STANDLEY TREES AND SHRUBS OF MEXICO. 857 



Tube of the perianth as long as the limb or 

 longer ; plants elongate, with white flowers. 



- Joints ribbed 14. NYCTOCEREUS. 



Joints angled 15. ACANTHOCERETJS. 



Tube of the perianth mostly shorter than the 

 limb ; plants bushy, usually with scarlet. 



flowers 16. HELIOCERETJS. 



Areoles of the ovary with wool or felt. 

 Perianth limb regular ; flowers white. 



17. CARNEGIE A. 

 Perianth limb oblique ; flowers scarlet 



18. RATHBUIIIA. 

 lil?. Plants with one or few joints, the joints usually short, some- 

 times clustered, ribbed or rarely tubercled. 



Flowers at lateral areoles 25. ECHHTOCEREITS. 



Flowers at central areoles. 

 Ovary and fruit naked. 

 Plants without spines. 

 Tubercles prominent, cartilaginous, flattened, more or less 



imbricate 26. ARIOCARPUS. 



Tubercles low, rounded above 27. LOPHOPHORA. 



Plants very spiny 28. EPITHELANTHA. 



Ovary and fruit with scales. 



Flowers funnelform 29. HAMATOCACTUS. 



Flowers mostly campanulate, at least not long and slender. 

 Areoles not arranged on ribs. 



Tubercles short, imbricate 30. STROMBOCACTITS. 



Tubercles much elongate, slender, finger-like. 



81. LEUCHTENBERGIA. 

 Areoles arranged on definite ribs. 



Axils of scales on ovary and fruit naked. 



Ribs usually continuous, rarely if ever tubercled ; 

 flowers with scarcely any tube. 

 Ribs usually numerous, much compressed, th'n. 



32. ECHINOFOSSULOCACTUS. 

 Ribs not so numerous, usually thick. 



33. FERGCACTtrS. 

 Ribs usually broad, tubercled ; flowers with a short 



but definite tube 34. ECHINOMASTUS. 



Axils of scales on ovary hairy, woolly, or bristly 

 Ribs several to many; plants very spiny. 



Fruit permanently woolly, nearly dry, dehiscent by 



a terminal pore 35. ECHHTOCACTITS. 



Fruit not so woolly, somewhat fleshy, bursting irregu- 

 larly 36. HOMALOCEPHALA. 



Ribs few, broad; plants mostly spineless. 



37. ASTROPHYTTJM. 

 AA. Flowers and spines borne at different areoles. Plants consisting 

 of a single joint. 

 Flowering areoles forming a central terminal cephallum. 



38. CACTUS. 



