WOOTON AND STANDLEY FLORA OF NEW MEXICO. 437 



Stems short, mostly ovoid, globose, or short-cylin- 

 dric, tuberculate; ovary and fruit smooth, 



neither scaly nor spiny 2. Mamillaria (p. 447). 



Stems mostly short or long-cylindric, ovoid or occa- 

 sionally globose, mostly larger than in the pre- 

 ceding genus; tubercles confluent into longi- 

 tudinal ridges; ovary and fruit not smooth. 

 Ovary and fruit scaly, not spiny; flowers borne 



in the center of the stem at the apex 3. Echinocactus (p. 451). 



Ovary and fruit spiny; flowers borne laterally 

 on the stem some distance from the apex. 

 Flowers usually brightly colored, red, yel- 

 low, or greenish, open during the day; 

 stems thick and very spiny, with 6 or 



more ribs 4. Echinocereus (p. 454). 



Flowers white, open at night; stems slender, 

 4 or 5-ribbed; spines very short and 

 inconspicuous 5. Peniocereus (p. 458). 



1. OPTJNTIA Mill. Prickly pear. 



Perennials with jointed stems, bearing small, terete or conic, fleshy, caducous 

 leaves; joints of the stems flattened ("prickly pears" or "nopales"), cylindric ("cho- 

 llas" or "cane cactij" inpart), or clavate or tumid, smooth to strongly tuberculate; 

 leaves usually 1 cm. long or less, to be seen only on the young joints or the young 

 ovary; areoles with numerous retrorsely barbed glochids 3 to 15 mm. long and 1 to 

 several slender or stout, long or short spines (in one section the spines covered by a 

 papery sheath) ; flowers mostly large, with numerous sepals and petals, very numer- 

 ous stamens, and a single thick style with several stigmas; fruit tuberculate or 

 smooth, with several to many areoles, these bristle-bearing or sometimes spine- 

 bearing, occasionally proliferous, dry or berry-like, with a thick rind (berry-like 

 fruits known as "tunas"). 



KEY TO THE SPECIES. 



Joints clavate, tumid, or cylindric, not conspicuously flattened. 



Joints clavate or tumid, smooth or tuberculate; plants low, 



30 cm. liigh or less, spreading; spines without sheaths. 



Joints tumid when fresh and growing, simulating some 



forms of the Platyopuntiae when dry, very small, 



2 to 5 cm. long, 2 to 3 cm. wide and nearly as 



thick; tubercles not conspicuous; spines of the 



fruit merely spreading. 



Joints elliptic-ovate, 3 to 5 cm. long and 2 to 3 cm. 



wide; bristles and spines very numerous, the 



latter white and small 3. 0. arenarin. 



Joints circular to short-obovate, 2 or 3 cm. long; 

 bristles few. 

 Spines white or whitish; joints short-obovate. . 1.0. brachyirlhrn. 

 Spines yellow or brownish; joints nearly orbicu- 

 lar 'J. O.fingUit. 



Joints clavate (in one species almost cylindric); tuber- 

 cles conspicuous, especially in dried specimens; 

 Hpines of the fniit in radiating clusters. 



