178 Proceedings of the Biological Society of Washington. 



and after harvest mature their fruits. They are formidable 

 weeds found on every farm. 



The classification of the species of Xanthium rests almost 

 entirely on the characters of the fruits or burs. These, like 

 most specific characters, are extremely variable, so that strict 

 definition of the groups ranked as species may be quite difficult. 

 After experience, however, the most dependable characters 

 or combinations of characters are easily appreciated and identi- 

 fication is robbed of most of its perplexities. The subjoined 

 key is based on the characters we have found most feasible 

 for identifying the local cockleburs. The plate illustrating 

 the burs we trust will be a useful adjunct to the key. For 

 numerous other illustrations see Millspaugh and Sherff's 

 "Revision." 



The present contribution lists 5 species of Xanthium from 

 the District of Columbia region, an increase of 2 over the num- 

 ber included in the recent "Flora." 1 X. americanum of that 

 work is X. chinense Miller and X. commune no doubt covers 

 both X. italicum Moretti and X. pennsylvanicum Wallroth 

 as listed in the present paper. 



Key to Species 



A. Burs with inconspicuous beaks; spines weak and curved like a 

 button-hook at apex; color usually greenish yellow. 



X. spinosum. 



AA. Burs with conspicuous beaks; spines strong, simply hooked; color 



usually reddish brown. 



B. Body of fruit broadly oblong, oblong-ovoid or ovoid-oblong; 



spines densely hispid-pubescent, up to and beyond the middle. 



C. Burs very large (2.8-4 cm. long and 2-3 cm. wide), broadly 



ovoid, oblong-ovoid or oval; spines stout, basal ones reflexed, 



thicker and longer than others and projecting much beyorfd 



stalk of bur. X. speciosum. 



CC. Burs smaller (less than 2.8 cm. long and 2 cm. wide), oblong 



or ovoid-oblong, often widest at base; spines more slender, 



basal ones not reflexed or projecting much beyond stalk of 



bur. X. italicum. 



BB. Body of fruit fusiform, ellipsoidal or narrowly oblong-ovoid; 



spines smooth, warty-glandular or if hispid usually only up 



to the middle. 



i Contr. Nat. Herb.. Vol. 21, p. 267, Sept., 1919. 



