180 



CONTRIBUTIONS FROM THE NATIONAL- HERBARIUM. 



Nodes, at least the lower, bearded. 



Spikelets 1.5 to 1.6 mm. long, glabrous (occasional in- 

 dividuals with pubescent spikelets) 102. P . microcarpon . 



Spikelets 2 mm. or more long. 



Spikelets glabrous, 2 mm. long; autumnal form top- 

 heavy-reclining 110. P. barbulatum. 



Spikelets pubescent. 



Blades all velvety; autumnal form sparingly 



branched 105. P. annulum. 



Blades glabrous, or only the lower pubescent or 

 velvety; autumnal form freely branching'. 

 Spikelets 2 mm. long; autumnal form pro- 

 fusely branching. 

 Fruits slightly exposed at ma,turity; 

 upper sheaths viscid-spotted; 

 autumnal form erect or reclining. . 103 . P. nitidum. 

 Fruits covered at maturity; sheaths not 

 viscid - spotted ; autumnal form 

 decumbent, with flabellate - 



fascicled branches; Mexican 104. P . muUirameum. 



Spikelets 2.2 mm. or more long; autumnal 

 form less profusely branching. 



Sheaths and upper nodes glabrous 107. P. clutei. 



Lower sheaths and all nodes pubescent. . 106. P. mattamusheet- 



ense. 

 Nodes not bearded. 



Spikelets pubescent. 



Culms erect, never becoming vine-like. 



Primary blades spreading; panicles purplish; 



fruit exposed at summit 107. P. clutei. 



Primary blades erect; panicles green; fruit cov- 

 ered (wood forms with spreading blades 

 may be distinguished from P. dichotomum 



by pubescent spikelets, 2.2 mm. long) 108. P. boreale. 



Culms soon prostrate, vine-like; branches divaricate. 

 Plants bright green, culms lax; spikelets not 



over 2.1 mm. long 114. P. lucidum. 



Plants grayish green, culms stiff; spikelets 2.5 



mm. long 115. P. sphagnicola. 



Spikelets glabrous. 



Culms soon prostrate. 



Plants bright green, culms lax; spikelets not 



over 2.1 mm. long 114. P. lucidum. 



Plants grayish green, culms stiff; spikelets 2.5 



mm. long 115. P. sphagnicola. 



Culms erect, or the autumnal form topheavy, never 

 prostrate. 

 Spikelets not over 1.6 mm. long; panicles nar- 

 row ; plants glaucous bluish green 113. P . caerulescens . 



Spikelets 2 mm. or more long; panicles open. 

 Blades erect, firm; spikelets turgid, strongly 



nerved; plants grayish olive green 112. P. roanokense. 



Blades spreading; spikelets not turgid. . 



