This section is from the book "An Illustrated Flora Of The Northern United States, Canada And The British Possessions Vol2", 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 branched erect or diffusely spreading glabrous or pubescent herbs, most of the species weeds, with alternate, petioled pinnately veined entire, undulate or crisped leaves and small monoecious polygamous or dioecious green or purplish mostly 3-bracteolate flowers in dense terminal spikes or axillary clusters. Calyx of 2-5 distinct sepals. Stamens 2-5; anthers 2-celled, longitudinally dehiscent. Styles or stigmas 2 or 3. Fruit an ovoid or oblong utricle, circumscissile, bursting irregularly or indehiscent, 2-3-beaked by the persistent styles. Embryo annular. [Greek, unfading flower, from the dry, unwithering bracts.]
About 50 species of wide geographic distribution. Besides the following some 22 others occur in the southern and western United States. Type species: Amaranthus caudatus L.
Utricle circumscissile, the top falling away as a lid.
Flowers, at least the upper, in dense terminal spikes.
Axils not spine-bearing.
Sepals oblong to lanceolate.
Spikes stout, 4"-7" thick.
Spikes slender, 2"-3" thick.
A pair of stout spines in each axil.
Flowers all in small axillary clusters, mostly shorter than the leaves.
Plant erect, bushy-branched; bracts subulate; utricle wrinkled.
Utricle indehiscent, membranous, coriaceous or fleshy.
Upper flowers in terminal, more or less elongated spikes.
Sepals 5, clawed; flowers dioecious; southwestern species.
Sepals 2 or 3, oblong or spatulate; flowers monoecious or polygamous; in waste places.
Utricle smooth, dry, scarious.
Utricle fleshy, 3-5-nerved.
Flowers all in small axillary clusters shorter than the leaves.
Plant not fleshy; stem prostrate; leaves crisped.
Sea-coast fleshy plant; stem short, erect; leaves not crisped.