328 AMBROSIACEAE. 



1. OXYTENIA Nutt. 



I. Oxytenia acerosa Nutt. On dry plains from Colo, to N. M. and Ariz. — 

 Alt. about 4S00 ft. — San Juan Valley. 



2. IVA L. Marsh Elder. 



Heads paniculate ; leaves ovate, canescent beneath. i . /. xanthifolia. 



Heads axillary ; leaves obovate or oblong, green. 2. I. axillaris. 



1. Iva xanthifolia Nutt. In moist soil, along streams and in waste places 

 from Mich., Sask. and Wash, to Neb. and N. M. — Alt. 4000-7000 ft. — Sun- 

 set Caiion; Cheyenne Mountain; Ft. Collins; Huerfano Valley, near Gardner; 

 Poudre River. 



2. Iva axillaris Pursh. In alkaline or saline meadows from Sask. and B. C. 

 to Ind. Terr, and Calif. — Alt. 4000-7000 ft. — Gunnison; near Greeley; Grand 

 Junction ; Calhan ; Ft. Collins ; Lamar. 



3. DICORIA T. & G. 



I. Dicoria Brandegei A. Gray. On sandy bottoms from Colo, and Utah to 

 Ariz. — San Juan River; between McElmo and Recapture Creeks. 



4. AMBROSIA L. Rag-weed, Hog-weed. 



Involucres of the staminate heads 3-ribbed ; leaves palmately 3-5-cleft or entire. 



1. A. trifida. 

 Involucres of the staminate heads not ribbed ; leaves once to thrice pinnatifid. 



Annual ; fruit with acute teeth. 2. A. artemisifolia. 



Perennial ; fruit with blunt teeth or unarmed. 3. A. psilostachya. 



I. Ambrosia trifida L. In moist soil from Que. and Ass. to Fla. and Colo. 

 — Alt. 4000-5000 ft. — East of Windsor; Ft. Collins; Dixon Caiion; Poudre 

 Cation. 



Ambrosia trifida integrifoUa (Muhl.) T. & G. A variety with entire leaves. 

 Together with the species. — Cache la Poudre River. 



a. Ambrosia artemisifolia L. In dry soil, waste places and fields from N. S. 

 and B. C. to Fla. and Colo. — Alt. 4000-5000 ft. — Ft. Collins ; banks of the 

 Poudre. 



3. Ambrosia psilostachya DC. On prairies and plains from Ills., Sask. and 

 Ida. to La. and Calif. ; also in Mex. — Alt. 4000-7000 ft. — Golden ; New Wind- 

 sor ; Garden of the Gods ; Pagosa Springs ; Boulder ; Lyons ; Ft. Collins. 



5. GAERTNERIA Med. 



Leaves twice or thrice pinnately dissected. 



Leaves regularly pinnate with linear or oblong divisions. 



Staminate involucres cleft below the middle ; root mostly annuals. 



1. G. acanthocarpa. 

 Staminate involucres not cleft to the middle ; perennials. 

 Divisions of the leaves oblong or oblong-linear, acute. 



i:. G. tenuifoUa. 

 Divisions of the leaves linear, obtuse. 3. G. linearis. 



Leaves interruptedly pinnate ; divisions ovate or triangular. 



4. G. tomentosa. 

 Leaves simply pinnate or simple. 5. G. Grayi. 



