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Published In: Species Plantarum 1: 67. 1753. (1 May 1753) (Sp. Pl.) Name publication detailView in BotanicusView in Biodiversity Heritage Library
 

Project Name Data (Last Modified On 8/28/2009)
Acceptance : Accepted
 

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77. Poa L. (bluegrass)

Plants annual or perennial, sometimes with rhizomes (sometimes appearing stoloniferous in P. palustris), forming tufts, clumps, or loose colonies. Flowering stems erect to spreading, sometimes weak or nodding, sometimes rooting at the lower nodes, often slender. Leaf sheaths open to below the middle (closed, with fused margins, toward the base), sometimes closed only at the very base. Leaf blades flat or folded, with boat‑shaped tips. Inflorescences open or narrow panicles with ascending to spreading or downwardly angled branches, these sometimes arched or curved, the spikelets single or in small clusters toward the branch tips, but not regularly paired, all similar in size and appearance and with fertile florets (except in P. bulbosa, with most of the florets replaced by vegetative bulblets). Spikelets elliptic‑lanceolate to ovate in outline, somewhat flattened, with 2–8 florets. Glumes much shorter than the rest of the spikelet, pointed at the tip, awnless, glabrous or more commonly roughened on the midnerve, the lower glume slightly shorter, 1‑ or 3‑nerved, the upper glume 3‑nerved. Lemmas sharply or bluntly pointed at the tip, awnless, usually somewhat keeled on the back, faintly to strongly 3‑ or 5‑nerved with the nerves converging (arched inward) toward the tip, glabrous or more commonly hairy on the back (sometimes minutely so) and usually also with a tuft of long, cobwebby hairs at the base. Paleas slightly shorter than the lemmas, elliptic. Stamens 3, the anthers yellow. Fruits elliptic in outline, slightly flattened on 1 side. About 500 species, worldwide, especially in temperate and boreal regions.

Species within the annual and perennial groups in Poa can be difficult to distinguish, and close attention must be given to rhizome, ligule, and spikelet characters.

 

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1 Spikelets with most of the florets replaced by vegetative bulblets with elongate, linear, pointed tips 2 Poa bulbosa
+ Spikelets all “normal,” none of them replaced by vegetative bulblets (2)
2 (1) Plants annual, without rhizomes, forming tufts (rarely rooting at the nodes under aquatic conditions in P. annua) (3)
+ Plants perennial, with short- or long-creeping rhizomes, forming small to large clumps or loose colonies (4)
3 (2) Lemmas lacking long, cobwebby hairs at the base; anthers 0.7–1.1 mm long, often exserted from the spikelet at maturity 1 Poa annua
+ Lemmas with a tuft of long, cobwebby hairs at the base; anthers 0.1–0.3 mm long, included in the spikelet at maturity 3 Poa chapmaniana
4 (2) Plants with well-developed rhizomes or stolons, forming clumps or loose colonies (5)
+ Plants without rhizomes, forming tufts (8)
5 (4) Flowering stems strongly flattened and 2-edged, at least in the apical 1/2; lemmas bluntly pointed at the tip 4 Poa compressa
+ Flowering stems circular in cross-section or very slightly flattened, not 2-edged; lemmas sharply pointed at the tip (6)
6 (5) Plants with well-developed rhizomes, the flowering stems erect or nearly so, not prostrate or rooting at the nodes; ligules 0.7–2.1 mm long 7 Poa pratensis
+ Plants without rhizomes, the flowering stems weak, sometimes prostrate and rooting at the nodes, thus appearing stoloniferous; ligules 2–8 mm long (7)
7 (6) Leaf sheaths glabrous, smooth; lemmas 3-nerved, the lateral nerves as well as the midnerve hairy, at least toward the base 6 Poa palustris
+ Leaf sheaths roughened with minute, downwardly pointing hairs; lemmas 5-nerved, glabrous except for the tuft of cobwebby hairs at the base and the minutely hairy, roughened midnerve 9 Poa trivialis
8 (4) Leaf sheaths roughened with minute, downwardly pointing hairs; lemmas glabrous except for the tuft of cobwebby hairs at the base and the minutely hairy, roughened midnerve 9 Poa trivialis
+ Leaf sheaths glabrous, smooth; lemmas with the lateral nerves (sometimes only the outer pair) as well as the midnerve hairy, at least toward the base (9)
9 (8) Lowermost nodes of the inflorescence with (1)2 or 3 branches; spikelets 4–6 mm long 10 Poa wolfii
+ Lowermost nodes of the inflorescence with (2–)4–8 branches; spikelets 2.7–4.5 mm long (10)
10 (9) Lemmas with 5 noticeable nerves, the lateral nerves all well developed, usually minutely hairy between as well as along the nerves 8 Poa sylvestris
+ Lemmas with 3 noticeable nerves, the second pair of lateral nerves absent or very faint, hairy only along the 3 main nerves (11)
11 (10) Ligules 0.6–2.0 mm long; inflorescences 5–15 cm long, the lowermost branches loosely ascending 5 Poa interior
+ Ligules (2.0–)2.5–5.5 mm long; inflorescences (8–)15–30 cm long, the lowermost branches spreading 6 Poa palustris
 
 
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