378 ASTERACEAE. 



clustered above the few axillary pistillate ones. Pistil- 

 late heads usually 2-flowercd, destitute of pappus and 

 corolla, completely enclosed by the more or less spiny 

 involucre and becoming a bur in fruit. Staminate 

 flowers many. Receptacle chaffy. Corolla present. 

 Anthers distinct or scarcely coherent. 



Involucral bracts of staminate heads united. 



Involucres of pistillate heads armed near the 



apex with a single row of prickles. 27. Ambrosia. 



Involucre of pistillate heads armed with several 



rows of prickles. 28. Gaertneria. 



Involucral bracts of staminate heads distinct. 29. Xanthium. 



27. AMBROSIA L. Ragweed. 



Monoecious branching herbs or shrubs, with alternate 

 or opposite, mostly lobed or divided leaves, and small 

 heads of green flowers, the staminate spicate or racemose, 

 the pistillate solitary or clustered in the upper axils. 

 Involucre of the pistillate heads globose-ovoid, closed, 

 1 -flowered, usually armed with 4-8 tubercles or spines; 

 corolla none; pappus none. Involucre of the staminate 

 heads hemispheric, 5-12-lobed, open, many-flowered; 

 corolla funnelform, 5-toothed; anthers scarcely coherent, 

 mucronate-tipped. 



1. A. psilostachya DC. Stems erect from horizontal root- 

 stocks, 5-8 dm. high, with strigose pubescence and somewhat 

 scabrous; leaves once or twice pinnatifid; fruit mostly solitary in 

 the axils, turgid-ovoid, about 3 mm. long, obtusely short-pointed, 

 rugose-reticulate, either unarmed or with 4 short or sharp tubercles. 



A common weed in low ground, especially in coast valleys. 

 June- September. 



28. GAERTNERIA Med. 



Hispid or tomentose branching herbs, sometimes 

 woody at the base, with mostly alternate lobed or divided 

 leaves, and small monoecious greenish heads of discoid 

 flowers, the staminate in terminal spikes or racemes, the 

 pistillate solitary or clustered in the upper axils. Invo- 

 lucre of the pistillate heads ovoid or globose, closed, 1-4- 

 celled, 1-4-beaked, armed with several rows of spines 

 and forming a bur in fruit; corolla none; style-branches 

 exserted; achenes obovoid, thick, solitary in the cells; 

 pappus none. Staminate heads sessile or short-ped- 



