Herbs, mostly annual. Leaves opposite or spuriously whorled, exstipulate. Flowers small, unisexual, sessile or pedicellate, in axillary cymes. Perianth sepaloid, green or slightly coloured, 5-partite; lobes equal. Stamens 5-15, free; filaments subulate, dilated at base. Carpels 3-5, free, 1-celled, sessile on a small torus; ovules solitary; styles short, recurved. Fruit a schizocarp of 5 free, 1-seeded, compressed achenes. Seeds crustaceous; embryo annular, enclosing endosperm.