812 OPUNTIACEAE 



1. CACTUS L. 



Plants globular to cylindric, covered with tubercles with apical spine-bearing 

 areolae. Flowers arising from near woolly and sometimes also bristly areolae in or 

 near the axils of terete or angled tubercles. Fruits fleshy, more or less immersed. 



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

 Tubercles 4-angled. 



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



Stems about ^ as high as broad. 



Radial spines 9 or more: tubercles densely crowded, much flat- 

 tened 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 58: flowers 1.5-2 cm. long. 5. C. Texanus. 

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



tubercles. 

 Flowers yellow. 



Flowers 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 2030, 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. 1 1 . 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: fruits club-shaped, 1.5-3 cm. long, 

 incurved: seeds decidedly rugose. 



In dry soil, Texas to Arizona and Sonora. 



2. Cactus hemisphaericus (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 : fruits club-shaped 2-3 cm. long, incurved : seeds slightly 

 rugose. [Mamillaria hemisphaerica E'ngelm.] 



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



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

 from a top-shaped base, 7.5-12.5 cm. broad, with numerous flattened tubercles vary- 

 ing 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 wanting; central solitary, snorter and stouter than the radials, 

 stout, rarely wanting: flowers reddish white, 2.8-3 cm. long: fruits 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: fruits not seen. 



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



5. Cactus Texanus (Engelm.) Small. Stems proliferous, forming a tufted 

 mass 2.5-6 cm. high, the branches ovoid to globose-obovoid, with tubercles varying 

 from 7-9 mm. in length, their axils furnished with wool and twisted bristles: spines 

 various; radials very numerous in many series, those of the outer series 30-50, white, 

 hair-like, 12-16 mm. long, more or less crisped, those of the inner series 10-12, 

 shorter, about i as long as the outer, puberulent, sometimes varying to yellow; central 

 spines 5-8, straight, rigid, white or darker above the base, pubescent: flowers yel- 

 lowish white, 1.5-2 cm. long: fruits club-shaped, 1.5-2 cm. long: seeds 1-2 mm. long, 

 black, shining, pitted. [Mamillaria pusilla var. Texana Engelm.] 



In sandy soil, Texas and adjacent Mexico. 



