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. hklentata. 

 Sterile heads short-pedicelled. 



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

 Leaves pinnately divided. 



Annual. 3. A . nrtemisiaefolia. 



Perennial. 4. A. psUostac.hya. 



1. A. bidentata Michx. Southeen 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 ShefBeld. July-October. 



2. A. trifida L. Horse weed. 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-nerved and 

 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'^ long or less. 



Fruit Q"-9" long. 1. X. Strumarium. 



Fruit S'^-\2'' long. 2. X. Canndense. 



Spines of fruit b'^ long. 3. X. spedosum. 



1. X. Strumarium L. Fruit Q'^-Q" long, contracted at base, slender, 

 puberulent : spines loosely disposed, V^-2V' 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, \V^-?/^ 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. speciosumi Kearney. Fruit \Q'^-V2'^ long, hairy : spines slen- 

 der, thickly disposed, 3^'-5^^ long, often exceeding the slender nearly 

 straight beaks. — Bottoms near Courtney. August-October. 



