Osodendron E.J.M. Koenen, 2022

Koenen, Erik J. M., 2022, Osodendron gen. nov. (Leguminosae, Caesalpinioideae), a new genus of mimosoid legumes of tropical Africa, PhytoKeys 205, pp. 453-470 : 453

Osodendron E.J.M. Koenen gen. nov.

Type.

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

Diagnosis.

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

Description.

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.

Distribution and habitat.

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.

Etymology.

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).

Notes.

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 ).

Figures

Figure 1. Confusing pods of Cathormion s. l. A pods of Osodendron altissimum that are very similar to B pods of the Australian species Albizia moniliformis (DC.) F. Muell. (syn. Cathormion umbellatum subsp. moniliforme (DC.) Brummit) C pods of Osodendron dinklagei and D pods of Osodendron leptophyllum that were likened to those of E Samanea (S. saman (Jacq.) Merr. is pictured here) and / or Cathormion F pods of Hydrochorea obliquifoliolata (De Wild.) E. J. M. Koenen (syn. Cathormion obliquifoliolatum) that are superficially similar to those of O. leptophyllum in particular; hence, this species having also been classified in a broadly circumscribed Cathormion in the past, but note that the pods become disarticulated while still on the tree rather than tardily as in Osodendron and Albizia moniliformis G and H pods of the closest relative of Osodendron, the Neotropical genus Robrichia (R. schomburgkii is pictured here). Images: A Jos Stevens, voucher unknown B Russel Cumming, voucher unknown C J. Alves Pereira 2664 (MA) D R. Gutzwiller 1865 (P) E J. F. Maxwell 91 - 300 (L) F B. Descoings 8278 (MPU) G Cristoph Moning, voucher unknown H Jose Benito Quezada Bonilla, J. B. Quezada Bonilla 623 (HULE, MO). A from www. africanplants. senckenberg. de B-H from www. gbif. org B-G distributed under a Creative Commons BY-NC-SA 4.0 Licence H distributed under a Creative Commons BY-NC-SA 3.0 Licence.