FLOWERING PLANTS AND FERNS OF ARIZONA 00? 



5. OPUNTIA * 9 



Shrubs or herbaceous perennials with short-jointed stems; joints 

 flattened or terete, often tubereulate but never ribbed; leaves small, 

 fleshy, subulate, caducous; areoies furnished with glochids (barbed 

 bristles); spines minutely barbed, at least at the tip, not hooked; 

 floriferous and spiniferous areoies combined in one organ; flowers diurnal ; 

 hypanthium bearing scales resembling the leaves, the tube short; 

 perianth segments usually broad, entire, spreading; fruit indehiscent, 

 fleshy or dry, glochidiate, often spiny; seeds bony, compressed or 

 angled, pallid. 



The flat-jointed kinds (pi. 25) are known as pricklypear, and 

 those with edible fruits particularly as tuna. The vernacular term, 

 cane cactus, is applied to species with cylindric joints (see pi. 

 26), and cholla to several of the especially spiny forms of this 

 kind. In the Arizona species the flowers apparently remain fresh 

 only one day. Some opuntias increase rapidly and become pes- 

 tiferous on range land subjected to prolonged overgrazing. On the 

 other hand, some of the species provide emergency feed. Cattle 

 can feed on opuntias without undue injury, provided the spines are 

 burned or the joints macerated. 



The genus presents many taxonomic difficulties, and satisfactory 

 classification of the Arizona species is far from an accomplished fact. 

 Diagnostic characters are often greatly altered by environment. 

 Natural hybrids are not rare. 



Key to the species 



1. Joints flattened; spines not sheathed (2). 



2. Fruit dry; plant small or low, rarely more than 0.5 m. high; areoies 5 to 15 

 mm. apart; joints inclined to be transversely marked or wrinkled (3). 

 3. Fruit not spin}' (4). 



4. Branches erect, 1-jointed; areoies deeply depressed; joints unarmed, 

 densely puberulent 1. O. basilaris. 



4. Branches prostrate or low, several-jointed; areoies depressed or level with 



the surface of the joint; joints unarmed or somewhat spiny, glabrous. 



2. O. AUREA. 

 3. Fruit spiny; joints inclined to be tubereulate (5). 



5. Joints readily detached, not more than 5 cm. long, often ovoid or sub- 



globose, spiny or unarmed; plant very small, low, with 1 to several 

 branches, these 1- to several-jointed; flowers yellow. 



3. O. FRAGILIS. 



5. Joints firmly attached, 5 to 20 cm. long (6). 



6. Areoies less than 10 mm. apart; spines appressed or strongly de- 

 flexed, 1 to 3 cm. long; joints usually suborbicular, spiny on most 



of the surface; flowers pale yellow 4. O. polyacamha. 



6. Areoies mostly more than 10 mm. apart; spines not appressed, 3 to 



15 cm. long; plants of the same species with either yellow or pink 



flowers (7). 



7. Spines bristlelike, obscuring the surface of the joint, those at base 



of old joints fiexuous and up to 20 cm. long 6. O. ursixa. 



7. Spines not bristlelike, of ten numerous but not obscuring the surface 



of the joint (8). 



8. Spines 4 to 12 cm. long, 4 or more at an areole; joints obovate, 



8 to 15 cm. long, not thickened, usually heavily armed over 



t lie entire surface . 5. O. hystricina. 



• Reference: Boisse\ ai.v, C. II.. and Davidson, c. Colorado cacti. Cactus and Succulent Jour. 

 12 <-'. (sup. L940. 



286744°— 42 



