STANDLEY — TREES AND SHRUBS OF MEXICO. 919 



Stems usually erect and stout ; ribs more or less tuberculate. 

 Plants forming large mounds, sometimes with 500 to 800 joints ; spines 



white, long and flexuous 6. E. mojavensis. 



Plants in much smaller clusters ; spines brownish or grayish, not 

 long and flexuous. 



Plant body with 12 to 14 ribs 7. E. leeanus. 



Plant body with 5 to 11 ribs (in one species 12). 

 Axils of flower scales filled with long cobwebby hairs. 

 Flowers 5 to 6 cm. long; spines yellowish at first. 



8. E. polyacanthus. 



Flowers 3 cm. long; spines reddish at first 9. E. pacificus. 



Axils of flower scales bearing short hairs. 



Stems elongate and thin 10. E. acifer. 



Stems short and thick 11, E. rosei. 



Flowers usually purple, sometimes yellow or greenish yellow, rarely pink 

 or nearly white, broad, rotate to campanulate. opening in sunlight, 

 closing at night. 

 Flowers yellow or greenish white. 



Plants densely cespitose 12. E. maritimus. 



Plants usually solitary. 

 Ribs very stout. 



Ribs 5 to 8; spines on flower tube and ovary short. 



13. E. subinerinis. 

 Ribs S or 9 : spines on flower tube and ovary acicular. 



14. E. luteus. 

 Ribs low. usually hidden by the spines. 



Flowers 2.5 cm. long or less 15. E. chloranthus. 



Flowers 5 to 10 cm. long. 



Flowers greenish white 16. E. grandis. 



Flowers yellow-red. 

 Central spines in more than one row__17. E. dasyacanthus. 



Central spines in one vertical row 18. E. ctenoides. 



Flowers purple. 

 Stems weak, slender, and creeping. 

 Stems 2 cm. thick or less. 

 Areoles distant ; spines not interlocking. 

 Perianth segments narrowly oblong or linear-oblanoeohitp. 



19. E. blanckii. 



Perianth segments oblong-erose 20. E. pentalophus. 



Areoles approximate; spines densely lnterlocking__21. E. sciurus. 



Stems 3 to 4 cm. thick 22. E. cinerascens. 



Stems stout, usually erect or ascending. 

 Areoles elliptic to circular, closely set, often with pectinate spines. 

 Areoles circular ; spines not pectinate. 



Areoles about 5 mm. apart; spines densely interlocking. 



28. E(. scopuloimm. 

 Areoles about 1 cm. apart; spines scarcely interlocking. 



29. E. rostteri. 

 Areoles elliptic; spines pectinate. 



Central spine often very long 23. E. adustus. 



Central spine, if present, short. 



Spines of ovary and flower tube slender and wp:i1c. the sur- 

 rounding hairs long and cobwebby__24. E. reichenbachii. 



