MONOPETALOUS EXOGENOUS PLANTS. 343 



aclienia in a single row, obovate, flattened, wingless, pubes- 

 cent on the inner face, the apex entire. — Perennial downy 

 or hoary herbs, with alternate leaves, solitary or corymbose 

 heads, and yellow rays. 



B. TOMEXTOSA, Torr. and Gray. Stem, leafy, hoary- 

 tomentose ; leaves oblong-obovate, crenate, hoary beneath, 

 closely pubescent above, the lowest tapering into a petiole, 

 the upper cordate, sessile; heads at length numerous, 

 corymbose-pan icled. Stem l°-3° high ; leaves 2-3' long. 



PARTHENIUM, L. 



Herls; heads many-flowered ; ranj-floivers 5, in a single 

 row, short obcordate, pistillate; disk-flowers tubular, 

 5-toothed, sterile ; anthers slightly united ; involucre scales 

 oval; recejjtade conical, chaffy, the chaff dilated upward; 

 achenia compressed, thick-margined ; flowers white ; 2)apims 

 of two roundish scales; alternate leaves. 



P. IXTEGRIFOLIUM, L. Stem l°-2° high, erect, rough ; 

 panicle dense, corymbose; involucre hoary; pappus awn- 

 like; conspicuous rays; leaves undivided, oblong-ovate, 

 serrate. 



IVA, L. 



Floivers tubular, marginal, with short corolla, pistillate, 

 fertile; central ones 5-toothed, sterile; anthers nearly dis- 

 tinct ; involucre scales 3-5 in a single row, oval or obovate, 

 distinct or partly united, or 6-9 and imbricated; chaff oi 

 the small receptacle linear or spatulate; heads few or 

 many-flowered ; achenia obovate ; pappus none. — Herhs or 

 shrubs; leaves opposite or alternate, mostly fleshy; flowers 

 small, nodding, axillary, white. 



* Scales of the involucre 3-5, single row. 



I. MICROCEPHALA, ]N"utt. Heads minute, 6-12 flowered; 



