MANUAL OF THE GRASSES OF THE UNITED STATES 577 



la. Rachis foliaceous, broad and winged. 



Racemes falling from the axis, rachis extending beyond the uppermost spike- 

 let 3. P. REPENS. 



Racemes persistent on the axis; rachis with a spikelet at the apex. 



Spikelets 2 mm long, obovate-oval 1. P. dissectum. 



Spikelets more than 3 mm long, pointed 2. P. acuminatum. 



lb. Rachis not foliaceous nor winged (slightly winged in P. boscianum). 



2a. Racemes 2, conjugate or nearly so at the summit of the culm, rarely a 

 third below. 

 Spikelets elliptic to narrowly ovate. 



Plants with creeping rhizomes or stoloDS. 



Second glume and sterile lemma glabrous; spikelets flattened. 



4. P. VAGINATUM. 



Second glume pubescent; spikelets relatively turgid- 5. P. distichum. 



Plants in dense tufts, without creeping rhizomes 11. P. almum. 



Spikelets suborbicular, broadly ovate or obovate. 



Spikelets concavo-convex, sparsely long-silky around the margin; plant 



stoloniferous ' 28. P. conjugatum. 



Spikelets plano-convex, not silky-margined; plants not stoloniferous. 



Spikelets 2.5 to 3 mm long 9. P. notatum. 



Spikelets less than 2.5 mm long 10. P. minus. 



2b. Racemes 1 to many, racemose on the axis, not conjugate. 



3a. First glume developed on at least one of the pair of spikelets (often 

 obsolete in some pairs in nos. 22 and 23). 



Spikelets turgidly biconvex 42. P. bipidum. 



Spikelets plano-convex. 



Plants without rhizomes; culms tufted; spikelets pubescent. 



24. P. LANGEI. 



Plants with stout scaly rhizomes, the culms mostly solitary; spikelets 

 glabrous. 



Blades flat, 8 to 15 mm wide 22. P. unispicatum. 



Blades folded at base, terete above, not more than 2 mm wide. 



23. P. MONOSTACHYUM. 



3b. First glume normally wanting (occasionally developed on 1 to few 

 spikelets in a raceme). 

 4a. Racemes terminal and axillary, the axillary sometimes hidden in the 

 sheaths; terminal inflorescence of 1 to 3, rarely to 6 racemes (see also 

 P. unispicatum and P. monostachyum) . 

 5a. Spikelets not more than 1.8 mm long (or sometimes 1.9 in P. debile 

 and P. propinquum), usually 1.5 to 1.7 mm (see also exceptional 

 P. ciliatifolium) . 

 Blades conspicuously ciliate, otherwise nearly glabrous. 



Blades relatively short, rounded at base and recurved-ascending; 

 foliage aggregate toward the base, the upper culm relatively 

 naked; spikelets glabrous, mostly 1.5 to 1.6 mm long. 



12. P. LONGEPEDUNCULATUM. 



Blades mostly elongate, suberect, not aggregate toward the base; 



spikelets pubescent, 1.7 to 1.9 mm long__ 20. P. propinquum. 

 Blades and sheaths conspicuously pubescent throughout. 



Culms slender, erect or suberect; foliage not aggregate at base; 



blades suberect, usually not more than 5 mm wide. 



13. P. SETACEUM. 



Culms stouter, mostly spreading; foliage more or less aggregate at 

 base; blades spreading, usually more than 5 mm wide. 



14. P. DEBILE. 



5b. Spikelets 2 to 2.5 mm long (or 1.8 to 1.9 mm in P. ciliatifolium 



and P. propinquum). 



Foliage, except margins, glabrous as a whole or nearly so (sparsely 



pubescent in exceptional P. ciliatifolium and lower sheaths 



usually pubescent in P. rigidifolium). 



Blades stiff, usually not more than 6 mm wide; spikelets mostly 



2.2 to 2.4 mm long 21. P. rigidifolium. 



Blades from lax to rather firm, if firm more than 6 mm wide; spike- 

 lets not more than 2.1 mm long. 

 Spikelets mostly 2 mm long, rounded at summit; blades mostly 

 more than 8 mm wide 19. P. ciliatifolium. 



