OPUNTIACEAE 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 bar : flowers arising from 
spine-bearing areolae, which also usually contain barbed bristles. 5. OPUNTIA. 
1. CÁCTUS 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 14 as high as broad. 1. C. Heyderi. 
Stems about 4% as high as broad. 
Radial spines 9 or more: tubercles densely crowded, much flattened at i 5 
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 em. long. 4. C. sphaericus. 
Radial spines 40 or more, the centrals 5-8: flowers 1.5-2 em. long. 5. C. Texanus. 
Flowers from the base of grooves on the uppersideof 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. : EUR 
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 urple, violet or reddish. i 
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 
em. 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 hemisphaéricus (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 meiacánthus (Engelm.) Kuntze. Stems hemispheric or nearly so, from 
a top-shaped base, 7.5-12.5 cm. broad. with numerous flattened tubercles varying from 7- 
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 em. long. 
In sandy soil, Texas to New Mexico and adjacent Mexico. 
4. Cactus sphaéricus (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. 
