[2.12.1]. Viola sect. Nosphinium subsect. Borealiamericanae

Viola subsect. Borealiamericanae (W. Becker) Gil-ad in Bossiera 53: 42. 1997 (“Boreali-Americanae”; Shenzhen Code Art. 41.8) 

≡ Viola [sect. Nomimium; unranked] Borealiamericanae W. Becker in Repert. Spec. Nov. Regni Veg. 19: 396. 1923 (“Boreali-Americanae”) 

≡ Viola [sect. Plagiostigma] subsect. Borealiamericanae (W. Becker) Brizicky in J. Arnold Arb. 42: 327. 1961, nom. inval. (Shenzhen Code Art. 41.5; “Boreali-Americanae”) 

≡ Viola [sect. Plagiostigma] subsect. Borealiamericanae (W. Becker) Val. Tikhom. in Bot. Zhurn. 100: 497. 2015, nom. inval. (Shenzhen Code Art. 41.8; “Boreali-Americanae”) 

≡ Viola sect. Borealiamericanae (W. Becker) Espeut in Botanica Pacifica 9(1): 35. 2020 (“Boreali-Americanae”)—Type (only species cited): Viola nuevoleonensis W. Becker

=Viola subg. Hesperion Nieuwl. & Kaczm. in Amer. Midl. Naturalist 3: 211. 1914

—Type: Viola palmata L.

Description.—Perennial herbs. Axes not morphologically differentiated; stem a perennial rhizome terminating in an apical rosette. Stipules narrow, free, glandular-lacerate. Laminas in some species lobed or dissected. Calycine appendages various. Petals violet (rarely whitish), lateral and often the spurred petal densely bearded. Style clavate with a pronounced thickened spreading broadly rounded sometimes weakly trilobate dorsolateral rim with sides or lateral lobes continuing to the ventrally oriented rostellum. Cleistogamous flowers produced, seasonal (in temperate species) or simultaneous (in subtropical species). Base chromosome number x = 27.

Diagnostic characters.—Habit strictly rosulate AND stipules free AND petals violet AND lateral (sometimes spurred) densely bearded AND style with pronounced thickened spreading broadly rounded sometimes weakly trilobate dorsolateral rim and ventrally oriented rostellum AND cleistogamy present AND base chromosome number x = 27.

Ploidy and accepted chromosome counts.—10x; 2n = 54.

Age.—Crown node at least 2.6 (0.7–5.0) Ma (Figure 6), stem node age 3.2–5.4 Ma [45].

Included species.—38. 

Viola affinis Leconte, 

Viola baxteri House, 

Viola brittoniana Pollard, 

Viola calcicola R. A. McCauley & H. E. Ballard, NMRP; iNat; McCauley and Ballard 2013

Viola chalcosperma Brainerd, 

Viola communis Pollard, 

Viola cucullata Aiton, 

Viola edulis Spach, 

Viola egglestonii Brainerd, 

Viola emarginata (Nutt.) Leconte, 

Viola fimbriatula Sm., 

Viola floridana Brainerd, 

Viola hirsutula Brainerd, 

Viola impostor R. Burwell & H. E. Ballard, ined. [H. E. Ballard 18-002], 

Viola langloisii Greene, 

Viola latiuscula Greene, 

Viola lovelliana Brainerd, 

Viola missouriensis Greene, 

Viola monacanora J. L. Hastings & H. E. Ballard, ined. [H. E. Ballard 21-015], 

Viola nephrophylla Greene, 

Viola novae-angliae House, 

Viola nuevoleonensis W. Becker, 

Viola palmata L., 

Viola pectinata E. P. Bicknell, 

Viola pedatifida G. Don, 

V. pedatiloba (Brainerd) Burwell & H. E. Ballard, ined., 

V. pratincola Greene, V. retusa Greene, 

V. rosacea Brainerd, 

V. sagittata Aiton, 

V. septemloba Leconte, 

V. septentrionalis Greene, 

V. sororia Willd., 

V. stoneana House, 

V. subsinuata (Greene) Greene, 

V. tenuisecta Zumwalde & H. E. Ballard, ined. [Ballard 21-017], 

Viola viarum Pollard, 

Viola villosa Walter

Distribution.—North America.

Discussion.—This endemic North American lineage retains the initial allodecaploid genome constitution of the ancestor to sect. Nosphinium. A suite of traits delimits the subsection, including a thickish rhizome, strictly rosulate habit, free stipules, undivided or lobed to dissected leaf laminas, large violet to dark violet, rarely whitish corolla, densely bearded lateral petals and often bearded bottom petal, and a style with a spreading conspicuously thickened dorsolateral rim and distinct rostellum. Species express a wide range of diagnostic features in cleistogamous capsule and seed morphology. The centre of diversity is in the Appalachian Mountain range and adjacent uplands. Ezra Brainerd and others conducted many studies of interspecific hybridisation in the subsection, including long-term garden observations and cultivation of F3 and F4 generations (summarised in Brainerd [264]). Hybridisation is extensive among locally co-occurring species, with hybrids, typically vigorous, failing in chasmogamous reproduction, commonly producing either underdeveloped capsules or capsules with a reduced proportion of viable seeds relative to parental species, and progeny of hybrids express recombinant phenotypic traits of the parental taxa in the plants derived from seeds of the cleistogamous capsules. All species but one occur north of Mexico, whereas V. nuevoleonensis is confined to northeastern Mexico.

Despite gradually increasing synonymy by specialists since Brainerd [69], recent studies by HEB and collaborators are revealing many overlooked new species (including some local and regional endemics) and resurrecting previously synonymised species, making it is one of the more diverse subsectional lineages in the genus, and the second largest in the Western Hemisphere (minimum 38 species, possibly as many as 60). Viola communis Pollard thrives in lawns and fencerows, and a few species have been inadvertently introduced into Europe