828 AMBRO3IACEAE 



Staminate and pistillate flowers in different heads, the latter 1-1, without corolla and 



enclosed in a nut-like or bur-like involucre. 

 Involucres of the staminate heads with united bracts; receptacles low; rudimentary 



styles penicillate or flmbriate at the apex. 



Involucres of the solitary pistillate flower with 9-12 dilated scarious wings; anther- 

 tips blunt. 5. HYMENOCLEA. 

 Involucres of the 1-4-flowered pistillate heads not winged, but usually armed with 



spines or tubercles; anther setiferous-acuminate. 

 Spines or tubercles of the 1-flowered pistillate heads in a single row. 



6. AMBROSIA. 

 Spines of the 1-4-flowered pistillate heads in more than one row. 



7. FRAXSERIA. 

 Involucres of the staminate heads with distinct bracts; receptacle cylindraceous; 



spines of the 2-flowered pistillate heads in several rows, uncinate. 8. XANTHIUM. 



1. OXYTENIA Nutt. 



Small shrubs. Heads small; involucres of 5 coriaceous-herbaceous, dilated, 

 ovate, rigidly acuminate bracts. Receptacle convex. Staminate flowers 10-20, 

 villous at the base, subtended by slender paleae, or these wanting in the central 

 flowers; pistillate flowers 5, without a trace of corolla. Achenes obovate-turgid, 

 very villous. Pappus none or of a single minute scale. 



1. O. acerosa Nutt. Shrub 1-2 m. high, canescent; branches erect, often 

 leafless and rush-like; leaves alternate, pinnately 3-7-parted, with filiform divi- 

 sions, 3-12 cm. long, or the upper entire and filiform; heads numerous, in dense 

 panicles, about 4 mm. high; bracts ovate, mucronate, canescent. Dry plains 

 and canons: sw Colo. N.M. se Calif. Son. Au. 



2. CYCLACHAENA Fresen. HORSEWEED, CARELESS WEED. 



Annual herbs, with mostly opposite petioled leaves. Heads paniculate, not 

 leafy-bracteate. Involucres of 5 obovate bracts. Receptacle chaffy, the paleae 

 subtending the pistillate flowers broad, nearly as large as the bracts, partly 

 embracing the achenes. Fertile flowers 5, marginal, their corollas none or rudi- 

 mentary. Staminate flowers 10-15, with funnelform corollas. Filaments mon- 

 adelphous. Achenes pyriform, without pappus. 



1. C. xanthifolia (Nutt.) Fresen. Stem 1-2 m. high, puberulent above; 

 leaf-blades ovate or ovate-lanceolate, cuneate to subcordate at the base, 5-10 

 cm. long, canescent beneath, scabrous above; heads 4-5 mm. broad, hemispheric; 

 bracts ovate. Iva xanthifolia Nutt. Waste places and along streams: Sask. 

 Mich. Neb. N.M. Wash. Alta. Plain Sub mont. Jl-S. 



3. IVA L. MARSH ELDER, BOZZLEWEED, SALT SAGE, 



POVERTY WEED. 



Annual or perennial herbs, or shrubs. Leaves opposite or alternate above, 

 glabrous or coarsely pubescent. Heads axillary or in terminal bracteate spikes. 

 Involucres campanulate or hemispheric, or turbinate; bracts 3-6, distinct or 

 partially united. Pistillate flowers 1-8, marginal; corolla a short truncate tube. 

 Disk-flowers perfect but sterile, with funnelform, 5-lobed corollas. Anthers 

 entire at the base, mucronate above. Achenes broadest above the middle, with- 

 out pappus. 



1. I. axillaris Pursh. Perennial, suffrutescent at the base; stems much 

 branched, 2-6 dm. high, sparingly hirsute or glabrate; leaves sessile, entire, 

 obovate, oblong, or linear-oblong, 1-3 cm. long, fleshy, pubescent or glabrate, 

 the upper alternate; heads axillary, 4-5 mm. broad, hemispheric; bracts of the 

 involucres 4-5, connate at least at the base; pistillate flowers 4-5; staminate 

 flowers 12-15. Alkaline or saline meadows: Man. Okla. N.M. Calif. B.C. 

 Plain Submont. My-S. 



4. DICORIA T. & G. 



Branched annuals. Lower leaves opposite, the upper alternate. Involucre 

 of 5, oval or oblong, herbaceous bracts; within these 1 or 2 large, broad, thin- 

 scarious paleae subtending the 1 or 2 pistillate flowers, or these lacking when 

 the heads are wholly staminate. Staminate flowers 6-12, w r ith rudimentary pis- 

 tils, subtended by a few narrow, hyaline paleae; corollas of the staminate flowers 



