[2.13.5] Viola sect. Plagiostigma subsect. Formosanae

Viola subsect. Formosanae (J.-C. Wang & T.-C. Huang) Marcussen, comb. et stat. nov.

—Basionym: Viola grex Formosanae J.-C. Wang & T.-C. Huang in Taiwania 35(1): 14. 1990.

—Type (only species listed): Viola formosana Hayata

Description.—Rhizome perennial; bulbils absent. Lateral stems present: aboveground stolons, most leaves in apical rosette. Stipules free or adnate at base, purplish-brown, lanceolate or narrowly ovate, long fimbriate-laciniate. Lamina broadly triangular-ovate or oblong-orbicular, base deeply cordate to rounded, apex acute to rounded or obtuse, margin crenate. Corolla white or pale violet. Sepals narrowly lanceolate to oblong; appendages short (0.5–1 mm), rounded. Lateral petals not bearded; bottom petal longer than the other petals (8–15 mm), apex deeply emarginate or shallowly cleft; spur long and slender (1.5–7 mm). Style at apex margined and flattened, not bilobate. Secondary base chromosome number x’ = 11.

Diagnostic characters.—Bottom petal longer than the other petals AND stolons with most leaves in apical rosette AND chromosome number 2n = 22.

Ploidy and accepted chromosome counts.—4x; 2n = 22.

Age.—unknown.

Included species.—2. 

Viola formosana Hayata, 

Viola  stoloniflora Yokota & Higa

Distribution.—Southeastern Asia: the islands of Taiwan (V. formosana) and Okinawa (V. stoloniflora).

Discussion.—Becker was familiar with Viola formosana ([286], page 167), the only of the two species known at the time, but he did not mention it or place it systematically in his revision of the genus [1]. The second species, V. stoloniflora, has been placed in subsect. Australasiaticae [229] or in its predecessor, subsect. Serpentes, “on account of its procumbent stolons, almost free fimbriate stipules, and deplanate obtriangular-dilatate styles” [97]. In their revision of the violets of Taiwan, Wang & Huang (1990 [75]) recognised the distinctness of V. formosana and placed it in a provisional group of its own, Formosanae, one of eight unranked greges; their delimitation of greges is reconcilable with our classification.

The phylogenetic placement of subsect. Formosanae is unresolved, but published chloroplast DNA sequences of Viola formosana place it among the other stoloniferous subsections [287].

The two species Viola formosana and V. stoloniflora have never been grouped together, despite their close geographical proximity and several synapomorphies that set them apart from all other subsections of sect. Plagiostigma, including the long and emarginate bottom petal, the shape of the stolons (reminescent of subsect. Diffusae), and the rare chromosome number 2n = 22 [75,97]. Viola stoloniflora is extinct in the wild; its only known locality in Okinawa Island was destroyed by the construction of the Benoki Dam, which was completed in 1987 [97].

Photogallery