MAN'S AID IN SPREADING WEEDS 531 



carrying weeds from place to place. Some weed seeds are 

 provided with hooks which cling to the wool or hair, as the 

 cockle bur, burdock, and beggar's ticks. Others are stored 

 as food by animals or by birds and are forgotten, springing 

 up as plants in new locations the following year. Ground 

 squirrels, prairie dogs, and other burrowing animals store 

 large quantities of grass and weed seeds, not all of which are 

 consumed, and some of which are not buried so deeply that 

 they fail to grow. Weed seeds are eaten by birds, carried 

 by them for long distances, and then, passing through their 

 digestive systems unharmed, are dropped in new localities. 

 Branches of weeds bearing seeds may be used by birds or 

 animals in building nests and thus disseminated. The 

 droppings of live stock furnish a local means of distribution 

 from one field of the farm to another when animals are 

 changed from pasture to pasture or worked in the field. 



701. The Activities of Man. The operations of human- 

 kind furnish many of the methods of weed distribution, some 

 of which are hardest to counteract. The movement of 

 vehicles along roads or from roads to fields often serves as 

 an agency of weed distribution, particularly in damp weather, 

 when mud sticks to the wheels and seeds are gathered up 

 with the mud, to be dropped off in some other place. Tillage 

 implements and the work of tillage furnish another means of 

 distribution. Weed seeds or the weeds themselves may be 

 carried from place to place on the implements, or may be 

 moved with the movement of earth in tillage. Roots of 

 perennial weeds are often carried by tillage tools; for this 

 reason, poor or occasional cultivation of fields infested with 

 quack grass, Johnson grass, or weeds that spread by similar 

 means is often worse than no cultivation at all. Thrashing 

 machinery furnishes a ready means by which weed seeds 

 are carried from farm to fnnn. 



