226 INTRODUCTION TO GENERAL SCIENCE 



3. Farmers' Bulletin No. 222 : 7-9. Formulas for Fertilizers. 



4. Farmers' Bulletin No. 255 : 11-12. Fertilizers for the Garden. 



5. Farmers' Bulletin, No. 278 : 14. Crops to Follow Green 



Manure. 



163. PLANT ROOTS 



The use of roots is to hold the plant erect and to supply it 

 with plant food, which is dissolved in soil water. The process 

 by which plant food is absorbed, is called osmosis, and the 

 pressure, which is produced in the plant, is called osmotic 

 pressure. See Section 119, Osmosis. Roots grow down- 

 ward on account of gravity, but later they also grow in the 

 direction of moisture. The length of the root system of a 

 single plant may run into miles, but the fine root-hairs die, 

 only to be replaced, in a few days, by thousands of new ones. 



Roots excrete a material which is capable of dissolving some 

 of the insoluble plant food in the soil. The excretion may 

 become harmful to the plant, however, and this is one of the 

 arguments in favor of rotation of crops, since each plant pro- 

 duces a different excretion, which does not seem to harm 

 other plants. The effect of this excretion may be neutralized 

 by the addition of lime. See Section 121, Acids, Bases, and 

 Salts. 



References : 



1. 1304 : 40-41. The Work of Roots. 



2. 1407 : 19-33. Roots. 



3. 1503 : 82-97. Roots and their Work. 



4. 1605 : 64-65. How the Plant Gets its Food. 



5. 1702 : 242. Osmosis in Roots. 



6. 1702 : 312-313. Roots and Tubers as Food. 



7. Bureau of Soils, Bulletin No. 73 : 17-25. Oxidation and 



Reduction by Roots, 

 a. 1401:36-61. Roots. 



