140 



The; cactaceae. 



Key to Species. 



More or less bushy plants. 



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

 Spines dark brown, stout, rigid. 



Plant pale green to purpUsh; spines up to 12 cm. long 138. O. macrocentra 



Plant dull dark green; spines 6 cm. long or less 139. O. tardospina 



Spines pale brown, flexible or subulate. 



Usually abundantly spiny 140. 0. gosseliniana 



Usually spineless or some areoles with i setaceous deflexed spine 141. O. santa-rita 



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 ^ ide 142. O. anguslata 



Joints broadly obovate to orbicular. 

 Flowers yellow. 



Spines subulate, brown at least in part. 



Plant light green 143. 0. atrispina 



Plant bluish green or grayish green. 



Plant erect, 2 meters high or less 144. 0. azurea 



Plant bushy, rarelj- over i meter high 145. 0. phaeacantha 



Plant prostrate ' 146. O. mojavensis 



Spines acicular, nearly white 147. O. covillei 



Flowers magenta 148. O. vaseyi 



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



Spines clear brown nearly throughout 149. 0. occidentalis 



Spines nearlj- white above or throughout. 



Spines with dark brown bases 150. 0. engelmannii 



Spines whitish throughout 151. O. discata 



Small creeping plants 152. O. rastrera 



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, 

 10 to 20 cm. long, often bluish 

 or purplish, sometimes spineless 

 but usually bearing spines at the 

 uppermost areoles; spines i or2, 

 rarely 3 together, usually brown- 

 ish or black but sometimes white 

 above, slender, erect or porrect, 

 4 to 7 cm. long ; flowers j^ellow, 

 often drying red, 7.5 cm. broad; 

 sepals ovate, acuminate; ovarv 

 with few areoles, these bearing 

 brown glochids; filaments ver}- 

 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, IMexico. 



This species, especially 

 the forms that have bluish 

 and purplish joints, are very 

 sho\yy. SeedUngs sometimes 

 produce long, silky hairs from 

 the areoles, in this respect 



resembling the Crinijcrae. pia. lye.-Opuntia macrocentra. X0.5. 



