252 



PASTORAL AND AGRICULTURAL BOTANY 



turned out and killed by the plow. Mowing infested fields before the 

 heads develop is another useful means of extermination (Figs. 106 and 

 107). 



Canada Thistle (Cirsium arvense). Nearly all of the states have laws 

 which make it an offense for their citizens to permit this weed to mature 

 and scatter its seeds. It is a perennial propagated by seeds and rootstocks. 

 The map shows its range (Fig. 108). 



FIG. 1 08. Map of the United States, showing the present 'distribution of the 

 Canada thistle. The heavy line indicates the approximate southern boundary of the 

 weed; the shaded area shows where the plant is most injurious. (After Hansen, Albert 

 A.: Canada Thistle and Methods of Eradication. Farmers' Bulletin 1002, 1918, p. 5.) 



CLASSIFICATION or WEEDS 



Weeds are conveniently classified according to the duration of their 

 underground parts into: 



1. Annual weeds, or those which complete their growth and mature 

 their seeds in one year. Such plants are easily destroyed by cultivation. 

 Here belong ragweed, crabgrass, purslane, pigweed and Russian thistle. 



2. Winter annuals. These plants drop their seeds in the fall which 

 germinate and give rise to plants which hold through the winter, finally 

 producing flowers and seeds in the spring. Such are the chickweed and 

 shepherd's purse. Here also is the prickly lettuce and dead nettle. 



