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 grasses with flat leaves, and spikelets borne in pairs or sometimes in 3' s, in secund racemes which are digitate, in whorls, or approximate at the summit of the culm. Racemes with the rachis angled or winged. Scales of the spikelet 4, sometimes 3 by the suppression of the lowest one; the fourth or innermost scale chartaceous, the margins hyaline and not inrolled, subtending a palet of similar texture and a perfect flower. Stamens 3. Stigmas plumose. [Greek, crop-making, in allusion to its abundance.]
Rachis of the racemes wingless; first scale of spikelet wanting, or rudimentary.
Racemes short, l' - 4' long; spikelets less than 1" long.
Racemes exceeding 4' long, rarely shorter; spikelets over 1" long.
Rachis of the racemes with the lateral angles broadly winged.
Pedicels terete, glabrous or nearly so; first scale wanting.
Leaves pubescent; second scale 1/2 as long as the spikelet or less.
Leaves glabrous; second scale nearly as long as the spikelet.
Pedicels sharply 3-angled, the angles strongly hispidulous; first scale minute.
Spikelets about 1 1/4" long; third scale with nerves mostly hispid.
Spikelets about 1 1/2" long; third scale with the nerves smooth.
Culms erect, 1°-4° tall, slender, smooth. Sheaths hirsute; blades 1'-8' long, 1/2"-2" wide, erect, hirsute or glabrous on the lower surface, rough on the upper; racemes 2-5, filiform, 1'-4' long, approximate at the summit of the culm, erect or nearly so; rachis 3-angled, very slender, not winged; spikelets less than 1" long, elliptic, pubescent, in pairs, occasionally in 3's; first scale rarely present; second three-fourths as long as or equalling the third, which equals the fourth.