600 MISC. PUBLICATION 200, U. S. DEPT. OF AGRICULTURE 



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. loxgepeduxculatum. 



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



pubescent, 1.7 to 1.9 mm. long 20. P. propexqutxi. 



Blades and sheaths conspicuously pubescent throughout. 



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

 erect, usually not more than 5 mm. wide 13. P. setacefm. 



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. propin- 



quum). 



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



exceptional P. ciliatifolium and lower sheaths usually pubescent in P. rigid' 



i folium). 



Blades stiff, usually not more than 6 mm. wide; spikelets mostly 2.2 to 2.4 mm. 



long 21. P. RIGIDIFOLFCM. 



Blades from lax to rather firm, if firm more than 6 mm. wide; spikelets not 

 more than 2.1 mm. long. 

 Spikelets mostly 2 mm. long, rounded at summit ; blades mostly more than 



8 mm. wide 19. P. ciLEvriFOLroi. 



Spikelets 1.8 to 1.9 mm. long, slightly pointed; blades not more than 8 mm. 



wide 20. P. propixqftcxi. 



Foliage conspicuously pubescent (or sparsely so in exceptional specimens of P. 

 pubescens) . 

 Culms erect or nearly so. 



Blades from sparsely to rather densely pilose, rather thin. 



18. P. PUBESCEXS. 



Blades puberulent on both surfaces with long hairs intermixed or the lower 

 surface nearly or quite glabrous except for a few long hairs along midrib 

 and margin, usually rather firm 17. P. stramineum. 



Culms widely spreading or prostrate. 



Foliage coarsely hirsute: plants commonly relatively stout. 



15. P. SUPIXUM. 



Foliage finely puberulent ; plants usually grayish olivaceous. 



16. P. PSAAIMOPHILUM. 



4b. Racemes terminal on the primary culm or leafy branches, no truly axillary 

 racemes. 

 6a. Spikelets conspicuously silky-ciliate around the margin, the hairs as long as 

 the spikelet or longer. 



Racemes commonly 3 to 5; culms geniculate at base 32. P. dilatatum. 



Racemes commonly 12 to 18; culms erect 33. P. urvillei. 



6b. Spikelets not ciliate. 



7a. Fruit dark brown and shining. 



Plants perennial; spikelets 2.2 to 2.8 mm. long, elliptic or obovate-oval. 

 Spikelets obovate, turgid, the sterile lemma wrinkled; culms erect, densely 



cespitose 43. P. plicatulum. 



Spikelets elliptic, depressed, not turgid; culms decumbent or floating at the 

 base. 



Plants terrestrial, culms decumbent at base. 44. P. texaxtjm. 



Plants aquatic, lower part of culms floating 45. P. hydrophilum. 



Plants annual; spikelets 2 to 3 mm. long, suborbicular or broadly obovate. 



Spikelets suborbicular, 2 to 2.2 mm. long, glabrous 46. P. bosciactjm. 



Spikelets broadly obovate, 2.2 to 3 mm. long, pubescent. 



47. P. COXVEXUM. 

 7b. Fruit pale to stramineous (brown but not shining in P. virgatum). 

 8a. Plants robust, 1 to 2 m. tall. 



Spikelets pubescent at least toward the summit; fruit brown at maturity. 



42. P. VIRGATUM. 



Spikelets glabrous; fruit pale. 



Culms ascending; leaves crowded toward the base.... 39. P. difforme. 

 Culms erect or suberect, leafy throughout. 



Glume and sterile lemma slightly inflated and wrinkled, green. 



40. P. FLORIDAXUM. 



Glume and sterile lemma not inflated and wrinkled, rusty-tinged. 



41. P. GIGAXTEU.M. 



8b. Plants not robust, if more than 1 m. tall, culms relatively slender. 



