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..
[Glyceria R. Br. Prodr. Fl. Nov. Holl. 1: 179. 1810.] Mostly perennial grasses, often tall, with flat leaf-blades and paniculate inflorescence. Spikelets few-many-flowered, terete or somewhat flattened. Two lower scales empty, obtuse or acute, 1-3-nerved; flowering scales membranous, rounded on the back, 5-9-nerved, the nerves disappearing in the hyaline apex. Palets scarcely shorter than the scales, rarely longer, 2-keeled. Stamens 2 or 3. Styles distinct. Stigmas plumose. Grain smooth, enclosed in the scale and palet, free, or when dry slightly adhering to the latter. [Latin, referring to the panicled spikelets.]
About 20 species, widely distributed in North America, a few in Europe and Asia. Type species: Poa aquatica L.
Spikelets ovate or oblong, 4" long or less.
Flowering scales very broad, obscurely or at least not sharply nerved.
Panicle open, the branches ascending or spreading, often drooping.
Spikelets 3-5-flowered; lowest flowering scale about 1" long.
Spikelets 5-12-flowered; lowest flowering scale about 1 1/2" long.
Panicle contracted, the branches erect.
Flowering scales narrow, sharply and distinctly 7-nerved.
Panicle elongated, its branches erect or appressed.
Panicle not elongated, open, its branches spreading or drooping, rarely erect.
Scales about 1" long, obtuse or rounded at the apex.
Spikelets 1 1/2" long or less; branches of the panicle often drooping.
Spikelets 2"-3" long; branches of the panicle ascending or spreading.
Scales 1 1/4"-1 1/2" long, truncate and denticulate at the apex.
Spikelet9 linear, 6" long or more.
Flowering scales 1 1/2"-2 1/2" long, obtuse, equalling or exceeding the obtuse palet.
Flowering scales firm, hispidulous all over, truncate at the apex.
Flowering scales thin, hispidulous on the nerves only, obtuse at the apex.
Flowering scales 3"-4" long, usually shorter than the acuminate palet.
Flowering scales obtuse, about 3" long, a little exceeded by the palet.
Flowering scales acute, about 4" long, much exceeded by the palet.