STANDLEY TREES AND SHRUBS OF MEXICO. 867 



Epidermis glabrous. 



Areoles bearing long soft hairs XIX. ORBICULATAE. 



Areoles without long hairs. 

 Joints green or bluish green. 

 Plants spineless, or with a few, usually short spines. 



XX. FICUS-INDICAE. 

 Plants spiny, at least on old joints. 



XXI. STEEPTACANTHAE. 

 Joints blue XXII. ROBUSTAE. 



I. LEPTOCAITLES. 



Ultimate joints short, usually at right angles to the branches, 4 to 7 mm. thick. 



Leaves ovoid to ovoid-subulate; young areoles long-hairy__l. O. mortolensis. 



Leaves linear ; areoles not long-hairy 2. O. leptocaulis. 



Ultimate joints longer, usually at an acute angle with the brandies, 8 to 

 15 mm. thick. 



Joints only slightly tuberculate 3. 0. arbuscula. 



Joints manifestly tuberculate 4. kleiniae. 



II. THURBERIANAE. 



Tubercles narrowly oblong, 1 cm. long or more. 



Spines, at least the larger ones, 2.5 cm. long or more 5, 0. recondita. 



Spines 2 cm. long or less 6. O. thurberi. 



Tubercles low, oblong, 6 to 8 mm. long 7. 0. clavellina. 



III. ECHINOCARPAE. 



Sheaths of the spines white or straw-colored 8. O. echinocarpa. 



Sheaths yellow-brown 9. O. serpentina, 



IV. BIGELOVIANAE. 



Larger spines numerous ; upper tubercles on fruit larger than the lower ones. 



10. 0. big'elovii. 

 Larger spines 4 to 6 ; tubercles on fruit all alike 11. 0. ciribe, 



V. IMBRICATAE. 



Joints clavate; tubercles not much flattened laterally 19, 0. molesta. 



Joints cylindric ; tubercles much flattened laterally. 

 Fruit smooth or but slightly tuberculate. 



Branches very stout, 5 cm. thick or more 12. 0. cholla. 



Branches relatively slender, 2 cm. thick or less. 



Plants glaucous; spines 4 at an areole 13. 0. calmalliana. 



Plants not glaucous; spines more than 4 at an areole 14. 0. versicolor. 



Fruit manifestly tuberculate. 

 nants tall, up to 2 to 4 meters high. 



Flowers small, the petals 1.5 cm. long 15. 0. lloydii. 



Flowers large, the petals 2 to 3 cm. long 16. 0. imbricata. 



Plants low, 60 cm. high or less. 



Flowers yellow 17. 0. tunicata. 



Flowers rose-colored 18. 0. pallida- 



