Vol. 33, pp. 177-180 December 30, 1920 



PROCEEDINGS 



OP THE 



BIOLOGICAL SOCIETY OF WASHINGTON 



NOTES ON COCKLEBURS (AMBROSIACEAE; XAN- 

 THIUM) OF THE DISTRICT OF COLUMBIA 



AND VICINITY. 



BY W. L. McATEE AND F. P. METCALF. 



Receipt of a copy of the "Revision of North American 

 Species of Xanthium " x by Drs. Charles F. Millspaugh and 

 Earl E. Sherff stimulated the writers to investigate the cockle- 

 burs of the District of Columbia and vicinity. Approximately 

 -150 samples of mature burs were collected chiefly in December 

 1919. These were studied with the assistance of the revision 

 just noted and reduced to a set of 24 samples illustrating the 

 principal variations. These have been examined and com- 

 mented upon by Dr. Earl E. Sherff whose kindness in this 

 respect we gratefully acknowledge. Specimens already on 

 hand in the National Herbarium also were taken into account 

 and copious representations of the species here recorded have 

 been deposited in that collection. 



Cockleburs strongly specialized for casual transportation 

 are typical waifs and reach all sorts of places where the wastes 

 of civilization and nature accumulate. The search for cockle- 

 burs takes the collector to such interesting spots as refuse- 

 heaps about freight-yards, factories and wharves, to ash, 

 trash and garbage dumps and the environs of the barnyard, 

 cattle-pens and pig-sties. Cockle-burs like ground not es- 

 pecially occupied with other plants. In eroded places, on 

 grades, anywhere there is new-turned earth they may be found. 

 They grow well among crops of rather open stand as corn 



i Field Museum Publ. 204, Bot. Ser. Vol. 4, No. 2, April 1919, pp. 9-49, pis. 7-13. 

 32 — Proc. Biol. Soc. Wash., Vol. 33, 1920. (177) 



