140 



THE CACTACEAE. 



KEY TO SPECIES. 



More or less bushy plants. 



Joints thin; spines, when present, very long and confined to the upper and middle areoles. 

 Spines dark brown, stout, rigid. 



Plant pale green to purplish ; spines up to 12 cm. long 



Plant dull dark green ; spines 6 cm. long or less 



Spines pale brown, flexible or subulate. 



Usually abundantly spiny 



Usually spineless or some areoles with i setaceous deflexed spine 



Joints thick ; spines not confined to the upper and middle areoles. 



Joints relatively small, seldom over 15 cm. broad; plants relatively low. 



Joints narrowly obovate, about twice as long as Side 



Joints broadly obovate to orbicular. 

 Flowers yellow. 



Spines subulate, brown at least in part. 



Plant light green 



Plant bluish green or grayish green. 



Plant erect, 2 meters high or less 



Plant bushy, rarely over i meter high 



Plant prostrate 



Spines acicular, nearly white 



Flowers magenta 



Joints relatively large, mostly over 15 cm. broad; plants relatively tall. 



Spines clear brown nearly throughout 



Spines nearly white above or throughout. 



Spines with dark brown bases 



Spines whitish throughout 



Small creeping plants 



138. Opuntia macrocentra En- 

 gelmann, Proc. Amer. 

 Acad. 3: 292. 1856. 



Somewhat bushy, with as- 

 cending branches, 6 to 9 dm. 

 high; joints orbicular to oblong, 

 or sometimes broader than long, 

 i o to 20 cm. long, often bluish 

 or purplish, sometimes spineless 

 but usually bearing spines at the 

 uppermost areoles ; spines i or 2, 

 rarely 3 together, usually brown- 

 ish or black but sometimes white 

 above, slender, erect or porrect, 

 4 to 7 cm. long; flowers yellow, 

 often drying red, 7.5 cm. broad; 

 sepals ovate, acuminate; ovary 

 with few areoles, these bearing 

 brown glochids; filaments very 

 short; fruit 3 to 6 cm. long, pur- 

 ple; seeds 4 to 4.5 mm. broad. 



Type locality: Sand hills 

 on the Rio Grande near El 

 Paso, Texas. 



Distribution: Western 

 Texas to Eastern Arizona 

 and Chihuahua, Mexico. 



This species, especially 

 the forms that have bluish 

 and purplish joints, are very 

 showy. Seedlings sometimes 

 produce long, silky hairs from 

 the areoles, in this respect 



resembling the CrinijeraC. FIG. 176. Opuntia macrocentra. Xo. 5 . 



138. O. macrocentra 



139. O. tardospina 



140. O. gosseliniana 



141. O. santa-rita 



142. O. angustata 



143. O. atrispina 



144. O. azurea 



145. O. phaeacantha 



146. O. mojavensis 



147. O. covillei 



148. O. vaseyi 



149. 0. occidentalis 



150. O. engelmannii 



151. O. discata 



152. O. rastrera 



