474 PRACTICAL BOTANY 



It is a well-known fact that new weeds are particularly 

 likely to be found in places where ballast from vessels is 

 dumped, where cargoes of foreign grain are cleaned, and 

 where foreign wool is scoured and cleaned from burs and 

 other seeds. 1 Some very troublesome weeds, as the common 

 carrot and the orange hawkweed, have been cultivated for use 

 or ornament and escaped into fields, meadows, and pastures. 



437. Weeds of various regions. Any two regions which 

 differ widely in soil or climate are sure to differ also in the 

 weeds which predominate there. Such tropical plants as the 

 sensitive plant 2 and the rosy periwinkle, 3 not uncommon in 

 our greenhouses and gardens, are troublesome weeds, the for- 

 mer in tropical South America and the islands of the South 

 Pacific, and the latter in the West Indies. But in our climate 

 it requires care and protection to keep them alive. Even in 

 the various climates afforded by the United States, there is 

 range enough to make one weed troublesome in one portion 

 of the country and another in another portion. The quack 

 grass, or couch grass, 4 so injurious from its creeping rootstocks 

 in fields and gardens from Maine to Minnesota, is replaced as 

 a weed in the southern states by the Johnson grass, 5 which 

 has still stronger and longer rootstocks. The wild gourd, 6 

 troublesome in the far Southwest, is not found as a weed 

 northeast of California and New Mexico, and the cacti, 7 an- 

 noying weeds in central and southern Kansas and westward 

 and southward, are of no importance farther east. 



The amount of moisture in the soil is an important factor 

 in the distribution of weeds. Such plants as the cacti just 

 mentioned, some cinquefoils, St.-John's-worts, lambkill, some 

 species of vervain, 8 the common mullein, rib grass, 9 and the 



1 A curious case of distribution of a bur is that of the grass Andropogon 

 acicularis. A buffalo with his hair filled with the needle-like fruits of this grass 

 was sent as a present to the so-called king of Ternate, in the Malay Archi- 

 pelago. From this one animal the grass soon spread over the whole island. 



2 Mimosa pudica. 3 Vinca rosea. 4 Agropyron repens. 6 Sorghum 

 halepense. 6 Cucurbita perennis. 7 Mamillaria, Opuntia, and other genera 

 in the Southwest. 8 Verbena. 9 Plantago aristata and P. lanceolate. 



