298 THE SOILS AND CEOPS OF THE FARM. 



Buch as the thistle, float in the air by means of their 

 pappus. The tumble-weed is a familiar example of a 

 whole plant being moved by the wind. Seeds drift 

 with the snow. 



Water is a common source of distribution. Far- 

 mers whose lands are over- flowed by spring freshets 

 find it impossible to keep their land free of weeds 

 grown by their neighbors farther up the stream. Co- 

 operative effort is here necessary. In 1882 the high 

 waters brought the horse-weed [Erigeron canadense) 

 to farms in the southern part of Ohio, where it was 

 unknown before. 



There are many devices of the fruit of plants by 

 which they stick to animals and are thus carried from 

 place to place. Cockle- burrs, burdock burrs andspan- 

 ish-needles are familiar examples. 



Seeds are canied in the mud, clinging to the feet of 

 birds and other animals. Many seeds are distributed 

 by being swallowed but not digested. Birds may 

 carry seeds long distances in this way. Doubtless 

 farm animals which are transported from place to 

 place may do the same. It may pay the careful far- 

 mer to look to this matter. 



Some plants have the power within themselves by 

 which seeds are disseminated to a slight extent. The 

 seed- pods of the wood sorrel ( Oxalis stricta) explode, 

 scattering the seeds. Plantain in moist weather ex- 

 udes a gelatinous substance which carries the seeds 

 to the ground and causes them to stick to passing ob- 

 jects. 



The seeds of the bunch grass (Stipa) have a sharp 

 point, and a long spiral awn which twists and un- 

 twists with the moisture of the air, thus causing the 



