

PV~- 



REVISION OF THE NORTH AMERICAN 

 SPECIES OF XANTHIUM 



CHARLES F. MILLSPAUGH, M.D., and EARL E. SHERFF, Ph.D 



The monographic study of the genus Xanthium is involved in 

 difficulties not only as to bibliography but also as to species concept. 

 Thanks to the kind and hearty co-operation of various botanists, we 

 have been able to settle very satisfactorily the numerous matters of 

 bibliography. The species concept in Xanthium, however, must long 

 remain a perplexing problem. In temperate regions, the plants do 

 not mature their fruits sufficiently for exact determination until after 

 frost comes and the majority of collecting botanists have ceased their 

 field-work. This renders good material in herbaria scanty in quantity 

 and inadequate in quality. Some of the species are known to exhibit 

 most striking variations in fruiting characters, variations that with 

 many botanists would be taken to represent varieties or subspecies. 

 In fact, several of the more pronounced of these forms have been made 

 the basis of new species by certain authors, notably Greene (e.g., 

 X. acutum, X. affine, X. calif ornicum, X. glanduliferum) . In the present 

 monograph the writers have endeavored to be neither hasty in the 

 proposal of new specific names nor radical in the reduction of old 

 names to synonomy. The taxonomic treatment has been made to 

 accord as strictly as possible with the observed data. We have retained 

 several of the less well known species (e.g., X. acerosum, X. cylindricum, 

 X. globosum, species that with some botanists might be reduced to 

 varietal or subspecific rank), because we have felt that only after fur- 

 ther field observations and breeding tests can satisfactory conclusions 

 as to their true status be reached. 



Many of the numerous references in literature have necessarily been 

 omitted in the main body of our text: for a large number of these 

 references the reader is directed to De Candolle's Prodromus 

 (6:522-524. 1836) and to Wallroth's Monograph of Xanthium (Beitr. 

 Bot. i": 229-244. 1844). Since the publications of De Candolle 

 and of Wallroth, several other more or less extended studies of the 

 genus have been made: In 1893, Rowlee (Bull. Torr. Bot. Club 20: 10), 

 writing upon the seedling development of Xanthium, noted that "both 



