dolichothele longimamma. This is an example of elongation, rather than proof that every tubercle has the perfectly conical outline drawn on the plate. [Britton and Rose 1923, pp. 61–63] [2-WWW]

Cylindrical and rounded

Example, Mammillaria bocasana. Slender tubercles with a rounded cross-section are partly obscured by hair-like spines in this species. Their shape is assessed on exposed green tissue, rather than the outline of the entire white covering. The boundary between a cylinder and an elongated cone is gradual in a living organ. [Britton and Rose 1923, p. 147] [2-WWW]

Slender and strongly tapering

Example, Mammillaria longimamma, comparison of elongation. The label “awl-shaped” in the drawing presents an extreme geometric model of slenderness and tapering. M. longimamma can be used to compare elongation. However, the source description read does not establish a separate “awl-shaped type” for this species. The label is therefore not assigned to the species as a diagnostic trait. This panel requires the most cautious interpretation. [Britton and Rose 1923, pp. 61–63] [2-WWW]

Blunt-tipped

Example, Mammillaria heyderi subsp. gaumeri. The taxon, originally described as Neomammillaria gaumeri, has short, almost rounded tubercles with a blunt tip. “Rounded” in the illustration should refer to the apex and outline, rather than treating the entire tubercle as a sphere. The subspecies designation preserves its scope in the current POWO treatment. [Britton and Rose 1923, pp. 72–73] [2-WWW]

Angular, approaching pyramidal

Example, Mammillaria polyedra. This species has been described with angular, pointed tubercles flattened on the dorsal side. Their shape can be compared with a pyramid, but this is a model. The same tubercle combines angularity and flattening. Such coexistence of traits shows why the plate's eight labels cannot be treated as a mutually exclusive classification. [Britton and Rose 1923, pp. 102–103] [2-WWW]

Laterally flattened

Example, Mammillaria compressa. The tubercles are described as short and laterally flattened, with a keel below and a more rounded upper side. The direction of flattening must therefore be specified. The word “flat” alone does not convey the asymmetry of the two surfaces. Views from above and from the side may give different impressions. [Britton and Rose 1923, pp. 90–92] [2-WWW]

Densely arranged

Example, Mammillaria evermanniana. In the original description of Neomammillaria evermanniana, the tubercles are rounded and closely set, almost covered by numerous spines. Density is an arrangement trait, rather than the shape of an individual tubercle. It can coexist with any of several preceding traits. [Britton and Rose 1923, p. 97] [2-WWW]

From historical name to current label

The 1923 monograph used placed most examples in the genus Neomammillaria and described the elongated tubercles of M. longimamma under Dolichothele. Current names were checked in POWO. The species studied by historical authors is not silently changed. The old combination is retained, and the name under which the same taxon can now be found is indicated. [Britton and Rose 1923] [2-WWW] [2-WWW]

Tubercle shape is one identification trait. Comparing taxa also requires areoles, spines, flowers, fruits, seeds and the material's origin. An illustration without specimen data, scale and variation is not treated as a standalone botanical key. [Mauseth 2006] [Britton and Rose 1923]