74 POACEAE 



Intermediate nerves of tlie lemma glabrous; plant usually over 3 dm. high; 

 siem-leaves several, flat and flaccid. 

 Hairs of the niidnerves and lateral nerves copious and spreading. 

 Lemma ovate, abruptly acute, usually purple. 13. P. rejlcza. 

 Lemma lanceolate, gradually acute, usually pale green. 



14. P. nervosa. 

 Hairs of the midnerves and lateral nerves few and appressed or none. 



15. P. leplocoma. 

 Cobweb lacking; internerves and the intermediate nerves glabrous; midnerves and 



lateral nerves hairy; habit like P. arctica. 16. P. alpicola. 



V. COMPRESSAE. 



One species. 17. P. compressa. 



VI. Apertae. 



Branches of the inflorescence short, usually in pairs. 18. P. aperla. 



Branches of tlie inflorescence very long, in 3's to 5's. 19. P. macroclada. 



VII. TRI FLORAE. 



Lemma 5 mm. long. 4. P. rhizomaia. 



Lemma 3 mm. long or less. 



Stem stout; leaves 2-5 mm. wide; ligule 3-4 mm. long, triangular; branches of the 

 panicle at last spreading; second glume narrower than the lemma, three- 

 fourths as long or more. 20. P. triflora. 

 Stem slender; leaves seldom over 2 mm. wide; ligule about 1 mm. long, trimcate; 

 branches of the panicle ascending or erect. 

 Flowers green; the second empty glume with broad, scarious margins and strong 

 lateral nerves 

 Inflorescence with erect branches; second empty glume narrower than the 



lemma. 21. P. subtrivialis. 



Inflorescence with ascending branches; second empty glume not narrower 

 than the lemma. 22. P. interior. 



Flowers usually purple-tinged; scarious margin of the empty glumes scarcely 

 evident and lateral nerves faint. 23. P. crocata. 



VIII. Alpinae. 

 One species. 24. P. alpina. 



IX. Wheelerianae. 



Lower sheaths retrorsely strigulose. 



Internerves of the acute lemma merely strigulose or scabrous. 

 Nerves of the lemma scabrous; ligules short, truncate. 



Branches of the inflorescence ascending. 25. P. Olneyae. 



Branches of the inflorescence refle.xed. 26. P. subreflexa. 



Nerves of the lemma silky or villous on the lower portion. 



Ligules 2 mm. long, truncate; leaf-blades narrow, ascending. 



27. P. Wheeleri. 

 Ligules 4-5 mm. long, lanceolate, acuminate; leaf-blades broad, spreading. 



28. P. Vascyana. 

 Internerves of the obtusish lemma villous, at least below. 29. P. tricholepis. 



Ijeaf-sheaths all glabrous and smooth. 



Ligules lanceolate, acute, 3 mm. long. 30. P. Tracyi. 



Ligules truncate, about 1 mm. long. 31. P. curta. 



X. Phoenice.-ve. 



Plant tall, 4 dm. high or more; nerves and internerves of the lemma villous. 



32. P. Grayana. 

 Plant low, usually less than 3 dm. high; internerves of lemma glabrous. 



16. P. alpicola. 



XI. Aridae. 



Internerves of the lemma pube.scent, at least below; stem stout; inflorescence dense; 

 ligules acute. 

 Intermediate nerves of the lemma strong; plant tall; glumes 5 mm. long. 



33. P. pratensiformis. 

 Intermediate nerves of the lemma weak; glumes 3-4 mm. long. 



Keel and the marginal nerves of the lemma v^illous; empty glumes equalling the 



lemma. 34. P. arida. 



Keel and marginal nerves of tlie lemma short-hairy, with appressed hairs; empty 

 glumes shortc ' than the lemma. 35. P. Sheldorii. 



Internerves of the lemma glabrous; stem slender; inflorescence open; ligule obtuse. 



36. P. ylaucifolia. 



XII. RUPICOLAE. 



Midnerve and lateral nerves of the lemma pubescent; plant strict, 1-5 dm. high. 



Cobweb at the base of the flowers scant; stem slender and leafy, usually 3-5 dm. 



high. 23. P. crocata. 



Cobweb none; stem 1-2 (seldom 3) dm. high, leafy mostly at the base. 



Flowering glumes 3 mm. long or less, firm, obtuse. 37. P. rupicola. 



Flowering glumes about 4 mm. long, acute, thin. 38. P. Patlersoni. 



Nerves of the lemma glabrous; plant seldom over 5 cm. high. 39. P. I.cllermani. 



