MIGRATION OF WEEDS 83 



weeds have spread westward some of our western plants 

 have spread eastward. Generally speaking, the move- 

 ment of weeds in our own country has been westward, 

 although niggerhead, squirrel-tail, marsh elder, and sun- 

 flower have moved eastward. In Gray's "Manual," sixth 

 edition, the distribution of marsh elder is given as follows : 

 Northwest Wisconsin to Minnesota and Kansas westward. 

 It must, indeed, originally have been quite local in many 

 places in this region. It is only recently that this weed 

 has attracted attention. Another illustration is buffalo bur. 

 In Gray's "Snyoptical Flora," the distribution is given 

 as "Plains of Nebraska to Texas and Mexico." In Gray's 

 "Manual," sixth edition, the statement is made, "spread- 

 ing eastward to Illinois and Tennessee." Britton, in his 

 "Manual of Botany," says, "On prairies, South Dakota to 

 Texas and Mexico. Occasionally in waste places as a 

 weed, Ontario to New Hampshire, Massachusetts and 

 New Jersey, adventive from the West." This plant has 

 certainly spread to many different points in the Eastern 

 States. 



The Canada thistle was early introduced in the East, 

 but now occurs, according to Dewey, in all of the North- 

 ern states and in Canada across the continent. Its dis- 

 tribution shows the lines of tension in its northern exten- 

 sion. The same may be said of the bull thistle, which is 

 as abundant in Montana and Utah as it is in Oregon and 

 Washington. Where new lines of railway are opened in 

 Canada these European immigrants follow. 



Few of the southern plants have spread northward. 

 Among these, however, we may cite the horse nettle 

 (Solatium carolinense). The record of sand brier or horse 

 nettle forms an interesting chapter in the migration of 

 perennial plants from one part of the country to another. 

 It is much easier for an annual to become acclimated 

 than a perennial. Throughout the Mississippi valley 



