Stellaria sect. Plettkea (Mattf.) Montesinos & Borsch

Montesinos-Tubée, Daniel B. & Borsch, Thomas, 2023, Molecular phylogenetics and morphology reveal the Plettkea lineage including several members of Arenaria and Pycnophyllopsis to be a clade of 21 South American species nested within Stellaria (Caryophyllaceae, Alsineae), Willdenowia 53 (3), pp. 115-148 : 127-128

Stellaria sect. Plettkea (Mattf.) Montesinos & Borsch , comb. & stat. nov.

≡ Plettkea Mattf. in Schriften Vereins Naturk. Unterweser, n.s., 7: reprint p. 13 (11, 17). 1934. – Type: Stellaria cryptantha (Mattf.) M. T. Sharples & E. A. Tripp (≡ Plettkea cryptantha Mattf. ). = Baretia Timaná in Adansonia, ser. 3, 45: 502. 2023, syn. nov. – Type: Baretia lanata (Phil.) Timaná (≡ Pycnophyllum lanatum Phil. ).

Morphological description — Annuals or perennials, mostly prostrate, cushion-forming or mat-like, sometimes in small and dense tufts. Stems often decumbent, rarely ascending, 2–60 cm high ( Stellaria galianoi having greatest height), stems glabrous with few exceptions ( S. apurimacensis ) specially at nodes of leaves where hirsute hairs may occur, often glandular in surface; internodes hirsute or glabrous, short as in most of species and longer in S. apurimacensis and S. galianoi . Leaves mostly ovate, but linear to lanceolate in some species, 2–10 mm long, with acute or mucronate apex, usually hirsute near base, margins glabrous or ciliate, rarely lanuginose. Flowers solitary, only terminal, or in some species also axillary on short peduncles, with 5 sepals, free to base, mostly ovate to lanceolate, acute to acuminate in apex, base hirsute and stiff in texture, 1.8–5 mm long; petals mostly 5, rarely 4 or 3, showy and white or cream in colour and then usually bifid at apex, 10–15 mm long, or reduced to various degrees, mostly obtuse at apex, cream to translucent, and only up to 5 mm long and not overtopping sepals; stamens 5, often adherent to petals, filaments filiform and terete throughout (in all species for which descriptions are provided here), rarely thickened at base ( P. keraiopetala , P. patagonica ), in dioecious species sometimes a few staminodes present in pistillate flowers; ovary ovoid to globose, with trifid or rarely bifid styles, very rarely style single and unbranched ( S. villasenori ), in dioecious species often rudimentary pistillodes present in staminate flowers, these not more than 1 mm long and seedless. Capsule 1–12-seeded; seeds more or less conspicuously tuberculate.

Notes — Until now there was no formal infrageneric name for the Plettkea clade. Sharples & Tripp (2019) annotated the lineage of Stellaria tetrasticha , corresponding to the Plettkea clade, in their trees with “Plettkeae” albeit without any reasoning for that name. The first infrageneric classification system by Fenzl (1840) recognized four entities, one of which, “ Eustellaria ”, was further subdivided. However, no specific nomenclatural rank was designated, until Pax & Hoffmann (1934) specified these entities as sections and subsections, respectively. We therefore suggest continuing the use of the sectional rank for the subdivision of Stellaria . No entity corresponding to a section Plettkea was included by Fenzl (1840) or Pax & Hoffmann (1934), so it is described here as a section of Stellaria .

As Sharples (2019) and Sharples & Tripp (2019) already pointed out, more work needs to be done to develop a consistent infrageneric classification for Stellaria , which includes testing if existing sectional/subsectional names are validly published. Considering that S. graminea was designated as the neotype for the monophyletic genus Stellaria ( Tikhomirov 2016) , and that the name “ Eustellaria ” is not permitted under Art. 21.3 of the Code ( Turland & al. 2018), the ‘Larbreae’ clade (sensu Sharples & Tripp 2019) would become the typical S. sect. Stellaria ( Sharples & Tripp 2019; Supplementary appendix S9).

Pax & Hoffmann (1934) validated the rank of Arenaria sect. Dicranilla (Fenzl) Pax & K. Hoffm. that was originally described by Fenzl (1840) based on A. dicranoides Kunth and A. bryoides , both described based on material from Mexico. In their treatment, Pax & Hoffmann (1934) also extended the concept of this section to include Andean species like A. bisulca (Bartl.) Fenzl & Rohrb. , A. pycnophylloides Pax, A. alpamarcae A. Gray , A. boliviana F. N. Williams, A. pedunculosa Wedd. , and A. pycnophylla Rohrb. However , the authors overlooked that these species do not match the original diagnosis by Fenzl (1840), who stated that the species of Dicranilla (Fenzl) Rchb. possess smooth, shiny, black seeds. On the contrary, members of the Andean Plettkea clade have tuberculate, brown seeds. Molecular phylogenetic data (T. Borsch, unpublished) further show that A. dicranoides belongs to the core Arenaria clade, underscoring that Pax & Hoffmann (1934) created an extended, morphologically heterogeneous and polyphyletic section. McNeill (1962) typified the genus name Dicranilla based on the section with Arenaria dicranoides , and commented on the misinterpretation of the seed characters by Pax & Hoffmann (1934). In summary, there was no name available for the ‘Plettkea’ clade at the level of section.

The publication by Timaná (2023), describing the new genus Baretia based on Pycnophyllum lanatum ( Stellaria villasenorii ), was received while correcting the proofs of this paper. We treat Baretia as a synonym here because to accept the new genus would make Stellaria paraphyletic.