MANUAL OF THE GRASSES OF THE UNITED STATES 27 



Spikelets all of one kind. 



Spikelets sunken in tne cavities of the flattened corky rachis. 



123. Stexotaphrum. 

 Spikelets not sunken in the rachis. 



la. Spikelets subtended or surrounded by 1 to many distinct or more or 

 less connate bristles, forming an involucre. 



Bristles persistent, the spikelets deciduous 135. Setaria. 



Bristles falling with the spikelets at maturity. 



Bristles not united at base, slender, often plumose__ 136. Pexxisetum. 

 Bristles united into a burlike involucre, the bristles retrcrsely barbed. 



137. Cexchrus. 

 lb. Spikelets not subtended by bristles. 



Glumes or sterile lemma awned (awn short and concealed in the silky 

 hairs of trie spikelet in Tricholaena; awn reduced to a point in 

 Echinochloa colonum). 



Inflorescence paniculate; spikelets silky 134. Tricholaexa. 



Inflorescence of unilateral simple or somewhat compound racemes along 

 a common axis; spikelets smooth or hispid, not silky. 



Blades lanceolate, broad, thin; culms creeping 132. Oplismexus. 



Blades long, narrow; culms not creeping 133. Echixochloa. 



Glumes and sterile lemma awnless. 



2a. Fruit cartilaginous-indurate, flexible, usually dark colored, the 

 lemma with more or less prominent w hite hyaline margins, these 

 not inrolled. 

 Spikelets covered with long silky hairs, arranged in racemes, these 



panicled 120. Trichachxe. 



Spikelets glabrous or variously pubescent but not long-silky (some- 

 what silky in Digitaria villosa). 

 Spikelets in slender racemes more or less digitate at the summit of 



the culms 121. Digitaria. 



Spikelets in panicles. 



Fruiting lemma boat-shaped; panicles narrow. 



119. AXTHAEXAXTIA. 



Fruiting lemma convex; panicles diffuse 122. Leptoloma. 



2b. Fruit chartaceous-indurate, rigid. 



Spikelets placed with the back of the fruit turned away from the 

 rachis of the racemes, usually solitary (not in pairs). 

 First glume and the rachilla joint forming a swollen ringlike callus 



below the spikelet 124. Eriochloa. 



First glume present or wanting, not forming a ringlike callus below 

 the spikelet. 

 First glume present (next to the axis) ; racemes racemose along the 



main axis 125. Brachiaria. 



First glume wanting; racemes digitate or subdigitate. 



126. Axoxopus. 



Spikelets placed with the back of the fruit turned toward the rachis 



(first glume, when present, away from the axis) of the spikelike 



racemes, or pedicellate in panicles. 



Fruit long-acuminate; both glumes wanting. 127. Reimarochloa. 



Fruit not long-acuminate; at least one glume present. 



First glume typically wanting; spikelets plano-convex, subsessile 



in spikelike racemes 128. Pasp alum. 



First glume present; spikelets usually in panicles. 



Second glume inflatecl-saccate, this and the sterile lemma much 



exceeding the stipitate fruit 131. Sacciolepis. 



Secona glume not inflated-saccate. 



Culms woody, bamboolike; fruit with a tuft of down at the 



apex 130. Lasiacis. 



Culms herbaceous; no tuft of down at the apex of the fruit. 



129. Paxicim. 

 TRIBE 13. ANDROPOGONEAE 



Spikelets in pairs along a rachis, the usual arrangement being one of 

 the pair sessile and fertile, the other pedicellate and staminate or 

 neuter, rarely wanting, only the pedicel present ; fertile spikelet con- 

 sisting of one perfect terminal floret and, below this, a staminate or 

 neuter floret, the lemmas thin or hyaline, and two awnless glumes, one 

 or usually both firm or indurate. 



