Arcoa Urb., Repert. Spec. Nov. Regni Veg. 19: 4. 1923.

Figs 7, 8

Type.

Arcoa gonavensis Urb.

Description.

Shrub or small tree, with well-developed brachylasts. Stipules spinescent, caducous. Leaves pinnate when juvenile, bipinnate with a terminal pinna when mature, leaflets opposite to subopposite, sessile. Inflorescence a sparsely branching panicle of spikes arising from a thickened woody brachyblast. Flowers unisexual, staminate flowers with a prominent pistillode, pistillate flowers with staminoidia; sepals free to a very short hypanthium; petals 5 (6); a short cupuliform disk present centrally; stamens (or staminodes) number more than twice sepal number (12 or more per flower); pollen markedly irregular and coarsely reticulate, porate with prominent pores; ovary with an appressed rust-coloured indumentum, stigma terminal and capitate. Fruits oblong-ellipsoid, subterete, thick-walled, indehiscent, 1-few-seeded. Seeds ovate, compressed, surrounded by copious pulp, pleurogram lacking (Fig. 7A).

Chromosome number.

Unknown.

Included species and geographic distribution.

Monospecific ( A. gonavensis), endemic to Hispaniola (Dominican Republic, Haiti, and the Island of Gonâve) (Fig. 8).

Ecology.

Arid tropical vegetation on limestone hills and cliffs.

Etymology.

Named for Count George von Arco (ca. 1903).

Human uses.

Unknown.

Notes.

Originally placed by Urban (1923) in the Euphorbiaceae, Arcoa was subsequently transferred to the Leguminosae by Urban (1928). The genus was placed in the eclectic Dimorphandra group of Caesalpinieae by Polhill and Vidal (1981), along with Tetrapetrocarpon, both with unisexual flowers and petals not covered by the sepals in bud, but phylogenetic analyses clearly resolved Arcoa as part of the " Umtiza clade", either sister to the Gleditsieae genera (Herendeen et al. 2003b) or sister to those of Ceratonieae (Bruneau et al. 2008), the latter as in Ringelberg et al. (2022).

Taxonomic references.

Lewis (2005b); Urban (1928).