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TEXTBOOK OF BOTANY 



FIG. 186. The quack grass. 



the few ferns that make themselves troublesome in this 

 way. 

 352. How Weeds are Introduced and Spread. As we 



have seen, most of our weeds, whether annual, biennial, or 

 perennial, produce great numbers 

 of seeds, and depend largely or 

 entirely upon seeds for their dis- 

 tribution. The most successful 

 weed, therefore, is the one that 

 has the most effective means of 

 scattering its seeds or fruits. An 

 excellent example of a very suc- 

 cessful weed is the dandelion, 



whose plumed fruits are so easily carried to great distances 



by the wind. Something was said in Chapter XVIII about 



the various ways in which seeds and fruits are scattered, 



and most of the means of distribution mentioned in that 



chapter are employed by some of the common weeds. 



That so many of the composite 



family are weeds is due to the 



excellent means of fruit distribu- 



tion especially distribution by 



the wind that the members of 



that family have developed. Be- 



sides composite weeds includ- 



ing the dandelion, thistles, and 



wild lettuce a good many 



members of other families de- 



pend likewise upon the wind ; 



examples are the milkweeds, 



FIG. rSy.- The Johnson grass. 



with tufts of hairs attached to their seeds, and many of the 

 grasses and sedges, whose fruits are very small and light. 

 Among the weeds whose fruits attach themselves to the 

 bodies of animals, some composites, too, such as the cockle- 

 bur and the burdock, are conspicuous. 



