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..
Mostly perennial tufted grasses, with flat leaf-blades and spike-like or open panicles. Spikelets 2-4-flowered, the flowers all perfect, or the uppermost staminate; rachilla glabrous or pilose, extended beyond the flowers. Scales 4-6, membranous, the 2 lower empty, unequal, acute, persistent; flowering scales usually shorter than the empty ones, deciduous, 2-toothed, bearing a dorsal awn below the apex, or the lower one sometimes awnless. Palet narrow, hyaline, 2-toothed. Stamens 3. Styles distinct. Stigmas plumose. Grain free, enclosed in the scale. [Latin, referring to the three bristles (one awn and two sharp teeth) of the flowering scales in some species.]
About 60 species, widely distributed in temperate or mountainous regions. Besides the following, about 8 others occur in the western parts of North America. Type species: Avena striata Lam.
Flowering scales all bearing long dorsal awns.
Panicle contracted, dense; flowering scales 2 1/2" long or less.
Panicle open, loose; flowering scales 2 l/2" long or more.
Lower flowering scale not bearing a long dorsal awn, a rudiment sometimes present.
Aira spicata L. Sp. PI. 64. I753. Aira subspicata L. Syst. Veg. Ed. 10, 673. 1759-Avena mollis Michx. Fl. Bor. Am. 1: 72. 1803. Trisetum subspicatum Beauv. Agrost. 180. 1812. T. spicatum Richter, PI. Europ. 1: 59. 1890.
Softly pubescent or glabrous, culms 6'-2° tall, erect, simple. Sheaths usually shorter than the internodes; ligule1/2"-1" long; blades 1'-4' long, ¥'-2" wide; panicle spike-like, 1'-5' in length, often interrupted below, its branches 1 1/2' or less long, erect; spikelets 2-3-flowered, the empty scales hispid on the keel, shining, the second about 2 1/2" long, the first shorter; flowering scales 2"-2 1/2" long, acuminate, scabrous, each bearing a long bent and somewhat twisted awn.
In rocky places, Labrador to Alaska, south on the mountains to North Carolina, New Mexico and California. Also in Europe and Asia. Downy Oat-grass. Aug.-Sept.
Avena flavescens L. Sp. PI. 809. 1753. Trisetum pratense Pers. Syn. 1: 97. 1805. T. flavescens Beauv. Agrost. 88. 1812.
Culms 1 1/2°-2 1/2° tall, erect, simple, smooth and glabrous. Sheaths shorter than the internodes, more or less pubescent; ligule 1/2" long; blades 1 1/2-5' long, 1"-3" wide, scabrous, sometimes sparingly hairy; panicle open, 2'-5' in length, the branches ascending or erect, somewhat flexuous, naked below, the lower 1'-2' long; spikelets 3-4-flowered; empty scales smooth and glabrous, the second acute, 2 1/2" long, the first about half as long, narrower, acuminate; flowering scales 2 1/2"-3" long, scabrous, bearing a long bent and twisted awn.
Avena pennsylvanica L. Sp. PI. 79. 1753.
Avena palustris Michx. Fl. Bor. Am. 1: 72. 1803.
Trisetum peunsylvanicum Beauv.; R. & S. Syst. 2: 658. 1817.
Trisetum palustre Torr. Fl. U. S. 1: 126. 1824.
Culms 1°-3° tall, erect, simple, slender and often weak, smooth and glabrous. Sheaths shorter than the internodes, sometimes scabrous; ligule 1/2" long; blades 1'-6' long, 1"-3" wide, rough; panicle 2'-8' in length, yellowish, narrow, the branches ascending, the lower 1'-2' long; spikelets 2-flowered; outer scales smooth, shining, subequal, the second 2"-2 1/2" long; flowering scales 2"-2i" long, scabrous, the lower not long-awned, but a rudimentary awn sometimes present, the upper with a long bent and twisted awn.