THE SOIL AS A CULTURE MEDIUM 643 



tion is controlled by the nature of the soil and other environmental 

 conditions. The magnitude of the population is especially controlled 

 by the supply of available energy. It may be controlled qualitatively 

 by the nature of the available energy. 



Very little is known concerning the role of the rhizosphere, or the 

 subterraneous part of the plant system, in controlling this population. 

 The concept of this system and its possible influence upon the soil 

 population was first introduced by Hiltner. 76 It is known (p. 792) 

 that plants actually secrete carbohydrates and phosphatides. 77 The 

 sloughed off portions of the root caps, the root hairs, etc., may influence 

 greatly the nature of the population developing in their neighborhood. 

 The nature of the gases formed by the plants also influences the 

 nature of the organisms developing in the particular locality. A certain 

 soil decomposes cellulose with varying rapidity according to the 

 kind of plants which have been growing in it; also, the nature of the 

 organisms taking part in the decomposition of the cellulose varies with 

 the plants grown in the soil. 78 Further studies in this direction are 

 much needed. 



76 Hiltner, 1904 (p. 128). 



77 Hansteen Cranner, B. Zur Biochemie und Physiologie der Grenzschichten 

 lebender Pflanzenzellen. Meld. Norges. Landbruks., 2. 1922. 



78 Rokitzkaia, A. Aerobic fermentation of cellulose under the influence of soil- 

 microflora in the root-zone of plants. Jour. Sci. Inst. Amelior. Leningrad., 13: 

 168-208. 1926. 



