OPUOTIACEAE 811 



Stems continuous or not conspicuously jointed : leaves obsolete : spines not barbed : spine-bearing and 



flower-bearing areolae distinct, with barbed bristles in the former. 



Flowers not arising from the tubercles, but from their axils : ovary naked. 1. CACTUS. 



Flowers arising from the tubercles or ribs : ovary scaly. 



Flowers arising from near to fully developed spine-bearing areolae. 



Stems mostly low : seeds tuberculate : embryo straight. 2. ECHINOCERUS. 



Stems elongated : seeds smooth or pitted : embryo curved. 3. CEREUS. 



Flowers arising near the areolae which later develop spines. 4. ECHINOCACTUS. 



Stems conspicuously jointed : leaves scale-like : spines barbed : flowers arising from 



spine-bearing areolae, which also usually contain barbed bristles. 5. OPUNTIA. 



1. CACTUS L. 



Fleshy plants, with globular cylindric or oval simple solitary or clustered stems whose 

 surface is completely covered with variously shaped tubercles, which are usually spirally 

 disposed and bear at the apex, spine-bearing areolae. Leaves obsolete. Flowers aris- 

 ing from near woolly and sometimes also bristly areolae in the axils of or near the base 

 of terete or angled tubercles. Hypanthium naked, funnelform or campanulate, prolonged 

 beyond the ovary. Petals in several rows. Ovary hidden between the tubercles : style 

 filiform. Berry fleshy, more or less emersed. [Mamillaria Haw.] 



Flowers from the axils of old or mature tubercles which are not grooved. 

 Tubercles 4-angled. 



Stems less than % as high as broad. 1. C. Heyderi. 



Stems about % as high as broad. 



Radial spines 9 or more: tubercles densely crowded, much flattened at 



the base. 2. C. hemisphaericus. 



Radial spines 7 or fewer : tubercles more loosely disposed, nearly as thick 



as wide at the base. 3. C. meiacanthus. 



Tubercles terete. 



Radial spines less than 15, the central solitary : flowers 3.5 cm. long. 4. C. sphaericus. 



Radial spines 40 or more, the centrals 5-8 : .flowers 1.5-2 cin. long. 5. C. Texanus. 



Flowers from the base of grooves on the upper side of young or developing tubercles. 

 Flowers yellow. 



FloweYs at first central and fruit soon pushed aside by the developing tubercles. 

 Radial spines 12 or more : stigmas 2-5. 



Stems simple or nearly so : seeds 1 mm. in diameter or smaller. 6. C. Missouriensis. 



Stems branched into dense tufts : seeds fully 1.5 mm. in diameter. 7. C. similis. 



Radial spines 12 or fewer : stigmas 7-8. 8. C. robustior. 



Flowers and fruit permanently terminal in the woolly top of the stem. 9. C. sulcatus. 



Flowers purple, violet or reddish. 



Radial spines 20-30, the centrals 4-5, tawny or yellowish : seeds fully 2 mm. 



long. 10. C. radiosus. 



Radial spines 12-40, the centrals 3-12, purplish : seeds 1.5 mm. long. 11. C. Neo-Mexicanus. 



1. Cactus Heyderi (Muhlenpf. ) Kuntze. Stems depressed and flat-topped, 8-12 

 cm. broad, 2.5-4 cm. high, with numerous elongated tubercles: spines various, radials 

 10-22, whitish, 5-12 mm. long, the upper mostly shorter more slender and paler than the 

 lower ; central solitary, yellowish brown, 4-8 mm. long, stout, straight or nearly so ; 

 flowers reddish white, 2-2.5 cm. long : fruit club-shaped, 1.5-3 cm. long, incurved : seeds 

 decidedly rugose. 



In dry soil, Texas to Arizona and Sonora. 



2. Cactus bemisphaericus (Engelm.) Small. Stems hemispheric round-topped, 

 7-12 cm. broad, with rather long-pyramidal tubercles : spines various ; radials 9-12, nearly 

 equal, 4-8 mm. long ; central solitary, 4-6 mm. long : flowers 2-5 cm. long and broad, 

 dirty white or reddish : fruit club-shaped 2-3 cm. long, incurved : seeds slightly rugose. 

 [Mamillaria hemisphaerica Engelm.] 



In sandy soil, northern Texas to New Mexico and adjacent Mexico. 



3. Cactus meiacanthus (Engelm.) Kuntze. Stems hemispheric or nearly so, from 

 atop-shaped base, 7.5-12.5 cm. broad, with numerous flattened tubercles varying from 7- 

 18 mm. in length : spines various ; radials 5-9 often 6, 6-10 mm. long, straight or slightly 

 curved, whitish or yellowish, the upper ones often shorter than the lower or rarely want- 

 ing ; central solitary, shorter and stouter than the radials, stout, rarely wanting : flowers 

 reddish white, 2.8-3 cm. long : fruit club-shaped, slightly incurved, 2-3 cm. long. 



In sandy soil, Texas to New Mexico and adjacent Mexico. 



4. Cactus sphaericus (Dietr.) Kuntze. Stems proliferous, becoming densely tufted, 

 the branches obovate to club-shaped, 5 cm. high or higher, with narrowly ovoid acutish 

 tubercles varying from 12-16 mm. in length, furnished with wool in their axils : spines 

 narrow ; radials 12-14, bristle-like from a thick base, 7-9 mm. long, white, often curved ; 

 central spine subulate, shorter than the radials, straight : flowers yellow, 3.5-5 cm. long : 

 fruit not seen. 



On sandy ridges along the Rio Grande, Texas and adjacent Mexico. 



