Trees or shrubs, dioecious or monoecious. Exudate (from cut stems) present, white. Armed (lateral shoots sometimes transformed into thorns) or unarmed? Glabrous throughout or indumentum present, hairs simple (multicellular, coloured). Stipules free*, simple, unlobed, entire, not spine-like*, not leaf-like*, with or without glands at base. Leaves distributed evenly along stem or crowded towards shoot tips, alternate, simple, unlobed, base symmetrical, margin toothed or entire, blade not pellucid-punctate, upper surface without glands, lower surface with a row of glands at or near margin, very rarely distributed elsewhere, also sometimes minutely papillose or whitish-powdery; venation pinnate; domatia absent*; stipel-like structures absent*. Petiole present, without swellings or bends at base or apex*, without glands. Inflorescence with axis, terminal and axillary, raceme- or panicle-like, with 0–3 female flowers in basal part, many male flowers in upper part, the male part of the inflorescence often falling away later; bracts of side branches small and undivided, floral bracts ( leaf-like or not?), with a pair of elliptical glands, rarely without glands, green; floral buds initially covered by large bracts; pedicels not articulated*. Male flowers: almost sessile or with distinct pedicel; sepals 3, free or partially fused, greenish, sometimes minute, sometimes 0; petals 0; disc 0; stamens (2–) 3–12, free or partially fused; anther without conspicuous modification, thecae extrorse in bud; pistillode absent. Female flowers: pedicel present, often elongating in fruit; sepals 3, free or partially fused , greenish; petals 0; disc 0; ovary locules 3, ovules per locule 1; styles 3, free* or partially fused*, entire, short, stigmas 3, entire, not flat*, surface smooth*; staminodes absent. Fruit often with very long pedicel, dehiscent (always in New World?), rarely indehiscent, smooth or horned, hairy or glabrous, greenish to brown; seed with or without small caruncle, with no other covering or membranous layer; embryo flat, not minute, cotyledons leaf-like.
An asterisk * indicates a state has been assumed but not yet confirmed.
Radcliffe-Smith (2001): about 45 spp; World Checklist: about 25 spp.
For this genus there is conflict between the genus description and the World Checklist. The description follows Esser’s modern, expanded concept of Gymnanthes in Radcliffe-Smith (2001), which includes many species previously treated as Sebastiania. The formal transfer of the Sebastiania species to Gymnanthes is still pending, hence the conflict with the World Checklist. In practical terms this means that where it is important to use technically correct names for species, many species that almost certainly belong to Gymnanthes must still be called Sebastiania. A similar conflict exists between Pleradenophora and Sebastiania (see under those genera). For further discussion see Esser, H.-J., Fátima de Araújo, M.F. & Alves, M. (2010). A new species of Gymnanthes (Euphorbiaceae) from northeastern Brazil. Willdenowia 40: 345-349.
With thanks to Hans-Joachim Esser (Botanische Staatssammlung München) for help with this genus.