TANZANIA. Katavi region: Mpanda district, 19 km on Mpanda-Uvinza road, seepage areas in tall Julbernardia paniculata, Terminalia mollis woodland, grey and sandy soils, 6°14'S, 30°59'E, 1100 m, 14 May 1997 (fl & fr) , S. Bidgood, D. Sitoni, K. Vollesen & C. Whitehouse 3935 (Holotype: K! [K000771858!]; isotypes: K! [K000771858!]; BR! [BR0000013189358!], C!, EA!, P! [P05222349!]).

Figure 1. 

Argyrella richardsiae, A habit B glandular trichomes C leaf D flower E stamens: inner stamen (left), outer stamen (right) F seed (drawn from Mrs H.M. Richards & S. Arasululu 26190 and Bidgood et al. 3935). Illustration by Doris Franke.

The new species differs from all other species of Argyrella by having only glandular trichomes throughout the whole plant (versus a mixture of stellate and glandular trichomes in the other species), secondary branches arising at each node and leaves generally pointing downwards. It resembles A. bambutorum (Gilg & Ledermann ex Engl.) Veranso-Libalah & G.Kadereit but differs by having leaf-blades with serrulate margins (versus entire margins in A. bambutorum) and the primary pair of lateral nerves disappearing half-way between the base and the apex (versus conspicuous lateral nerves percurrent from the base to the apex in A. bambutorum). Argyrella richardsiae also differs from the widely distributed A. canescens and the Angolan endemic A. angolensis (Cogn.) Veranso-Libalah & G.Kadereit by having many new branches or buds arising at each node (versus unbranched herb in A. angolensis and A. canescens) and leaves sessile with amplexicaul bases generally pointing downwards (versus leaves petiolate with rounded to cordate bases and generally pointing upwards in A. angolensis and A. canescens) and only glandular trichomes on the hypanthium (versus dense stellate and glandular trichomes on the hypanthium of A. angolensis and A. canescens).

Erect herb up to 1 m tall with branches arising at each node (Fig. 1A); stems quadrangular, covered with glandular trichomes (0.3–0.7 mm) (Fig. 1B). Leaves sessile, broadly ovate, generally pointing downwards; lamina 15–35 × 7–22 mm, covered with sparse glandular trichomes on both surfaces, apex acute, base amplexicaul, margins serrulate; principal nerves 5–7, lateral nerves fading about half-way from the base and never reaching the apex on the adaxial surface but reaching the apex on the abaxial surface (Fig. 1C). Inflorescence a terminal panicle of cymes with 15–25 flowers or axillary with 5–10 flowers (Fig. 1D). Two caducous bracts, 4–6 × 3–5 mm, pink–mauve, covered with glandular trichomes and enclosing the calyx-tube. Calyx-tube campanulate, 2.5–6 mm in diameter, covered with glandular trichomes. Calyx-lobes 5, triangular, 4.5–6 mm long, persistent, margins and dorsal surface covered with glandular trichomes. Petals 5, pink, 9–13 × 7–8 mm, obovate. Stamens 10, markedly unequal, anthers mauve-purple, filaments yellow, pedoconnectives pink-mauve, appendages yellow. Outer stamens 16–18 mm long, anthers 5–7 mm, filaments 5–7 mm, pedoconnective 7–8.5 mm, strongly curved, appendage ventrally tri-cuspidate, 1–2 mm (Fig. 1E). Inner stamens 9–13 mm long, anthers 3.5–5 mm, filaments 4–5 mm, pedoconnective ca. 1.5 mm long, appendage ventrally bilobed, ca. 0.5 mm (Fig. 1E). Style 22–25 mm long, glabrous. Stigma punctate. Fruit a capsule, dehiscent, ca. 6 mm long. Seeds cochleate, ca. 0.6 mm in diameter, numerous (Fig. 1F).

ANGOLA. Huambo province: Longonjo, Lépi, Caála 1700m, 3 August 1940, (fr), J. Gossweiler 12147 (LISC030751!, LISC030752!, LISC030753!, LISC030754!, LISC030755!, LISC030756!). TANZANIA. Mpanda district: Uruwira-Tabora road, Brachystegia woodland, Kambisama river, 1400 m, 30 September 1970 (fl & fr), Mrs H.M. Richards & S. Arasululu 26190 (K!, BR!); Mlele beekeeping reserve, riverine woodland, Iloba river, 6°47'56"S, 31°37'33"E, 1562 m, 7 May 2004, N.A. Mwangulango 1193 (MO, MJG!).

The species epithet is in honour of Mary Alice Eleanor Richards (also known from her collection labels as Mrs H.M. Richards), who collected extensively in Africa from 1951 to 1974 (Polhill and Polhill 2015). Of our new species Argyrella richardsiae, she made a collection which is cited above in ‘Additional specimens examined’.

This species is evidently endemic to the wet miombo woodlands of Mpanda district, southwestern Tanzania and Huambo province, central Angola (Fig. 2). Miombo woodland is a significant biome covering about 10% of the African landmass (White 1983; Campbell et al. 1996, 2007). Miombo woodlands are mainly found in southern and central African countries, and are the dominant vegetation component of Angola, Zambia, Tanzania, Malawi, Mozambique and Zimbabwe (Malmer 2007, White 1983, Campbell et al. 1996, 2007). They are mainly dominated by Brachystegia Benth., Julbernardia Pellegr. and Isoberlinia Craib & Stapf trees of the subfamily Caesalpinioideae, Leguminosae. The wet miombo is found in areas of more than 1000 mm annual rainfall with an elevation of 1000–2500 m. A predominant wet miombo woodland vegetation is composed of riverine woodland along watercourses and marshes in poorly drained and/or low-lying areas, mainly characterised by alluvial soils (Campbell et al. 2007, Lupala et al. 2015). Also, wet miombo has higher tree height (typically > 15 m) and higher floristic diversity which mainly occurs in the northern part of miombo distribution: eastern Angola, northern Zambia, southwestern Tanzania and central Malawi (Frost 1996). It is likely that A. richardsiae also occurs in wet miombo woodlands of southern Democratic Republic of Congo (DRC), northern Zambia and southern Malawi and not just disjunctly between Angola and Tanzania. Argyrella richardsiae like the other Argyrella species grows in marshes.

Figure 2. 

Distribution map of Argyrella species.

Proposed IUCN Red List Category: Endangered (EN): B2ab (ii,iii) EOO ~79 km², AOO 32 km². This species is only known from four collections in seepage, marshes or riverine woodlands (wet miombo woodlands) of the Mpanda district, Tanzania and Huambo province of Angola. Although cited on the collection label Mwangulango 1193 as a locally frequent herb in the Mpanda district, it is still a poorly collected species. As earlier suggested, A. richardsiae may also occur in wet miombo woodlands of southern DRC, northern Zambia and southern Malawi, but at the moment we think it is better to treat it as endangered until we are certain that this species is found in other places. Moreover, this species is only known from one collection in Angola since 1940. Generally, miombo woodlands are an important source of livelihood because they provide social, economic, and environmental benefits such as firewood, timber, medicinal plants, food, and catchment protection, among others. According to Campbell et al. (2007) over 75 million people inhabit areas covered, or formerly covered, by miombo woodland, with an additional 25 million urban dwellers relying on miombo wood or charcoal as a source of energy. As a result, these woodlands have been and are being depleted for the harvesting of timber used for charcoal production, conversion to farmlands and fuel-wood extraction (Campbell et al. 2007, Lupala et al. 2015).

KX889285 (ITS), KY248410 (psbK-psbL), KY284711 (accD-psaI) (see Veranso-Libalah et al. 2017).

Argyrella richardsiae is similar to A. bambutorum but differs by having serrulate leaf margins, intersepalar appendages absent, and stamens dimorphic (versus entire leaf margins, intersepalar appendages present, and stamens isomorphic in A. bambutorum). The new species also differs from A. amplexicaulis, A. sessilis (Hutch. ex Brenan & Keay) Veranso-Libalah & G.Kadereit and A. angolensis by its many new branches or buds arising at each node (versus unbranched in A. amplexicaulis, A. sessilis and A. angolensis). Argyrella richardsiae has broadly ovate leaves versus lanceolate to linear in Argyrella canescens and A. linearis (Jacq.-Fél.) Veranso-Libalah & G.Kadereit. Also, A. richardsiae has only glandular trichomes on the entire plant and leaves generally pointing downwards versus a mixture of stellate and glandular trichomes with leaves pointing upwards in the other species.