KEY TO THE SPECIES 51 



XLII. CACTACEiE (Cactus Family) 



Green, thickened-fleshy, spiny and mostly leafless plants 

 which in outline are g-lobular cylindric or flattened ; the flow- 

 ers of numerous sepals, petals and stamens which cohere by 

 their bases and form a tube or cup above the many ovuled 

 inferior ovary. 



1. Echlnocereus. Stems globular-ovate or oblong, ribbed ; the spines on the 

 ribs and the lowers just above the bristle-bearing cushions (areolae) ; flowers 

 green. 



2. Echinocactus. Stems globose, with tubercles (in spiral rows) bearing 

 spines in apical areolEe ; flowers greenish-pink, borne close to the areolee. 



3. Cactus. Stems globose to oblong, with spine-bearing tubercles; flowers 

 borne between the tubercles. 



4. Opnntia. Stems jointed ; the joints flat or sub-cylindric ; flowers yellow. 



1. ECHINOCEREUS (Cerehs) 



Stems singly or clustered, ribs 13, vertical ; calyx-tube also spine bearing. 



1. Echlnocereus vlridiflorus Engelm. (Green-plowered Cebeus). Mostly 

 less than 1 dm. high ; spines radiating, 12-18, with 2-6 superior setaceous ones ; 

 the lower brown, the others white or purplish ; seeds tubercled. Plains. 



2. ECHINOCACTUS 



Stems singly or clustered ; rows of tubercles 8-13 ; calyx-tube covered with 

 sepal-like scales, passing into the sepals and the sepals into the petals. 



1. Echinocactus Slinpsunl Engelm, (Simpson's Cactus). Outer spines 

 20-30, whitish ; the inner 8-10, stouter and darliier ; seeds black, tuberculate. Fre- 

 quent in valleys and on rocky slopes. 



3. CACTUS (Cactus) 



Stems oval to oblong or in some species columnar ; the tubercles cylindric, 

 woolly and spiny at summit ; the flowers from areolae at the base of the tubercles. 



1. Cactus viviparus Nutt. (Purple Cactus). Tubercles slightly grooved, 

 with 8-8 reddish-brown and 12-25 shorter white spines ; flowers purple, 3-4 cm. 

 long ; ovary naked ; seeds brown, pitted. Plains and prairies. 



4. OPUNTIA (Pkicklt Pear) 



Stems jointed and branching ; flowers lateral, with many petals and very 

 numerous stamens ; fruit pear-shaped. 



1. Opuntia polyacantha Haw. (Mant-spined Opuntia). Prostrate-spread- 

 ing, the joints oval or obovate, variable in size ; flowers large, 4-6 cm. broad ; 

 fruit dry and prickly. Abundant ; exceedingly variable. 



2. Opuntia fragilis (Nutt.) Haw. (Brittle Opuntia). Usually in broad 

 dense beds : the stems prostrate, of many small ovate, sub-terete joints ; flowers 

 yellow, much smaller than in the preceding, rarely produced— the plant spreading 

 from the joints, which become broken oflE and scattered and then rooting. 



