1907] Hall. Compositae of Southern California. 119 



39. AMBROSIA L. RAGWEED. 



Ours coarse perennial monoecious herbs with mainly alternate 

 pinnatifid leaves and inconspicuous greenish flowers. Staminate 

 heads nodding, in erect ament-like leafless racemes ; involucral 

 bracts united into a broadly turbinate cup; receptacle with 

 slender bracts subtending at least the outer flowers; corollas 

 funnelform, 5-lobed. Pistillate heads in the axils of the upper 

 leaves at the base of the staminate racemes; involucre oblong or 

 turbinate, closed, containing but a single flower; corolla none; 

 pappus none ; fruit an achene-like bur which is beaked or pointed 

 and commonly armed near the top with a single row of prickles. 



Herbage rough-pubescent: leaf -lobes lanceolate, acute....!. A. psilostachya. 



Herbage soft-pubescent: leaf -lobes crowded, short-oblong, obtuse 



2. A. pumila. 



1. A. psilostachya DC., Prodr. v. 526 (1836). WESTERN 

 RAGWEED. 



Stems simple, erect, commonly 5 to 10 (2 to 15) dm. high 

 from slender running rootstocks : herbage scabrous or short- 

 hirsute, somewhat strigose : leaves once or the lower twice pin- 

 natifid, with acute lobes: fruit an obovoid turgid bur, about 3 

 mm. long, mostly solitary in the axils, pubescent, rugose-reticu- 

 lated, bearing 4 protuberances or sometimes unarmed. 



A common weed along roadsides and in waste places through- 

 out western North America. 



2. A. pumila (Nutt.) Gray, Proc. Am. Acad. xvii. 217 (1882). 

 Franseria pumila Nutt., Trans. Am. Philos. Soc. ser. 2, vii. 344 

 (1841). 



Stems often branching, erect, 4.5 dm. or less high from slender 

 running rootstocks : herbage canescent with a dense soft pubes- 

 cence : leaves mainly alternate, long-petioled, crowded, as also 

 are their numerous short-oblong or narrower obtuse lobes : fruit 

 said to be 2 mm. long, obovoid, muticous, the surface pubescent. 



Known only from the vicinity of San Diego : Nutt all, Cleve- 

 land, etc., ace. to Gray : Purpus, Miss Rose Smith, Mrs. Brandegee. 



