CACTUS FAMILY 159 



1. OPIJNTIA Hill. PiiiCKLY Pear, Cholla., Indian Fkj, Tkeii; Cactus. 



luternodes cylindrical; spines covered with a delicate sheath. 



Internodes very fleshy, turgid, easily detached. 1. O. echinocarpa. 



Internodes less fleshy, not easily detached. 



Fuit dry; seeds angular. 2. O. acanlhocarpa. 



Fruit fleshy; seeds not angular. 



Plants tall (1 to 3 meters high); flowers red. 3. O. arborescens. 



Plants low and spreading (less than 1 meter iiigh). 



Spines yellow; flowers olive-green. '• 4. O. Davisii. 



Spines wliite; flowers yellow. 5. O. Whipplei. 



Internodes flat and broad; spines not covered by a sheatli. 

 Pulvini not densely woolly. 



Fruit dry when mature, usually very spiny. 



Internodes very fleshy, often terete in section, the terminal one easily break- 

 ing loose. 6. O. fragilis. 

 Internodes never terete in section, but always much broader than thick. 

 Internodes somewhat turgid; spines very long. 7. O. rutila. 

 Internodes flat; spines medium length. 

 Petals normally yellow. 



Spines stout, dark brownish. 8. O. polyacanlha. 



Spines weak, white. 9. O. Schweriniana. 



Petals red. 



Filaments red. 10. O. rhodantha. 



Filaments yeUow. 11. O. xanthoslemma. 



Fruit juicy, usually naked or nearly so. 



Internodes pubescent, always spineless. 12. O. basilaris. 



Internodes glabrous. 



Internodes often naked or sometimes bearing long, stout spines. 



Internodes pale, somewhat glaucous. 13. O. humifusa. 



Internodes deep green. 14. O. Greenei. 



Internodes generally bearing several spines from each areole. 



Spines twisted. 15. O. tortispina. 



Spines not evidently twisted. 



Low and procumbent; internodes 5 to 10 cm. broad. 



16. O. camanchica. 

 Stout and somewhat ascending; internodes 12 cm. or more broad. 



17. O. utahensis. 

 Pulvini densely long-woolly. 



Plant erect; one spine deflexed. 18. O. Palmeri. 



Plant prostrate; all spines defle.Ked. 19. O. rubrifolia. 



2. ECHINOCEREUS Engelm. Hedgehog Cereus. 



Corolla greenish; ribs of the stem about 13. 1. E. viridiflorus. 



•Corolla red or purple; ribs usually less than 13. 

 Ribs of the stem 8-12. 



Flowers scarlet; central spines 2-7. 



Central spines all terete; flowers 4-6 cm. long, yellowish inside. 



2. E. aggregatus. 

 Lower central spine quadrangular; flowers 8-10 cm. long. 3. E. Roemeri. 

 Flowers purple or violet. 



Central spines 4-5, straight. 4. E. Engelmanni. 



Central spines solitary, bulbous at the base. 



Central spine terete; radials 12-25 mm. long, the lower stouter. 



5. E. Fendleri. 

 Central spine angular; radials 2-5 cm. long, the lateral ones strongest. 



6. E. mojavensis. 

 Ribs of the stem 5-7. 



Central spine present, 8 cm. long, angled and grooved; radials 6-8, strongly 



angled. 7. E. goniacanthus _ 



Central spine wanting; radials 3-59, almost terete. 8. E. pauci spinas] 



3. ECHINOCACTUS Link & Otto. Hedgehog Cactus, Barrel Cactus. 



Stem with defuiite ridges, not tuberculate. 



Scales of the hypantliium subulate, copiously woolly in their axils; fruit dry and 

 clothed with wool. 

 Uppermost central spine the broadest; ribs of the stem 15-20. 



1. E. polycephalus. 

 Lowermost central spine the broadest; ribs of the stem 13. 2. E. xeranthemoides. 

 Scales of the hypantliium ovate, orbicular, or cordate, not woolly in their axils; fruit 

 not woolly. 

 One or more of the central spines strongly hooked, neither annulate nor strongly 

 compressed. 

 Central spines except the hooked one, flattened, glabrous. 



3. E. Whipplri. 

 Central spines aU terete, pubescent. 4. E. pubispinus. 



On or more of the central spines distinctly annular, strongly broadened, flattened 

 above, slightly if at all hooked. 

 Radial spines 9-13; upper central spine angled. 5. E. cylindraceus. 



Radial spines over 20; upper three spines terete. 6. E. Wislizeni. 



