HOW WEEDS ARE SPREAD S8i 



and thistle, are rendered buoyant by a ring of downy hairs 

 (the pappus) attached to the fruits containing them. They are 

 readily wafted long distances even by a gentle breeze. A few 

 thistles allowed to flower and seed are soon able to stock a 

 parish with their progeny. Seeds of grasses are often carried 

 about by the wind. 



A few weeds, such as cleavers and burdock, have hooks upon 

 their fruits, which cling to the bodies of animals, and especially 

 to the wool of sheep. They subsequently become disentangled 

 and rubbed off in other localities than those in which they are 

 grown. The hairy seeds of grasses are not unfrequently distri- 

 buted over the land in the same manner. 



{b) Besides these natural methods of weed distribution, a large 

 number are spread over the farm by the use of impure seed. 

 Imperfectly cleaned samples of barley, wheat, and oats are 

 sometimes responsible for the introduction of wild oats, 

 cleavers, charlock, and other obnoxious plants to what was 

 previously clean land. Samples of smaller seeds, such as those 

 of grasses and clovers, very often contain a large number of 

 weed seeds, the chief of which are given in the chapters on 

 "Farm Seeds." 



{c) Another fertile source of mischief is the use of impure farm- 

 yard and stable dung. Weeds are frequently thrown.on the manure 

 heap under the impression that the heat of fermentation and 

 the chemical changes going on in it will destroy the germinating 

 power of any seeds which may be present. Many weed-seeds 

 are killed by this treatment, but a large number remain quite 

 uninjured, and when dung containing the latter is spread on the 

 land, weeds spring up in abundance. It must be remembered 

 also that many plants pulled out of the ground in an immature 

 state are capable of ripening their seed without any contact with 

 the soil. This is particularly the case with biennials and per- 

 ennials which contain rich stores of food in their thick tap 

 roots and stems. If these are cast upon the manure heap, 



