352 



4. Panicle somewhat open, the branches elon- 

 gated, ascending (13) P. aiiritum 



4. Panicles dense, cylindrical. 



5. Spikelets 4 to 5 mm. long, the outer 



glumes strongly acuminate; aquatic (12) P. amplcxicaule 

 5. Spikelets 3 mm. long or less ; empty 

 glumes acute or only slightly acu- 

 minate. 

 6. Panicles dense, cylindrical, 

 about 5 mm. thick ; spikelets 



3 mm. long.. (14) P. indicum 



6. Panicles slender, caudiform, 

 about 3 mm. thick ; spikelets 



1.5 mm. long (15) P. inyosuroUlcs 



3. Spikelets small, in pairs or fascicles on the 



filiform branches of a lax simple panicle, the 



first and second glumes much shorter than the 



third and fourth. (§ Breviglume.) 



4. Panicle branches solitary or fascicled, 



elongated, filiform ; spikelets in scattered 



fascicles (10) P. nodosum 



3. Spikelets solitary, rarely binate, sessiled or pedi- 

 celled on the slender branches of a decompound 

 open panicle. (§Effusae.) 

 4. First glume equaling the spikelet or nearly 

 so. 

 5. Spikelets obtuse, 1.5 to 2 mm. long. 



6. Plant 30 to 40 cm. high ; leaves 

 ovate-lanceolate to broadly 

 lanceolate, the base cordate 

 and amplexicaul ; spikelets 



1.5 mm. long (17) P. brcvifolium 



6. Plant 1.5 to 2 m. high, subscan- 

 dent leaves lanceolate ; spike- 

 lets 2 mm. long (25) P. luxitrians 



5. Spikelets strongly acuminate, 2 to 

 3.5 mm. long. 

 6. Spikelets 3 to 3.5 mm. long ; 

 leaves about 20 cm. long, 



tuberculate-pilose (27) P. caudir/lume 



6. Spikelets 2 to 2.5 mm. long ; 

 leaves 5 cm. long or less, 



glabrous or nearly so (28) P. windnuacnsc 



4. First glume much shorter than the spikelet. 

 5. Annual erect leafy grasses. 



6. Glabrous (20) P. htimilc 



6. More or less pubescent or 

 pilose. 

 7. Spikelets 2 mm. long. 



8. Sheaths strongly 



tuberculate - h i s- 

 pid ; leaves and 

 panicles 20 cm. 



long or more (21) P. varniKiti 



8. Sheaths slightly 

 pilose to subgla- 

 brous ; leaves and 

 panicles 15 cm." 



long or less (22) P. liizonicnse 



7. Spikelets 3 to 4 mm. long. 

 8. Spikelets turgid, 

 ovate-obl oiig, 

 acute or cuspi- 

 date - acuminate ; 

 panicles thyrsi- 

 form ; cultivated ... (IS) P. niilidiciini 



