MIGRATION OF WEEDS 8 1 



in the character of the weed flora in various parts of the 

 United States. 



Of the cosmopolitan weeds, shepherd's purse (Capsella 

 Bursa-pastoris), occurs everywhere in Europe, Persia, In- 

 dia, Japan, United States, Chili, South Africa and Can- 

 ada. Pusley occurs in India, Egypt, Europe, South 

 Africa, Japan, China, Java and Philippine Islands; horse 

 weed (Erigeron canadensis), everywhere in the United 

 States from Maine to California, Brazil, South Italy, 

 Russia, Sweden, Persia, North India, South Africa. 

 It is sometimes a very difficult matter to decide 

 whether plants are strictly native to a country or have been 

 introduced, so thoroughly have they established them- 

 selves. Pusley is an excellent illustration. It is believed 

 to be a native to the southwest. 



Lines of Travel. Weeds in their migration have fol- 

 lowed certain well-defined lines of travel. The floras of 

 our Atlantic states mention the occurrence of certain 

 weeds commonly found where the ballast material was 

 discharged. The list of ballast plants on the Atlantic 

 and Pacific coast is constantly increasing. 



In speaking of the means of transportation of weeds, 

 Dewey says that the routes of transportation are indi- 

 cated by the names, ballast plants, roadside weeds, weeds 

 along the towpath, and railway weeds. Mr. Dewey 

 says : "One hundred and three species were taken in bal- 

 last from Buenos Ayres to New Zealand within a period 

 of a few years." 



There is no doubt that the Crusaders brought many 

 weeds back from western Asia into Europe. Thus plants 

 like the horse-radish, mustard or charlock, hemp (Can- 

 nabis sativa), are Asiatic plants. Kabsch notes that most 

 of the weeds of cereal crops like Centaurea Cyanus, Ag- 

 rostemma Githago, Raphanus Raphanistrum and Myagrum 

 sativum are foreign to Europe. But Europe has also re- 

 ceived a number of American weeds from us, like the 



