FLOWERING PLANTS AND FERNS OF ARIZONA 945 



indurate, 1 -flowered, with 5 to 12 transverse scarious wings near the 

 middle, completely enclosing the achene; staminate heads with a 

 flattish 4- to 6-lobed involucre. 



Characteristic and abundant shrubs of sandy stream bed-- find 

 washes, tending to form thickets. Their forage value is small. The 

 conspicuously winged fruiting involucres make these plants rather 

 attractive. 



Key to the species 



1. Wings of the fruiting involucre spirally arranged, 5 to 8 mm. wide. 



1. H. SALSOLA. 

 1. "Wings of the fruiting involucre in a single whorl (rarely with 1 or 2 additional 

 wings above or below the middle), 1 to 4 mm. wide (2). 

 2. Wings 5 to 7, flabellate or reniform-orbicular, 2.5 to 4 mm. wide. 



2. H. PEXTALEPIS. 



2. Wings 7 to 12, mostly cuneate or obovate, 1 to 2 mm. wide. 



3. H. MOXOGYRA. 



1. Hymenoclea salsola Torr. and Grav in A. Gray, Amer. Acad. Arts 



and Sci. Mem. ser. 2, 4: 79. 1849. 

 Mohave County to Pima and Yuma Counties, 4,000 feet or usually 

 lower, sandy washes and rocky slopes, sometimes in saline soil, 

 March and April. Southern Utah, Arizona, and southern California. 



2. Hymenoclea pentalepis Rydb., North Amer. Fl. 33: 14. 1922. 



Hymenoclea hemidioica A. Xels., Amer. Jour. Bot. 25: 117. 1938. 



Topock (Mohave County), and Graham, Maricopa, Pima, and 

 Yuma Counties, 2,700 feet or lower, February to April, type of H. 

 hemidioica from the Mohawk Mountains, Yuma County (-4. and R. 

 Nelson 1340, 1341). Western and southern Arizona, southeastern 

 California, and adjacent Mexico. 



3. Hymenoclea monogyra Torr. and Grav in A. Grav, Amer. Acad. 



Arts and Sci. Mem. ser. 2, 4: 79. " 1849. 

 Gila, Pinal, Santa Cruz, and Pima Counties, 2,000 to 4,000 feet, 

 usually in sandy soil, September. Western Texas to southern 

 California and northern Mexico. 



49. AMBROSIA. Ragweed 



Weedy monoecious herbs; leaves opposite or alternate, lobed or 

 dissected; pistillate heads mostly axillary, 1 -flowered, their involucres 

 more or less turbinate, short-beaked, indurate, armed with a few 

 tubercles in a single series around the middle, completely enclosing 

 the achene; staminate heads naked-racemose above the pistillate ones, 

 terminating the stem and branches. 



Ragweed pollen is one of the commonest causes of hay fever. 



Key to the species 



1. Leaves large, palmately 3- to 5-lobed with serrate lobes, rarely ovate, not lobed. 



and merely serrate; fruiting involucre 4 to 7 nun. long 1. A. aptera. 



1. Leaves smaller, pinnatifid or bipinnatifid; fruiting involucre 3 to 3.8 mm. long 

 (2). 

 2. Perennial, with a running rootstock; leaves thickish, mostly only once 



pinnatifid 2. A. psilost.u uya. 



2. Annual; leaves thin, the lower ones usually twice pinnatifid. 



3. A. ARTEMIS1IFOLIA. 



