608 MISC. PUBLICATION 42 3, IT. S. DEPT. OF AGRICULTURE 



8. Spines 2 to 6 cm. iong (9) ■ 



9. Spines 4 or more at an areole, siender, acicular; joints 8 to 

 20 cm. long, elliptic or oblong, armed over the entire sur- 

 face, usually thickened and even subterete. 



7. O. ERINACEA. 



9. Spines 1 to 4 at an areole, stout; joints 5 to 12 cm. long, 



elliptic to obovate, spiny only on the upper half, often 



somewhat thickened 8. O. rhodantha. 



2. Fruit fleshy; flowers yellow (10). 



10. Plant small or low, at most not more than 0.5 m. high; joints rarely more 



than 15 cm. long, obovate or rarely orbicular, usually lead green, 



transversely marked or wrinkled, not tuberculate; areoles 15 to 25 



mm. apart (11). 



11. Roots definitely tuberous; spines 1 to 3 at an areole, mostly confined 



to the upper areoles 9. O. plumbea. 



11. Roots not tuberous; spines 1 to several at an areole, often borne well 



down on the sides of the joint 10. O. rafinesqtjei. 



10. Plant commonly large, bushy, more than 0.5 m. high; joints mostly more 

 than 20 cm. long, not tuberculate, not transversely marked (12). 



12. Areoles 15 to 30 mm. apart; joints prevailingly orbicular, usuallv 6 to 



10 mm. thick (13). 

 13. Principal spines deflexed or appressed, yellow, translucent, subseta- 

 ceous, 2 to 4 cm. long; joints yellowish green, usually crowded 

 and orbicular, the areoles soon abundantly filled with large 



glochids 11. O. CHLOROTICA. 



13. Principal spines spreading, never appressed, opaque, reddish brown 

 or darker, coarse, 4 to 7 cm. long; joints purple-tinged (14). 

 14. Central spines wanting, or few and confined almost entirely to 

 the upper margin; plant of erect bushy habit; joints crowded, 



orbicular or ovate 12. O. santa-rita. 



14. Central spines never wanting, produced from the sides as well as 

 the margin of the joint; plant of rather open habit; joints not 



crowded, orbicular or obovate 13. O. macrocentra. 



12. Areoles 30 to 50 mm. apart; joints usually broadest above the middle, 



seldom less than 10 mm. thick (15). 



15. Joints yellowish green, the spines short and few, or wanting; plant 



bushy (16). 



16. Joints obovate-oblanceolate, nearly or quite spineless, asymmetric, 



25 to 30 cm. long, often purple around the areoles; fruits mostly 



with a long stipelike base 14. O. laevis. 



16. Joints relatively broader, not entirely spineless; fruit not con- 

 tracted at base, or shortly so (17). 

 17. Joints obovate, spiny on the upper margin and more or less so 

 on the sides, rich yellow green, often purple around the 

 areoles, inclined to be asymmetric, about 30 cm. long; spines 

 at first yellowish, soon whitened, on old joints darkened, 

 deflexed, often curved, mostly less than 2.5 cm. long. 



15. O. gilvescens. 

 17. Joints orbicular-obovate, usually unarmed except along or near 



the upper margin, symmetrical, not purple around the areoles, 

 20 to 25 cm. long; principal spines reddish brown toward the 

 base, in age whitened, 2 to 3 (rarely 4.5) cm. long. 



16. O. FLAVESCENS. 



15. Joints commonly gray green, with more numerous and longer spines, 

 obovate (18). 

 18. Branches prostrate or decumbent; joints 15 to 25 cm. long; princi- 

 pal spines not appreciably flattened or angled, straight, spread- 

 ing, often spirally twisted, 5 to 6 cm. long, reddish brown, 

 variegated, or whitened; center of the flower darkened. 



17. 0. PHAEACANTHA. 



18. Branches ascending or erect, or the lowermost prostrate, forming 

 a bushy plant; joints commonly more than 25 cm. long; principal 

 spines stout, more or less angled or flattened, slightly curved, 

 2 to 4 cm. long, the lower or lowest deflexed and whitened, 

 those at the facial areoles arranged in a characteristic figure 

 somewhat resembling a bird's foot; center of the flowers only 

 slightly darkened 18. 0. engelmannii. 



