Distribution and t^ropagation. 49 



root-stocks embedded in the earth that is 

 thus carried from place to place are depos- 

 ited in new centers, where they spring up 

 to carry on the work of further multipli- 

 cation. The distributing power of the 

 implements of tillage under favorable con- 

 ditions furnishes us with one explanation 

 of the rapid multiplication in our fields of 

 such weeds as quack grass and the Canada 

 thistle, when once these pests have obtained 

 a footing in them. 



8. Thrashing machines. Thrashing ma- 

 chines, during the season in which they 

 are used, are continually bearing the seeds 

 of noxious weeds from farm to farm, and 

 from district to district ; for at every thrash- 

 ing, if weed seeds are present at all, some 

 of them are sure to become lodged in the 

 various parts of the machines, and to 

 remain long enough to be carried away. 

 In this manner, from year to year, a very 

 general local distribution is sure to be 

 effected of every form of noxious weed 

 w^hich matures its seeds in a grain crop. 

 Farms that are managed in the best man- 

 ner often become thus infected with noxious 

 weed life. 



