blades mostly 3-6 dm. long, 2-5 cm. broad or larger, often glaucous, firm; 

 inflorescence a thick narowly ellipsoid buffy-white panicle 3-6 dm. long; spike- 

 lets 13-15 mm. long, usually 3-flowered; rachilla glabrous, with zones of abscis- 

 sion at lower part of each node; glumes subequal, cymbiform, thin, nearly as long 

 as the spikelet, each 3-nerved; lemmas (including the small calloused base) long- 

 pilose, 3-nerved, thin, cymbiform, obscurely round-keeled, long-attenuate to very 

 fine points. 



Established along irrigation ditches and streams (occasional in marshes), on 

 sand bars and levees of the Rio Grande in Rio Grande Plains, w. to the Trans- 

 Pecos and near rivers and lakes in s.e. and cen. Tex., summer-fall; widespread 

 in warmer areas, nat. to Old World, adv. in Tex. 



Planted for erosion control along roads in dune areas. 



14. Phragmites Trin. 



A genus of 3 species, cosmopolitan; we have one. 

 1. Phragmites communis Trin. Common reed. Figs. 93 and 94. 



Perennial reed with thick rhizomes; culms 1-3 m. tall, 5-15 mm. thick; ligule 

 a short tough lacerate fringe; blades flat. 1-4 cm. broad; panicle a large terminal 

 plume, many-branched and densely flowered; spikelets few-flowered, the lower 

 flowers empty or merely staminate, the rest perfect; rachilla abscising at the 

 upper part of each node, the fragments thus consisting of one floret with a 

 portion of the densely long-silky-villous rachilla below (not above) the node; 

 glumes lanceolate, shorter than the lowest lemma; lemmas lanceolate, glabrous, 

 about 1 1 mm. long. 



Locally abundant in marshes, seeps, along rivers, at streamsides and canal 

 banks, scattered throughout our region, fall; in most of the warmer parts of the 

 world. 



15. Tridens R. & S. Trtoens 



Tufted or rarely shortly rhizomatous perennials; culms erect (or in one species 

 scandent); ligule a white fringe or short fringed scale; blades mostly flat, elongate; 

 panicles terminal, diffuse or spikelike; spikelets not much laterally compressed, 

 several-flowered, all the florets perfect or the pistil of the uppermost usually abor- 

 tive; rachilla abscising just below the lemma nodes; lemmas broad, mostly apically 

 obtuse, emarginate and/or very shallowly cleft, 3-nerved (the midnerve and/or 

 the laterals in some species minutely excurrent), usually pubescent on the lower 

 half to two-thirds of the nerves (glabrous in T. albescens); paleas either glabrous 

 or short silky-hairy on the nerves and dorsally. 



A North American genus of about a dozen species, in some works enlarged 

 to include the related genus Erioneuron. 



1. Glumes as long as the spikelets or nearly so 1. T. strictus. 



1. Glumes about equaling the lowest lemma (2) 



2(1). Lemmas essentially glabrous (hair, if present, only at the basal callus of 

 the lemma); panicles spikelike, 8-23 cm. long 2. T. albescens. 



2. Lemmas pubescent, at least at the base of the lateral nerves 



3. T. ambiguus. 



1. Tridens strictus (Nutt.) Nash. 



Culms 8-17 dm. long, erect; sheaths not keeled; blades 3-8 mm. broad; panicle 

 spikelike, 1-3 dm. long, 9-15 mm. thick, with a few short appressed branches 

 near the base; glumes as long as to longer than the rest of the spikelet, viscid, 

 acuminate to a fine point, conspicuous in the panicle; lemmas about 3 mm. long, 

 the lateral nerves reaching the distal margin and in some specimens excurrent, 

 all 3 nerves hairy in the distal two-thirds the length. 



205 



