Viola singularis J.M.Watson

Watson, John Michael & Flores, Ana Rosa, 2009, A new and rare rosulate species of Viola (Violaceae) from Argentina, Phytotaxa 2, pp. 19-23 : 20-22

Viola singularis J.M.Watson et A.R. Flores, sp. nov. ( Fig. 1 )

Herba pusilla , acaulis, probabiliter perennis, glandulis instructa, rosulas distinctas formans. Pars aliquantum distincta sectionis Andinium , verosimiliter e turma Violae volcanicae vel proxima, characteribus foliorum plus minusve inter haec et eos V. montagnei in mentem vocantibus. Ab illis, praeter V. montagnei , indumento sparso ad densissimo in superficiebus ambabus folii, et a V. montagnei laminis crenatis, nunquam integeris differt. Facie florali externa affinitatem V. volcanicae simulans , sed ab omnibus crista styli utrimque appendicibus magnis, decurvatis distinguitur.

Type:— ARGENTINA: Mendoza: Río del Nacimiento , entre rocas, en la orilla del Río. Mimético, 21.01.1992, anon. (holotype: MERL, acc. no. 55055) .

Diminutive, acaulous, pubescent herb, apparently perennial, forming lax but depressed rosettes to 2 cm across, one terminal, one subfasciculate-lateral-branched, on ca. 1 cm length of stem and primary tap-root segment (feeder root system unknown). All indument mixed pilose to lanose, with simple, white, slightly rigid hairs. Base of rosette with some remnants of old leaf bases. Leaves to 9 mm long, abruptly widened from narrower base, pseudopetiole to 6 mm, linear, narrow, more or less densely pubescent. Lamina 2.5–3.5 x 1.75–3.5 mm, semi-orbicular to rhomboid-spathulate, truncate or subtruncate at base, with 5–6 indistinct crenulations on both sides, apex obtuse; adaxial surface alveolate-reticulate, sparsely pubescent; abaxial surface smooth, densely pubescent, with many small, dark, shortly oblong glands around base of blade. Stipules 1 mm long, linear-oblong, obtuse, white-hyaline with sparse indumentum, apparently often caducous. Flowers arising singly from leaf axils, peduncle equal to leaves, firm, 1 mm wide at base, pubescent at apex to basally glabrescent, hairs longer above, much shorter and straight below. Bracteoles identical to stipules, on basal half of peduncle. Sepals 3 mm, lanceolate, acute, pubescent, with several equal-sized, linear, dark glands, notably larger than those of leaves. Corolla 6.5–7 mm long, apparently white or pale lilac with fine, darker veins, more closely spaced towards base of petals. Upper petals slightly exceeding calyx, oblong, recurved, apex obtuse, glabrous; lateral petals 4–5 mm long, oblong, apex obtuse, short-bearded basally and with longer clavate hairs along basal half of lower margin; lowest petal 6–7 mm long, spurred, emarginate with two rounded apical lobes, short-bearded with pollen-trap hairs along centre-line; spur ca 1.5 mm, bilobed, constricted at neck. Stamens not examined (inaccessible). Style clavate, stigma beaked, directed forwards; crest as lateral, tapered, down-curved, narrowly subtriangular appendages or flanges, one on each side. Mature fruit and seeds unknown; remnants of voided capsules glabrous, length ca 5 mm.

Habitat:—Between rocks, at the edge of the river. Cryptically camouflaged.

Etymology:— The epithet singularis was considered particularly appropriate in view of its notable rarity and the consequent general lack of basic knowledge that is so characteristic of many species of its section. It also further indicates that it is only known from a single specimen.

Observations:—The number of rosettes as for the type specimen, and their disposition, is highly unlikely to be a constant character for the species.

The diagnostic style crest was observed in the only flower suitable for examination and was analysed with considerable difficulty. However, even if damaged or anomalous, it is clearly distinct from the crests of its putative closest allies. That of V. volcanica is flabellate, that of V. congesta Gillies ex Hooker & Arnott (1833: 144) trilobate. All other comparative species possess one or other of those forms except any that lack a distinct crest altogether.

The adaxial leaf surface is alveolate-reticulate, that is to say honeycombed by an intricate network of raised veins with irregular slight depressions. Alveolate-reticulate is used here, following the definition of Stearn (1966), rather than the term foveolate employed by Becker (e.g. 1922), which seems misapplied. This is a usual and probably significant defining feature for an apparently broad major alliance under investigation, as represented by V. volcanica . Other species in V. sect. Andinium (e.g. V. subandina J.M.Watson in Hoffmann et al. (1998: 66)) also have adaxially raised leaf veins which are not distinctly anastomosing.

The specimen of V. singularis presents a mixture of patterned and smooth leaves due to part of the rosette being “ironed flat” by the strong force applied during the pressing process. As a result species wellrepresented in herbaria often comprise a mixture of specimens, some with all leaves veined, and others with mixed smooth and veined leaves, and some with all leaves smooth. This may even lead to inaccurate morphological interpretation ( Rossow, 1988). Our extensive field experience indicates that all leaves of any living specimen of any species from V. sect. Andinium with raised veins always possess that feature. The reticulate leaf-venation and corolla facies of V. singularis evidently relate most closely to V. volcanica and its immediate allies. The presence of dense pubescence on the abaxial leaf surface is also found in the more distantly related V. montagnei , a species that possesses a similar venation pattern as the V. volcanica group, but otherwise differs markedly in configurations of the style crest and the corolla. It forms an alliance with two other rather similar species, V. canobarbata Leybold (1866: 285) and V. evae Hieron. ex Becker (1922: 182) . Viola singularis therefore appears to be intermediate between the V.volcanica and V. montagnei groups.

Figures

FIGURE 1. Viola singularis J.M.Watson & A.R.Flores. Anon. (MERL-55055, holotype).