WOOTON AND STANDLEY FLORA OF NEW MEXICO. 635 



2a. Xanthium commune wootoni Cockerell, Proc. Biol. Soc. Washington 16: 9. 

 1903. 



Type locality: "At Espaiiola, N. M., and Las Vegas, N. M.'' 



Range: New Mexico. 



New Mexico : Las Vegas; Albuquerque. Valleys and cultivated fields, in the 

 Lower and Upper Sonoran zones. 



This seems distinct enough from X. commune to be regarded as a species. It 

 certainly is more easily separable from that than are most of the eastern species from 

 each other. Professor Cockerell states, however, that he has found both forms of 

 fruit on the same plant, hence we hesitate to raise the subspecies to specific rank. 

 The occurrence of both forms of fruit on a single plant would not necessarily invalidate 

 either as a species but would rather seem to be a result of hybridization. Ordinarily 

 the two plants are distinct enough. 



5. AMBROSIAL. Ragweed. 



Coarse annual or perennial herbs with lobed or dissected, opposite or alternate 

 leaves and small inconspicuous flowers; sterile heads racemose, bractless; fertile 

 flowers mostly glomerate in the lower axils; involucre of staminate flowers hemi- 

 spheric to turbinate, 5 to 12-lobed or truncate; receptacle flat, with filiform chaff 

 among the outer flowers; involucre of the solitary pistillate flower nutlike, beaked 

 at the apex, usually with 4 to 8 tubercles or stout spines in a row below the beak. 



KEY TO THE SPECIES. 



Leaves mostly 3 or 5-cleft; involucre of staminate heads 3 or 



4-ribbed 1. A. aptera. 



Leaves once to thrice pinnatifid; involucre of staminate flowers 

 not ribbed. 

 Annual; fruit with sharp tubercles; leaves mostly twice 



parted 2. A. artemisiaefolia. 



Perennial; fruit with blunt tubercles or unarmed; leaves 



mostly once pinnatifid 3. A. psilostachya. 



1. Ambrosia aptera DC. Prodr. 5: 527. 1836. Great ragweed. 

 Ambrosia trifida texana Scheele, Linnaea 22: 156. 1849. 



Type locality: Near San Antonio, Texas. 

 Range: Texas to southern Arizona. 



New Mexico: Vermejo Peak; Mangas Springs; Brockrnans Ranch; Cliff; Grains 

 Ranch. Wot ground. 



2. Ambrosia artemisiaefolia L. Sp. PI. 987. 1753. I !omho . ragweed. 

 Type locality: "Habitat in Virginia, Pennsylvania." 



Range: British America to Mexico and South America. 

 New Mexico: Santa Fe; Ogle; Agricultural College. Waste ground. 

 The common ragweed of the Eastern Stales is, so tar, a rare introduction inio Now 

 Mexico. 



3. Ambrosia psilostachya DC. Prodr. 5: 526. 1836. Western ragwi i p. 

 Type locality: "In Mexico inter San-Fernando et Matamoros." 



Ranoe: Illinois and Saskatchewan to Arizona and California, south into Mexico. 



New Mexico: Chanui; Shiprock; Pecas; Santa Fe; Clayton; Nam Visa; Malaga; 

 Albuquerque; Brockmans Ranch; Kingston; Dog Spring; Meedlla Valley. Plains, 

 in the Sonoran and Transition zon< 



