3(2). Panicle narrow, the branches ascending or appressed; lemmas scabrous to 

 pubescent on all the nerves but not webbed 3. P. glaucifolia. 



3. Panicle open, the branches mostly spreading; lemmas often webbed at base 



or glabrous on the internerves (4) 



4(3). Plants alpine or subalpine; lemmas pubescent over the back and silky on 

 the 5 nerves, usually webbed 4. P. Grayana. 



4. Plants often of lower elevations; lemmas sometimes glabrous on the internerves 



or not webbed (5) 



5(4). Anthers mostly 0.5-0.9 mm. long; lower panicle branches mostly in twos 

 6. P. leptocoma. 



5. Anthers at least 1 mm. long; lower panicle branches usually in threes or fives 



(6) 



6(5). Lemmas not webbed at base 3. P. glaucifolia. 



6. Lemmas more or less strongly webbed at base (7) 



7(6). Ligules mostly not over 1.5 mm. long, rarely with the uppermost much 

 longer, truncate; lemmas more than 3 mm. long 2. P. pratensis. 



7. Ligules mostly (2-) 3-5 mm. long, often acute; lemmas 2.5-3 mm. long 



7. P. palustris. 



8(1). Lemmas with long tangled cobwebby hairs at base (9) 



8. Lemmas not cobwebby at base (13) 



9(8). Panicle loose, the lower slender branches 1 to 3 per node and spreading 

 or reflexed; spikelets usually purplish; ligules glabrous, usually over 

 1 mm. long; anthers not over 1 mm. long (10) 



9. Panicle very narrow, the branches ascending or more than 3 per node; spike- 



lets usually greenish; ligules often puberulent-scabridulous, some- 

 times less than 1 mm. long; anthers often over 1 mm. long (11) 



10(9). Lower panicle branches reflexed, 1 to 3 per node 5. P. reflexa. 



10. Lower panicle branches usually not reflexed, generally in pairs 



6. P. leptocoma. 



11(9). Spikelets averaging about 6 mm. long, the lemmas 4-5 mm. long 



4. P. Grayana. 



11. Spikelets usually less than 5 mm. long, the lemmas less than 4 mm. long (12) 



12(11). Plants of the lowlands or low mountains; culms decumbent and usually 



stoloniferous, 4-12 dm. tall; ligules (2-) 3-5 mm. long 



7. P. palustris. 



12. Plants montane to subalpine; culms erect, not at all stoloniferous; ligules 



rarely as much as 2 mm. and never so much as 3 mm. long 



8. P. interior. 



13(8). Spikelets compressed, at anthesis usually less than twice as long as broad; 

 lemmas rather strongly keeled (14) 



13. Spikelets only slightly compressed, at anthesis over twice as long as broad; 



lemmas rounded on the back or only slightly keeled (15) 



14(13). Ligules usually truncate, 0.3-1 (or rarely to 2) mm. long; spikelets 



mostly 2- to 3- (4-) flowered; second glume 2.5-3.5 mm. long 



8. P. interior. 



14. Ligules usually obtuse to acute, generally at least 1 (the longest 4) mm. 



long; spikelets mostly 3- to 5-flowered; second glume rarely less 

 than 4 mm. long 4. P. Grayana. 



15(13). Ligules thickish, strongly ciliolate, truncate to roundish, those of the 

 innovations and basal culm leaves scarcely visible from the side, 

 mostly not over 0.5 (to 1 ) mm. long, those of the upper culm 

 leaves mostly 1-1.5 (rarely to 2.5 mm.) long 10. P. juncifolia. 



187 



