299 
Key to the Species. 
Involucre consisting of two spine-bearing valves, enclosing the 2-6 spikelets. 
Involucre armed at base. 
With shorter, generally reflexed spines; pedicels smooth; involucre 2-flow- 
ered, 
Involucre globose, pubescent. 1, C. tribuloides. 
Involucre ovate, smooth, 2. C. gracillimus. 
With erect barbed bristles; pedicels villous; involucre 4-6 flowered. 
3. C. echinatus. 
Involucre naked at base. 4. C. incertus. 
Involucre consisting of 2-4 rows of erect slender spines and bristles, at the base of the 
single spikelet. 5. C. myosuroides. 
I. CENCHRUS TRIBULOIDES L. Sp. Pl. 1050. 1753. 
C. Carolinianus Walt. Fl. Car. 79. 1788. Teste Pursh. 
Stems erect or ascending, generally decumbent at base, robust, 
2-6 dm, long, branching freely, sheaths generally strongly inflated, 
smooth, striate, compressed, the lower ones overlapping, the upper 
shorter than internodes, 3-8 cm. long; leaves 6-12 cm. long, 4-8 
mm. wide, smooth or scabrous, flat or slightly complanate ; spikes 
3-6 cm. long, generally partially included in upper sheaths, densely 
6-20-flowered; internodes of scabrous rachis 3-5 mm. long; invo- 
lucre 2-flowered, globose, pubescent to villous, yellowish to light 
green in color, smooth at base, spines 3-4 mm. long, slightly to 
Strongly ciliate at base, acutely barbulate at apex; spikelets 6-7 
mm. long, generally not exserted beyond the involucre; Ist glume 
hyaline, triangular, acute, about one-half as long as spikelet, spar- 
ingly scabrous, strongly I-nerved; 2d glume of firm texture, 
ovate, obtuse, about three-fourths as long as spikelet, scabrous, 
faintly 5-nerved at base, strongly so at apex; 3d glume firm, 
ovate, somewhat acute, about equaling spikelet, scabrous, 5-nerved; 
4th glume chartaceous, broadly ovate, acuminate, 5-7 mm. long, 
scabrous at apex, 5-nerved ; palet of first flower about as long as 
glume, 2-nerved, strongly scabrous between the nerves especially 
toward the apex, empty or enclosing 3 stamens; palet of per- 
fect flower chartaceous, broadly ovate, slightly shorter than glume, 
faintly 2-nerved, scabrous toward the apex; seed oblong-obovate, 
3 mm. long, 2—2.5 mm. broad, compressed. 
Extensively distributed in the United States ranging from 
Massachusetts to Ontario, Minnesota and Nebraska, south to Flor- 
ida and Mexico. Extremely variable, but its robust habit, gener- 
ally strongly inflated sheaths and light colored globose involucres 
will distinguish it from its nearest relatives. 
