WEEDS 



179 



or as immigrants from other parts of the world. The orange 

 hawkweed (Hieradum aurantiacum), which within a generation 

 has spread over large areas in the Eastern states, is a case in 

 point, and the Russian thistle, which appeared somewhat 

 earlier, is another. The rapidity with which these weeds have 

 spread is accounted for by their methods of seed distribution. 

 Plants with wind-dis- 

 tributed seeds are 

 usually good travel- 

 ers, but those whose 

 seeds lack special 

 means of distribution 

 are often very slow 

 in conquering new 

 territory. The ox eye 

 daisy, which has 

 proved such a pest 

 in New England, is 

 still rare or absent 

 in the north -central 

 states, while the yel- 

 low daisy, originally 

 a Western plant, has 

 spread to the East in 

 comparatively recent 

 times. New weeds 



are likely to be first found along traveled ways, especially if 

 their seeds are not modified in some way for distribution, and 

 a new line of traffic is usually responsible for their introduc- 

 tion. Galinsoga parviflora, a harmless Mexican plant, was 

 unknown in the Northern and Eastern states until the inhabit- 

 ants began railway traffic with Mexico. Now it is common in 

 many places as far north as Canada. The teasel is an Old 

 World plant that has long been known as a weed in America, 



Photograph from American Steel and Wire Co. 



FIG. 129. Yarrow, a nearly harmless weed of 

 wide distribution 



