Annual or perennial herbs with stinging hairs and opposite petioled leaved with 3-7 nerves, stipitate. Our species dioecious or monoecious, the flowers clustered in geminate racemes or heads. Staminate calyx deeply 4-parted and with 4 stamens. Pistillate calyx with unequal sepals, the longer inner ones enclosing the flattened achene. Stigma sessile, tufted. (Munz, Flora So. Calif. 847).
Upwards of 40 genera and about 500 species are recognized in many parts of the world, chiefly trop.; a very few are grown for ornament and fibre. (Bailey 342).