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TEXTBOOK OF BOTANY 



Fig. 127. Effects of dry soil (lighter color) on the growth of roots. Photo by 



F. H. Norris. 



lar literature that the horizontal spread of the root system of a tree is 

 the same as the diameter of the crown is erroneous. Isolated trees on a 

 lawn or campus may have a horizontal spread of roots as much as 4 to 

 5 times the diameter of the crown. A young pear tree was found to have 

 a horizontal spread of roots 9 times the diameter of its crown. 



Plants closely crowded together do not have as extensive root systems 

 as isolated ones. The more extensive root system may or may not occur 

 with the larger crown. The root system of a corn plant at Lincoln, Ne- 

 braska, commonly has a horizontal spread of about 7 feet and a vertical 

 penetration of about 6 feet. At Wooster, Ohio, the horizontal spread of 

 the root system of a corn plant is generally about 3 feet, and the depth 

 about 2 feet. These figures represent differences in the growth of root 

 systems due to the combined effects of heredity, climate, and soil. 



Many factors affect the development of root systems. The striking dif- 

 ferences in form of the root systems of different species of plants are 

 expressions of different heredities in similar environments. But expres- 



