186 NATUEAL HISTOEY BULLETIN 



Euphorbia hexagona Nutt. 

 Gilia linearis (Nutt.) Gray. 

 Hosackia americana (Nutt.) Piper. 

 Lepachys columnaris (Sims.) T. & G. 

 Malvastrum coccineum (Pursh) Gray. 

 Mentzelia oligosperma Nutt. 

 Opuntia fragilis (Nutt.) Haw. 

 Oxybaphus linearis (Pursh) Eob. 

 Pentstemon hirsutus (L.) Willd. 

 Solanum rostratum Dunal. 

 Yucca glauca Nutt. 



Several of these species are gradually being carried eastward 

 along the railways. 



4. Introduced weeds. These are chiefly European and have 

 been incidentally introduced by man. Quite a number of these 

 weeds have become established on the prairie and even invade 

 unbroken areas. Some of the species are included in the table 

 and are marked with an asterisk. The following may be added : 



Asparagus officinalis L. 

 Brassica arvensis (L.) Ktze. 

 Brassica nigra (L.) Koch. 

 Bromus racemosus L. 

 Cannabis sativa L. 

 Capsella bursa-pastoris (L.) Medic. 

 Chrysanthemum leucanthemum L. 

 Digitaria sanguinalis (L.) Scop. 

 Echinochloa crus-galli (L.) Beauv, 

 Melilotus officinalis (L.) Lam. 

 Trifolium hybridum Muhl. 

 Trifolium pratense L. 

 Verbena urticifolia L. 



These species occur most frequently on or near lands which 

 have been cultivated, and their abundance is probably deter- 

 mined in large part by the accident of seed-dispersal. 



The list does not include the numerous species of weeds which 

 have everywhere invaded cultivated lands, but is confined to the 

 species which seem to be able to establish themselves even in 

 competition with the native prairie flora. 



The table of plants brings out the fact that neither topog- 

 raphy nor geological formation determines the character of the 



