GENERA OF GRASSES OF THE UNITED STATES. 19 



The spikelets are usually awnless, but the glumes and sterile lemma 

 are awned in Echinochloa and Oplismenus, and the second glume and 

 sterile lemma in Tricholaena. In Eriochloa and in some species of 

 Brachiaria the fertile lemma is awn-tipped. In Chaetochloa there 

 are, beneath the spikelet, one or more bristles, these representing 

 sterile branchlets. In Pennisetum similar bristles form an involucral 

 cluster, falling with the spikelet. In Cenchrus the bristles are 

 united, forming a bur. The spikelets are of two kinds in Amphi- 

 carpon, aerial and subterranean. The culms are woody and perennial 

 in Lasiacis and Olyra. 



Key to the genera of Paniceae. 



la. Spikelets of two kinds 2 



2a. Spikelets all perfect, but those of the aerial panicle not perfect- 

 ing grains; the fruitful spikelets borne on subterranean 



branches 127. AMPHICARPON. 



2b. Spikelets unisexual, the pistillate above, the staminate below on 

 the branches of the same panicle ; blades broad, elliptic. 



OLYRA. (See p. 252.) 



Ib. Spikelets all of one kind 3 



3a. Spikelets sunken in the cavities of the flattened corky rachis. 



112. STENOTAPHEUM. 



3b. Spikelets not sunken in the rachis 4 



4a. Spikelets subtended or surrounded by 1 to many distinct or 



more or less connate bristles, forming an involucre 5 



5a. Bristles persistent, the spikelets deciduous- 124. CHAETOCHLOA. 



5b. Bristles falling with the spikelets at maturity G 



6a. Bristles not united at base, slender, often plumose. 



125. PENNISETUM. 

 6b. Bristles united into a burlike involucre, the bristles 



retrorsely barbed 126. CENCHRUS. 



4b. Spikelets not subtended by bristles 7 



7a. Glumes or sterile lemma awned (awn short and con- 

 cealed in the silky hairs of the spikelet in Tricholaena, 



awn reduced to a point in Echinochloa colonuni) 8 



8a. Inflorescence paniculate ; spikelets silky_ 123. TRICHOLAENA. 

 8b. Inflorescence of unilateral simple or somewhat com- 

 pound racemes along a common axis; spikelets 



smooth or hispid, not silky 9 



9a. Blades lanceolate, broad and thin ; glumes 2- 

 lobed, awned from between the lobes. 



121. OPLISMENUS. 

 9b. Blades long and narrow ; glumes awned from 



the tip 122. ECHINOCHLOA. 



7b. Glumes and sterile lemma awnless 10 



lOa. Fruit cartilaginous-indurate, flexible, usually dark 

 colored, the lemma with more or less prominent 



white hyaline margins, these not inrolled 11 



Ha. Spikelets covered with long silky hairs, ar- 

 ranged in racemes, these panicled 109. VALOTA. 



lib. Spikelets glabrous or variously pubescent but 



not long-silky 12 



