Ambrosiae 411 



late; heads glomerate, leafy-bracted ; involucres about 2 mm. 

 high, embedded in loose wool; bracts linear, obtuse, brownish- 

 green, the tips white. 



Occasional along river bottoms and on the margins of ponds. May- 

 October. 



** Pappus united at the base, deciduous in a ring. 



5. G. purpureum L. Biennial, simple or branching, erect 

 or decumbent at the base, 2-3 dm. high, canescent with a dense 

 close wool ; leaves spatulate, obtuse, usually becoming glabrate 

 and green above; heads crowded in an elongated more or less 

 interrupted spiciform inflorescence ; involucre brownish ; achenes 

 sparsely scabrous. 



Lincoln Park, Davidson. 



Tribe 4. AMBROSIAE. RAGWEED TRIBE. 



Herbs with mostly alternate leaves and greenish or 

 white unisexual flowers. Staminate heads racemose or 

 clustered above the few axillary pistillate ones. Pistil- 

 late heads usually 2-flowered, 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. 26. AMBROSIA. 



Involucre of pistillate heads armed with several rows of prickles. 



27. GAERTNERIA. 

 Involucral bracts of Staminate heads distinct. 28. XANTHIUM. 



26. 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 ; 



