642 MISC. PUBLICATION 200, U. S. DEPT. OF AGRICULTURE 



Spikelets 2 to 2.1 mm. long; panicle rather dense, branches 



ascending 68. P. addisonii. 



Spikelets not more than 1.7 mm. long. 



Blades white-margined; spikelets 1.6 to 1.7 mm. long, elliptic. 



76. P. TENUE. 

 Blades not white-margined; spikelets 1.3 to 1.4 mm. long, nearly- 

 globular 71. P. COLTJMBIANTJM Var. THINIUM. 



16b. Sheaths glabrous or puberulent only. 



17a. Spikelets spherical, not more than 1.8 mm. long. Blades cordate, 



ciliate at base See 10. Sphaerocarpa. 



17b. Spikelets not spherical. 



18a. Culms soon prostrate, vinelikc; branches divaricate. 



Plants bright green; culms lax; spikelets not more than 2.1 mm. 



long... _ 38. P. lucidum. 



Plants grayish green; culms stiff; spikelets 2.5 mm. long. 



39. P. SPHAGNICOLA. 



18b. Culms not vinelike; branches not divaricate. 



19a. Spikelets asymmetrically pyriform, strongly nerved; culms 



wiry See 12. Lancearia. 



19b. Spikelets not pyriform. 

 20a. Blades elongate, especially the upper, about 20 times as 

 long as wide. Spikelets about 2.5 mm. long, on long pedicels. 



24. P. BICKNELLII. 

 20b. Blades not elongate. (See continuation.) 

 (Continuation.) 

 21a. Spikelets 2 mm. long or more. 



Spikelets 2.5 to 3 mm. long; blades cordate, usually 1 cm. or more wide. 



Plants glaucous; basal blades conspicuously ciliate 110. P. mutabile. 



Plants not glaucous; basal blades ciliate at base only. 



Culms crisp-puberulent; blades rarely more than 1 cm. wide; spikelets about 2.5 mm. 



long 103. P. ashei. 



Culms glabrous or obscurely puberulent; blades usually 1.5 cm. wide or more; spike- 

 lets 2.7 to 3 mm. long 109. P. commutatum. 



Spikelets not more than 2.3 mm. long; blades not cordate, usually less than 1 cm. wide. 



Blades conspicuously ciliate, soft, lax, crowded at the base 11. P. ciltatum. 



Blades not ciliate or at base only, not crowded at the base. 



Blades not more than 6 mm. wide; plants not branching or rarely branching from 



near the base 8. P. werneri. 



Blades 7 mm. wide or more; plants branching from middle and upper nodes. 

 Primary blades spreading; panicle purplish; fruit exposed at summit. 



31. P. CLUTEI. 



Primary blades erect; panicle green; fruit covered 32. P. boreale. 



21b. Spikelets not more than 1.7 mm. long. 



Culms crisp-puberulent; spikelets turgid 71. P. columbianum. 



Culms glabrous. 



Blades white-margined, firm. 



Blades puberulent beneath, often above 76. P. tenue. 



Blades glabrous. 



Uppermost blades much reduced; culms branching from lower nodes only, the 



branches repeatedly branching 77. P. albomarginatum. 



Uppermost blades about as long as the others; culms bearing short branches from 



middle and upper nodes 78. P. trifolium. 



Blades not white-margined or very obscurely so (or if white margin is evident, spikelets 

 only 1.1 mm. long). 



Culms branching only at base; plants soft, light green 82. P. vernale. 



Culms branching at the nodes. 



Spikelets 1.1 mm. long; winter blades bluish green, not glossy. 



80. P. CONCINNIUS. 

 Spikelets 1.3 to 1.5 mm. long. 



Blades involute, falcate, with long stiff hairs on margin near base; plants stiff 



and wiry 86. P. breve. 



Blades not involute or at tip only, not falcate. 



Plants bright green; winter blades conspicuous, glossy green. 



79. P. FLAVOVIRENS. 



Plants olive; winter blades not conspicuous nor glossy.... 81. P. ensifolium. 



