218 PROCEEDINGS BOSTON SOCIETY NATURAL HISTORY. 



c. Spikelets 3 mm. or more long. 



d. Leaves linear-elongated, not over 5 mm. wide P. strictum. 



d'. Leaves oblong-lanceolate to ovate-lanceolate, more than 1 dm. 

 wide, 

 e. Spikelets 3 mm. long; at least the lower sheaths papillose-hispid. 



P. clandestinum. 

 e'. Spikelets 3 . 5 to 4 mm. long. 



Panicle spreading; blades 2.5 cm. or more wide. . . .P. latifoKum. 

 Panicle narrow; blades rarely over 1.8 cm. wide. 



P. xanthophysum. 

 c'. Spikelets less than 3 mm. long. 



/. Spikelets glabrous P. dichotomum. 



f. Spikelets pubescent. 



g. Blades elongated, not over 5 mm. long; secondary panicles 

 from the base only or none. 



Sheaths glabrous P. Werneri. 



Sheaths pilose P. KnearifoKum. 



g'. Blades usually not conspicuously elongated; secondary 

 panicles not at the base. 

 h. Sheaths, or all but the lowest glabrous. 



i. Cuhns crisp-puberulent P. umbrosum. 



i'. Culms glabrous. 



Spikelets 1.5 to 1.8 mm. long; in dry, sandy ground, 



in the valley P. sphaerocarpon. 



Spikelets 2.2 to 2.8 mm. long; in moist upland. 



P. boreale. 

 h'. Sheaths pubescent. 



j. Sheaths puberulent, not pilose P. umbrosum. 



j'. Sheaths spreading, or appressed pilose or velvety. 

 k. Pubescence spreading. 

 I. Blades stiff, glabrous above, or with a few hairs. 



P- tennesseense. 

 I'. Blades pubescent above, or if glabrous lax. 

 m. Upper surface of blades with erect hairs 3 to 5 

 mm. long, axis of panicle long pilose. 



P. implicatum. 

 m'. Upper surface of blades with short or some- 

 what appressed pubescence. 

 Blades stiff; spikelets obovate; rare. 



P- huachucae. 

 Blades lax; spikelets elUptical; common. 



P. huachucae, var. fasciculatum. 

 k'. Pubescence crisp-appressed. 

 Spikelets 1.9 mm. long; blades 5 to 6 cm. long; plants 



blue-green P. tsugetorum. 



Spikelets 1.7 mm. long; blades 3 to 5 cm. long; plants 

 gray-green P. heterophyllum. 



