The Concept of Khizospliere 



249 



other possible causes were eousidered, iiaineK, lowerinu; ol the con- 

 centration of certain mineral nutrients in* the soil due to absorption, 

 partial desiccation of the soil by absorption of water, increase in soil 

 carbonates following root excretion of carbon dioxide, contribution 

 of microbial foods by sloughed-off root portions or excretions. 

 Thom and Humfeld found that alfalfa, rye, and vetch stimulated 







Beans 



Cucumbers 



Tomatoes, 



vegetative 



growith 



Tomatoes, 

 fruit 



0.1 



0.02 0.04 0.06 0.08 



Per cent carbon dioxide in the atmosphere 



Fic. 99. Influence of concentration of carbon dioxide in the atmospliere on 

 growth of plants (from Lnndegardh). 



the bacteria more than the fungi, and the latter more than the actino- 

 mycetes. In other studies, the rhizosphere population of manured 

 soil was reported to be much greater than that of unmanured soil, 

 although the effect of manuring was much greater upon the non- 

 rhizosphere population. 



Garrett emphasized that it is necessary to distinguish between 

 rhizosphere effects characteristic of living roots and the increase in 

 the microflora of the root region in diseased roots and in healthy 

 but senescent roots; the latter phenomena are associated with the 

 initial stages of the microfloral succession occurring in moribund 

 and dead roots, whether a pathogen is the primary colonizer or not. 

 Thom and Humfeld observed a marked increase in numbers of root 



