Introduction

With 17 genera and approximately 85 species, Dialioideae is the second smallest of the legume subfamilies. Dialium is a pantropical genus of c. 30 species, Labichea includes approximately 14 species, but otherwise the subfamily mostly comprises small genera with few species, eight of which are monospecific. Several of these small genera are considered threatened and have rarely been collected in nature.

All recent phylogenetic analyses place Dialioideae as sister to Caesalpinioideae + Papilionoideae. Recent analyses suggest an Oligocene crown age but no fossils have been clearly attributed to the subfamily and because of this the age of the clade has been difficult to assess with certainty. Although studies are in progress, generic level relationships within the subfamily remain poorly resolved, except for the position of the New World Poeppigia, initially considered a close relative of taxa now placed in Caesalpinioideae, which is clearly resolved as sister to the rest of the subfamily.

Key Features

Dialioideae are mostly unarmed trees or shrubs (Australian Labichea and Petalostyles). Their flowers are highly diverse, displaying multiple symmetries and widely varied numbers of floral organs. Organ loss is frequent in the subfamily. The subfamily is also unusual in that many Dialioideae have thyrsoid inflorescences, a rare condition in the primarily racemose Leguminosae. The fruit is often indehiscent and drupaceous or samaroid, also less typical of the legumes. Finally, most species of the subfamily lack vestured pits in their xylem (present in Poeppigia and Mendoravia), a feature that is otherwise present in all Leguminosae except Cercidoideae and Duparquetioideae.

Distribution and Ecology

Dialioideae occur throughout the world tropics. They are native to South and Central America, Africa, Madagascar, South and Southeast Asia, south China, Australia, New Guinea and some Pacific islands. Most species of Dialioideae occur in the Rainforest biome, but Poeppigia (New World), Eligmocarpus and Baudouinia (both Madagascan) are Succulent biome plants, and Labichea and Petalosytles occur in the Savanna biome of Australia.

Formal Botanical Description

As published in LPWG (2017), Taxon 66: 44-77, doi.org/10.12705/661.3

Subfam. Dialioideae Legume Phylogeny Working Group, stat. nov. ≡ Dialiinae H.S.Irwin & Barneby in Polhill & Raven, Adv. Legume Syst. 1: 100. 1981.

Type: Dialium L.

Unarmed trees or shrubs, rarely suffruticose (Labichea Gaudich. ex DC., Petalostylis R.Br.); specialised extrafloral nectaries lacking on petiole and leaf rachis and on leaflet surface. Stipules in lateral position, free or absent. Leaves imparipinnate, rarely paripinnate (Eligmocarpus Capuron, Poeppigia C.Presl), 1-foliolate (Baudouinia Baill., Labichea, Mendoravia Capuron, Uittienia Steenis) or palmately compound (Labichea), leaflets alternate, rarely opposite (Eligmocarpus, Poeppigia), exstipellate. Inflorescences highly branched, thyrsoid, less commonly racemes with distichous anthotaxy (Labichea, Petalostylis), borne in both terminal and axillary positions, or reduced to one axillary flower (Petalostylis); bracteoles small or absent. Flowers bisexual (polygamous in Apuleia Mart.), radially or slightly to strongly bilaterally symmetrical, hypanthium rarely present, receptacle may be broad and flattened, bearing nectary-like bodies; sepals commonly 5, reduced to 4 (Labichea, Storckiella Seem.) or 3 (Apuleia, Dialium), rarely 6 (Mendoravia), free, equal to sub-equal; petals 5 or fewer (0, 1, 3, 4), rarely 6 (petal number often equivalent to sepal number), free, equal to subequal, imbricate, the adaxial petal innermost; fertile stamens 5 or fewer, rarely 6–10 (some Dialium spp., Poeppigia), usually only antesepalous whorl present, free, uniform, rarely dimorphic (Eligmocarpus), anthers basifixed, rarely dorsifixed (Poeppigia), dehiscing via longitudinal slits, often reduced to a short apical, poricidal slit, staminodes present or absent; pollen in tricolporate monads with punctate or finely reticulate, rarely striate (some Dialium) sculpture patterns; gynoecium 1-carpellate (sometimes bicarpellate in scattered flowers of Dialium), ovary stipitate or sessile, ovules frequently 2 (1–many). Fruits commonly indehiscent drupaceous or samaroid, rarely dehiscent (Eligmocarpus, Labichea, Mendoravia, Petalostylis) or the drupaceous fruit with indurating endocarp breaking up in one seeded segments (Baudouinia). Seeds 1–2, rarely more; embryo straight. Vestured pits absent in the secondary xylem, rarely present (Poeppigia, Mendoravia); silica bodies sometimes present (Apuleia, Dialium, Dicorynia Benth., Distemonanthus Benth.); septate fibres rarely present (Apuleia, Distemonanthus, Poeppigia); storeyed rays often present. Root nodules absent. 2n = 28 (most genera unsurveyed).

To learn more

Falcão MJA, Mansano VF, Pinto RB. 2016. A taxonomic revision of the genus Dialium (Leguminosae: Dialiinae) in the Neotropics. Phytotaxa 283: 123–142.

Falcão MJA, JV Paulino, FJ Kochanovski, RC Figueiredo, JP Basso-Alves, VF Mansano. 2020. Development of inflorescences and flowers in Fabaceae subfamily Dialioideae: an evolutionary overview and complete ontogenetic series for Apuleia and Martiodendron. Botanical Journal of the Linnean Society 193: 19–46. https://doi.org/10.1093/botlinnean/boz098

Zimmerman E., Herendeen PS, Lewis GP, Bruneau A. 2017. Floral evolution and phylogeny of the Dialioideae, a diverse subfamily of tropical legumes. American Journal of Botany 104: 1019–1041.

Zimmerman, E., G. Prenner & A. Bruneau. 2013. Floral ontogeny in Dialiinae (Caesalpinioideae: Cassieae), a study in organ loss and instability. South African Journal of Botany 89: 188–209.

List of genera

Below is an alphabetical list of all genera accepted by the LPWG with links out to the taxonomic pages on our portal, GBIF and World Checklist of Vascular Plants (Kew). Over time this list will be updated to reflect the evolving taxonomy.

Please see the Species List and Synonyms and Legume Taxonomy Working Group pages for more taxonomic information. The current taxonomy is accessible by Browse or Advanced Search.