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 flat or convolute leaves and contracted or open panicles. Spikelets 2-6-flowered, compressed, the rachilla usually glabrous; flowers perfect. or rarely dioecious. Scales membranous, keeled; the 2 lower empty, 1-3-nerved; the flowering scales longer than the empty ones, generally with a tuft of cobwebby hairs at the base, 5-nerved, the marginal nerves usually pubescent, often also the dorsal one; palets a little shorter than the scales, 2-nerved or 2-keeled. Stamens 3. Styles short, distinct. Stigmas plumose. Grain free, or sometimes adherent to the palet. [Name Greek, for grass or herbage.]
A genus of about 150 species, widely distributed in all temperate and cold regions. The English name Meadow-grass is often applied to most of the species. Besides the following some 50 others occur in the western parts of North America. Type species: Poa pratensis L.
Flowering scales distinctly 5-nerved, not webby at the base.
Flowering scales 3-nerved, or obscurely 5-nerved, webby at base.
Innovations extravaginal, plants often bearing rootstocks or stolons.
Culms tufted, usually densely so.
Rootstock slender; plants often stoloniferous.
Flowering scales pubescent all over.
Flowering scales glabrous, or slightly pubescent.
Culms exceeding 8' tall.
Flowering scales not webby at the base.
Flowering scales glabrous below between the nerves.
Flowering scales pubescent below between the nerves.
Panicle narrow, contracted, its branches short, erect.
Panicle open, branches long, flexuous, spreading.
Flowefing scales webby at the base.
Flowering scales glabrous; culms manifestly compressed
Flowering scales somewhat pubescent; culms round or little compressed.
Lateral nerves of the flowering scale glabrous.
Plant yellowish green; flowering scales 1 1/4" long
Plant green; flowering scales 1 3/4" long.
Lateral nerves of the flowering scale pubescent.
Lower half of the flowering scales densely villous between the nerves;
Lower half of the flowering scales glabrous between the nerves (sometimes somewhat pubescent in No. 14).
Spikelets 2" long or less: panicle-branches dividing and spikelet-bearing at or below the middle.
Intermediate nerves of the flowering scale obscure.
Panicle erect, rarely exceeding 5', branches ascending.
Panicle drooping, up to 1°, or more, branches spreading.
Intermediate nerves prominent.
Midnerve of the flowering scales pubescent only below; spike-
lets crowded on the branches.
Midnerve pubescent its whole length; spikelets scattered on
the spreading often reflexed branches.
Spikelets 2 1/2" long or more; panicle-branches usually dividing and spikelet-bearing only at the end.
Flowering scales very webby at the base, nerves strongly pilose;
innovation leaves much shorter than culms.
P. Wolfii. .
Flowering scales little webby; nerves sparsely pilose; innovation
leaves equalling or exceeding culms.
Rootstocks short and stout; no stolons.
Ligule acute, 2" long; flowering scales lanceolate, 1 1/2" long.
Ligule truncate, 1" long; flowering scales ovate, 2" long.
Culms not tufted; rootstocks long, creeping.
Flowering scales naked or with short hairs at the base.
Culms compressed, slender; plant bluish green; spikelets 1 1/2" - 3"
Culms not compressed, stout; spikelets above 5" long.
Flowering scales with webby hairs at the base longer than scale.
Innovations intravaginal, hence plants without rootstocks or stolons.
Flowering scales strigose below, hispidulous above.
Flowering scales hispidulous all over.
Ligules 1 1//2"-2" long, glabrous.
Ligules 1/2"-1" long, hispidulous on the outside.