524 



FIELD CROPS 



up from them. Some spread by means of both seeds and 

 running rootstocks. Perennials which spread only by seeds 

 include the dandelion, docks, and plantains. The Canada 

 thistle in many sections spreads only by its rootstocks and 

 does not produce seeds; elsewhere it seeds abundantly. 



Fig. 156. Quack grass, or couch grass. 



Other weeds which spread by both means are Johnson grass, 

 quack grass, sow thistle, and ox-eye daisy. 



THE DAMAGE DONE BY WEEDS 



690. Weeds Lower Crop Yields. Weeds occupy space 

 which is needed by crops, thus crowding them out and shad- 



