108 POACEAE 



niwly ovoid, iiciite, o mm. long, tlie flowering scale acute, narrowly ovoid, finely trans- 

 versely I'ugose. 



On shell islands or keys, or sometimes in old fields, southern Florida and Texas. Summer to fall. 



15. Chaetochloa villosissima Scribn. & Mer. Perennial. Stems branching at the 

 base, 4-10 dm. tall, comiiressed, glabrous : leaf-sheatbs loose, compressed, rough above, 

 nearly smooth below, densely villous above ; blades linear, 1.5-3 dm. long, 6-8 mm. wide, 

 tapering toward the base, pubescent on botli surfaces with white spreading hairs : panicle 

 about 2 dm. long, 2-3 cm. in diameter, tapering to the apex : bristles single, green, 1.5-2.5 

 cm. long: spikelets ovate-lanceolate, acute, 2.5-3 mm. long, the flowering scale abruptly 

 short-apiculate at the incurved tip, rather finely transversely rugose below and punctate 

 above. 



In dry soil, Texas. Spring and summer. 



16. Chaetochloa Grisebachii (Fourn. ) Scribn. Annual. Stems 1.5-8 dm. tall, 

 Ijranching at tiie base, slender, glabrous : leaf-sheaths loose, compressed, sparingly strigose, 

 the margins ciliate ; blades lanceolate, slightly narrowed at the cordate base, 5-10 cm. 

 long, 5-10 nun. wide, rough and sparingly short-pubescent : bristles single or in pairs, 

 widely spreading, purple or sometimes green, 5-15 mm. long : spikelets ovoid, 2 mm. long, 

 acute, the flowering scale nearly 2 mm. long, ovoid, acute, very finely transversely rugose 

 below. 



In dry soil, Texas to Arizona. Also in Mexico. Spring to fall. 



17. Chaetochloa polys«'achya (Scheele) Scribn. & Mer. Perennial. Stems stout, 

 H-12dm. tall, branching at the base, glabrous: leaf-sheaths glabrous or sparingly pubes- 

 cent, the margins ciliate : blades 1.5-3 dm. long, 1.5-2 cm. wide, abruptly narrowed at the 

 V)ase, rough, the lower ones sometimes strigose : panicle lax, pale green, 1.5-3 dm. long, 

 1.5-3 cm. in diameter : bristles single, 8-12 mm. long, green, slender, flexuous : spikelets 

 narrowly ovoid, acute, 2 mm. long, the flowering scale narrowly ovoid, acute, transversely 

 undulate-i'ugose at the base. 



In dry soil. Texas. Summer and fall. 



18. Chaetochloa caudata (Lam.) Scribn. Annual. Stems 3-6 dm. tall, much 

 branched from the l)ase, glabrous, slender : leaf -sheaths pubescent, ciliate on the margin ; 

 blades linear, 1-3 dm. long, 3-5 mm. wide, pubescent, filiform-attenuate at the apex : 

 panicle 5-15 cm. long, attenuate, the rachis slender and flexuous, pilose : bristles single, 

 flexuous, 4-10 mm. long: spikelets ovoid, acute, the flowering scale ovoid, acute, short- 

 apiculate, transversely undulate-rugose below. 



In dry soil, New Jersey to Florida and Alabama, Also in tropical America. Spring to fall. 



36. CENCHRUS L. 

 Annual or perennial grasses, with flat, convolute or complanate leaf-blades and terminal 

 spikes. Spikelets 2-6, in an ovate or globose involucre, consisting of two thick hard 

 valves which are exteriorly armed with stout spines and sometimes also with basal bristles 

 which are thickened at the base, the involucres articulated to the rachis and readily de- 

 ciduous, carrying the persistent spikelets with them. Scales 4, awnless, the first and sec- 

 ond empty, the first small or minute, the third equalling or longer than the second, enclos- 

 ing a palet and also sometimes a staminate flower, the foui'th scale chartaceous, firmer, 

 enclosing a palet of similar texture and a perfect flower. Stamens 3. Styles often connate 

 at the very base. Stigmas plumose. BuR Grass. Sand Bur. Sand Spur. Cockspur. 



Involucre armed at the base : 



With shorter generally retiexed spines, the base of the involucre glabrous : in- 

 volucres containing 2 spikelets. 

 Involucre pubescent : 



Usually less than 5 mm. broad. 1. C. tribuloidrs. 



6-8 mm. broad. 2. C. macrocrphalus. 



Involucre glabrous. 3. C. gracilUmus. 



With erect barbed bristles, the base of the involucre villous: involucres con- 

 taining 4-6 spikelets. 4. C. echinatus. 

 Involucre naked at the base. 5. C.incerius. 



1. Cenchrus tribuloides L. Stems at first erect, later prostrate and forming mats, 

 2-6 dm. long, branching : leaf-sheaths compressed ; blades 6-12 cm. long, 4-8 mm. wide, 

 smooth or rough, usually Hat : spikes 3-6 cm. long : involucres 6-20, 3-5 mm. broad, en- 

 closing 2 spikelets, pubescent, tlie spines 3-4 mm. long : spikelets 6-7 mm. long, usually 

 not exserted bevond the involucre. 



In dry sandy places, Massachusetts, Ontario, Minnesota and Nebraska to Florida, Texas and 

 Mexico. Very variable. Spring to fall. 



