This section is from the book "An Illustrated Flora Of The Northern United States, Canada And The British Possessions Vol1", 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 grasses with usually flat leaves. Inflorescence in spikes. Spikelets subtended by a spiny involucre which is deciduous with them at maturity. Scales 4; the first hyaline; the second and third membranous, the latter sometimes having a palet and staminate flower in its axil; the fourth chartaceous, subtending a palet of similar structure which encloses a perfect flower. Stamens 3. Styles united below. Stigmas plumose. Grain free, enclosed in the scales. [Ancient Greek name for some grass, probably millet.]
About 20 species, in tropical and temperate regions. Type species: Cenchrus echinatns L.
Body of the involucre 3"-4" broad, pubescent with very long hairs, the spines commonly 2 1/2"-4" long.
Body of the involucre rarely exceeding 2 1/2" broad, pubescent, the spines 1 l/2"-2" long.
Culms at first erect, up to 1° tall, later elongated, trailing and much-branched; sheaths glabrous, excepting the ciliate margin, compressed, very loose, the upper one often partly enclosing the inflorescence; blades flat or complanate, smooth and glabrous, 4' long or less, 2"-4" wide; spikes stout, i'-2' long; involucres 5-12, the body 3"-4" broad, pubescent with very long hairs, the spines usually 2 1/2"-4" long; spikelets 3"-3 1/2" long, not exserted beyond the involucre.
Cenchrus carolinianus Walt. Fl. Car. 79. 1788.
Culms at first erect, later prostrate and forming mats, 8'-2° long or more, branched; sheaths glabrous, excepting the ciliate margin, compressed; blades 2 1/2'-5' long, 2"-4" wide, smooth or rough, usually flat; spikes i'-3' long; involucres 6-20, the body rarely exceeding 2\" broad, pubescent with relatively short hairs, the spines 1 1/2"-2" long; spikelets 3"-3 1/2" long, usually not exserted beyond the involucre.
In dry sandy places, Maine to Wisconsin and California, south to Florida and Mexico; also in the Bahamas and tropical America. Figured for C. tribuloides L. in our first edition. June-Sept.