ECONOMIC IMPORTANCE OF GREEN PLANTS 129 



do the greatest damage are weeds. Those plants which provide 

 best for their young are usually the most successful in life's 

 race. Plants which combine with the ability to scatter many 

 seeds over a wide territory the additional characteristics of rapid 

 growth, resistance to dangers of extreme cold or heat, attacks of 

 enemies, inedibility, and peculiar adaptations to cross-pollina- 

 tion or self-pollination, are usually spoken of as* weeds. They 

 nourish in the sterile soil of the roadside and in the fertile soil of 

 the garden. By means of rapid growth they kill other plants of 

 slower growth by usurping their territory. Slow-growing plants 

 are thus actually exterminated. Many of our common weeds 

 have been introduced from other countries and have, through 

 their numerous adaptations, driven out other 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 in 1873, it spread so rapidly that in twenty years it had 

 appeared as a common weed over an area of some twenty-five 

 thousand square miles. It is now one of the greatest pests in our 

 Northwest. 



REFERENCE BOOKS 



ELEMENTARY 



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



Gannet, Commercial Geography. American Book Company. 



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



Toothaker, Commercial Raw Materials. Ginn and Company. 



U. S. Dept. of Agriculture, Farmers' Bulletin 86, Thirty Poisonous Plants of thf 



United States, V. K. Chestnut. Bulletin 17. Two Hundred Weeds, How 10 



Know Them and Ho'w to Kill Them, L. H. Dewey. 



ADVANCED 



Bailey, Cyclopedia of American Agriculture. The Macmillan Company. 



HUNTER, CIV. BI. 9 



