1. Basal leaves usually distinctly diflferent from those of the culm, forming a 

 winter rosette; plants perennial, in spring producing simple culms 

 with mostly narrowly lanceolate blades and terminal panicles with 

 numerous spikelets (in most species these not producing viable 

 seed); later culms often much-reduced and much-branched, produc- 

 ing an autumnal phase usually quite different from the vernal 

 phase and with reduced axillary panicles (2) 



1. Basal leaves similar to though usually smaller than those of the stem leaves; 



winter rosette absent; plants annual or perennial; spikelets usually 

 nearly all fertile (9) 



2(1). Spikelets blunt and strongly nerved; blades rarely as much as 20 mm. 

 broad 6. P. Ravenelii. 



2. Spikelets rarely if ever both blunt and strongly nerved (3) 



3(2). Ligule (or at least a ligulelike tuft at the extreme base of the blade) of 

 conspicuous hairs usually (2-) 3-5 mm. long (4) 



3. Ligule less than 2 mm. long to obsolete (5) 



4(3). Spikelets 0.9-1.3 mm. long; "ligule" 2-3 mm. long 2. P. leucothrix. 



4. Spikelets 1.3-2.9 mm long; "ligule" (2-) 3-5 mm. long 3. P. lamiginosum. 



5(3). Spikelets nearly spherical at maturity; blades glabrous, firm, cordate; ligule 

 obsolete 5. P. polyanthes. 



5. Spikelets usually obovoid or ellipsoid; ligule usually developed (obsolete in 



P. commutatum and allies) (6) 



6(5). Spikelets 2.3-3.2 mm. long (7) 



6. Spikelets 1.2-2.2 mm. long (8) 



7(6). Ligule obsolete; blade only 5-10 (-15) cm. long, 8-25 mm. wide, glabrous, 

 at base cordate and ciliate; spikelets slenderly elHjwoid, 2.4-3.1 mm. 

 long, never turgid 8. P. commutatum. 



I. Ligule a muticous scale or a short fringe; blades 10-23 cm. long, 9-30 rrfrn. 



wide, glabrous or scabrous, at base only slightly if at all cordate; 

 spikelet turgidly ellipsoid to obovoid or ovoid, 2.3-3.2 mm. long 

 7. P. scoparium. 



8(6). Culms delicate, usually less than 30 (rarely to 40) cm. tall; blades filmy, 



usually only 1-3 cm. long and 1-3 mm. wide, often reflexed 



4. P. ensifolium. 



8. Culms delicate or not so delicate, (15-) 20-100 cm. tall; blades firmer (mem- 



branous or more firm), 3-12 cm. long, 3-15 mm. wide, reflexed or 

 ascending 1. P. dichotomwn. 



9(1). Plants annual (doubtful cases should be keyed under both alternatives) 

 (10) 



9. Plants perennial (11) 



10(9). First glume about a fourth as long as the spikelet 



21. P. dichotomiflorum. 



10. First glume usually proportionately longer 20. P. capillare. 



11(9). Spikelets short-pedicelled along one side of the rachises to form spikelike 

 racemes; fertile lemma transversely rugose (except in P. hemi- 

 tomum) (12) 



II. Spikelets in open or sometimes contracted or congested panicles (somewhat 



1 -sided in P. anceps and P. rigidulum) (16) 



12(11). First glume nearly equaling the sterile lemma (13) 

 12. First glume much shorter than the sterile lemma (14) 



13(12). Racemes spreading; fertile lemma not more than one third the total 

 length of the spikelet 11. P. gymnocarpon. 



306 



