184 COMPOSITE. 



3 6 inches long, and nearly of the same width. Heads in short axillary 

 racemose clusters. Var. Canadense Torr. 4- Gr. has the stem spotted and the 

 fruit-bearing involucre scabrous pubescent. X. strumarium Mich. Introduced 1 



Common Clot-weed. Small Burdock. 



2. X. echinatum Murr. : fruit-bearing involucre oval, very densely clothed 

 with rigid slender prickles and with the incurved beaks strongly hispid ; 

 leaves rough, broad-cordate, irregularly sinuate-toothed, obscurely lobed. 

 ( Torr. fy. Gr.) X. macrocarpon Beck Hot. 1st. Ed. X. orientate Muhl. 

 X. maculalum Raf. 



Near salt water. Mass, and N. Y. to Car. W. to Miss. Aug. Oct. . 

 Stem marked with purple spots and stripes, roughly pubescent. Leaves very 

 rough. Fruit very large, woolly. Sea Clot-weed. 



3. X. spinosum Linn. : spines 3-parted, slender ; leaves ovate-lanceolate, 

 cuneate at base, entire or somewhat 3-lobed, acuminate, minutely-pubescent 

 above, the under surface and the veins of the upper canescent. 



Waste grounds. N. Y. to Geor. Sept., Oct. (T).Stem 23 feet high, 

 pubescent, branched. Leaves entire or repand-denticulate, at length often 

 3-lobed. Heads few, axillary, solitary. A troublesome weed. Introduced from 

 Europe. Spiny Clot-weed. 



28. AMBROSIA. Linn. Rag-weed. 



(Ambrosia was the food of the Gods ; but it is difficult to determine the appli- 

 cation to the plants of this genus.) 



Heads monoecious ; the fertile at the base and the sterile at 

 the top of the spike. STERILE FL. Involucre hemispheric or 

 turbinate ; scales few. Receptacle naked. Corolla tubular, 

 short. FERTILE FL. Involucre 1 -flowered, incurved and often 

 armed with several tubercles or horns. Corolla none. Achenia 

 ovoid or obovoid. 



* Upper leaves undivided. 



1. A. inlegrifolia Muhl. : leaves opposite, ovate, sessile, acuminate, ser- 

 rate, hispid on both sides, ciliate at base ; racemes terminal and mostly ter- 

 nate. A. trifida,\ax. Torr. <$ Gr. 



Near ponds and ditches. Penn. and Virg. (I). Pursh. It is said to have the 

 lower leaves sometimes 3-lobed. Probably a variety of the next, as suggested by 

 Torrey and Gray. Simple-leaved Rag-weed. 



** Leaves all 3 5-lobed. 



2. A. trijida Linn. : hirsute, rough ; leaves 3 5-lobed, serrate ; the lobes 

 oval-lanceolate, acuminate ; fruit 6-spined below the summit. 



Banks of streams. Can. to Geor. W. to Miss. July Sept. . Stem 48 

 or 10 feet high, angular, branched above. Leaves very large and rough. Heads 

 small ; the sterile ones in long paniculate racemes ; the fertile in small clusters 

 at the base of the racemes. Three-lobed Raff weed. 



*** Leaves singly or doubly pinnatifid. 



3. A. artemisiaifolia Linn. : stem pubescent, often much branched ; 

 leaves bi-pinnatifid, rough, hoary beneath, the petioles ciliate with long 

 hairs; racemes paniculate, terminal. A. elatior Linn. A. 



Mich. 



