AMBROSIACEAE 187 



2. AMBROSIA L. 



Herbs with alternate or opposite more or less lobed leaves. Staminate 

 heads numerous in spike-like racemes, the involucre 5-12-lobed and con- 

 taining from 5-20 flowers. Fertile involucres few, at the base of the 

 sterile, 1-flowered, tuberculate near the top. 



Sterile heads sessile. 1. A. bidentata. 

 Sterile heads short-pedicelled. 



Leaves entire or 3-5-lobed. 2. A. trifida. 

 Leaves pinnately divided. 



Annual. 3. A . artemisiaefolia. 



Perennial. 4. A. psilostachya. 



1. A. bidentata Michx. SOUTHERN RAGWEED. Annual, l-3 high, 

 hirsute : leaves lanceolate, sessile, alternate, with two lobes near the base : 

 one of the lobes of the staminate involucre extended into a long appen- 

 dage. -Sparingly introduced south of the Union Depot in Kansas City and 

 at Sheffield. July-October. 



2. A. trifida L. HOKSEWEED. Annual, rough-hairy, 2-20 high : 

 leaves opposite, short-petioled, 3-lobed, serrate. A very common weed. 

 The form with undivided leaves is not uncommon. July-October. 



3. A. artemisiaefolia L. RAGWEED. HOGWEED. Annual, l-5 

 high, pubescent : leaves thin : pistillate involucre with 4-6 spines. Our 

 most abundant weed. July-October. 



4. A. psilostachya DC. WESTERN RAGWEED. Perennial, l-3 

 high, grayish-pubescent and rough : leaves thick : pistillate involucre 

 with very small spines or spineless. Rather common in dry grounds 

 throughout. July-October. 



3. XANTHIUM L. COCKLE-BUR. 



Annuals with alternate, petioled, cordate- ovate, dentate, 3-uerved aud 

 lobed leaves. Staminate heads in short racemes. Pistillate heads axil- 

 lary, 2-celled, 2-flowered, the involucre covered with hooked spines, 2- 

 beaked. 



Spines of fruit 3 /x long or less. 



Fruit 6 // -9 // long. 1. X. Strumarium. 



Fruit 8"-12" long. 2. X. Canademe. 



Spines of fruit 5 /x long. 3. X. speciosum. 



1. X. Strumarium L. Fruit &"-9" long, contracted at base, slender, 

 puberulent : spines loosely disposed, l/'-Sj" long, exceeded by the nearly 

 straight spreading beaks. Occasional in waste places. August-October. 



2. X. Canadense Mill. Fruit 8 // -12 // long, rounded at base, sub- 

 glabrateto hairy : spines thickly disposed, 1^ // -3 // long, usually exceeded 

 by the stout incurved beaks. Common in waste places. August-October. 



Var. echinatum Gray. Fruit strongly hispid. Frequent with the type. 



3. X. speciosum Kearney. Fruit 10 // -12 // long, hairy : spines slen- 

 der, thickly disposed, 3 // -o // long, often exceeding the slender nearly 

 straight beaks. Bottoms near Courtney. August-October. 



