18 GRAMINBAE 



Spikelets lanceolate, about 0.6 mm. wide. 



3. P. Philadelphicum. 

 Spikelets elliptic, about 0.8 mm. wide. 



4. P. Gatlingeri. 

 Spikelets 3 mm. long, acuminate. 5. P. flexile. 



Spikelets ovate, 5 mm. long or more. 6. P. miliaceum. 



Sheaths glabrous. ?• P. proH/erum. 



** Stems simple, from long stout rootstocks. 

 Stems tufted : rootstocks scaly. 8. P. virgatum. 



Stems not tufted, rootstocks not scaly. 9. P. amarum. 



b. I<ower leaf-sheaths much compressed, broad, keeled, often equitant. 

 Plants with no long rootstocks. 

 Fourth scale of the spikelet sessile. 

 Ligule naked : stems finally much branched. 



10. P. agrostoides. 

 lyigule ciliate : stems simple or nearly so. 11. P. longifolium. 

 Fourth scale of the spikelet distinctly stalked. 12. P. sHpitatum. 

 Plants with long, scaly rootstocks. 13. P. anceps. 



B. Basal and stem leaves dissimilar, the former generally much shorter and 

 broader than the latter : rosulate tufts of leaves often present in the fall : 

 spikelets oval to obovate or globose, obtuse, or rarely acutish (acute in 

 no. 15). 

 a. Middle leaf -blades of the main stem less than 1.5 cm. broad, usually less 

 than I cm., the base rounded or truncate, rarely subcordate, sometimes 

 narrowed. 

 Leaf-blades elongated, erect or nearly so, narrow, distinctly narrowed 

 toward the base, especially the lower ones. 

 I,eaf-blades narrowly linear : secondary branches on short basal 

 branches. 

 Spikelets less than 3 mm. long, obtuse or acutish, pubescent. 



14. P. linearifolium. 

 Spikelets 3-4 mm. long, acute, glabrous. 15. P. depauperatum. 

 I,eaf-blades lanceolate : no basal panicles. 16. P. BicknelUi. 

 I<eaf-blades not elongated, lanceolate, not manifestly narrowed toward 

 the base. 

 Stems, leaf-blades and sheaths glabrous, or the latter two parts merely 

 ciliate on the margins. 

 Spikelets glabrous. 

 Nodes of the stem naked. 17. P. dichotomum. 



Nodes of the stem densely barbed. 18. P. barbulatum. 



Spikelets pubescent. 

 I,eaf-blades smooth above or nearly so, thin, naked at the base. 



19. P. boreale. 

 I^af-blades very rough above, thick, ciliate at the base. 



28. P. sphaerocarpon. 

 Stems, leaf-blades or sheaths, or all of them pubescent. 

 * Spikelets 2.5 mm. long or less. 

 Leaf-blades glabrous, or sometimes minutely pubescent beneath. 

 Spikelets about 1.25 mm. long : stems glabrous. 



20. P. niiidum. 

 Spikelets about 2.5 mm. long : stems puberulent. 



31. P. Ashei, 

 Leaf-blades strongly pubescent beneath, usually also above. 



