THE WORK OP ROOTS 79 



stance, it must make room for the surplus gained in the 

 transaction. In the experiment just perfonned, it crawls 

 up the tube. 



The roots of plants, having within them contents which 

 are denser than the soil water, draw this soil water with its 

 dissolved minerals into themselves, forcing it upward 

 through the stems into the leaves. 



It -sometimes happens that the soil water contains 

 enough dissolved material to make it denser than the 

 liquid contents of the roots. In this case, in which direc- 

 tion would the greater flow of liquid be? 



Fields of cotton, com and other crops wilt during times 

 when the rainfall is light if too much of commercial fer- 

 tiUzers has been previously added to them. Why is this 

 true? 



Tell in your own words how roots gather moisture and 

 plant food from the soil. Also, tell under what conditions 

 roots may lose their moisture. 



66. Roots are Able to select the Minerals which they 

 need. — The roots of a given plant not only draw from 

 the soil their mineral plant food dissolved in water, but in 

 some way which we do not yet understand, they choose, 

 or select, those elements which they need, leaving others 

 unabsorbed. This is shown by the fact that the clover 

 plant when in flower and the barley plant in flower contain 

 about the same amount of mineral matter, yet the clover 

 contains almost six times as much lime as the barley, 

 while the barley contains about eighteen times as much 

 of the mineral of which white sand is composed as does the 

 clover. 



Grain crops remove large quantities of phosphorus 

 from the soil, while the root crops remove much potassium. 

 Timothy requires plenty of nitrogen. The best system 



