Osodendron altissimum (Hook. f.) E.J.M. Koenen.

Osodendron differs from Robrichia by pods being either straight to twisted or coiled, but not contorted and ear-shaped.

Trees , or rarely shrubs, unarmed or sometimes with spine-like projections on twigs or spine-like outgrowths on adventitious roots, resting buds perulate with scales distinctly different from stipules. Indumentum of usually dense ferruginous pubescence on twigs, petioles, rachis and pinna rachises, stipules, bud scales and peduncles. Leaves with (3–)5–30(–34) pairs of pinnae, opposite or sometimes (the lowermost pairs) sub-opposite, with a single petiolar nectary usually present, as well as nectaries between at least some of the pinna pairs on abaxial surface of the rachis and often also between the upper 1–many leaflet pairs on the pinna-rachises, the lowermost pair of pinnae usually distinctly shorter than others, pinnae with (7–)13–40(–48) leaflet pairs, one of the two leaflets of the lowermost pair usually reduced to a small paraphyllidium or lacking. Inflorescences sub-globose capitula, dimorphic, borne on peduncles arising from axillary fascicles, sometimes arranged in short compound pseudoracemes with the leaves suppressed (not fully developing) and caducous as apparent from the presence of leaf scars in the pseudoracemes, these pseudoracemes developing below the foliage. Flowers sessile or shortly stipitate, 4- or 5-merous, with fused calyx and corolla, androecium consisting of 10–25 stamens that are fused in the lower part to form a staminal tube, pollen compound in 32-celled polyads, central flowers more robust with a broader nectariferous base and longer staminal tube exserted beyond corolla tube. Fruits septate, indehiscent or tardily breaking up into articles, either lomentiform, twisted and strongly curved to coiled or weakly to not articulate and slightly curved to straight, seeds with a hard testa and open or closed pleurogram.

Three species in tropical Africa, from Senegal in the west to the Democratic Republic of Congo in the east and Zambia and Angola in the south. Typically occurring in rainforest and extending into the savannah zone in gallery forest.

The genus is named after “Oso”, a food that is prepared in West Africa (Ghana and Nigeria) by fermenting the seeds of the type species O. altissimum and grinding them into a protein-rich paste that is subsequently cooked and eaten as either a main food, a delicacy or as a condiment to flavour soups and stews (Popoola et al. 2004; Jolaoso et al. 2014).

Osodendron is closely related to the Central and South American Robrichia, which was originally described as a section of Enterolobium by Barneby and Grimes (1996) and later segregated as a new genus by Souza et al. (2022). In the phylogeny of Ringelberg et al. (2022), the sampled species of both genera form sister-lineages. In habit, leaves and flowers, the two genera are very similar; however, the numbers of pinnae per leaf and leaflets per pinna and leaflet dimensions show greater variation in Osodendron with especially relatively large leaflets in the type species compared to Robrichia. The clearest difference between the two genera is to be found in the fruits, where those of Robrichia are contorted (or “ear-shaped”) and indehiscent, while those of Osodondron are either straight to falcate and indehiscent or twisted to spirally-coiled and lomentiform, only tardily breaking up into articles.

Species of Osodendron can be easily distinguished from African Albizia species by the sub-opposite lower few pinna-pairs of which the lowermost is usually distinctly shorter (half or two thirds the length of the next few pairs of pinnae), as well as the strong reduction or lack of one of the two leaflets of the lowermost leaflet pair on each pinna (in Albizia, the lowermost pair of pinnae is only slightly shorter at most and the lowermost leaflet pair is not reduced or lacking one leaflet). The fruits are also notably different, with the inertly dehiscent pods of continental African Albizia spp. never woody nor articulate and always flat and papery (although in several Asian, Australian and Madagascan Albizia spp., indehiscent woody or tardily dehiscent articulate pods occur, for example, Fig. 1B; Table 1).

Albizia altissima Hook. f. Niger Fl. [W. J. Hooker]. 332. 1849.

Ghana, Cape Coast, 7/41, T. Vogel 18 (lectotype here designated from amongst the syntypes: K!); Nigeria, Aboh, 8/41, T. Vogel 28 (syntype: K!).

Figure 2. 

Osodendron altissimum A distribution of the two subspecies as per the legend, based on data from Gbif.org with the addition of occurrence records for O. altissimum subsp. busiraensis from specimens at BR B trunk on riverbank C bipinnate leaf D close-up of leaflets E close-up of rachis apex showing nectary F inflorescences G unripe pod H mature pod. Images: B, F, H William Hawthorne (FHO) C–E, G David Harris (E), from www.africanplants.senckenberg.de.

Tree or rarely shrub, unarmed or sometimes with blunt spine-like projections on young twigs (W. Hawthorne, personal communication), 5–35 m tall, stem to 40 cm in diameter. Branches covered with many minute round lenticels 0.1–0.2 mm, twigs, stipules, bud scales, all leaf axes and peduncles brown pubescent, often densely so. Foliage consisting of bipinnate leaves with opposite to sub-opposite pinnae with closely-spaced, distinctly discolourous leaflets, slightly wider in the middle with the apical pair distinctly distally projected, in the basal pair often one or both leaflets reduced to paraphyllidia. The inflorescences consisting of sub-spherical capitula emerging in axillary fascicles, often aggregated into pseudoracemes emerging from the perulate resting buds, with the leaves not fully developing and caducous. Stipules deltoid to linear, 2–8 ×1.0–1.8 mm, caducous, perulae deltoid, 2–4 × 1.8–3 mm. Leaves with (3–)5–7(–8) pairs of pinnae, apical pairs usually slightly longer and the basal pair sometimes much shorter with fewer leaflets, petiole 1.3–2.5 cm, pulvinate, adaxially shallowly canaliculate and often laterally and abaxially grooved, with a circular to elliptic sessile cupular nectary ca. 0.8–2.1 mm in diameter located ca. mid-petiole, rachis (1.8–)4.5–11(–14.5) cm, adaxially shallowly canaliculate and often laterally and abaxially grooved, with 2–5 circular to elliptic sessile cupular nectaries between or just below the distal pairs of pinnae ca. 0.5 mm in diameter, pinna-rachises (2.4–)3.5–8(–10.2) cm, proximally pulvinate, laterally compressed, with elliptic to circular sessile cupular nectaries of 0.2–0.8 × 0.2–0.5 mm between the upper 1–4(–15) leaflet pairs, pinnae with (7–)13–20(–25) leaflet pairs, very often the abaxial leaflet of the lowermost pair reduced to a paraphyllidium, leaflets sessile, pulvinate, inequilaterally hastate or weakly sigmoid, with acute or sometimes obtuse apex or (in subsp. busiraensis) weakly rhombic with oblique base and mostly obtuse apex or sometimes apiculate, (4–)7–14(–18) × (1.5–)3–4 mm, asymmetrically palmately-pinnate secondary venation with 7–10 veins starting from leaflet base next to the mid-rib and (7–)15–22 major and intercalary secondary veins laterally from the mid-rib, brochidodromous to craspedodromous, distinctly prominent on both surfaces when dry (or obscure on adaxial surface in subsp. busiraensis), tertiary venation finely reticulate, hardly visible on adaxial surface. Capitula ca. 35–50-flowered, on peduncles 1.8–3.2 cm, dimorphic with 3–9 central flowers with broader base and longer staminal tubes, bracts lanceolate to lanceolate-spatulate, ca. 0.9–1.2 × 0.1–0.2 mm, ciliate in upper half, caducous. Peripheral flowers slenderly campanulate with pale green or greenish-white calyx and corolla, subsessile to shortly pedicellate, pedicel to ca. 0.5 mm, sparsely pubescent. Calyx 5-merous, 2–3.2 mm long, the deltoid lobes ca. 0.5 × 0.5 mm, ciliate at the apex and often also a few scattered short hairs on the outer surface of calyx tube and lobes, otherwise glabrous, corolla 5-merous, 5–7.5 mm long, glabrous, androecium of ca. 20–25 stamens, filaments white, ca. 12–17 mm long, fused into a tube at the base for ca. 2–3 mm, anthers yellow, basifixed, pollen aggregated into 32-celled flattened polyads. Pistil ca. 13–18 mm long, the ovary ca. 3 mm with a few scattered hairs on distal half and the funnel-shaped stigma extending ca. 1 mm beyond the anthers. Central flowers similar, but sessile, broadly campanulate and staminal tube ca. 5.5–8.5 mm long (i.e. the stamens are approximately the same length, but the fused tube is longer than in peripheral flowers), the tube exserted beyond the corolla. Fruit a spirally-curved, twisted or coiled lomentiform pod, usually articulate between the seeds, breaking up tardily after abscission in water or on the forest floor, often with arenchymous swollen mesocarp for flotation, (10–)18–29(–30) × (0.8–)1.1–1.7 × 0.2–0.6 cm, (3–)9–20(–23) seeded. Seeds plano-compressed, nearly round or slightly elliptic, ca. 5–7.5 × 4–6 × 1.5–2 mm, chestnut brown with a small 2.1–3.2 × 1–1.5 mm elliptic to obovate or oblong open pleurogram.

A specimen seen at Kew, of Espírito Santo 1858, collected in Guinée-Bissau, is doubtfully placed in this species. This specimen is highly unusual in the number of pairs of pinnae (13) and leaflet pairs (14–35) and the leaflets appear relatively small and elongate (even though they appear not fully expanded), while having the typical asymmetrical hastate shape. Furthermore, the fruit is not clearly articulate and also differs in being only somewhat curved rather than twisted or coiled. Brenan and Brummitt (1965) also cited this material as being unusual and speculated that it may be a distinct taxon. I do not share this opinion as I do not think it is sufficiently morphologically distinct. It is perhaps more likely of hybrid origin with O. dinklagei, which occurs sympatrically, as the other parent. However, this unusual specimen resembles O. altissimum much more closely than O. dinklagei and the number of pinnae and leaflets and dimensions of the leaflets clearly distinguish it from that species.

The holotype of Pithecellobium stuhlmannii was destroyed at B and no surviving isotypes are known. According to the Flora of East Tropical Africa (Brenan 1959), the original material was collected in the Democratic Republic of Congo, west of Lake Albert. The specimen of Bequaert 4887 from BR is from the same region, includes leaves, flowers and immature fruit and matches the description in the protologue and is here selected as the neotype of P. stuhlmannii.

The holotype of Albizia passargei was also destroyed at B, but Villiers (1989) placed the name in synonymy with C. altissimum, based on the description in the protologue. A specimen of Letouzey 2562 from P, comprising leaves and flowers, is here chosen as the neotype as it was collected in N Cameroon (ca. 150 km SW of the type locality) and matches the description in the protologue.

Widespread across Upper and Lower Guinea and Congolia, from Guinea-Bissau in the west to the Democratic Republic of Congo in the east and Angola in the south (Fig. 2A), also reported from Uganda and Zambia (Brenan 1959).

Fresh-water swamp forest and along rivers in primary and secondary, evergreen or semi-deciduous rainforest.

Guinée-Bissau: Espírito Santo 3810 & 3832 (frts., BR); Côte d’Ivoire: Oldeman 968 (fls, BR); Cameroon: A.J.M. Leeuwenberg 8868 (fr., BR), B. Sonké 5499 (fr., BR); Gabon: N. Hallé 2119 (fls, frts, BR); Democratic Republic of Congo: F. Seret 461 (fls, BR);

Arthrosamanea altissima var. busiraensis G.C.C. Gilbert & Boutique, Fl. Congo Belge 3: 183. 1952.

Democratic Republic of Congo, Eala surroundings, Wangata-Watsiko, October 1943, Germain 1604 (holotype: BR! [BR0000015846686], isotype: P [P00418269] digital image!).

This subspecies was originally described as an infraspecific variety. I have considered treating it as a distinct species, but after studying material held at BR, I concluded it cannot be reliably separated from the typical subspecies across its entire range. The material stands out in its combination of slightly different leaf shape and smaller pods, but material outside the Busira river catchment in the Democratic Republic of Congo, as well as in other parts of Africa, is rather variable in leaf shape and pod size. Given that both these differences are consistently present in most of the material from that region, i.e. the variation is geographically structured within this variable and widespread species, subspecies is more appropriate than varietal rank and I treat it here as such.

Local to the Busira river catchment in the western Democratic Republic of Congo (Fig. 2A).

As the typical subspecies.

Democratic Republic of Congo: Boyekoli Ebale Congo Expedition 2010 104 (fls, BR); J. Louis 2052 (BR) & 13196 (BR); Robyns 653 (BR); Ghesquiere 2787 (BR); G. Hulstaert 607 (BR).

Mimosa dinklagei Harms, Bot. Jahrb. Syst. 26(3–4): 253. 1899.

Liberia, Grand Bassa, 21 May 1897, Dinklage 1827 (holotype: B†, isotype: K! [K000044068]).

Figure 3. 

A–F Osodendron dinklagei: A trunk of adult specimen in terra firme forest B foliage C detail of pinnae D detail of resting bud and petiolar nectary E inflorescence F pods G distribution map of O. dinklagei and O. leptophyllum as per the legend based on data from Gbif.org H–K O. leptophyllum: H trunk I bipinnate leaf J close-up of leaflets K twig and pods. Images: A–D, F William Hawthorne (FHO) E Xander van der Burgt (K) H–J David Harris (E) K Paul Latham H–K from www.africanplants.senckenberg.de.

Tree , up to 30 m tall, twigs with rows of lenticels and becoming shallowly grooved with age. Twigs, stipules, perulate resting buds, all leaf-axes and peduncles covered in a densely villose ferruginous indumentum. Foliage consisting of very finely divided bipinnately compound leaves, the tiny leaflets ciliate and somewhat discolourous. The inflorescences consisting of sub-spherical capitula emerging from axillary fascicles of coeval leaves, sometimes in compound ramiflorous pseudoracemes with the leaves not fully developing and caducous. Stipules narrowly deltoid or lanceolate to linear 3.5–4.3(–9) × ca. 1 mm, caducous leaving conspicuous glabrous scars, perulae deltoid, ca. 2 × 1.5 mm. Leaves with 10–34 pairs of pinnae, often with one or a few much reduced pinnae at the base, petiole 5–11 mm, pulvinate and adaxially flattened at the base, usually with a sessile or shortly stipitate cupular nectary 0.5–0.7 mm in diameter just below the lowermost pinna pair, rachis grooved and adaxially canaliculate although obscured by dense indumentum, with a 1–1.5 mm long deciduous mucro, 6.5–16(–21) cm, with 3–8(–11) interpinnal subsessile cupular nectaries apically, these sometimes button-shaped and on a 0.7 mm long stipe, pinnae obscurely pulvinate, with an adaxial ridge and a tiny 0.3–0.4 mm long mucro at the apex, 1.0–2.5(–4.5) cm, usually without nectaries, except on larger leaves (of juveniles or coppice shoots) with cupular nectaries between the upper 1–3 leaflet pairs, pinnae with (9–)27–40(–48) leaflet pairs, the abaxial leaflet of the lowermost pair usually reduced to a paraphyllidium, leaflets inequilaterally linear, sessile and pulvinate, with strongly oblique base and acute apex, 0.8–3 × 0.2–0.8 mm, venation obscure on the adaxial surface, palmately-pinnate brochidodromous, with three secondary veins emerging from the base next to the clearly stronger mid-rib on the adaxial side, 2–3 further secondary veins on each side and sparse reticulate tertiary venation. Capitula 1–3 per leaf axil, ca. 40–60-flowered, on peduncles 1.8–3.5(–4.1) cm, dimorphic with 1(–3) central flower(s) that are more robust and with a longer, exserted staminal tube. Peripheral flowers ca. 0.7 mm broad at the base, the basal ones on a pedicel of ca. 0.4–0.6 mm, the others sub-sessile to sessile closer to the apex. Calyx slenderly campanulate, green, 5-merous, 1.6–3.1 mm long, densely puberulous on outer surface, the deltoid lobes ca. 0.4 × 0.4 mm, corolla green, 4- or 5-merous, 3–5.5 mm long, pubescent, the lobes 1.8–2 mm long, stamens 10–14, filaments white, 14 mm long, basally fused into a tube ca. 3 mm long, anthers light yellow, basifixed, pollen released in 32-celled polyads, pistil ca. 18 mm long with a sessile ovary ca. 2.5 mm long, puberulent on upper half with white hairs, stigma shallowly funnel-shaped. Central flower(s) similar to peripheral flowers, but ca. 1.8 mm broad at the base, calyx 2.0–2.4 mm long, corolla ca. 4.5 mm long, staminal tube ca. 6 mm long (i.e. the stamens are approximately the same length, but fused for a greater part than in peripheral flowers). Fruit an indehiscent woody pod up to 13–18 × 2–2.2(–2.5) × 0.3–0.6 cm, straight or slightly curved and with uniformly thick valves, except for the distinctly thickened margins, not swollen over the seeds, 16–24 seeded when well-fertilised, seeds rounded elliptical, only slightly laterally compressed, greenish light brown, 6–7.5 × 4–5 × 3–3.5 mm when dry, with a hard testa and an elongate closed pleurogram 5–6 × 2–2.5 mm.

Upper Guinea, from Guinea-Bissau to Ghana (Fig. 3G).

Rainforest, gallery forest, wooded grassland, edges of mangrove swamp.

Guinée-Bissau: Espírito Santo 1747 (K) & 1864 (K) & 1871 (K) & 2697 (K). Sierra Leone: X. van der Burgt 1994 (K).

Albizia leptophylla Harms, Bot. Jahrb. Syst. 53(3–5): 455. 1915.

Democratic Republic of Congo, Kimuenza, 17 km S of Leopoldville, August 1910, Mildbread 3520 (lectotype (designated by Villiers (1989) as holotype, here corrected): HBG [HBG519160] digital image!, isolectotypes: HBG [HBG519161] digital image!, P [P00418364] digital image!).

Tree or more rarely a shrub, unarmed or with spine-like outgrowths near the base of the bole perhaps associated with adventitious roots (W. Hawthorne, personal communication), to 30 m tall and to 1.25 m in stem diameter, twigs dark with densely scattered small pustular lenticels or sometimes with lighter coloured corky bark. Twigs, stipules, perulate resting buds, all leaf-axes and peduncles covered in dense ferruginous villose indumentum. Foliage consisting of finely divided bipinnate leaves, the leaflets variably ciliate, sometimes with only very few hairs on the margins, the lamina usually glabrous, but sometimes also with a few scattered hairs along the mid-rib on lower surface or rarely pilose or with appressed long hairs on both surfaces especially in lower half, usually distinctly discolourous. The inflorescences consisting of sub-spherical capitula emerging from axillary fascicles of coeval leaves, usually a few below the leaves in the axils of caducous leaves on the same shoots or sometimes also with a few short ramiflorous pseudoracemes, with all leaves caducous, lower down on the branch. Stipules elliptically oblong to asymetrically oblanceolate or falcate, 5–8(–9) × 2–3 mm, more scarcely pubescent or nearly glabrous on adaxial surface, caducous leaving conspicuous scars, perulae deltoid to ovoid, ca. 1.5–5 × 1.5–3.5 mm. Leaves with (10–)12–30(–35) pairs of pinnae, the lower 1–2 pairs of pinnae often shorter with fewer leaflets, petiole pulvinate and flattened on adaxial side at the base, (0.5–)0.8–2.2(–3) cm long, petiolar nectary usually present, cupular and circular, sessile or shortly stipitate and 0.6–1 mm in diameter, in variable position ranging from just above the pulvinule to just below the basal pair of pinnae, rachis canaliculate although often obscured due to dense pubescence, 8–15(–22) cm long, with 1–4(–6) cupular nectaries between the upper pairs of pinnae, these 0.3–0.6 mm in diameter, pinnae pulvinate and with an adaxial ridge, (0.5–)2.5–5(–7.5) cm long, nearly always the abaxial leaflet of the lowermost pair reduced to a paraphyllidium and the adaxial one somewhat smaller than the other leaflets, with elliptical nectaries ca. 0.1–0.25 mm in diameter between the upper 1–3 leaflet pairs, pinnae with (17–)24–35(–42) leaflet pairs, leaflets sessile, pulvinate, asymmetrically oblong to oblanceolate, base oblique to sometimes hastate, apex rounded to acute, (2–)3–6(–9) × (0.3–)0.5–1.2(–2) mm, venation palmately-pinnate brochidodromous, with 2–4 basal veins adaxially next to the mid-rib and 5–8 further secondary veins on each side and reticulate tertiary venation, apart from the mid-rib, all venation often obscure in smaller leaves, otherwise the secondary venation prominent, sometimes only on the lower surface. Capitula 2–4 arranged in fascicles, on peduncles ca. 2.7–4.5 cm, dimorphic with ca. 50–70 peripheral flowers and a single elongated central flower. Bracteoles spatulate, ca. 0.5 mm long, pubescent on outer surface. Flowers white, the peripheral ones on pedicels 0.75–2.5 mm. Calyx slenderly campanulate, green, 5-merous, 2–2.5 mm long, densely puberulous on outer surface, the deltoid lobes ca. 0.25 mm long, corolla 4–4.5 mm long, pubescent on outer surface, the lobes 1.8–2 mm long, stamens 10–14, the filaments white, 11–14 mm long, basally fused into a tube 1.5–2.5 mm long, anthers basifixed, pollen released in 32-celled polyads, pistil on a ca. 0.25–0.75(–1.25) mm long stipe, ovary ca. 1–1.5 mm long, pubescent, style 12–16 mm long, stigma shallowly funnel-shaped. Central flower(s) similar to peripheral flowers, but sessile and longer, calyx ca. 2.5 mm long, corolla ca. 6–6.5 mm long, staminal tube 4–6 mm long, exserted beyond corolla tube. Fruit a dark brown to black indehiscent pod that may tardily disintegrate into 1–multiple-seeded parts, the slightly thickened margins usually straight or also often articulate especially around aborted seeds, the valves papery and thin or with somewhat thickened mesocarp, but not really ligneous, when ripe, swollen over the seeds, when well-fertilised, 12–28(–32)-seeded, (4.5–)10–16(–20) × 0.9–1.3 × 0.4–0.7 cm. Seeds yellowish-brown, 5.5–7.5 × 3–4.5 × 2–3 mm with hard testa and a darker closed pleurogram of 4.5–5.5 × 1.2–1.5 mm.

Lower Guinea and Congolia in Cameroon, Central African Republic, Gabon, Republic of the Congo, Democratic Republic of Congo, Angola (Fig. 3G).

Forest edges and gallery forest.

Cameroon: J. Mildbread 8584 (K); Democratic Republic of Congo: Luidi, Thysville, Lusolo, 16 October 1959, P. Compere 610 (BR), Isangi, 21 February 1950, Callens 2363 (BR), Petite vallées d’affluente de la Belanzovi, près son confluent avec la Lubimbe, 26 January 1949, A. Michelson 877 (BR, K).

Osodendron leptophylla is highly variable in number of pinnae per leaf, number and size of leaflets and density and length of the indumentum. No consistent correlations between any of these variations have been found suggesting that the material referred to two heterotypic synonyms is best included under O. leptophyllum. The first of these is Albizia eriorhachis, described by Harms in the same publication as A. leptophylla. Specimens from Cameroon, Central African Republic and Gabon, identified as Cathormion eriorhachis, appear to be similar to typical O. leptophyllum, but with relatively large leaflets and denser indumentum. However, the material is not distinct enough to merit recognition as a separate species, given the large variation in leaf dimensions and indumentum in O. leptophyllum across its range and is here placed in its synonymy.

The second is Samanea guineensis, which was originally described as a variety of Arthrosamanea leptophylla, based on more numerous pairs of pinnae and leaflets per leaf, but Brenan and Brummitt (1965) found it to be distinctive enough to recognise it at species rank. Apart from the difference in foliage, they noted differences in the indumentum of the calyx and corolla (glabrous to subglabrous, instead of densely puberulent) and a longer exserted staminal tube. I have observed material that fits their observations (e.g. Jean Louis 10051, BR), but in other collections with the leaf type of “guineensis”, I have observed dense pubescence on the calyx (e.g. Michelson 877, BR) and vice versa specimens with ca. 15 pairs of pinnae and a subglabrous calyx (Letouzey 9816). Villiers (1989) also judged S. guineensis to be conspecific with S. leptophylla, having also seen numerous specimens with intermediate foliage. I follow Villiers (1989) and place this name in the synonymy of O. leptophyllum.