This section is from the book "An Illustrated Flora Of The Northern United States, Canada And The British Possessions Vol3", by Nathaniel Lord Britton, Addison Brown. Also available from Amazon: An Illustrated Flora of the Northern United States, Canada and the British Possessions. 3 Volume Set..
Annual or perennial herbs, monoecious, or sometimes dioecious, many of them weeds, some shrubby, with alternate leaves, or the lower opposite, and small heads of greenish or white flowers subtended by an involucre of few, separate or united bracts, the pistillate heads sometimes larger and nut-like or bur-like. Staminate and pistillate flowers in the same heads, or in separate heads. Receptacle chaffy. Pistillate flowers with no corolla, or this reduced to a short tube or ring; calyx adnate to the i-celled ovary, its limb none, or a mere border; style 2-cleft. Staminate flowers with a funnelform tubular or obconic 4-5-lobed corolla; stamens mostly 5, separate, or their anthers merely connivent, not truly syngenesious, with short inflexed appendages; ovary rudimentary; summit of the style often hairy or penicillate.
Eight genera and about 60 species, mostly natives of America, a few only of the Old World. Staminate and pistillate flowers in the same heads; involucre of a few rounded bracts.
Staminate and pistillate flowers in separate heads, the staminate mostly uppermost; involucre of the pistillate heads bur-like or nut-like. Involucral bracts of the staminate heads united.
Involucre of the pistillate heads with several tubercles or prickles in a single series.
Involucre of the pistillate heads with numerous prickles in several series.
Involucral bracts of the staminate heads separate; involucre of pistillate heads an oblong bur.