Monoecious shrubs or small trees, glabrous throughout, the stipules small; leaves along main branches spirally arranged and reduced, distichous along ultimate branchlets, short-petiolate, simple, the blades entire; inflorescences axillary, the lower axils bearing fascicles of 3-many male flowers, the higher axils bearing solitary female flowers, the flowers lacking petals and disk; male flowers with the calyx fleshy, obovoid-turbinate, shortly 6-lobed, the stamens 3, the filaments connate into a column bearing 2-locular anthers apically, these not apiculate; female flowers with the calyx campanulate to cupuliform, 6-lobed or "toothed, persistent and sometimes accrescent, the ovary 3-locular, the walls thickened distally, each locule with 2 ovules, the stigmas small, subulate-dentiform, incurved; fruit a capsule dehiscing into 2-seeded cocci, the seeds with a somewhat fleshy testa, ventrally invaginated.
Breynia are of special note in the fields of pollination biology and coevolution because they have a specialized mutualism with moths in the genus Epicephala (leafflower moths), in which the moths actively pollinate the flowers—thereby ensuring that the tree may produce viable seeds—but also lay eggs in the flowers' ovaries or in the space between the tepals and the carpel walls, from where their larvae consume a subset of the developing seeds as nourishment.[4][5] Other species of Epicephala are pollinators, and in some cases, non-pollinating seed predators, of certain species of plants in the genera Phyllanthus[6][7] and Glochidion,[8][9][10] both closely related to Breynia.[11] This relationship is similar to those between figs and fig wasps and yuccas and yucca moths.
