292 CONTRIBUTIONS FROM THE NATIONAL HERBARIUM. 



47. ARCTIUM L. 

 1. Arctium minus Schkuhr. Burdock. 



Waste ground; common. May-July. Native of Eur.; widely naturalized in the 

 U. S. {Lap-pa officinalis of Ward's Flora.) 



Arctium tomentpsum (Lam.) Schkuhr, with very woolly heads, was collected in 

 waste ground about Washington, June, 1896 (Steele). Native of Eur. 



48. cmSITJM Mill. Thistle. 



Leaves glabrous or nearly so, green. 

 Heads 2.5 cm. broad or smaller; perennial, with rootstocks; bracts with very 



short spiny tips 1. C. arvense. 



Heads 5-7 cm. broad; biennial, without rootstocks; bracts with slender spiny tips. 



2. C. odoratum. 



Leaves densely white-woolly, at least beneath. 

 Bracts not ending in spines. Jjeaves pinnately lobed, white- woolly beneath when 

 young, sometimes nearly glabrous in age; heads on naked stalks. 



3. C. muticum. 

 Bracts ending in spines. 



Bracts all ending in spines. Leaves pinnately lobed, the lobes triangular, end- 

 ing in sharp spines 4. C. lanceolatum. 



Bracts only in part (the outer ones) ending in spines, the inner acuminate. 



Peduncles naked or nearly so. Leaves deeply lobed 5. C. vlrginianum. 



Peduncles leafy, the heads subtended by leaves. 



Leaves deeply lobed, the lobes lanceolate or linear 6. C. discolor. 



Leaves merely toothed, or the lowest sometimes shallowly lobed. 



7. C. altissimum. 



1. Cirsium arvense (L.) Scop. Canada thistle. 

 Waste ground; infrequent. June-Aug. Native of Eur.; widely naturalized in the 



U. S. and often a troublesome weed. {Carduus arvensis Robson; Cnicus arvetisis 

 Hoffm.) 



2. Cirsium odoratum (Muhl.) Britton. Pasture thistle. 

 In woods; Woodley. June. Me. to W. Va. (Carduus odoratus Porter.) 



3. Cirsium muticum Michx. ' Swamp thistle. 

 Above, Cowdens, Seipt., 1914 (Steele). Eastern N. Amer. (Carduus muticusFers.) 



4. Cirsium lanceolatum (L.) Hill. Common thistle. 

 Fields and pastures; infrequent. July-Oct. Native of Eur. and Asia; widely 



naturalized in the U. S. (Carduus lanceolatus L.; Cnicus lanceolatus Willd.) 



6. Cirsium virginianum (L.) Michx. Virginia thistle. 



Wooded hillside along Difficult Run, June, 1915 (Standley). Southeastern U. S. 

 (Carduus virginianus L.) 



6. Cirsium discolor (Muhl.) Spreng. Field thistle. 

 Dry woods; frequent. Aug. -Sept. Eastern U. S. 



7. Cirsium altissimum (L.) Spreng. Tall thistle. 

 Moist woods and copses; common. July-Oct. Eastern U. S. 



Carduus nutans L., musk thistle, was collected in waste ground in southwest Wash- 

 ington in 1897. Native of Eur. and Asia; naturalized locally in the northeastern U. S. 



49. ONOPORDON L. 



1, Onopordon acanthium L. Scotch thistle. 



Waste ground about Washington iud Alexandria, frequent about the latter place 



and well established. Native of Eur. and Asia; adventive in the northeastern U. S. 



