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..
Perennial grasses with usually long narrow leaves, and terminal and axillary racemes. Spikelets in pairs at each node of the jointed hairy rachis, one sessile and perfect, the other with a pedicel, staminate, empty, or reduced to a single scale, or sometimes wanting. Perfect spikelet consisting of 4 scales, the outermost coriaceous, 2-keeled, the second keeled and acute, the two inner hyaline, the fourth more or less awned and subtending a palet and perfect flower. Stamens 1-3. Grain free. [Greek, in allusion to the bearded rachis.]
About 150 species, widely distributed in tropical and temperate regions. Besides the following, some 25 others in southern and western North America Type species: Andropogon hirtum L.
Pedicellate spikelets empty, of 1 or 2 scales, much smaller than the sessile spikelets, or wanting.
Stamen 1; racemes, or some of them, included in the spathes; rachis internodes slender.
Sheaths at the upper part of the culm not enlarged; racemes equally exserted.
Inflorescence oblong; branches divided into corymbiform masses.
Inflorescence long, linear, little divided, not corymbiform.
Sheaths at the summit or upper part of the stem much enlarged; racemes on one of
branches exserted much beyond the others.
Stamens 3; racemes usually exserted beyond the spathes; rachis internodes stouter.
Intercarinal space of the first scale of the sessile spikelet nearly nerveless; terminal hairs
of the internodes about twice their length.
Intercarinal space with 2 or 3 nerves running the length of the scale; terminal hairs of the
internodes about equaling them in length.
Pedicellate spikelets staminate, of 3 or 4 scales, equaling or exceeding the sessile spikelets.
Rachis internodes copiously pubescent with long hairs.
Awn perfect, with a defined column; culms tufted, or with short rootstocks.
Sessile spikelets hispidulous; rachis hairs 1" long or less.
Sessile spikelets mostly glabrous; rachis hairs 1 1/2" - 2" long, yellow.
Awn imperfect, rarely spiral at the base; rootstocks long, horizontal.
Rachis internodes glabrous, or with a few weak crimped hairs.