68 UNIVERSITY OF MISSOURI STUDIES [2IO 



too acid for the alfalfa. Besides Medicago sativa, certain weeds 

 that are altogether unique in the region, occur only in alfalfa 

 fields, notably Crepis setosa, Centaurea solstitialis, Cichorium 

 Iniybus, Lolium Italicum, and Malva moschata. 



3. Ruderales. Weeds of cultivated and waste grounds. 

 Weeds are susceptible of division into associations from many 

 points of view. Thus they may be divided into lowland, midland, 

 and upland weeds. The lowland weeds consist mainly of native 

 plants such as Bidens frondosa. Coreopsis involucrata, Helian- 

 thus tuberosus subcanescens. Ambrosia spp., Polygonum spp., 

 and Xanthium spp. The midland weeds, for the most part ex- 

 otic, are mainly Chenopodiateae and Amarantaceae, with many 

 Gramineae and Compositae. The uplands and sands have the 

 weedy grasses such as Panicum sanguiimle, Eragrostis spp., 6"^- 

 taria spp., and Cenchrus tribuloides (rare). Weeds may be 

 classified also according to the cultural conditions in which 

 they grow. It has been seen that certain weeds accompany 

 certain crops. The weeds of cornfields are not those of 

 pastures and meadows. Those about houses and those about 

 barns dififer. Those along streets of the city are not those 

 along country roads. A superficial, yet useful division is into 

 weeds of pastures, meadows and lawns. These are suited 

 to biennials and perennials, since their habitats remain un- 

 broken for a few years at least. Weeds of grain and stubble 

 fields. Such weeds are usually winter annuals, for only 

 plants that can grow and mature with the grain can insure 

 their seeds being sown with the grain in autumn. Weeds 

 of gardens, and hoed and cultivated crops. Such are mainly 

 annuals, as the crops in which they grow exist but for the 

 season. Weeds along roads, streets, and railroads. Weeds 

 about houses and barns. Weeds in waste unswarded fields, etc. 

 But whatever principle be adopted, there are many species com- 



