34 FLOSA OF PHILADELPHIA. 



Spikelets 3 mm. long, acuminate, 

 4. P. flexile. 

 Spikelets ovate, 5 mm. long. 5. P. miliaceum. 

 Sheaths glabrous. 6. P. proliferum. 



Culms simple, from stout, often sealy rootstocks. 



Culms tufted; rootstocks scaly. 7. P. virgatum. 



Culms not tufted; rootstocks not scaly; leaves glaucous 

 and very thick. 8. P. amarum. 



Lower sheaths much compressed, broad, keeled, often equitant. 

 Fourth scale of spikelet sessile. 



Spikelets 3-3.5 mm. long, the apex curved. 



11. P. rostratum. 

 Spikelets less than 3 mm. long, the apex not curved. 



Ligule naked; 'Culms finally much branched. 



9. P. agrostoides. 

 Ligule ciliate; culm simple or nearly so. 



, ,.. 10.. P. longifolium. 

 Fourth scale of spikelet distinctly stalked. 



12. P. stipitatum. 



(B) Basal and culm leaves dissimilar, the former generally much shorter 

 and broader than the latter; rpsulate tufts of leaves often pres- 

 ent in the fall; spikelets oval to obovate or globose, obtuse, or 

 rarely acutish (acute in No. 13). 

 (1) Middle leaves of the main culm less than 1.5. em. wide, usually 

 less than 1 cm. in width, the base rounded or truncate, rarely 

 subcordate, sometimes narrowed toward the base. 



(a) Leaves elongated] erect, narrow, 1 distinctly narrowed to- 



ward the base, especially the lower ones. 



Spikelets less than 3 mm. long, obtuse or acutish, pubes- 

 cent. 14. P. MnearifoUum. 



Spikelets 3-4 mm. long, acute, glabrous. • , ., 



13. P. depauperatum. 



(b) Leaves not elongated, lanceolate, not manifestly narrowed 



toward the base. 

 Culms, leaves and sheaths, glabrous, or the latter two 

 merely ciliate on the margins. 

 Spikelets glabrous. 



Nodes of the culm naked. 



Leaves divaricate; culms erect, short, the 

 later branches branched. 



15. P. dichotomum. 

 Leaves erect; culms acciimbent, weak, finally 

 elongated, and the later branches scat- 

 tered. 16. P. lucidum. 

 Nodes of the culm densely barbed. : 



17. P. barbulatum. 

 Spikelets pubescent: 



Spikelets elliptic; leaves smooth above or nearly 



so, naked at the base. 18. P. boreale. 

 Spikelets globose, or nearly so ; leaves very rough 

 above, thick ciliate at the base. 



30. P. sphaerocarpon. 

 Culms, sheaths, or leaves, or all of them, pubescent. 

 Spikelets 2.5 mm. or less long. 



Leaves glabrous, or if pubescent beneath, the 

 hairs short and soft. 

 Culms villous with short hairs, at least below. 

 Leaves minutely villous beneath. 



20. P. Columbianwm. 

 Leaves glabrous beneath. 



33. P. Ashei. 

 Culms not villous. 



