Microorganisms and Plant Life 185 



organisms. Some of these can be expected to affect root 

 development in appropriate concentrations. 



In some respects the root surface of the plant, the "rhizo- 

 plane" of Clark (5), resembles the digestive tract of ani- 

 mals. Both are the principal regions of absorption of 

 various substances from the external substrate; in both 

 cases there is extensive development of saprophytic bac- 

 teria, and in both cases organic materials affecting the 

 microorganisms are liberated from the absorbing system 

 into the region of microbial development. The micro- 

 organisms act on the substances in the external substrate, 

 decompose some of the compounds, produce others, and 

 affect the nutrient value of the substrate. 



Development of Microorganisms within Plant 

 Tissues 



Among the more intimate associations of microorgan- 

 isms and higher plants are symbiotic phenomena desig- 

 nated as "bacteriorrhizae," when bacteria are involved, 

 and "mycorrhizae," when filamentous fungi are concerned. 

 In both cases the microbial associate penetrates the plant 

 tissues, where it is supported in part or entirely by the 

 plant, and the plant generally benefits from the associative 

 development of the microorganism. 



BACTERIORRHIZAE 



The association between legumes and bacteria of the 

 genus Rhizobium is the best known and virtually the only 

 well defined example of bacteriorrhizae. The bacteria be- 

 come located within the plant in the tissues of the spe- 

 cialized root nodules, and they depend entirely on the con- 

 ditions of environment and food supply provided by the 

 plant. Once they have entered the plant, they are affected 

 by the soil conditions only indirectly, through the influ- 

 ences of these conditions on development of the plant. 



