[2.13] Viola sect. Plagiostigma

Viola sect. Plagiostigma Godr., Fl. Lorraine, ed. 2, 1: 90. 1857 

≡ Viola [unranked] (“Gruppe”) Plagiostigma (Godr.) Kupffer in Oesterr. Bot. Z. 53: 329. 1903 

≡ Viola [sect. Nomimium] subsect. Plagiostigma (Godr.) J. C. Clausen in Ann. Bot. (Oxford) 43: 751. 1929; (Godr.) P. Y. Fu, Fl. Pl. Herb. Chin. Bor.-Or. 6: 89. 1977 (isonym)

—Type: Viola palustris L.

Description.—Perennial herbs, very rarely annuals. Axes morphologically differentiated in a rhizome and lateral stems; sometimes only a rhizome present. Rhizome densely or sometimes remotely noded, with an apical leaf rosette. Lateral stems annual aerial stems or stolons. Stipules free or adnate to petiole. Lamina extremely variable, entire or deeply divided, petiolate. Corolla violet, pink or white, with violet striations, and a white or yellow-green throat. Spur short and saccate to very long and slender. Style clavate, at apex flattened above, laterally and distally margined, or bilobate, beardless. Cleistogamous flowers produced; cleistogamy seasonal. Allotetraploid (CHAM + MELVIO). ITS sequence of CHAM type. Secondary base chromosome number x’ = 12.

Diagnostic characters.—Corolla violet, pink or white with a white or yellow-green throat AND style clavate, at tip flattened above, laterally and distally margined, or bilobate AND base chromosome number x’ = 12.

Ploidy and accepted chromosome counts.—4x, 8x, 12x; 2n = 20, 22, 24, 26, 44, 48, 72, 74.

Age.—Crown node age 16.6 (15.4–17.0) Ma [28].

Included species.—142.

Distribution.—Throughout the northern temperate region, with a few species south of the equator in Australasia and South America; centre of diversity in eastern Asia (Figure 1).

Discussion.—Sect. Plagiostigma is phylogenetically an allotetraploid CHAM + MELVIO lineage; unlike all other sections except sect. Danxiaviola it has retained the CHAM homoeolog for ITS (Figure 6). It is karyologically characterised by the secondary base chromosome number x = 12, and morphologically by the clavate, margined or bilobate, beardless style, and the occurrence of seasonal cleistogamy. Here, we recognise a narrowly circumscribed sect. Plagiostigma that comprises the six Beckerian greges [1] having a ‘plagiostigmate’ style shape and a secondary base chromosome number x’ = 12, i.e., sect. Nomimium greges Adnatae p.p., Bilobatae, Diffusae, Serpentes p.p., Stolonosae, and Vaginatae. In this respect our classification approaches Clausen’s [29,59] but we further exclude the North American allodecaploid lineage, herein transferred to sect. Nosphinium [28,45,61].

With its 139 known species and a crown node of 16.6 (15.4–17.0) Ma, sect. Plagiostigma is both the oldest and the most species-rich of all Viola sections. It could be justified to treat subsect. Diffusae and subsect. Patellares as separate sections. We keep them within sect. Plagiostigma because of at least two synapomorphies, the style shape and the base chromosome number x = 12. We recognise seven subsections within sect. Plagiostigma (Figure 27), each monophyletic and morphologically characterised, i.e., subsect. Australasiaticae, subsect. Bilobatae, subsect. Bulbosae, subsect. Diffusae, subsect. Patellares, and subsect. Stolonosae. Diffusae and Patellares are sisters (or sister) to the lineage comprising Bilobatae, Bulbosae, and Stolonosae. The phylogenetic placement of subsect. Australasiaticae within the section is unknown, as this taxon is represented by ITS sequences only and this marker (Figure 6) poorly reflects the genome phylogeny (Figure 2).

While 2n = 24 is retained in most of the subsections, 2n = 22 is apomorphic in subsect. Formosanae and, possibly, 2n = 46 in subsect. Austalasiaticae.

There is little agreement between Becker’s [1] greges and the subsections proposed herein. This is discussed briefly under each subsection

subsect. Australasiaticae 10

subsect. Bilobatae 8

subsect. Bulbosae 2

subsect. Diffusae 17

subsect. Formosana 2

subsect. Patellares 62

subsect. Stolonosae 41