Page No. 



la. Head with both ray and disc flowers; pappus of several to many scales 



Xanthooephaltov 63 



(matchbrush, snakeweed) 



2a. Leaves linear-filiform, less than 1 mm wide; heads tiny, cylin- 

 drical, 1 to 1.5 mm thick; ray flowers 4 to 5; disc flowers 



1 to 3 X. microoephala 64 



(small -headed matchweed) 



2b. Leaves linear, 1 to 2 mm wide; heads larger, flaring; ray flowers 



3 to 9, disc flowers 3 to 8 X. sarothrae 64 



(broom snakeweed) 



lb. Heads with only disc flowers or if ray flowers present then pappus of 

 capillary bristles or lacking. 



3a. Pappus of capillary bristles, at least in part. 



4a. Involucral bracts in a single series Tetradymia 58 



(horsebrush) 



5a. Stem vestiture evenly pannose; primary leaves forming rig- 

 id spreading or recurved spines; flowers 5 to 9 per head; 

 involucral bracts 5 to 6; heads solitary or paired in 

 upper axils; achenes with abundant long hairs that obscure 

 the pappus; pappus of small scales.... T. spinosa 62 



(spiny horsebrush) 



5b. Stem vestiture interrupted by linear streaks; spines, if 

 present, not spreading; flowers 4 per head; involucral 

 bracts usually 4; heads arranged in corymbose clusters at 

 ends of branches; achenes glabrous or with hair shorter 

 than the pappus; pappus of numerous bristles. 



6a. Primary leaves forming rigid spines.. nuttallii 61 



(Nuttall horsebrush) 



6b. Primary leaves not forming rigid spines, but may be 

 spinulose- tipped . 



7a. Primary leaves linear-subulate, 0.5 to 1 cm 

 long, mucronate or spinulose-tipped; herbage 



often becoming glabrate T. glabrata 60 



(little-leaf horsebrush) 



7b. Primary leaves linear-lanceolate to spatulate, 

 1 to 3 cm long, not mucronate or spinulose- 

 tipped; herbage permanently canescent or 



toraentose T. "anesoens 59 



(gray horsebrush) 



4b. Involucral bracts of 2 or more series Chrysothamnus 36 



(rabbitbrush) 



74 



