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CONTROL OF WEEDS 333 



been introduced from other countries and have, through their 

 numerous adaptations, driven out plants which stood in their way. 

 Such is the Russian thistle. A single plant of this kind will give 

 rise to over 20,000 seeds. First introduced from Russia with 

 imported flaxseed in 1874, by 1898 it had appeared as a pest in all 

 states east of the Rocky Mountains. 



Weeds are introduced into lawns and fields often because their 

 seeds are mixed with the good seeds which are sown there. Farm- 

 ers should be sure of the quality of seed before planting it. Har- 

 rowing and cultivating land helps to destroy weeds. We should 

 use every method possible to prevent the weeds from producing 

 seeds, because they are so numerous. Poisons are used in some 

 cases ; sheep, which seem to prefer some weeds to grass, are also 

 a great aid in keeping down these pests. 



Summary. — This chapter has explained the very great impor- 

 tance of green plants in food production for man and the lower 

 animals. Man is provided also with shelter, material for clothing, 

 drugs, and many other commodities by the green plants. Some 

 green plants do harm, as is proved by the presence of weeds and 

 poisonous plants. 



Problem Questions 



1. Tabulate the useful plants of your locality. The harmful plants. 



2. How does the value of the present wheat or corn crop compare with the 

 cost of building Panama Canal or a college ? 



3. What plants are used economically in your locality? Are there any 

 useful plants not made use of there? 



4. What are weeds? How would you go to work to control them? 



Problem and Project References 



Hunter, Laboratory Problems in Civic Biology. American Book Company. 



Atwood, Civic and Economic Biology. P. Blakiston's Son and Company. 



Whitbeck, Industrial Geography. American Book Company. 



Sargent, Plants and Their Uses. Henry Holt and Company. 



Toothaker, Commercial Raw Materials. Ginn and Company. 



Transeau, General Botany. World Book Company. 



United States Dept. of Agriculture, Farmers' Bulletins 17, 86, and others 





