Decorsella arborea, a second species in Decorsella (Violaceae), and Decorsella versus Rinorea

Abstract: A new species of Violaceae, Decorsella arborea Jongkind, is described and illustrated. The new species differs from the only other species in the genus, D. paradoxa A. Chev., by the larger size of the plants, smaller leaves, more slender flowers, and stamen filaments that are free for a much larger part. Both species are from the Guineo-Congolian forest of tropical Africa. The differences between Decorsella and Rinorea are discussed. Confirming recent reports, some species of Rinorea can have zygomorphic flowers and some of these can be almost equal in shape to Decorsella flowers. Citation: Jongkind C. C. H. 2017: Decorsella arborea, a second species in Decorsella (Violaceae), and Decorsella versus Rinorea. — Willdenowia 47: 43–47. doi: https://doi.org/10.3372/wi.47.47105 Version of record first published online on 13 February 2017 ahead of inclusion in April 2017 issue.


Introduction
The genus Decorsella A. Chev. (Violaceae) is restricted to the Guineo-Congolian region in tropical Africa and was, until now, represented only by D. paradoxa A. Chev. When flowering, Decorsella at first sight resembles Rinorea Aubl., a genus with many species in the Guineo-Congolian forests, because several Rinorea species have almost similar zygomorphic flowers. However, the fruit of Decorsella is not a capsule as in Rinorea. Instead, the ovary of Decorsella dehisces shortly after fertilization, with each seed developing a fleshy testa and resembling an orange berry at maturity (Fig. 1). Keay (1953: 75-77) tried to capture this character in the genus name Gymnorinorea Keay, a name that became a synonym of the earlier-published Decorsella. According to Ballard & al. (2014: 304), the ovary wall of Decorsella disintegrates shortly after fertilization, leaving the placental traces behind. The status of Decorsella as a distinct genus was recently confirmed when many genera and species in the Violaceae worldwide were compared in molecular phylogenetic studies (Tokuoka 2008;. At first, Decorsella paradoxa was known only from Liberia, Ivory Coast and Ghana (Keay 1953: 76). Later, a comparatively small number of plants from Lower Guinea (sensu White 1979) were also identified as belonging to this species (Sosef & al. 2006). While these Upper and Lower Guinean plants all undoubtedly represent Decorsella, there are also conspicuous differences between them. The Lower Guinean plants are larger, the flowers are more slender, the sepals are smaller, also compared to the petals, and the free part of the stamen filament is longer (Table 1, Fig. 2). In this publication, these differences are illustrated and the plants from Lower Guinea are described as D. arborea Jongkind, sp. nov. After this, D. paradoxa will again be endemic to Upper Guinea (sensu White 1979).

Material and methods
The comparison of the two Decorsella species was based on dried and pickled herbarium specimens kept at the BR, P and WAG herbaria (herbarium codes according to Thiers 2017+). Decorsella para doxa was studied and collected by the author in the forest in Liberia.

Decorsella versus Rinorea
When flowering it is difficult to separate Decorsella from Rinorea. In several earlier publications all Rinorea species are said to have actinomorphic flowers (Munzinger & Ballard 2003: 346, 350;Tokuoka 2008: 258). Most illustrations and photographs from Rinorea indeed show more or less actino morphic flowers. This character would make it easy to separate this genus from the zygomorphicflowered Decorsella. However, more recent reports have documented a zygomorphic corolla in the Latin American Rinorea zygomorpha H. E. Ballard & Wahlert (Wahlert & Ballard 2009) and also in a few African species of Rinorea (Wahlert & Ballard 2012). The zygomorphic corollas of R. longicuspis Engl. and of R gabonensis Engl. (Fig. 4 & 5) also show without doubt that Rinorea flowers can in some species be conspicuously zygomorphic and, in the case of R. longicuspis, almost equal to Decorsella. With its diadelphous stamens, R. calcicola Velzen & Wieringa shows that the androecium in certain species of Rinorea can also be zygomorphic (van Velzen & Wieringa 2014). Decorsella still differs in flower bud from all African Rinorea species in the shape of the anterior petal. While in the bud stage, the anterior petal in Decorsella is hooded at the apex and covers the tips of the other petals. However, this character is shared by the Latin American R. zygomorpha (Ballard, pers. comm.). The number of ovules in Decorsella is much larger than in Rinorea, especially compared to Rinorea from Africa. African Rinorea species have, as far as known, 3 or 6 ovules per ovary (Wahlert & Ballard 2012;van Velzen & al. 2015). Diagnosis -Decorsella arborea differs from D. paradoxa by its larger size, smaller leaves and more slender flowers with smaller sepals and with a longer free part of the stamen filaments (Table 1).
Conservation status -Data deficient (DD). The new species is (or was) clearly relatively widespread and, being an understorey tree, it is probably undercollected. It could be threatened if the forest where it grows is disappearing on a large scale, but I have not enough data to support this.
Etymology -The specific epithet arborea refers to the more robust habit of the new species compared to Decorsella paradoxa.
Remarks -Between the two species of Decorsella there is a gap of c. 1500 km. The Decorsella specimen that was sampled as representative of the genus by Tokuoka (2008) and later used also by  was de Wilde & al. 495 (MO), a specimen here included in D. arborea. In the line drawing with the two Decorsella species (Fig. 2), only the lower petal of D. arborea shows an emarginate apex. The illustration of D. paradoxa published by Keay (1953: 77) suggests that the lower petal of the other Decorsella species can also be emarginate.