594 MISC. PUBLICATION 4 2 3, U. S. DEPT. OF AGRICULTURE 



1. Eucnide urens Parry, Amer. Nat. 9: 144. 1875. 



Along the Colorado River, from above Boulder Dam (Mohave 

 County) to Ehrenberg (northern Yuma County), 300 to 1,500 feet, 

 dry rocky slopes and in canyons, April and May, type from the Colorado 

 River valley (Bigelow). Southern Utah and Nevada to western 

 Arizona and southeastern California. 



Plant remarkable for the large size of the flowers and the stinging 

 hairs. 



85. CACTACEAE. 88 Cactus family 



The Arizona species fleshy-stemmed, perennial, mostly spiny and 

 xerophytic plants with mucilaginous or rarely milky juice, character- 

 ized by complex cushionlike organs (areoles) from which spines, 

 branches, or flowers arise; stems of 1 or more joints, these flattened, 

 cylindric, or globose, often tuberculate or ribbed; leaves wanting or 

 rudimentary; flowers perfect, mostly regular and solitary; perianth 

 segments several to many, more or less united at base, inserted on a 

 hypanthium; stamens indefinitely numerous, inserted within the 

 hypanthium tube; ovary inferior, 1 -celled, the ovules parietal, numer- 

 ous; style 1; stigma lobes several; fruit several- to many-seeded, 

 fleshy or dry, indehiscent or dehiscent, spiny, scaly, or smooth. 



The cactus family is characteristic of the desert regions of Arizona, 

 where the plants are abundant and conspicuous. The cactus most 

 outstanding in scenic appeal is the gigantic sahuaro, the largest 

 succulent in the United States. 



Key to the genera 



1. Areoles furnished with glochids (barbed bristles) ; spines barbed or scabrous. 



5. Opuntia. 

 1. Areoles not furnished with glochids; spines not barbed or scabrous (2). 



2. Flowers borne in the axil of the tubercle or at base of the groove, at some 

 distance from the spiniferous areole; tubercles distinct, disposed in 



spiral rows 4. Mammillaeia. 



2. Flowers borne at apex of the tubercle, contiguous with or actually on the 



spiniferous areole; tubercles coalesced to form vertical or spiral ribs, 



except in one section of Echinocactus (3). 



3. Flowers nocturnal, borne on the spiniferous areoles; stems greatly elongate, 



many times longer than thick; plant more or less branched above the 



base 1. Ceretts. 



3. Flowers diurnal, not borne on the spiniferous areoles; stems not greatly 

 elongate; plant cespitose or simple (4). 



4. Hypanthium spiny; flowers lateral 2. Echinocereus. 



4. Hypanthium devoid of spines, commonly scaly; flowers terminal. 



3. Echinocactus. 

 1. CEREUS 



Plants large or small; stems greatly elongate, more or less branched 

 above the base of the plant; spines not hooked; flowers funnelform or 

 salverform, borne on the mature spiniferous areoles, nocturnal, lasting 

 less than 24 hours; scales of the hypanthium usually entire, acute, 

 rather persistent ; ovary or fruit more or less spiny, the latter a berry ; 

 seeds usually tessellate and shining. 



88 Reference: Britton, N. L., and Rose, J. N. the cactaceae. Carnegie Inst. Wash. Pub. 248. 

 1919-23. 



The synonyms given by Britton and Rose are not repeated in treating the Arizona 

 species except to indicate a different opinion. The segregate genera employed by Britton 

 and Rose and many other modern authorities have not been adopted herein, although 

 such names are indicated in the synonymy for the convenience of readers who are accus- 

 tomed to that nomenclature. 



The authors are greatly indebted to A. R. Leding for much helpful advice and material 

 from New Mexico, and to A. A. Nichol for the use of his splendid maps showing distri- 

 bution of cacti in Arizona. 



