648 



WEED FLORA OF IOWA 



of leaves and thus pass the winter stage. In the spring stems 

 are thrown up frora these leaves and seed is produced. Our com- 

 mon shepherd's purse {Caxpsella bursa-pastoris) is an example. 



The third class is the biennials or those which expend their energy 

 the first season in forming a root system, usually fleshy, and the 

 second season in maturing seed. We have numerous examples 

 among the garden crops, such as the beet, turnip, carrot, etc., while 

 among the weedy plants we have the burdock {Arctium lappa), 

 the sweet clover, figure 505, the wild parsnip and others. If 

 these plants are prevented from forming seed the second year they 

 perish. 



Fig. 508. Roots of Buokhorn (Plantago lanceolata). This perennial weed is 



easily destroyed by cultivation. 



(Photographed by Colbum.) 



In the fourth class belong the perennials which live year after 

 year and for this reason become our most noxious weeds. When 

 the roots once become well established it is very difficult to eradicate 

 them, as is shown by the lawns, fields and pastures which are in- 

 fested with such weeds as dandelions, quack grass, Canada thistle. 



