MANUAL OF THE GRASSES OF THE UNITED STATES 603 



Spikelets nearly spherical at maturity; blades glabrous, firm, cordate. 



Plants sparingly branching 10. Sphaerocarpa. 



Spikelets usually obovate or elliptic. 



Blades of midculm elongate, less than 1.5 cm -wide. Culms 

 usually tall; spikelets pointed, abruptly so in the velvety 



P. scoparium 15. Scoparia. 



Blades of midculm not elongate (somewhat so in P. equilaterale) . 

 Blades cordate, 1 to 3 cm wide (5 to 12 mm in P. ashei). 

 Spikelets pubescent. 

 Spikelets 2.5 to 3 mm long. Sheaths glabrous or minutely 



puberulent 16. Commutata. 



Spikelets 3 to 5 mm long (sometimes but 2.7 mm long in the 



hispid-sheathed P. clandestinum) 17. Latifolia. 



Blades not cordate, less than 1 cm wide. 



Sheaths crisp- or appressed-pubescent. Blades firm; spike- 

 lets pubescent 9. Columbiana. 



Sheaths glabrous or ciliate only in autumnal phase (sparsely 



pilose in P. curtifolium and the lower velvety in P. 



mattamuskeetense) . 



Vernal culms delicate (sometimes scarcely so in P. albo- 



marginatum and P. tenue); spikelets 1.5 mm or less 



long (1.6 to 1.7 mm in P. tenue) 11. Ensifolia. 



Vernal culms slender but not delicate, rarely less than 



40 cm tall; spikelets 2 to 2.9 mm long (1.5 mm in 



P. microcarpon and P. caerulescens). 



Lower internodes short, upper elongate, producing a 



nearly naked culm, leafy at base; spikelets narrowly 



ovate, 2.7 to 2.9 mm long 5. Ntjdicaulia. 



Lower internodes not shorter, the vernal culms about 

 evenly leafy throughout; spikelets elliptic or 

 obovate, not more than 2.5 mm long. 



6. DlCHOTOMA. 



1. Depauperata 



Spikelets about 3.5 mm long, beaked 5. P. depauperatum' 



Spikelets 3 mm long or less, (sometimes 3.2 mm long in P. perlongum) not beaked' 

 Culms single or few in a tuft; spikelets turgid, blunt, 2.7 to 3.2 mm long- 

 prairie plants 6. P. PERLONGUM. 



Culms in large tufts; spikelets not turgid, 2.2 to 2.7 mm long; plants of dry 

 woods. 

 Sheaths pilose; spikelets 2.2 to 2.7 mm long, pilose, . 7. P. linearifolium. 

 Sheaths glabrous; spikelets 2.2 to 2.3 mm long, glabrous or sparingly pilose. 



8. P. werneri. 

 2. Laxiflora 



Sheaths retrorsely pilose; spikelets papillose-pilose. 



Blades ciliate and more or less pilose on the surface; spikelets 2 mm long. 



10. P. xalapense. 

 Blades glabrous or nearly so on the surface and margin; spikelets 2.2 mm long. 



9. P. laxiflorum. 

 Sheaths not retrorsely pilose; spikelets pubescent or glabrous. 



Spikelets pubescent, about 2 mm long 11. P. ciliatum. 



Spikelets glabrous. 



Blades glabrous on the surface 12. P. polycaulon. 



Blades pilose on the surface 13. P. strigosum. 



3. Angustifolia 



Nodes bearded; plants grayish-villous; autumnal blades flat. 



Spikelets 2 mm long 15. P. chrysopsidifolium. 



Spikelets 2.5 to 2.8 mm long 16. P. consangtjineum. 



Nodes not bearded; plants villous only at base, or nearly glabrous; autumnal 

 blades involute or flat. 

 Autumnal blades flat; lower panicle branches spreading or reflexed. 



17. P. angustifolium. 



