42 POACEAE. 



Hairs of the mid-nerves and lateral nerves few and appressed or none. 



9. P. leptocoma. 



Cobweb lacking ; internerves and the intermediate nerves glabrous ; mid-nerves 

 and lateral nerves hairy; habit like P. arctica. 10. P. alpicola. 



IV. PLATYPHYLLAE. 

 One species. n. P. platyphylla. 



V. COMPRESSAE. 



One species. 12. P. compressa. 



VI. APERTAE. 



Branches of the inflorescence short, usually in pairs. 13. P. aperta. 



Branches of the inflorescence very long, in 3's to s's. 14. P. macroclada. 



VII. SEROTINAE. 



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 flowering glumes, 

 y 4 as long or more. 15. P. serotina. 



Stem slender ; leaves seldom over 2 mm. wide ; ligule about i mm. long, truncate ; 

 branches of the panicle ascending; second glume as wide as the flowering 

 glumes and */$ as long. 



Flowers green; empty glumes, especially the second, with broad, scarious mar- 

 gins and strong lateral nerves. 16. P. interior. 

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

 evident and lateral nerves faint. 17. P. crocata. 



VIII. RUPICOLAE. 



Mid-nerve and lateral nerves of the flowering glumes pubescent ; plant strict, 1-5 



dm. high. 



Empty glumes shorter than the flowering glumes ; their lateral nerves indis- 

 tinct. 

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



dm. high. 17. 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. 18. P. rupicola. 

 Flowering glumes about 4 mm. long, acute, thin. 19. P. Pattersonii. 



Empty glumes equalling or longer than the flowering glumes, t. e., their tips 

 almost as high as the tip of the subtended flowers ; their lateral nerves more 

 prominent. 20. P. Gray ana. 



Nerves of the flowering glumes glabrous ; plant seldom over 5 cm. high. 



21. P. Letter manii. 

 IX. ALPINAE. 



One species. 22. p. alpina. 



X. PHOENICEAE. 



One species. 23. P. phoenicea. 



XI. WHEELERIANAE. 



Leaf-sheaths retrorsely stigose. 



Internerves of the very acute flowering glumes merely strigulose or scabrous. 

 Nerves scabrous. 24. P. Wheeleri. 



Nerves silky or villous on the lower portion. 25. P. Vaseyana. 



Internerves as well as nerves of the obtusish flowering glumes villous at least 



below. 26. P. tricholepis. 



Leaf-sheaths glabrous. 



Internerves of the lanceolate flowering glumes scabrous or strigulose ; innova- 

 tions very few and consisting of wholly extravaginal runners. 



27. P. occidentalis. 



