HITCHCOCK AND CHASE — NORTH AMERICAN GRASSES. 51 



In all our species the bur varies in size and in the length of the spines. This 

 variation is not so important, systematically, as it would seem at first sight, 

 since the bur is only a fascicle of branchlets and as such varies relatively 

 much less than do the branchlets of an ordinary panicle. The spikelets in a 

 single bur are unequally developed ; usually one is larger with plumper grain 

 than the rest. In the illustrations it is these better-developed spikelets that 

 are shown and their measurements that are given in the descriptions. 



In three specimens of C. pauciflorus (Pammel's no. 657 from Des Moines, Iowa, 

 a plant collected by Jones at Grinnell, Iowa, and Hitchcock's no. 6128, from 

 Oaxaca, Mexico,) the lowest burs are undeveloped, the well-developed spikelet 

 being naked or having a few rudimentary bristles below it on the very short 

 peduncle. 



KEY TO THE SPECIES. 



Involucral lobes united at the base only ; spikes dense. 



Plants perennial ; involucral lobes terete, scabrous. . . . 1. C. myosuroides. 

 Plants annual ; inner involucral lobes sulcate down the outside, densely 



villous-ciliate within 2. C. catharticus. 



Involucral lobes united above the base. 



Blades involute, squarrose, numerous, conspicuously distichous, not over 



» 2.5 cm. long, about 1 cm. apart 3. C. distichophyllus. 



Blades not involute and squarrose, nor conspicuously distichous, much 

 longer and farther apart. 

 Involucre with a ring of slender bristles at base. Plants annual. 



Bristles antrorsely scabrous, much exceeding the involucral lobes. 



4. C. pilosus. 

 Bristles retrorsely barbed, not much exceeding the involucral lobes. 

 Burs, excluding the bristles, not over 4 mm. wide, numerous, 

 crowded in a long spike; lobes of the involucre inter- 

 locking, not spinelike 5. C. viridis. 



Burs, excluding the bristles, 5 to 7 mm. wide, not densely 



crowded ; lobes of the involucre erect or nearly so or rarely 



one or two lobes loosely interlocking, the tips spinelike. 



Spikelets about 5.5 mm. long; involucral lobes villous at 



base within 6. C. echinatus. 



Spikelets 6.5 mm. long ; involucral lobes long-ciliate except 



at summit 7. C. insularis. 



Involucre with flattened spreading spines, no ring of slender bristles 

 at base. 

 Body of the bur ovate, not over 3.5 mm. wide, tapering at base ; 

 plants perennial. 

 Burs glabrous ; spines 4 to 6 mm. long. . . . 8. C. gracillimus. 

 Burs pubescent ; spines rarely over 4 mm. long, usually shorter. 

 Body of bur 3 to 3.5 mm. wide ; spines 3 to 4 mm. long. 



9. C. incertus. 

 Body of bur less than 3 mm. wide ; spines 2 to 3 mm. long. 



10. C. microcephalus. 

 Body of the bur globose, 5 mm. wide or more, not tapering at base ; 

 plants annual. 

 Burs, including spines, 7 to 8 mm. wide, finely pubescent. 



11. C. pauciflorus. 

 Burs, including spines, 10 to 40 mm. wide, densely woolly. 



Burs several to many; spines not over 8 mm. long. 



12. C. tribuloides. 

 Burs 1 to 4; spines 1 cm. long or more. . . 13. C. p umerL, 



