FLOWERING PLANTS AND FERNS OF ARIZONA 609 



1. Joints not flattened (19). 



19. Spines sheathed only at the tip, strongly flattened, scabrous; plant low, 

 creeping; joints more or less clavate; base of the fruit contracted (20 . 

 20. Principal spines flattened, but slender and not daggerlike; joints 2.5 to 



7 cm. long, narrowly clavate; flowers purple 19. O. pui/heli.a. 



20. Principal spines usually daggerlike, sharply 2-edged; joints broader; 

 flowers yellow (21). 

 21. Tubercles tending to be confluent in more or less distinct rows; joints 

 subcylindric, narrowed only toward the base, 10 to 20 cm. long, 3 



to 4 cm. thick 23. O. wrightiaxa. 



21. Tubercles all distinct; joints tapering from near the apex to the base 

 (22). 



22. Joints mostly 3 to 7 cm. long 20. O. parishii. 



22. Joints mostly more than 7 cm. long (23). 



23. Joints very stout, 3 to 5.5 cm. thick, 10 to 20 cm. long; tubercles 

 25 to 4o mm. long, 8 to 11 mm. wide, 10 to 15 mm. high. 



21. O. STAXLYI. 



23. Joints not so stout, 2 to 3.5 cm. thick, 8 to 10 or 15 cm. long; 

 tubercles 10 to 30 mm. long, 5 mm. wide, 5 to 10 mm. high. 



22. O. KUXZEI. 



19. Spines completely invested with a loose, papery sheath, minutely barbed 



but not scabrous; plant not creeping (except in low forms of 0. ivhipplei), 



bushy or arborescent; joints cylindric (24). 



24. Ultimate joints rarely more than 1.5 cm. thick; spines usually fewer than 



6 at an areole (25). 



25. Joints tessellate with somewhat rhombic tubercles; fruit dry, bristly; 



joints gray, often purple-tinged 24. O. ramosissima. 



25. Joints not tessellate; fruit fleshy (26). 



28. Fruit conspicuously tuberculate, 2.5 to 4 cm. long; plant erect, or 

 low and creeping; joints prominently tuberculate; principal spines 



1 to 4; flowers greenish yellow 35. O. whipplei. 



26. Fruit not tuberculate when fully mature (27). 



27. Flowers purplish; spines 1 to 6, commonly 4, at an areole; tubercles 

 long, narrow, rather prominent but not crowded; plant an 

 openly branched shrub about 1 m. high without a definite 

 trunk; fruit 2 to 2.5 cm. long, scarlet-tinged. 



26. O. TETRACAXTHA. 



27. Flowers yellow (28). 



28. Flowers greenish yellow; principal spines 1 to 4 at an areole; 

 joints strongly tuberculate, the tubercles crowded and only 



about twice as long as wide 35. O. whipplel. 



28. Flowers bronze yellow; spines usually solitary at the areoles of 



the ultimate joints; tubercles low, not prominent, not 



crowded (29). 



29. Fruit 2.5 to 4 cm. long, greenish yellow, obovoid or clavate, 



often proliferous; ultimate joints 7 to 10 mm. in diameter. 



25. O. ARBUSCULA. 



29. Fruit 10 to 18 mm. long, obovoid, scarlet; branches 3 to 5 

 mm. thick; plant less than 1 m. high, sparingly branched 



except toward the base 27. O. leptocaulis. 



24. Ultimate joints more than 1.5 cm. thick; spines 6 to 30 at an areole (30). 

 30. Fruit not persisting for more than 1 season (31). 



31. Fruit fleshy, bristly with numerous setaceous, readily detached 

 spines; joints 3 to 5 cm. thick, clavate, impenetrably armed, 

 readily detached; tubercles nearly as broad as long, somewhat 

 4-sided; spines and sheaths straw-colored; flowers greenish yellow, 



the outer segments pink-tinged 28. O. bigelovii. 



31. Fruit drv, the spines stout, firmlv attached; joints less than 3 cm. 



thick (32). 



32. Ultimate joints mostly less than 10 cm. long; tubercles about 



twice as long as wide, not strongly compressed except in new 



growth; plant typically with a well-developed trunk and very 



dense crown, 1 to 1.5 m. high; flowers greenish yellow. 



29. O. ECHIXOCARPA. 



32. Ultimate joints mostly 15 to 30 cm. long; tubercles twice or thrice 

 as long as wide, stronglv compressed; trunk rarelv well de- 

 veloped (33). 



